WO2008000069A1

WO2008000069A1 - Methods and systems for transmit diversity

Info

Publication number: WO2008000069A1
Application number: PCT/CA2007/001124
Authority: WO
Inventors: Aaron Callard; Jianglei Ma; Ming Jia; Hua Xu; Peiying Zhu; Wen Tong
Original assignee: Nortel Networks Limited
Priority date: 2006-06-26
Filing date: 2007-06-26
Publication date: 2008-01-03

Abstract

Systems and methods of applying transmit diversity to the synchronization channel (SCH) and broadcast channel (BCH) in OFDM systems are provided. Depending on a given implementation, various advantages may be realized such as enhanced synchronization performance, enhanced cell search performance, and improved P-BCH coverage. Supporting transmit diversity for the SCH further enables coherent cell searching, signalling a transmit antenna configuration, improving the unicast channel estimation for P-SCH and enabling frame synchronization if there are multiple P-SCHs in a given frame. Systems and methods of applying transmit diversity to the SCH using cyclic delay diversity are also provided.

Description

Methods and Systems for Transmit Diversity

Related Applications

This application claims the benefit of U.S.

Provisional Patent Application Nos. 60/805,815 filed on^' June 26, 2006, 60/870,987 filed on December 20, 2006 and 60/894,019 filed on March 9, 2007 , which are all hereby incorporated by reference in their entirety.

Field of the Invention

The invention relates to OFDM systems employing transmit diversity.

Background of the Invention

Orthogonal frequency division multiplexing (OFDM) is a form of multiplexing that distributes data over a number of carriers that have a very precise spacing in the frequency domain. The precise spacing of the carriers provides several benefits such as high spectral efficiency, resiliency to radio frequency interference and lower multi-path distortion. Due to its beneficial properties and superior performance in multi- path fading wireless channels, OFDM has been identified as a useful technique in the area of high data-rate wireless communication, for example wireless metropolitan area networks (MAN) . Wireless MAN are networks to be implemented over an air interface for fixed, portable, and mobile broadband access systems .

Communication between mobile terminals and base stations includes synchronization channels and other channels for broadcasting to all receivers within a cell. In some systems at least two synchronization channels are used. A first synchronization channel, also known as a primary synchronization channel (P-SCH) is used for timing synchronization for a frame structure which aids in locating a secondary synchronization channel (S-SCH) . A signal transmitted on the P-SCH is often the same for the base stations in each cell. The S-SCH includes additional information that is cell specific. For example it may include a cellID for identifying the cell.

A broadcast channel is defined in some systems to transmit static system information, for example the transmission bandwidth (BW) and antenna configuration of the base station.

Figure 1 is an example of a portion of a frame structure used for transmitting synchronization channel and broadcast channel (SCH and BCH) information for SISO (single input, single output) or SIMO (single input, multiple output) systems. In either type of system, there is only a single transmit antenna being used. The portion of the frame structure consists of a sub-frame 150 with seven orthogonal frequency division multiplexing (OFDM) symbols in a time dimension (vertical direction) and a set of sub-carriers in a frequency dimension (horizontal direction) . In a particular communication signalling protocol two such sub-frames form a "transmission time interval" (TTI) and 10 TTIs form a 10 ms radio frame.

In the example of Figure 1, the first four OFDM symbols are used for data and reference symbols (RS) . An example of an RS is a pilot symbol. The fifth OFDM symbol is used for a primary broadcast channel (P-BCH) and additional reference symbols. The sixth OFDM symbol is used for S-SCH. Finally, the seventh OFDM symbol is used for P-SCH, but only some of the sub-carriers are used, with the remaining sub- carriers unused. This structure is only used for sub- frames/TTIs that transmit SCH and BCH. According to an agreed working assumption in UMTS LTE (long term evolution) :

a) P-SCH (primary synchronization channel) and S-SCH (secondary synchronization channel) are transmitted in different OFDM symbols, i.e. using TDM (time division multiplexing); and

b) P-SCH and S-SCH are located in the same sub-frame in a first TTI and sixth TTI of a frame including 10 TTI, as described above .

In addition, a P-BCH is defined to transmit static system information, for example the transmission bandwidth (BW) and the antenna configuration.

In existing schemes, coherent cell searching is supported without the application of transmit diversity. Cell searching involves a mobile station searching for a cell and may include determining characteristics of the cell such as frame synchronization information of the cell. Transmit diversity involves using two or more antennas for transmission of the same transmit information. Transmit diversity is typically only applied for non-coherent cell searching. Also in existing systems, antenna configuration information is transmitted in the S-SCH, which introduces additional complexity to the cell search.

In existing schemes, multiple P-SCH sequences are used to improve the performance of unicast channel estimation required by the coherent detection of S-SCH. This results in an increase in complexity for initial access, and requires additional network planning.

In existing schemes, frame synchronization can be done together with timing synchronization, when there is only a single P-SCH in each frame. However, when there are more than one P-SCH in a frame, frame synchronization is required.

In existing methods of S-SCH sequence mapping the same S-SCH sequences are mapped such that more than one S-SCH is included in each frame. This results in poor inter-cell inference rejection.

Existing methods of frame synchronization are based on using the S-SCH. In one method, different cell specific sequences are used by different S-SCH symbols, which are located adjacent to P-SCH symbols. In another method, an additional signalling channel is used.

Summary of the Invention

According to a first aspect of the invention, there is provided a method of transmit diversity comprising transmitting a broadcast channel and at least one synchronization channel from a plurality of antennas such that channel information obtained from the at least one synchronization channel can be used to coherently detect the broadcast channel.

In some embodiments the method further comprises: for each of a plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that: one OFDM symbol duration carries a primary broadcast channel (P-BCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the antennas for the P-BCH/ one OFDM symbol duration carries a secondary synchronization channel (S-SCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the plurality of antennas for the S-SCH; one OFDM symbol duration carries a primary synchronization channel (P-SCH) for the plurality of antennas. In some embodiments the transmitting a set of OFDM symbols comprises transmitting a set of OFDM symbols with a repeating pattern.

In some embodiments the method further comprises inserting the P-SCH, S-SCH and P-BCH in positions such that channel information obtained by a receiver from the P-SCH can be used for coherent detection of the S-SCH and/or the channel information obtained by a receiver from the S-SCH can be used to assist the detection of the P-BCH.

In some embodiments the P-SCH and the S-SCH are transmitted on sequentially adjacent OFDM symbols and the S-SCH and P-BCH are transmitted on sequentially adjacent OFDM symbols.

In some embodiments the method comprises transmitting a frame structure comprising 10 transmission time intervals (TTIs), each TTI comprising two sub-frames, each sub-frame comprising seven OFDM symbols, wherein each of said sets of OFDM symbols occupies a respective sub-frame.

In some embodiments a sub-frame in the first TTI and the sixth TTI are used to transmit the P-BCH, S-SCH and P-SCH.

In some embodiments the plurality of antennas is a number of antennas equal to 2N, N=>1.

In some embodiments the method further comprises: for each of a first plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that: one OFDM symbol duration carries a primary broadcast channel (P-BCH) for a first subset of the plurality of antennas with each sub-carrier frequency being used on at least one of the antennas for the P-BCH; one OFDM symbol duration carries a secondary channel (S-SCH) for the first subset of antennas with each sub-carrier frequency being used on at least one of the antennas of the first subset for the S-SCH; one OFDM symbol duration carries a primary synchronization channel (P-SCH) for the first subset of the plurality of antennas; for each of a second plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that: one OFDM symbol duration carries the P-BCH for a second subset of antennas equal to the plurality of antennas minus the first subset of antennas, with each sub-carrier frequency being used on at least one of the antennas for the P-BCH; one OFDM symbol duration carries the S-SCH for the second subset of antennas with each sub-carrier frequency being used on at least one of the antennas of the second subset for the S-SCH; one OFDM symbol duration carries the P-SCH for the second subset of antennas .

In some embodiments for each of the first and second plurality of sets of OFDM symbol durations, transmitting a respective set of OFDM symbols comprises transmitting a respective set of OFDM symbols with a repeating pattern.

In some embodiments the method further comprises inserting the P-SCH, S-SCH and P-BCH in positions such that channel information obtained by a receiver from the P-SCH can be used for coherent detection of the S-SCH.

According to a second aspect of the invention, there is provided a method of transmit diversity comprising transmitting a first synchronization channel and a second synchronization channel from a plurality of antennas such that channel information obtained from the first synchronization channel can be used to coherently detect the second synchronization channel. In some embodiments the method comprises: transmitting the second synchronization channel in locations proximal the first synchronization channel and/or reference symbol locations; performing channel estimation on the first synchronization channel to generate channel estimates; performing coherent detection of the second synchronization channel using the channel estimates.

In some embodiments the method further comprises: for each of a plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that: one OFDM symbol duration carries a secondary synchronization channel (S-SCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the plurality of antennas for the S-SCH; one OFDM symbol duration carries a primary synchronization channel (P-SCH) for the plurality of antennas.

According to a ninth aspect of the invention, there is provided a performing channel estimation on the first synchronization channel to generate channel estimates further comprises using the ireference symbol locations in conjunction with the first synchronization channel to generate channel estimates .

According to a third aspect of the invention, there is provided a method operable to perform antenna structure and framing signalling using a primary synchronization channel (P- SCH) .

In some embodiments the method comprises : transmitting using one of a plurality of different antenna configurations, each antenna configuration having a different number of antennas; transmitting the P-SCH on a sub-set of an available set of sub-carriers on each of a plurality of OFDM symbols within a frame; signalling which antenna configuration is being used through selection of which sub-carriers to include in the sub-set.

In some embodiments transmitting and signalling comprises: transmitting the P-SCH on odd sub-carriers to indicate a first antenna configuration; transmitting the P-SCH on even sub-carriers to indicate a second antenna configuration .

According to a fourth aspect of the invention, there is provided a method comprising: a plurality of base stations transmitting synchronization information using a common P-SCH, with each base station using a respective set of sub-carriers with or without network planning.

According to a fifth aspect of the invention, there is provided a method operable to perform framing structure signalling through a primary synchronization channel (P-SCH) .

In some embodiments the method comprises : transmitting the P-SCH on a sub-set of an available set of sub- carriers on each of a plurality of OFDM symbols within a frame; signalling framing information through selection of which sub- carriers to include in the sub-set.

In some embodiments transmitting and signalling comprises: transmitting the P-SCH on a first OFDM symbol within a frame and a second OFDM symbol within the frame; transmitting the P-SCH on odd sub-carriers within the first OFDM symbol; transmitting the P-SCH on even sub-carriers within the second OFDM symbol.

According to a sixth aspect of the invention, there is provided a method using an antenna mapping scheme which allows the coherent detection of a secondary synchronization channel (S-SCH) . According to a seventh aspect of the invention, there is provided a transmitter comprising: a plurality of antennas; processing logic for inserting a broadcast channel (BCH) and at least one synchronization channel (SCH) into a plurality of sets of OFDM symbol durations such that: one OFDM symbol duration is used for reference symbols and a primary broadcast channel (P-BCH) for the plurality of antennas with each sub- carrier frequency being used on only one of the antennas for one of the reference symbols and the P-BCH; one OFDM symbol duration is used for a secondary synchronization channel (S- SCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the plurality of antennas for the S-SCH; one OFDM symbol duration is used for a primary synchronization channel (P-SCH) for the plurality of antennas; transmitting circuitry configured to transmit for each of the plurality of sets of OFDM symbol durations, a set of OFDM symbols from each of the plurality of antennas .

According to an eighth aspect of the invention, there is provided a method comprising: transmitting a first sequence of synchronization information from at least one antenna of a plurality of antennas on a first sub-carrier group in a first OFDM symbol; and transmitting a second sequence of synchronization information from at least one antenna of a plurality of antennas on said first sub-carrier group in a second OFDM symbol.

In some embodiments a mapping relation for cell related information is different for the first sequence of synchronization and the second sequence of synchronization.

According to a ninth aspect of the invention, there is provided a method comprising: transmitting a first sequence of synchronization information from at least one antenna of a plurality of antennas according to a first mapping scheme in a first OFDM symbol/ and transmitting a second sequence of synchronization information from at least one antenna of a plurality of antennas according to a first mapping scheme in a second OFDM symbol .

According to a tenth aspect of the invention, there is provided a method comprising: receiving a first sequence of synchronization information from a transmitter in a first OFDM symbol; receiving a second sequence of synchronization information from said transmitter in a second OFDM symbol;

⁾ remapping at least one of a first sequence index for said first sequence or a second sequence index for said second sequence such that said first and second sequence index can be combined for transmitter index identification purposes.

In some embodiments said remapping is based on a known permutation formula.

According to another aspect of the invention, there is provided a transmitter comprising: a plurality of antennas; processing logic configured to: insert a first sequence of synchronization information from a first antenna on a first sub-carrier group in a first OFDM symbol; insert a second sequence of synchronization information from a second antenna on said first sub-carrier group in a second OFDM symbol; transmitting circuitry configured to transmit the first and second OFDM symbols.

According to a further aspect of the invention, there is provided a receiver comprising: a plurality of antennas; receiving circuitry configured to: receive a first sequence of synchronization information from a transmitter in a first OFDM symbol; receive a second sequence of synchronization information from said transmitter in a second OFDM symbol; processing logic configured to: remap at least one of a first sequence index for said first sequence or a second sequence index for said second sequence such that said first and second sequence indices can be combined for transmitter index identification purposes.

According to yet another aspect of the invention, there is provided a method comprising: transmitting a synchronization sequence from multiple transmit antennas using cyclic delay diversity, with a respective cyclic delay for each antenna.

In some embodiments the respective cyclic delays being selected such that orthogonal synchronization sequences are created at a receiver receiving the transmitted synchronization sequence.

In some embodiments in a two transmit antenna system the cyclic delays are 0 and N/2, where N is the FFT size.

In some embodiments in a four transmit antenna system the cyclic delays are 0 N/4, and N/2 3N/4, where N is the FFT size .

In some embodiments the method further comprises using an orthogonal property of the synchronization sequences to estimate channel state information for data coherent detection purposes.

According to still another aspect of the invention, there is provided a method comprising: receiving a plurality of synchronization sequences, each received sequence with a respective cyclic delay; performing synchronization using a respective synchronization sequence of the plurality of synchronization sequences tuned to each of the respective cyclic delays.

In some embodiments a respective synchronization sequence comprises:

where p(k) is a known synchronization sequence, r is a delay, and N is a size of a Fast Fourier transform (FFT) .

In some embodiments, the method further comprises: using the received plurality of synchronization sequences to blindly detect a number of transmit antennas which transmitted the plurality of synchronization sequences.

In some embodiments using the received plurality of synchronization sequences to blindly detect a number of transmit antennas comprises: generating a respective tuned synchronization sequence for each possible cyclic delay; performing a respective correlation for each such synchronization sequence; determining the number of transit antennas according to the number of correlations that produce correlation peaks .

According to another aspect of the invention, there is provided a method comprising: transmitting a respective OFDM signal from each of a plurality of antennas, the OFDM signals collectively containing a CDD-based synchronization channel.

According to yet another aspect of the invention, there is provided a method comprising: receiving an OFDM signal containing a CDD-based synchronization channel on at least one receive antenna; performing synchronization using the CDD-based synchronization channel.

According to yet another aspect of the invention, there is provided a transmitter comprising: a plurality of antennas; processing logic configured to: generate a synchronization sequence for multiple transmit antennas using cyclic delay diversity, wherein the synchronization sequence allocated to each antenna has a respective cyclic delay; transmitting circuitry configured to transmit the synchronization sequence.

According to still another aspect of the invention, there is provided a receiver comprising: a plurality of antennas; receiving circuitry configured to receive a plurality of synchronization sequences; processing logic configured to: perform synchronization using a respective synchronization sequence of the plurality of synchronization sequences tuned to each of the respective cyclic delays.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. >

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the attached drawings in which:

Figure 1 is a schematic diagram of a portion of an example frame structure used for SISO (single input, single output) or SIMO (single input, multiple output) communications systems;

Figure 2 is a schematic diagram of a portion of an example frame structure for use with two transmit antennas according to an embodiment of the invention;

Figure 3 is a schematic diagram of a portion of a frame structure for use with four transmit antennas according to an embodiment of the invention; Figure 4 is a schematic diagram of a portion of a frame structure for use with four transmit antennas according to another embodiment of the invention;

Figure 5 is a schematic diagram of a portion of an example frame structure for use with four transmit antennas according to a further embodiment of the invention;

Figure 6A is a schematic diagram of a portion of an example frame structure illustrating how a sub-carrier pattern in the frequency domain can be used to determine a number of transmit antennas according to an embodiment of the invention;

Figure 6B is a schematic diagram of a portion of an example frame structure illustrating how a repetitive pattern in the time domain can be used to determine a number of transmit antennas according to an embodiment of the invention;

Figure 7 is a schematic diagram of a portion of an example frame structure for use with two or four transmit antennas according to an embodiment of the invention based on a frequency domain approach for providing transmit diversity;

Figure 8 is a schematic diagram of a portion of an example frame structure for use with two or four transmit antennas according to another embodiment of the invention based on a frequency domain approach for providing transmit diversity;

Figure 9 is a schematic diagram of a portion of an example frame structure for use with two or four transmit antennas according to an embodiment of the invention based on a time domain approach for providing transmit diversity;

Figure 10 is a block diagram of a cellular communication system; Figure 11 is a block diagram of an example base station that might be used to implement some embodiments of the present invention;

Figure 12 is a block diagram of an example wireless terminal that might be used to implement some embodiments of the present invention;

Figure 13 is a block diagram of a logical breakdown of an example OFDM transmitter architecture that might be used to implement some embodiments of the present invention;

Figure 14 is a block diagram of a logical breakdown of an example OFDM receiver architecture that might be used to implement some embodiments of the present invention;

Figure 15 is a block diagram of an example transmitter that might be used to implement some embodiments of the present invention; and

Figure 16 is a block diagram of an example receiver that might be used to implement some embodiments of the present invention.

Detailed Description of the Embodiments of the Invention

Methods and systems are described herein for configuring frame structures to enable transmit diversity for the P-SCH, S-SCH and P-BCH and to enable the coherent detection of S-SCH and P-BCH.

For example, one implementation may include transmitting a broadcast channel and at least one synchronization channel from a plurality of antennas such that channel information obtained from the at least one synchronization channel can be used to coherently detect the broadcast channel . Another implementation may include transmitting a first synchronization channel and a second synchronization channel from a plurality of antennas such that channel information obtained from the first synchronization channel can be used to coherently detect the second synchronization channel.

In some embodiments, the P-BCH is located in a same sub-frame that transmits the primary synchronization channel (P-SCH) and the secondary synchronization channel (S-SCH) . When this is the case, the channel information obtained from the S-SCH can be applied to aid in detection of the P-BCH. Reference symbols (RS) may also be used to assist the detection of the P-BCH during and subsequent to initial access, if a transmit antenna configuration of a base station is known.

In some embodiments, the P-SCH may be used as a phase reference to enable coherent detection of the S-SCH.

In some embodiments, the P-SCH may have a repetitive structure in the time domain. For example, the P-SCH may be repeated twice in separate sub-frames of a frame. Such a repetitive structure may allow detection of the P-SCH by methods such as auto-correlation based timing/frequency synchronization or a hybrid of auto-correlation and cross- correlation timing-frequency synchronization. A particular example with respect to the UMTS LTE frame mode is that the P- SCH occurs in the first TTI and the sixth TTI. In some embodiments, to simplify correlation based detection of the SCH, the P-SCH does not use a repetitive structure in the time domain so as to avoid multiple peaks that would be generated during the correlation based detection.

Embodiment-1: Transmit Diversity Scheme for SCH/BCH

Two Transmit Antennas A first embodiment supports transmit diversity for the P-SCH, S-SCH and P-BCH for up to two transmit antennas using frequency switched transmit diversity (FSTD) . This means that multiple transmit antennas transmit during the same OFDM symbol duration, but different sub-carrier frequencies are used by each antenna. In a two transmit antenna case, a first transmit antenna is TxI and a second transmit antenna is Tx2.

A specific example of a frame structure for this embodiment is shown in part in Figure 2. In the illustrated example, two particular portions of the frame structure are shown. Each portion consists of a sub-frame with seven OFDM symbols in the time dimension (vertical direction) and a set of 24 sub-carriers in the frequency dimension (horizontal direction) .

In a particular implementation, according to the agreed working assumption in UMTS LTE, two such sub-frames form a TTI, and 10 TTIs form a 10 ms radio frame. However, it is understood that more generally, the invention can be applied to various other forms of communications signalling beyond that defined for UMTS LTE.

In the example of Figure 2, a first single sub-frame is indicated at 210 and a second single sub-frame is indicated at 220. Each of the sub-frames includes antenna mappings for the P-BCH, S-SCH and P-SCH. In some embodiments, the respective sub-frames may be one of a pair of sub-frames that form a first TTI and a sixth TTI of a group of 10 TTI.

Figure 2 is a short hand way of presenting antenna mapping information for the frame structure for multiple antennas. While Figure 2 is specific to the example of two antennas, the manner of presentation is equally applicable to more than two antennas. As such, the same manner of presentation will be used in subsequent figures pertaining to four antennas. For data locations, which are indicated in Figure 2 with a "d", data can be included for each antenna. For the remaining locations (used to transmit RS, P-BCH, S-SCH, and P-SCH) , contents of the location that are shown are included for only the transmit antenna that will transmit information for that location, as indicated in the legend. The remaining transmit antennas do not transmit at that location.

In Figure 2, in sub-frame 210, the first four OFDM symbols are used for data and RS. The fifth OFDM symbol 212 is used for P-BCH and as well as RS. Different sub-carrier locations are used within the fifth OFDM symbol 212 for P-BCH on each antenna. For example, the first sub-carrier of the fifth OFDM symbol 212 in sub-frame 210 is allocated for a RS for Tx2. The second sub-carrier of the fifth OFDM symbol 212 is allocated for a P-BCH for TxI. The third sub-carrier of the fifth OFDM symbol 212 is allocated for a P-BCH for Tx2. The fourth sub-carrier of the fifth OFDM symbol 212 is allocated for an RS for TxI. This pattern is repeated for the remainder of the sub-carriers in the fifth OFDM symbol 212.

The sixth OFDM symbol 214 is used for S-SCH.

Different sub-carrier locations are used within the sixth OFDM symbol 214 for S-SCH for each antenna. For example, the first sub-carrier of the sixth OFDM symbol 214 in sub-frame 210 is allocated for a S-SCH for TxI. The second sub-carrier of the sixth OFDM symbol 214 is allocated for a S-SCH for Tx2. This pattern is repeated for the remainder of the sub-carriers in the sixth OFDM symbol 214.

Finally, the seventh OFDM symbol 216 is used for P- SCH, but only some of the sub-carriers are used, with the remaining sub-carriers unused. Furthermore, of the sub- carriers that are used for the P-SCH, different sub-carriers are used for each antenna. For example, the first sub-carrier of the seventh OFDM symbol 216 in sub-frame 210 is not used. The second sub-carrier of the seventh OFDM symbol 216 is allocated for a P-SCH for TxI. The third sub-carrier of the seventh OFDM symbol 216 is also not used. The fourth sub- carrier of the seventh OFDM symbol 216 is allocated for a P-SCH for Tx2. This pattern is repeated for the remainder of the sub- carriers in the seventh OFDM symbol 216.

Sub-frame 220 has a similar antenna mapping structure to sub-frame 210.

The number of OFDM symbols, number of sub-carriers, and particular mapping of sub-carriers to antenna and particular grouping of sub-frames to form a frame are all implementation specific factors that may vary from the example described in Figure 2. For example, while the order of the broadcast channel and synchronization channels is illustrated to be the P-BCH, S-SCH and P-SCH in the fifth, sixth and seventh OFDM symbols, other channel orderings are possible in which the P-BCH, S-SCH and P-SCH are otherwise arranged. In addition, while the broadcast channel and primary and secondary synchronization channels are illustrated to be only a single OFDM symbol each, it is to be understood that any or all of the channels could be multiple OFDM symbols in duration. In an implementation in which P-BCHs are included in multiple OFDM symbols in a frame, RS may or may not be included in one or more of the additional P-BCHs. In some embodiments where a P- BCH occupies a single OFDM symbol, it may occur that no RS are included in the P-BCH. Furthermore, while the above example suggests that the P-BCH, S-SCH and P-SCH are transmitted in the first and sixth TTIs, the vsub-frames which contain the P-BCH, S-SCH and P-SCH can be located elsewhere in a frame.

In some embodiments, the P-BCH, S-SCH and P-SCH are located in different respective locations in different frames. In some embodiments, the P-BCH, S-SCH and P-SCH are located in a same location in each frame of a sequence of frames, but the location of the P-BCH, S-SCH and P-SCH varies from one sequence to the next. In some embodiments, when sychronization channel information is transmitted multiple times per frame, each of the P-SCH, S-SCH and P-BCH may not be transmitted simultaneously during each of the multiple times per frame. For example, with reference to the LTE example structure described above, P-SCH and S-SCH may be transmitted in both the first and sixth TTIs, however P-BCH is only transmitted in the first TTI.

Four Transmit Antennas

Another embodiment of the invention supports up to four transmit antenna transmit diversity using FSTD and time switched transmit diversity (TSTD) for the P-SCH, S-SCH, and P- BCH. This means that different transmit times are used for each of multiple antennas, and different sub-carrier frequencies are used for each of the antennas .

In a specific example, a first transmit antenna (TxI) and a second transmit antenna (Tx2) transmit the P-BCH, S-SCH and P-SCH in a first sub-frame, in a manner similar to that described for the two transmit antenna example. A third transmit antenna (Tx3) and a fourth transmit antenna (Tx4) transmit the P-BCH, S-SCH and P-SCH in a subsequent sub-frame in a manner similar to that described for the two transmit antenna example. In some embodiments, the respective sub-frames may be one of a pair of sub-frames that form a first TTI and a sixth TTI of a group of 10 TTI.

The S-SCH in the sub-frame can be detected with the help of channel specific information. In some embodiments, for example during an initial cell search, the S-SCH can be detected with the aid of the P-SCH. This assumes that the S- SCH and the P-SCH locations in the sub-frame are close together so that channel estimates obtained from the P-SCH are relevant.

In some embodiments, the S-SCH can be detected with the aid of the RS.

In some embodiments, a single P-SCH sequence is used in multiple sub-frames of the frame. For example, in the case of a frame that includes 10 TTI, the same P-SCH sequence may be used in both the first TTI and the sixth TTI. This means that some other mechanism will need to be used to perform framing at the receiver, i.e. to detect which TTI is the first TTI and which is the sixth TTI. Various example mechanisms are provided below.

More generally, the location of the sub-frames that include the P-SCH, S-SCH and P-BCH is not limited to the first and sixth frames, but is implementation specific.

A receiver receiving information using the example frame structure does not need antenna configuration information to be able to receive the contents of the frame. When the antenna configuration is unknown separate channel estimators based on the P-SCH may be used for coherently determining information transmitted on the odd indexed S-SCH tones and even indexed S-SCH tones. Furthermore, two separate correlators may be used in the receiver.

A specific example of a frame structure for this embodiment is shown in Figure 3. The physical structure is similar to Figure 2, in terms of the 2 dimensional nature of the frame being composed of multiple OFDM symbols each using multiple sub-carriers. In the illustrated example, a first single sub-frame is indicated at 310 and a second single sub- frame is indicated at 320. In some embodiments, the respective sub-frames may be one of a pair of sub-frames that form a first TTI and a sixth TTI of a group of 10 TTI.

In sub-frame 310, the first four OFDM symbols are used for data and RS. Specifically, the first OFDM symbol 312 includes some data, and RS for all of the antennas. For example, an RS for Tx2 is located in the first sub-carrier of the first OFDM symbol 312, an RS for Tx3 is located in the first sub-carrier of the first OFDM symbol 312, an RS for Tx4 is located in the first sub-carrier of the first OFDM symbol 312 and an RS for TxI is located in the first sub-carrier of the first OFDM symbol 312.

The fifth OFDM symbol 314 is used for P-BCH and RS for TxI and Tx2. Different sub-carrier locations are used within the fifth OFDM symbol 314 for P-BCH on each antenna. The fifth OFDM symbol 314 for sub-frame 310 is the same as sub- frame 210 of Figure 2.

The sixth OFDM symbol 316 is used for S-SCH.

Different sub-carrier locations are used within the sixth OFDM symbol 316 for S-SCH for each antenna. The sixth OFDM symbol 316 for sub-frame 310 is the same as sub-frame 210 of Figure 2.

Finally, the seventh OFDM symbol 318 is used for P- SCH, but only some of the sub-carriers are used, with the remaining sub-carriers unused. Furthermore, of the sub- carriers that are used for the P-SCH, different sub-carriers are used for each antenna. The seventh OFDM symbol 318 for sub- frame 310 is the same as sub-frame 210 of Figure 2

Sub-frame 320 is similarly laid out to sub-frame 310, except that the P-BCH, S-SCH and P-SCH are allocated for transmit antennas Tx3 and Tx4. For example, in the fifth OFDM symbol 324, an RS for Tx2 is located in the first sub-carrier of the fifth OFDM symbol. The second sub-carrier of the fifth OFDM symbol 324 is allocated for a P-BCH for Tx3. The third sub-carrier of the fifth OFDM symbol 324 is allocated for a P- BCH for Tx4. The fourth sub-carrier of the fifth OFDM symbol 324 is allocated for an RS for TxI. This pattern is repeated for the remainder of the sub-carriers in the fifth OFDM symbol 324.

The first sub-carrier of the sixth OFDM symbol 326 in sub-frame 320 is allocated for a S-SCH for Tx3. The second sub- carrier of the sixth OFDM symbol 326 is allocated for a S-SCH for Tx4. This pattern is repeated for the remainder of the sub- carriers in the sixth OFDM symbol 326.

The first sub-carrier of the seventh OFDM symbol 328 in sub-frame 320 is not used. The second sub-carrier of the seventh OFDM symbol 328 is allocated for a P-SCH for Tx3. The third sub-carrier of the seventh OFDM symbol 328 is also not used. The fourth sub-carrier of the seventh OFDM symbol 328 is allocated for a P-SCH for Tx4. This pattern is repeated for the remainder of the sub-carriers in the seventh OFDM symbol 328.

In some embodiments, all of the sub-carriers of the seventh OFDM symbol can be occupied by the P-SCH.

A method of transmit diversity for a broadcast channel (BCH) and at least one synchronization channel (SCH) according to aspects of the invention can be described as for each of a plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of a plurality of antennas such that: one OFDM symbol duration carries a primary broadcast channel (P-BCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the antennas for the P-BCH; one OFDM symbol duration carries a secondary synchronization channel (S-SCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the plurality of antennas for the S-SCH; one OFDM symbol carries a primary synchronization channel (P-SCH) for the plurality of antennas.

Embodiment-2 : Transmit Diversity Scheme for SCH/BCH

With this embodiment, a four transmit antenna FSTD scheme is used for the four transmit antenna case. This provides additional transmit diversity gain, as for any given P-BCH transmission all four antennas are used, while only two antennas are used in any one transmission. The frame structure is similar to the frame structure described above in Figures 2 and 3, but the antenna mapping scheme is different. In some embodiments, antenna configuration information is not required for non-coherent cell search since no channel estimation is required during the search.

Coherent cell ID detection (combining S-SCH information from multiple frames or multiple instances in the same frame in a coherent manner) can be used to improve the cell search performance.

Option-1: In some implementations, the channel information is obtained from P-SCH without prior knowledge of antenna configuration. The FSTD diversity scheme for four transmit antenna transmit diversity described above may be used for the P-SCH. Also, four separate channel estimators and correlators may be used in a mobile station. In some embodiments, by using four correlators to perform cell searching, the mobile station does not need transmit antenna configuration information, even if it is inherent to the signal.

A specific example of a frame structure for this embodiment is shown in Figure 4. The physical structure is similar to Figure 2, in terms of the 2 dimensional nature of the frame being composed of multiple OFDM symbols each using multiple sub-carriers. In the illustrated example, a single sub-frame is indicated at 410. In some embodiments, the sub- frame may be one of a pair of sub-frames that form a first TTI and a sixth TTI of a group of 10 TTI.

In sub-frame 410, the first four OFDM symbols are used for data and RS. The first OFDM symbol 412 for example, includes some data, and RS for all of the antennas, in a similar manner to sub-frames 310 and 320 of Figure 3.

The fifth OFDM symbol 414 is used for P-BCH and RS for the first, second, third and fourth antennas (TxI, Tx2, Tx3 and Tx4) . Different sub-carrier locations are used within the fifth OFDM symbol 414 for P-BCH on each of four antennas. For example, an RS for Tx2 is located in the first sub-carrier of the fifth OFDM symbol 414. The second sub-carrier of the fifth OFDM symbol 414 in sub-frame 410 is allocated for a P-BCH for TxI. The third sub-carrier of the fifth OFDM symbol 414 is allocated for a P-BCH for Tx2. The fourth sub-carrier of the fifth OFDM symbol 414 is allocated for an RS for TxI. The fifth sub-carrier of the fifth OFDM symbol 414 is allocated for a P- BCH for Tx3. The sixth sub-carrier of the fifth OFDM symbol 414 is allocated for a P-BCH for Tx4. This pattern is repeated for the remainder of the sub-carriers in the fifth OFDM symbol 414.

The sixth OFDM symbol 416 is used for S-SCH.

Different sub-carrier locations are used within the sixth OFDM symbol 416 for S-SCH for each of the four antennas. The first sub-carrier of the sixth OFDM symbol 416 in sub-frame 410 is allocated for a S-SCH for TxI. The second sub-carrier of the sixth OFDM symbol 416 is allocated for a S-SCH for Tx2. The third sub-carrier of the sixth OFDM symbol 416 is allocated for a S-SCH for Tx3. The fourth sub-carrier of the sixth OFDM symbol 416 is allocated for a S-SCH for Tx4. This pattern is repeated for the remainder of the sub-carriers in the sixth OFDM symbol 416.

Finally, the seventh OFDM symbol 418 is used for P- SCH, but only some of the sub-carriers are used, with the remaining sub-carriers unused. Furthermore, of the sub- carriers that are used for the P-SCH, different sub-carriers are used for each of the four antennas. The first sub-carrier of the seventh OFDM symbol 418 in sub-frame 410 is not used. The second sub-carrier of the seventh OFDM symbol 418 is allocated for a P-SCH for TxI. The third sub-carrier of the seventh OFDM symbol 418 is not used. The fourth sub-carrier of the seventh OFDM symbol 418 is allocated for a P-SCH for Tx2. The fifth sub-carrier of the seventh OFDM symbol 418 is not used. The sixth sub-carrier of the seventh OFDM symbol 418 is allocated for a P-SCH for Tx3. The seventh sub-carrier of the seventh OFDM symbol 418 is not used. The eighth sub-carrier of the seventh OFDM symbol 418 is allocated for a P-SCH for Tx4. This pattern is repeated for the remainder of the sub-carriers in the seventh OFDM symbol 418.

Option-2 : In some implementations, channel information is obtained from the P-SCH and RS. In this case, antenna configuration information is used to enable a coherent cell search. In some embodiments, this information can be obtained from the P-SCH, assuming that only up to two transmit antenna FSTD is applied for the P-SCH.

In some embodiments, improved channel estimation performance is possible because the P-SCH can be used as RS. Therefore, the P-SCH or a combination of actual RS and the P- SCH being used as RS results in a higher RS density for the purposes of channel estimation.

In some embodiments, channel estimations for TxI and Tx2 are based on RS located in the OFDM symbol with the P-BCH, while the channel estimations for Tx3 and Tx4 are obtained from the P-SCH.

A specific example of a frame structure for this embodiment is shown in part in Figure 5, and is indicated by sub-frame 510. The physical structure is similar to Figure 2, in terms of the 2 dimensional nature of the frame being composed of multiple OFDM symbols each using multiple sub- carriers .

The first six rows of sub-frame 510 are the same as sub-frame 410 of Figure 4. In the seventh OFDM symbol 512, which is used for the P-SCH, only some of the sub-carriers are used, with the remaining sub-carriers unused. Furthermore, only two of the four antennas are allocated sub-carriers for the P-SCH. Only the antennas which are not transmitting the RS in the fifth OFDM symbol, which includes the P-BCH, are used to transmit the P-SCH. In the illustrated example these are antennas Tx3 and Tx4. The remaining antennas, in this example antennas TxI and Tx2, do not transmit the P-SCH.

The same approach can be used for a two transmitter case. In such a case, sub-carriers allocated to Tx3 and Tx4 are replaced with TxI and Tx2, respectively. Such an approach may result in an improved result, for example, by allowing better interpolation from additional channel estimates .

A method of transmit diversity for a broadcast channel (BCH) and at least one synchronization channel (SCH) , according to aspects of the invention can be described as for each of a first plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of a plurality of antennas such that: one OFDM symbol duration transmits reference symbols for each of the plurality of antennas and a primary broadcast channel (P-BCH) for a first subset of the plurality of antennas with each sub-carrier frequency being used on only one of the antennas of the first subset of antennas for one of the reference symbols and at least one of the antennas for the P-BCH; one OFDM symbol duration is used for a secondary channel (S-SCH) for the first subset of antennas with each sub-carrier frequency being used on at least one of the antennas of the first subset for the S-SCH; one OFDM symbol is used for a primary synchronization channel (P-SCH) for the first subset of the plurality of antennas. For each of a second plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of a plurality of antennas such that: one OFDM symbol duration is used for reference symbols for each of the plurality of antennas and the P-BCH for a second subset of antennas equal to the plurality of antennas minus the first subset of antennas, with each sub- carrier frequency being used on only one of the antennas of the second subset for one of reference symbols and at least one of the antennas for the P-BCH; one OFDM symbol duration is used for the S-SCH for the second subset of antennas with each sub- carrier frequency being used on at least one of the antennas of the second subset for the S-SCH; one OFDM symbol is used for the P-SCH for the second subset of antennas .

Embodiment-3 : Antenna Configuration Signalled by P-SCH

In accordance with a further aspect of the invention, two methods of signalling antenna configuration information using the P-SCH will now be described.

A first scheme, Scheme-1, is based on the transmission of additional P-SCH cell common sequences. A different sequence can be associated with each antenna configuration. However, this increases initial access complexity.

A second scheme, Scheme-2, is based on P-SCH sub- carrier locations. To allow fast, coarse timing/frequency synchronization, a repetitive time domain P-SCH structure can be used. In some embodiments, only half of the assigned sub- carriers in the P-SCH are modulated. The location of modulated sub-carriers can be used to signal the number of transmit antennas. For example, in some embodiments if the locations of the modulated sub-carriers are odd indexed sub-carriers, then there are one or two transmit antennas. If the locations of the sub-carriers are even indexed sub-carriers, then there are four transmit antennas. In other embodiments, if the locations of the modulated sub-carriers are odd indexed sub-carriers, then there is a single one transmit antenna. If the locations of the sub-carriers are even indexed sub-carriers, then there are more than one transmit antenna.

In some embodiments, blind detection is performed to determine the number of transmit antennas. Blind detection can be performed in both the frequency domain and in time domain.

For example, Figure βA illustrates the example described above in which different locations of occupied sub- carriers are used to indicate the number of transmit antennas. In Figure βA, a P-SCH is represented by a single OFDM symbol 610, 620 which uses multiple sub-carriers for transmission. In OFDM symbol 610 the even indexed sub-carriers 612 are modulated indicating there are one or two transmit antennas. In OFDM symbol 620 the odd indexed sub-carriers 614 are modulated indicating there are four transmit antennas .

When using blind detection in the time domain, the phase value between two repeated portions of the frame can be used to indicate the number of transmit antennas. In Figure 6B, frame portion 630, sub-portions 632,634 of the frame portion 630 have a different phase values indicating that there are one or two transmit antennas. In frame portion 640, sub-portions 642,644 of the frame portion 640 have the same phase values indicating that there are four transmit antennas .

Embodiment-4 : Unicast Channel Estimation Performance Improvement Based on Broadcast Channel

In the scenario where no antenna configuration information is carried by the location of the modulated sub- carriers in the P-SCH, each cell can randomly select the location of the modulated sub-carriers . Allowing each cell to randomly select the location of the modulated sub-carriers for the cell results in an improved channel estimation from the broadcast P-SCH channel for the decoding of S-SCH. The randomization may result in only half of the cells in a given geographical area transmitting the same P-SCH sequence on the same sub-carriers. Therefore, the observed combined channel from multiple cells is closer to the channel of the S-SCH that transmits the cell specific sequence. Furthermore, there is no need to introduce another P-SCH sequence. This can be done with or without network planning.

Embodiment-5 : Framing Signalling by P-SCH

In another embodiment of the invention, the location of the modulated sub-carriers in the P-SCH can also be used to signal the frame boundary.

A P-SCH in a first OFDM symbol of a first sub-frame occupies a different respective set of sub-carriers than does a P-SCH of a second OFDM symbol in a second-sub-frame. Therefore, the set of sub-carriers used in the first sub-frame, as opposed to the set of sub-carriers used in the second sub- frame, can be used to distinguish between the two sub-frames, notwithstanding the fact that the same P-SCH sequence may be used in each sub-frame. For example, a P-SCH in an OFDM symbol in a first TTI of a set of 10 TTI occupies a set of sub-carriers including odd indexed sub-carriers and a P-SCH sequence in an OFDM symbol in a sixth TTI of the set of 10 TTI occupies a set of sub-carriers including even indexed sub-carriers .

For this case, no additional framing signalling is required during the cell search. In some embodiments, this reduces the cell search complexity.

An example of a transmitter that could be used to implement methods described above will be described with regard to Figure 15. Figure 15 illustrates a transmitter 1500 that includes a plurality of antennas 1510, processing logic 1520 and transmitting circuitry 1530.

The processing logic 1520 for example may be used to insert one of more of the following: a broadcast channel (BCH) ; at least one synchronization channel (SCH) ; data and reference symbols such as pilots into a plurality of sets of OFDM symbol durations .

In some embodiments, one OFDM symbol duration is used for reference symbols and a primary broadcast channel (P-BCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the antennas for one of the reference symbols and the P-BCH.

In some embodiments, one OFDM symbol duration is used for a secondary synchronization channel (S-SCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the plurality of antennas for the S-SCH.

In some embodiments, one OFDM symbol duration is used for a primary synchronization channel (P-SCH) for the plurality of antennas. The transmitting circuitry 1530 is configured to transmit for each of the plurality of sets of OFDM symbol durations, a set of OFDM symbols from each of the plurality of antennas 1510.

In some embodiments, the processing logic 1520 is implemented by computer readable programmable code instructions stored on a computer readable medium in the transmitter 1500. As such, the processing logic 1520 in Figure 15 is one or more computer algorithm for implementing the functionality of the processing logic 1520. However, other implementations are possible. For example, the functionality of the processing logic 1520 may be implemented as software, hardware, firmware, or as any appropriate combination thereof.

The examples above have assumed a specific sub-frame structure with 7 OFDM symbols. More generally, the concepts described can be applied to sub-frames with arbitrary size. In addition, the sub-framing approach used in the detailed examples (two sub-frames per TTI, 10 TTIs per radio frame) is also just one approach, and an arbitrary number of sub-frames (from one to some number N) can be grouped together and considered a radio frame, optionally with further combination of some number of sub-frames into TTIs.

Throughout this description, P-SCH, S-SCH, and P-BCH are referred to. More generally, the embodiments are applicable to systems in which there is a first synchronization sequence that is common to multiple transmitters (such as is the case with the P-SCH) and a second transmitter specific synchronization sequence (such as is the case with the S-SCH) . The P-BCH is a specific example of the more general concept of broadcast information transmitted from all of the antennas. Examples of MIMO OFDM systems within which these methods can be implemented are provided in co-pending PCT application nos. PCT/CA2005/000506 filed April 5, 2005, and PCT/CA2005/000387 filed March 15, 2005 both hereby incorporated herein by reference in their entirety. More generally, multiple antenna OFDM transmitter can be configured to implement these methods.

In accordance with another embodiment of the invention, Figure 7 illustrates portions of two separate sub- frames 710,720 of a frame structure. The two separate sub- frames illustrate an antenna mapping for P-SCH and S-SCH where sub-carriers used for the S-SCH associated with respective antennas switch sub-carrier locations in the two sub-frames .

In the illustrated example of Figure 7, each sub- frame 710,720 includes two consecutive ODFM symbols in the time dimension (vertical direction) and a set of sub-carriers in the frequency dimension (horizontal direction) . Each sub-frame includes other OFDM symbols which may be allocated for data, a broadcast channel and reference symbols, such as pilot symbols.

In each sub-frame 710,720 first and second OFDM symbols are used for a S-SCH 712,722 and a P-SCH 714,724, respectively. In some embodiments, these two OFDM symbols in each sub-frame may correspond to the sixth and seventh OFDM symbols of a seven OFDM symbol sub-frame, as described above in relation to Figures 1 to 5. More generally, the OFDM symbols could be any two adjacent or near adjacent OFDM symbols in a sub-frame having any given number of OFDM symbols.

For a two transmit antenna case, in the first OFDM symbol 712 of the first sub-frame portion 710, a first sub- carrier is allocated for a S-SCH for TxI. A second sub-carrier of the first OFDM symbol 712 is allocated for a S-SCH for Tx2. This pattern is repeated for the remainder of the sub-carriers in the first OFDM symbol.

For the two transmit antenna case, in the second OFDM symbol 714 of the first sub-frame portion 710, a first sub- carrier is allocated for a P-SCH for TxI. A second sub-carrier of the second OFDM symbol 714 is allocated for a P-SCH for Tx2. This pattern is repeated for the remainder of the sub-carriers in the second OFDM symbol.

For the two transmit antenna case, in the first OFDM symbol 722 of the second sub-frame portion 720, a first sub- carrier is allocated for a S-SCH for Tx2. A second sub-carrier of the first OFDM symbol 722 is allocated for a S-SCH for TxI. This pattern is repeated for the remainder of the sub-carriers in the first OFDM symbol.

For the two transmit antenna case, in the second OFDM symbol 724 of the second sub-frame portion 720, a first sub- carrier is allocated for a P-SCH for Tx2. A second sub-carrier of the second OFDM symbol 724 is allocated for a P-SCH for TxI. This pattern is repeated for the remainder of the sub-carriers in the second OFDM symbol .

The same mapping pattern can be used with four transmit antennas by replacing locations in the above example which are allocated for TxI with allocations for, for example, TxI and Tx3 and replacing locations in the above example which are allocated for Tx2 with allocations for, for example Tx2 and Tx4.

Even though the SCH sub-carriers are shown alternating for the respective transmit antennas on a single consecutive sub-carrier basis, this need not be the case. For example, for a two transmit antenna case, a repeating pattern of at least two adjacent sub-carriers allocated to a first antenna followed by at least two adjacent sub-carriers allocated to a second antenna could work as well. Furthermore, there need not be an equal number of sub-carriers associated with each antenna.

As described above, when applying FSTD to the SCH, sub-carriers are allocated to particular transmit antennas. According to an embodiment of the invention, a single long sequence of synchronization information to be transmitted may be inserted into a frame where, in a two transmit antenna case, each transmit antenna transmits half of the sequence. For example, a single long sequence may be represented by Sl, S2, S3, S4,... where a first transmit antenna TxI transmits Sl, S3,... and a second transmit antenna Tx2 transmits S2, S4,....

More generally, each transmit antenna transmits a portion of the overall single long sequence. In some embodiments, for N transmit antenna, each transmit antenna transmits a 1/N of the long sequence.

According to another embodiment of the invention, two or more short sequences may be used for providing of synchronization information, in which each transmit antenna transmits a respective short sequence. For example, in a two transmit antenna case, a sequence Sl, Sl*, S2, S2*,... includes a first short sequence Sl, S2... and a second short sequence Sl*, S2*,.... A first transmit antenna TxI transmits the first short sequence Sl, S2,... and a second antenna Tx2 transmits the second short sequence Sl*, S2*,.... The short sequences may be equal in length or the short sequences may vary in length for transmission by respective antennas.

In some embodiments, the long and/or short sequences are orthogonal sequences of pseudo noise (PN) carried by the P- SCH and the S-SCH. In accordance with an embodiment of the invention, Figure 8 shows portions of two separate sub-frames 810,820 of a frame structure. The sub-frames 810,820 are similar to Figure 7 except that 2x repetition is used on the P-SCH. The use of 2x repetition means that every second sub-carrier is used for the P-SCH. For example, for a two antenna case, in the second OFDM symbol 814 of the first sub-frame portion 810, a first sub- carrier is not used. A second sub-carrier of the second OFDM symbol 814 is allocated for a P-SCH for Tx2. A third sub- carrier of the second OFDM symbol 814 is not used. A fourth sub-carrier of the second OFDM symbol 814 is allocated for a P- SCH for Tx2. This pattern is repeated for the remainder of the sub-carriers in the second OFDM symbol. In the second OFDM symbol 824 of the second sub-frame portion 820, the same pattern is used except locations for P-SCH for TxI and Tx2 are reversed.

In accordance with an embodiment of the invention, Figure 9 shows a time domain approach to providing diversity similar to the frequency domain approach set out in Figure 7.

In Figures 7, 8 and 9, the number of OFDM symbols, number of sub-carriers, and particular mapping of sub-carriers to antenna are all implementation specific factors that may vary from the example described in Figures 7, 8 and 9. Furthermore, while the above examples suggest that the S-SCH and P-SCH are transmitted in the first and sixth TTIs, the sub- frames which contain the S-SCH and P-SCH can be located elsewhere in a frame.

In some embodiments of the invention, a P-SCH detection scheme may utilize an auto-correlation based approach, cross-correlation based approach or a hybrid approach .

An auto-correlation based approach can be supported for one or more transmit antenna (s).

A cross-correlation based approach can be performed in either the time-domain or the frequency-domain. Improved detection performance may be obtained if two or more consecutive P-SCH transmissions are combined. The P-SCH will in general be transmitted periodically, therefore the mobile station can continue to combine consecutive transmissions until correct reception is obtained.

In some embodiments, when implementation occurs in MIMO systems, the impact of multiple peaks during correlation based detection of the SCH is reduced. By combining multiple transmissions with the FSTD/TSTD method proposed above, improved performance may be achieved for cases where the channel characteristics vary slowly because the combined P-SCH signal does not have repetition structure and therefore there is no multiple peak issue when detecting P-SCH by correlation.

In the hybrid approach, auto-correlation is performed for a single time domain P-SCH sequence for a given OFDM symbol and cross-correlation is performed for combined P-SCH transmissions from two adjacent P-SCH symbols.

In some embodiments beamforming gain can be exploited by applying different weighting factors when combining multiple P-SCHs. In some embodiments, time domain interpolation of P- SCH symbols with the same mapping sequence can be employed so that signals combined are considered to be from the same channel. In this way, cell search performance can be improved.

In some embodiments, P-SCH search performance can be improved by blind detection of the antenna configuration. Blind detection can be performed based on correlation of two received short sequences of the P-SCH to determine the number of transmit antennas. For example, correlating the two received short sequences determines if the system is a single input multiple output (SIMO) or a multiple input multiple output (MIMO) system. If it is determined that the system is a SIMO system, the two correlation values obtained from the two short sequences can then be combined coherently.

S-SCH detection can be performed in the time domain or in the frequency domain;

In some embodiments, for initial cell search, the cell search performance is improved by combining multiple S- SCHs where beamforming gain can be exploited with different weighting factors. In some embodiments, to make selection of neighbouring cells more reliable, no beamforming operation is applied during cell search of neighbouring cells .

In some embodiments, improved S-SCH search performance can be obtained by blind detection of the antenna configuration. Blind detection can be performed based on the correlation of two received short sequences, for either the P- SCH or S-SCH to determine the number of transmit antennas. If it is determined that the system is a SIMO system, the two correlation values obtained from the two short sequences can then be combined coherently. - • - U C U U

In accordance with an embodiment of the invention, to further improve the initial cell search and neighbour cell search performance, a pre-defined permutation can be used to change a mapping relation for S-SCH information transmitted in different sub-frames of the same frame or different frames cyclically, on a per cell basis. An example of a pre-defined permutation may be the inverse of the original mapping Therefore, S-SCH information in a first sub-frame of the frame is related to S-SCH information in a second sub-frame of the frame by the pre-defined permutation.

As opposed to transmitting SCH information, which for example includes the cellID of the cell, in two sequences during a first sub-frame and the same two sequences during a second sub-frame, the pre-defined permutation is used to change the mapping relation of the SCH information in the second sub- frame .

In a synchronous environment, a mobile terminal often receives signals from multiple base stations simultaneously. In some implementations, these base stations transmit the same SCH every half frame and therefore interference resulting from all of the base stations every half frame is identical. According to some embodiments of the invention, since each cell has a different permutation, the mapping relation for different sub-frames in different cells results in a reduced probability that interference from neighbouring cells will interfere coherently.

According to an implementation of the invention based on the UMTS LTE frame model, the P-SCH and S-SCH are transmitted twice every 10ms frame, that is every 5ms. A first P-SCH/S-SCH transmission occurs during a first sub-frame and a second P-SCH/S-SCH transmission occurs during a second sub- frame. The mapping relation is permuted between the first and second sub-frames based on a cell specific pre-defined permutation.

In some embodiments of the invention, transmit diversity schemes described herein are useful for the P-SCH for one or more of the following reasons. The transmit diversity- schemes may provide a transmit diversity gain, may allow the flexible deployment of different timing/frequency synchronization algorithms, may reduce the impact of multiple peaks caused by correlation based detection of the SCH information to the cell search performance resulting from frequency switched transmit antenna mapping, and may optimize timing synchronization performance for both single transmit antennas and multiple transmit antennas .

In some embodiments, a method for implementing the invention includes a first step of transmitting a first sequence of synchronization information from at least one antenna of a plurality of antennas on a first sub-carrier group in a first OFDM symbol. A further step includes transmitting a second sequence of synchronization information from at least one antenna of a plurality of antennas on said first sub- carrier group in a second OFDM symbol.

In some embodiments, the at least one antenna is the same antenna for both the first and second sequences.

In some embodiments, a mapping relation for cell related information is different for the first sequence of synchronization and the second sequence of synchronization.

In some embodiments, another method for implementing the invention includes a first step of transmitting a first sequence of synchronization information from at least one antenna of a plurality of antennas according to a first mapping scheme in a first OFDM symbol. A second step in this method includes transmitting a second sequence of synchronization information from at least one antenna of a plurality of antennas according to a first mapping scheme in a second OFDM symbol .

At a receiver that receives the sequences of synchronization information a method may include a first step of receiving a first sequence of synchronization information from a transmitter in a first OFDM symbol. A further step includes receiving a second sequence of synchronization information from said transmitter in a second OFDM symbol. A next step includes remapping at least one of a first sequence index for said first sequence or a second sequence index for said second sequence such that said first and second sequence index can be combined for transmitter index identification purposes .

In some embodiments, the remapping is based on a known permutation formula.

In a particular example, a receiver receives signals from two cells, Cell 1 and Cell 2. In a first sub-frame, the base stations of both cells transmit short sequences (si, S₂) and (si, s₇) , respectively. Table 1 below illustrates the contents of the first sub-frame and a second sub-frame based on four different approaches to generating the contents of the second sub-frame. Two approaches do not use remapping and two approaches do use remapping, in which each cell use a predefined permutation which is different from the other cell. The table also indicates interference between signals received from the base stations of the two cells in the form of "Number of collisions of short sequences". For example, for "Repetition with Switch", Si would be received in both first and second sub- frames in the same location resulting in two collisions. However, in "Remapping", si would be received in only the first sub-frame in the same location resulting in only one collision.

Table 1

As shown in Table 1, with an appropriate remapping of the two short sequences in one of the two sub-frames the maximum number of collisions between the base stations in Cell 1 and Cell 2. However if the same S-SCH structure or BPSK modulation is applied to the first and second sub-frames, the number of collisions can be equal to two.

Implementation examples

Three specific examples of mapping schemes which reduce the number of collisions will now be described. For the three examples, there are N hypotheses, with N equal to 170, 340 or 510 depending on the final decision of how much antenna information is transmitted. Within a frame four short codes are transmitted (s_a, Sj₃) in a first sub-frame and (s_c, sa) in the second sub-frame.

A first example of a remapping scheme does not use any specific formula, but can be found by iterative search. This example would require a lookup table.

A second example of a remapping scheme is based on a formula and was designed specifically for the N=340 case. The variable Groupχ_D is represented by a number from 0-170, and TxNum is an element of the set {0,1}. The formula is

b = m.od(2i+a,M) c =mod(-i,M) d =mod(a+b-mod(c+1,2),M)

A third method for remapping involves encoding the sequence numbers (a,b) by a rate 1/2 Reed Solomon code to produce (c,d) . By the properties of the Reed Solomon encoder the distance between the various sequences is maximized. Thus the chosen sequence is

ό =mod(z,M) (c,d) = rsenc(a,b)

Where "rsenc" is a function which represents a [4,2] Reed Solomon encoder based on a Galois field of size 32 (i.e. integers 1 to 32) . However, this encoder implicitly assumes that all sequences of (a,b) are viable inputs and thus for small values of N, it has marginally worse performance than the lookup table method. In some embodiments of the invention, transmit diversity schemes described herein may be useful for the S-SCH for the following reasons. The transmit diversity schemes may optimize cell search performance for both single transmit antennas and multiple transmit antennas, may improve both the initial cell search performance and neighbour cell search performance, and may reduce a false alarm rate during the neighbour cell search caused by coherent interference of S-SCH information from neighbouring cells.

The transmitter of Figure 15 could also be used to implement additional embodiments of the invention. For example, the processing logic 1520 may be used for inserting a synchronization channel from the plurality of antennas 1510 in a first OFDM symbol and a second OFDM symbol on a first sub- carrier group and a second sub-carrier group and transmitting circuitry 1530 may be configured for transmitting the respective OFDM symbols on the first and second sub-carrier groups .

In some embodiments, the processing logic 1520 inserts a first sequence of synchronization information from a first antenna on a first sub-carrier group in a first OFDM symbol and inserts a second sequence of synchronization information from a second antenna on said first sub-carrier group in a second OFDM symbol.

The transmitting circuitry 1530 is configured to transmit the first and second OFDM symbols.

An example of a receiver that could be used to implement methods described above will be described with regard to Figure 16. Figure 16 illustrates a receiver 1600 including: a plurality of antennas 1610; receiving circuitry 1620 configured to receive synchronization information and processing logic 1630 for processing the received synchronization information.

In some embodiments, the receiving circuitry 1620 receives a first sequence of synchronization information from a transmitter in a first OFDM symbol and receives a second sequence of synchronization information from said transmitter in a second OFDM symbol.

In some embodiments, the processing logic 1630 is configured to remap at least one of a first sequence index for said first sequence or a second sequence index for said second sequence such that said first and second sequence indices can be combined for transmitter index identification purposes.

In some embodiments, the processing logic 1630 is implemented by computer readable programmable code instructions stored on a computer readable medium in the receiver 1600. As such, the processing logic 1630 in Figure 16 is one or more computer algorithm for implementing the functionality of the processing logic 1630. However, other implementations are possible. For example, the functionality of the processing logic 1630 may be implemented as software, hardware, firmware, or as any appropriate combination thereof.

Cyclic delay diversity (CDD) as a transmit diversity approach is becoming a serious contender to other conventional choices, such as STTD, in UMTS Long Term Evolution (LTE) . CDD was originally proposed for data transmission. One property of CDD includes transparency - the receiver does not need to know how many antennas are used at a transmitter. This works well for legacy handsets. Another property is reduced overhead - the number of pilot tones needed for channel estimation is independent of the number of transmit antennas . CDD is also referred to by other names, one example being cyclic shift diversity (CSD) . CDD transmits the same OFDM symbol simultaneously from all transmit antennas. However, to avoid possible destructive signal superposition within the transmission range of the transmitter, a different cyclic shift for the same OFDM symbol is introduced for each individual antenna.

For the i-th transmit antenna, the time domain signal is cyclically shifted by ti samples before a cyclic prefix is attached to the signal. One of the operating principles of CDD is that ti is selected in such a way so that ti is larger than di/C, where C is the speed of light, and di is the distance between the i-th and (i-l)-th transmit antennas.

With the above condition being met, ti can be selected as small as possible so as to facilitate improved and or easier to implement channel estimation.

Current proposals for CDD involve its adoption for data transmission to provide transmit diversity for flat fading channels. When this is the case, the impact of using CDD for data can be viewed as channel phase shifts in some of the paths which result in frequency diversity.

Embodiments of the invention enable CDD based synchronization (sync) channel implementations. A sync channel works differently from a data channel in that the signal is known. For this reason, there is freedom to attribute the phase shift effect to either the channel or the signal. By taking advantage of this freedom, it is possible to collect all the transmitted energy resulting from CDD and thus achieve a similar level of performance as when time switched transmit diversity/frequency switched transmit diversity (TSTD/FSTD) is implemented for the sync channel.

Four aspects of the invention will be described below, any one or more of which may be implemented in a given implementation. A first aspect is determining how to select the amount of cyclic delay at the transmitter. A second aspect is determining how to collect the transmitted energy at the receiver. A third aspect is determining how to reuse the sync channel for channel estimation purposes. A fourth aspect is determining how to detect a base transmitting station (BTS) antenna configuration using the sync channel.

In the examples that follow, most of the description focuses on two antenna implementations, but it is to be understood that all of the embodiments generalize to an arbitrary number of transmit antennas.

Synchronization using CDD based Sync Channel

Synchronization can involve one or multiple stages. In some implementations, a coarse synchronization is performed first in the time domain followed by a fine synchronization in the frequency domain.

Synchronization in the Frequency Domain Using CDD based Sync Channel

For a two transmit antenna, one receive antenna [2Tx, IRx] CDD system, a received signal in the frequency domain for a given sub-carrier k can be described as

j2πτk ^' r(k)= A₁ (k)+ h₂{k)i ^» Uk) Eq. (1 )

where hi (k) is the channel response in the frequency domain from the first transmit antenna to the receive antenna, h₂(k) is the channel response in the frequency domain from the second transmit antenna to the receive antenna, N is a Fast Fourier Transform (FFT) size, p{k) is the sync sequence, and τ is the cyclic delay. In Eq. (1) the sync sequence p(k) has been applied to an Inverse Fast Fourier Transform (IFFT) in the transmitter to produce a series of time domain samples for transmission. These time domain samples are transmitted by both antennas, but with a cyclic shift of τ applied to one of the transmit antennas with respect to the other. Where additional transmit antennas are present, each would have its own term in Eq. (1) with a respective value for the cyclic shift τ .

At the receiver, the received signal, which is the sum of the received signals for each sub-carrier k of the frequency band for each transmit antenna, is correlated with the known sync sequence p(k) resulting in:

∑r(k)p^*(k)=a_ι+η₂ Eq. (2) k

where r(k) is defined in Eq. (1), p*(k) is a compl-ex conjugate of p (k) and

In Eq. (3) , a_x is the correlation output for the first transmit antenna. The maximum value of a_l as a function of different delays applied to the received signal reflects the synchronization value to be detected for the first transmit antenna.

In Eq. (4), η₂ is the correlation output for the second transmit antenna. The maximum value of η₂ as a function of different delays applied to the received signal reflects the synchronization value to be detected for the second transmit antenna. The channel response h_∑(k) for the second antenna is

multiplied by a phase shift equal to e ^N . When using CDD, t is selected large enough so that within a band of N sub- carriers several cycles of phase shift occur. For this reason, η₂.approaches zero, and hence is unable to provide a reliable estimate of ^Th₂(k) . Since η₂ is unable to provide a reliable k estimate of ∑h₂(k) , it is not useful in generating a k synchronization value for the second transmit antenna.

Creating sync sequences that tune to the delay

In a first embodiment, separate sync sequences that tune to the delay are created at the receiver so that Vh₂(k) is k detected for synchronization purposes, ∑hfø) is still computed

as above using the normal synchronization sequence. However, in performing the correlations for the second antenna (and subsequent antennas) , a modified sync sequence for use by the

2πτk receiver is defined as follows: p₂ ^*(k)- p^*(k)e ^N .

Then ∑h_{2 \}k) is determined according to the following: k

where

In Eq. (6) , a₂ is the correlation output for the second transmit antenna. The maximum in a₂ as a function of different delays applied to the received signal reflects the synchronization value to be detected for the second transmit antenna.

In Eq. (7), 77, is the correlation output for the first antenna. The maximum value of η_x as a function of different delays applied to the received signal reflects the synchronization value to be detected for the first transmit antenna. The value of η_x in Eq. 7 approaches to zero and hence is unable to provide a reliable estimate of ∑\{k) . Since η_x is

* unable to provide a reliable estimate of ]TA₁(Y), it is not k useful in generating a synchronization value for the first transmit antenna. However this is not problematic as a synchronization value has already been determined for the first transmit antenna using Eqs . (1) and (2).

Thus, using a combination of a_∑ and ai introduced previously, synchronization values for both antennas can be determined reliably.

In the modified sync sequence, the value of τ is equal to the value of τ used by a respective transmit antenna. Therefore, in some embodiments the value of τ for a given antenna is a previously define value which is known by receiver or provided to the receiver during the synchronization process.

Time domain synchronization

There are two main approaches to do time domain synchronization: a first approach uses a cyclic prefix (CP); a second approach uses a version of the synchronization symbols from the frequency domain are converted to the time domain to perform correlation in the time domain. A maximum value resulting from the correlation will correspond with the synchronization value.

In the first approach, where the cyclic prefix is used for time domain synchronization, the fact that CDD is employed at the transmitter does not change the cyclic prefix property in any way. In the second approach, two frequency domain sequences are needed in the time domain: one sequence corresponds to p{k) , and another sequence corresponds to P₂{k) • If p{n) and p₂{n) are mutually uncorrelated (n is the time domain index) , due to the cyclic delay of r , the peaks corresponding to A₁ and Jt₂ can be detected in the time domain.

Selecting cyclic delay to form orthogonal sequences

In another embodiment, the cyclic delay is selected at the transmitter such that orthogonal sync sequences are achieved. It is desirable to create orthogonal sync sequences at the receiver to simplify the synchronization process, to reduce inter-code interferences, and to facilitate easy and reliable channel estimation (from sync channel for data) .

In some embodiments, for two transmit antennas, a delay is selected to be N/2 (N=FFT size) , so that the second sequence is the same as the first sequence, but with an opposite phase value.

In some embodiments, for four transmit antennas, the delays are selected to be at N/4, N/2, and 3N/4.

In some embodiments, the delay selection for a CDD based sync channel is performed independently of delay selection for the CDD based data channel. Estimating channel state information with sync channel

In another embodiment of the invention, a method of estimating channel state information with the sync channel is provided. Assuming the transmit delay selection described

> above has been performed, the sync sequences may be orthogonal over a very short period. In particular, for the examples given above, for two transmit antennas with a delay selected to be N/2, the sync sequences are orthogonal over every two consecutive code symbols; for four transmit antennas, they are i orthogonal over every four consecutive code. Channel state information can be estimated over each orthogonal period, and be used for data channel coherent detection.

Blindly detecting the number of transmit antennas

Another embodiment of the invention provides a method of blindly detecting a number of transmit antennas by using the above discussed method of tuning the synchronization sequence to the delay at a receiver. Assuming a set of possible delays used by transmit antennas is known to the receiver, correlations can be computed using a respective tuned synchronization sequence for each such delay. Within the search window, each different sync sequence will be able to detect a channel response corresponding to a respective transmit antenna. If a correlation peak is detected for a given delay, that means that an antenna transmitted with that delay. The collective number of such peaks that are detected relates to the number of transmit antennas employed.

The transmitter of Figure 15 could also be used to implement additional embodiments of the invention. For example, processing logic 1520 may be configured to generate a synchronization sequence for multiple transmit antennas using cyclic delay diversity, wherein the synchronization sequence allocated to each antenna has a respective cyclic delay and transmitting circuitry 1530 may be configured to transmit the synchronization sequence.

The receiver of Figure 16 could also be used to implement additional embodiments of the invention. For example, receiving circuitry 1620 may be configured to receive a plurality of synchronization sequences and processing logic 1630 may be configured to perform synchronization using a respective synchronization sequence of the plurality of synchronization sequences tuned to each of the respective cyclic delays.

Description of components of an example communication system

For the purpose of providing context for embodiments of the invention for use in a communication system, Figure 10 shows a base station controller (BSC) 10 which controls wireless communications within multiple cells 12, which cells are served by corresponding base stations (BS) 14. In general, each base station 14 facilitates communications using OFDM with mobile and/or wireless terminals 16, which are within the cell 12 associated with the corresponding base station 14. The movement of the mobile terminals 16 in relation to the base stations 14 results in significant fluctuation in channel conditions. As illustrated, the base stations 14 and mobile terminals 16 may include multiple antennas to provide spatial diversity for communications. Also shown are relay stations 17.

A high level overview of the mobile terminals 16 and base stations 14 upon which aspects of the present invention are implemented is provided prior to delving into the structural and functional details of the preferred embodiments. With reference to Figure 11, a base station 14 is illustrated. The base station 14 generally includes a control system 20, a baseband processor 22, transmit circuitry 24, receive circuitry 26, multiple antennas 28, and a network interface 30. The receive circuitry 26 receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals 16 (illustrated in Figure 10) . A low noise amplifier and a filter (not shown) may co-operate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams.

The baseband processor 22 processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor 22 is generally implemented in one or more digital signal processors (DSPs) or application-specific integrated circuits (ASICs) . The received information is then sent across a wireless network via the network interface 30 or transmitted to another mobile terminal 16 serviced by the base station 14.

On the transmit side, the baseband processor 22^V receives digitized data, which may represent voice, data, or control information, from the network interface 30 under the control of control system 20, and encodes the data for transmission. The encoded data is output to the transmit circuitry 24, where it is modulated by a carrier signal having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas 28 through a matching network (not shown) . Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the base station and the mobile terminal.

With reference to Figure 12, a mobile terminal 16 configured according to one embodiment of the present invention is illustrated. Similarly to the base station 14, the mobile terminal 16 will include a control system 32, a baseband processor 34, transmit circuitry 36, receive circuitry 38, multiple antennas 40, and user interface circuitry 42. The receive circuitry 38 receives radio frequency signals bearing information from one or more base stations 14. A low noise amplifier and a filter (not shown) may co-operate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams.

The baseband processor 34 processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The baseband processor 34 is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs) .

For transmission, the baseband processor 34 receives digitized data, which may represent voice, data, or control information, from the control system 32, which it encodes for transmission. The encoded data is output to the transmit circuitry 36, where it is used by a modulator to modulate a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas 40 through a matching network (not shown) . Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station.

In OFDM modulation, the transmission band is divided into multiple, orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used.

OFDM modulation utilizes the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal recovers the transmitted information. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT) , respectively. Accordingly, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands within a transmission channel. The modulated signals are digital signals having a relatively low transmission rate and capable of staying within their respective bands. The individual carrier waves are not modulated directly by the digital signals. Instead, all carrier waves are modulated at once by IFFT processing.

In operation, OFDM is preferably used for at least down-link transmission from the base stations 14 to the mobile terminals 16. Each base station 14 is equipped with "n" transmit antennas 28, and each mobile terminal 16 is equipped with "m" receive antennas 40. Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labelled only for clarity.

With reference to Figure 13, a logical OFDM transmission architecture will be described. Initially, the base station controller 10 will send data to be transmitted to various mobile terminals 16 to the base station 14. The base station 14 may use the channel quality indicators (CQIs) associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be directly from the mobile terminals 16 or determined at the base station 14 based on information provided by the mobile terminals 16. In either case, the CQI for each mobile terminal 16 is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band.

Scheduled data 44, which is a stream of bits, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic 46. A cyclic redundancy check (CRC) for the scrambled data is determined and appended to the scrambled data using CRC adding logic 48. Next, channel coding is performed using channel encoder logic 50 to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal 16. Again, the channel coding for a particular mobile terminal 16 is based on the CQI. In some implementations, the channel encoder logic 50 uses known Turbo encoding techniques. The encoded data is then processed by rate matching logic 52 to compensate for the data expansion associated with encoding.

Bit interleaver logic 54 systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic 56. Preferably, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QpSK) modulation is used. The degree of modulation is preferably chosen based on the CQI for the particular mobile terminal. The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic 58.

) At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. When spatial diversity is desired, blocks of symbols are then processed by space-time block code (STC) encoder logic 60, which modifies the symbols in a fashion making the transmitted signals more resistant to interference and more readily decoded at a mobile terminal 16. The STC encoder logic 60 will process the incoming symbols and provide "n" outputs corresponding to the number of transmit antennas 28 for the base station 14. The control system 20 and/or baseband processor 22 as described above with respect to Figure 11 will provide a mapping control signal to control STC encoding. At this point, assume the symbols for the "n" outputs are representative of the data to be transmitted and capable of being recovered by the mobile terminal 16.

For the present example, assume the base station 14 has two antennas 28 (n=2) and the STC encoder logic 60 provides two output streams of symbols. Accordingly, each of the symbol streams output by the STC encoder logic 60 is sent to a corresponding IFFT processor 62, illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein. The IFFT processors 62 will preferably operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT processors 62 provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by prefix insertion logic 64. Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to- analog (D/A) conversion circuitry 66. The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry 68 and antennas 28. Notably, pilot signals known by the intended mobile terminal 16 are scattered among the sub-carriers. The mobile terminal 16, which is discussed in detail below, will use the pilot signals for channel estimation.

Reference is now made to Figure 14 to illustrate reception of the transmitted signals by a mobile terminal 16. Upon arrival of the transmitted signals at each of the antennas 40 of the mobile terminal 16, the respective signals are demodulated and amplified by corresponding RF circuitry 70. For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog- to-digital (A/D) converter and down-conversion circuitry 72 digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (AGC) 74 to control the gain of the amplifiers in the RF circuitry 70 based on the received signal level.

Initially, the digitized signal is provided to synchronization logic 76, which includes coarse synchronization logic 78, which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic 80 to determine a precise framing starting position based on the headers. The output of the fine synchronization logic 80 facilitates frame acquisition by frame alignment logic 84. Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time domain to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic 86 and resultant samples are sent to frequency offset correction logic 88, which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. Preferably, the synchronization logic 76 includes frequency offset and clock estimation logic 82, which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic 88 to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic 90. The results are frequency domain symbols, which are sent to processing logic 92. The processing logic 92 extracts the scattered pilot signal using scattered pilot extraction logic 94, determines a channel estimate based on the extracted pilot signal using channel estimation logic 96, and provides channel responses for all sub-carriers using channel reconstruction logic 98. In order to determine a channel response for each of the sub-carriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM sub-carriers in a known pattern in both time and frequency. Examples of scattering of pilot symbols among available sub-carriers over a given time and frequency plot in an OFDM environment are found in PCT Patent Application No. PCT/CA2005/000387 filed March 15, 2005 assigned to the same assignee of the present application. Continuing with Figure 14, the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub- carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel.

The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each receive path are provided to an STC decoder 100, which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder 100 sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols

The recovered symbols are placed back in order using symbol de-interleaver logic 102, which corresponds to the symbol interleaver logic 58 of the transmitter. The de- interleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic 104. The bits are then de-interleaved using bit de-interleaver logic 106, which corresponds to the bit interleaver logic 54 of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic 108 and presented to channel decoder logic 110 to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic 112 removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic 114 for de- scrambling using the known base station de-scrambling code to recover the originally transmitted data 116.

In parallel to recovering the data 116, a CQI, or at least information sufficient to create a CQI at the base station 14, is determined and transmitted to the base station 14. As noted above, the CQI may be a function of the carrier- to-interference ratio (CR) , as well as the degree to which the channel response varies across the various sub-carriers in the OFDM frequency band. The channel gain for each sub-carrier in the OFDM frequency band being used to transmit information is compared relative to one another to determine the degree to which the channel gain varies across the OFDM frequency band. Although numerous techniques are available to measure the degree of variation, one technique is to calculate the standard deviation of the channel gain for each sub-carrier throughout the OFDM frequency band being used to transmit data.

Figures 10 to 14 each provide a specific example of a communication system or elements of a communication system that could be used to implement embodiments of the invention. It is to be understood that embodiments of the invention can be implemented with communications systems having architectures that are different than the specific example, but that operate in a manner consistent with the implementation of the embodiments as described herein.

Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein.

Claims

CLAIMS :

1. A method comprising:

transmitting a synchronization sequence from multiple transmit antennas using cyclic delay diversity, with a respective cyclic delay for each antenna.

2. The method of claim 1 wherein the respective cyclic delays being selected such that orthogonal synchronization sequences are created at a receiver receiving the transmitted synchronization sequence.

3. The method of claim 1 wherein in a two transmit antenna system the cyclic delays are 0 and N/2, where N is the FFT size.

4. The method of claim 1 wherein in a four transmit antenna system the cyclic delays are 0 N/4, and N/2 3N/4, where N is the FFT size.

5. The method of claim 1 further comprising using an orthogonal property of the synchronization sequences to estimate channel state information for data coherent detection purposes .

6. A method comprising:

receiving a plurality of synchronization sequences, each received sequence with a respective cyclic delay;

performing synchronization using a respective synchronization sequence of the plurality of synchronization sequences tuned to each of the respective cyclic delays.

7. The method of claim 6 wherein a respective synchronization sequence comprises:

where p(k) is a known synchronization sequence, τ is a delay, and N is a size of a Fast Fourier transform (FFT) .

8. The method of claim 6 further comprising:

using the received plurality of synchronization sequences to blindly detect a number of transmit antennas which transmitted the plurality of synchronization sequences.

9. The method of claim 8 wherein using the received plurality of synchronization sequences to blindly detect a number of transmit antennas comprises:

generating a respective tuned synchronization sequence for each possible cyclic delay;

performing a respective correlation for each such synchronization sequence;

determining the number of transit antennas according to the number of correlations that produce correlation peaks.

10. A method comprising:

transmitting a respective OFDM signal from each of a plurality of antennas, the OFDM signals collectively containing a CDD-based synchronization channel.

11. A method comprising:

receiving an OFDM signal containing a CDD-based synchronization channel on at least one receive antenna; performing synchronization using the CDD-based synchronization channel.

12. A transmitter comprising:

a plurality of antennas;

processing logic configured to:

generate a synchronization sequence for multiple transmit antennas using cyclic delay diversity, wherein the synchronization sequence allocated to each antenna has a respective cyclic delay;

transmitting circuitry configured to transmit the synchronization sequence.

13. A receiver comprising:

a plurality of antennas;

receiving circuitry configured to receive a plurality of synchronization sequences;

processing logic configured to:

perform synchronization using a respective synchronization sequence of the plurality of synchronization sequences tuned to each of the respective cyclic delays.

14. A method of transmit diversity comprising transmitting a broadcast channel and at least one synchronization channel from a plurality of antennas such that channel information obtained from the at least one synchronization channel can be used to coherently detect the broadcast channel.

15. The method of claim 14, further comprising: for each of a plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that:

one OFDM symbol duration carries a primary broadcast channel (P-BCH) for the plurality of antennas with each sub- carrier frequency being used on only one of the antennas for the P-BCH;

one OFDM symbol duration carries a secondary synchronization channel (S-SCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the plurality of antennas for the S-SCH;

one OFDM symbol duration carries a primary synchronization channel (P-SCH) for the plurality of antennas.

16. The method of claim 15, wherein the transmitting a set of OFDM symbols comprises transmitting a set of OFDM symbols with a repeating pattern.

17. The method of claim 15 further comprising inserting the P-SCH, S-SCH and P-BCH in positions such that channel information obtained by a receiver from the P-SCH can be used for coherent detection of the S-SCH and/or the channel information obtained by a receiver from the S-SCH can be used to assist the detection of the P-BCH.

18. The method of claim 17, wherein the P-SCH and the S- SCH are transmitted on sequentially adjacent OFDM symbols and the S-SCH and P-BCH are transmitted on sequentially adjacent OFDM symbols.

19. The method of claim 15 comprising transmitting a frame structure comprising 10 transmission time intervals (TTIs) , each TTI comprising two sub-frames, each sub-frame comprising seven OFDM symbols, wherein each of said sets of OFDM symbols occupies a respective sub-frame.

20. The method of claim 19 wherein a sub-frame in the first TTI and the sixth TTI are used to transmit the P-BCH, S- SCH and P-SCH.

21. The method of claim 15 wherein the plurality of antennas is a number of antennas equal to 2N, N=>1.

22. The method of claim 14 further comprising:

for each of a first plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that:

one OFDM symbol duration carries a primary broadcast channel (P-BCH) for a first subset of the plurality of antennas with each sub-carrier frequency being used on at least one of the antennas for the P-BCH;

one OFDM symbol duration carries a secondary channel (S-SCH) for the first subset of antennas with each sub-carrier frequency being used on at least one of the antennas of the first subset for the S-SCH;

one OFDM symbol duration carries a primary synchronization channel (P-SCH) for the first subset of the plurality of antennas;

for each of a second plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that:

one OFDM symbol -duration carries the P-BCH for a second subset of antennas equal to the plurality of antennas minus the first subset of antennas, with each sub-carrier frequency being used on at least one of the antennas for the P- BCH;

one OFDM symbol duration carries the S-SCH for the second subset of antennas with each sub-carrier frequency being used on at least one of the antennas of the second subset for the S-SCH;

one OFDM symbol duration carries the P-SCH for the second subset of antennas.

23. The method of claim 22, wherein for each of the first and second plurality of sets of OFDM symbol durations, transmitting a respective set of OFDM symbols comprises transmitting a respective set of OFDM symbols with a repeating pattern.

24. The method of claim 22 further comprising inserting the P-SCH, S-SCH and P-BCH in positions such that channel information obtained by a receiver from the P-SCH can be used for coherent detection of the S-SCH.

25. A method of transmit diversity comprising transmitting a first synchronization channel and a second synchronization channel from a plurality of antennas such that channel information obtained from the first synchronization channel can be used to coherently detect the second synchronization channel.

26. The method of claim 25 comprising:

transmitting the second synchronization channel in locations proximal the first synchronization channel and/or reference symbol locations;

performing channel estimation on the first synchronization channel to generate channel estimates; performing coherent detection of the second synchronization channel using the channel estimates.

27. The method of claim 25, further comprising:

for each of a plurality of sets of OFDM symbol durations, transmitting a set of OFDM symbols from each of the plurality of antennas such that:

28. The method of claim 26 wherein performing channel estimation on the first synchronization channel to generate channel estimates further comprises using the reference symbol locations in conjunction with the first synchronization channel to generate channel estimates.

29. A method operable to perform antenna structure and framing signalling using a primary synchronization channel (P- SCH) .

30. The method of claim 29 comprising:

transmitting using one of a plurality of different antenna configurations, each antenna configuration having a different number of antennas/

transmitting the P-SCH on a sub-set of an available set of sub-carriers on each of a plurality of OFDM symbols within a frame; signalling which antenna configuration is being used through selection of which sub-carriers to include in the subset.

31. The method of claim 30 wherein transmitting and signalling comprises:

transmitting the P-SCH on odd sub-carriers to indicate a first antenna configuration;

transmitting the P-SCH on even sub-carriers to indicate a second antenna configuration.

32. A method comprising:

a plurality of base stations transmitting synchronization information using a common P-SCH, with each base station using a respective set of sub-carriers with or without network planning.

33. A method operable to perform framing structure signalling through a primary synchronization channel (P-SCH) .

34. The method of claim 33 comprising:

transmitting the P-SCH on a sub-set of an available set of sub-carriers on each of a plurality of OFDM symbols within a frame;

signalling framing information through selection of which sub-carriers to include in the sub-set.

35. The method of claim 34 wherein transmitting and signalling comprises:

transmitting the P-SCH on a first OFDM symbol within a frame and a second OFDM symbol within the frame; transmitting the P-SCH on odd sub-carriers within the first OFDM symbol;

transmitting the P-SCH on even sub-carriers within the second OFDM symbol.

36. A method using an antenna mapping scheme which allows the coherent detection of a secondary synchronization channel (S-SCH) .

37. A transmitter comprising:

a plurality of antennas;

processing logic for inserting a broadcast channel (BCH) and at least one synchronization channel (SCH) into a plurality of sets of OFDM symbol durations such that:

one OFDM symbol duration is used for reference symbols and a primary broadcast channel (P-BCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the antennas for one of the reference symbols and the P-BCH;

one OFDM symbol duration is used for a secondary synchronization channel (S-SCH) for the plurality of antennas with each sub-carrier frequency being used on only one of the plurality of antennas for the S-SCH;

one OFDM symbol duration is used for a primary synchronization channel (P-SCH) for the plurality of antennas;

transmitting circuitry configured to transmit for each of the plurality of sets of OFDM symbol durations, a set of OFDM symbols from each of the plurality of antennas.

38 . A method comprising : transmitting a first sequence of synchronization information from at least one antenna of a plurality of antennas on a first sub-carrier group in a first OFDM symbol; and

transmitting a second sequence of synchronization information from at least one antenna of a plurality of antennas on said first sub-carrier group in a second OFDM symbol .

39. The method of claim 38 wherein a mapping relation for cell related information is different for the first sequence of synchronization and the second sequence of synchronization.

40. A method comprising:

transmitting a first sequence of synchronization information from at least one antenna of a plurality of antennas according to a first mapping scheme in a first OFDM symbol; and

transmitting a second sequence of synchronization information from at least one antenna of a plurality of antennas according to a first mapping scheme in a second OFDM symbol .

41. A method comprising:

receiving a first sequence of synchronization information from a transmitter in a first OFDM symbol;

receiving a second sequence of synchronization information from said transmitter in a second OFDM symbol;

remapping at least one of a first sequence index for said first sequence or a second sequence index for said second sequence such that said first and second sequence index can be combined for transmitter index identification purposes.

42. The method of claim 41 wherein said remapping is based on a known permutation formula.

43. A transmitter comprising:

a plurality of antennas;

processing logic configured to:

insert a first sequence of synchronization information from a first antenna on a first sub-carrier group in a first OFDM symbol;

insert a second sequence of synchronization information from a second antenna on said first sub-carrier group in a second OFDM symbol;

transmitting circuitry configured to transmit the first and second OFDM symbols.

44. A receiver comprising:

a plurality of antennas;

receiving circuitry configured to:

receive a first sequence of synchronization information from a transmitter in a first OFDM symbol;

receive a second sequence of synchronization information from said transmitter in a second OFDM symbol;

processing logic configured to:

remap at least one of a first sequence index for said first sequence or a second sequence index for said second sequence such that said first and second sequence indices can be combined for transmitter index identification purposes.