KR20090107092A - Synchronisation in multicarrier cdma systems - Google Patents

Abstract

A method and apparatus are provided for performing acquisition, synchronization and cell selection within an MIMO-OFDM communication system. A coarse synchronization is performed to determine a searching window. A fine synchronization is then performed by measuring correlations between subsets of signal samples, whose first signal sample lies within the searching window, and known values. The correlations are performed in the frequency domain of the received signal. In a multiple-output OFDM system, each antenna of the OFDM transmitter has a unique known value. The known value is transmitted as pairs of consecutive pilot symbols, each pair of pilot symbols being transmitted at the same subset of sub-carrier frequencies within the OFDM frame.

Description

SYNCHRONISATION IN MULTICARRIER CDMA SYSTEMS

TECHNICAL FIELD The present invention relates to cellular wireless communication systems, and more particularly to system access in cellular communication systems using OFDM or OFDM-like techniques, and physical layer packet and preamble designs.

In a wireless communication system having one or more transmitters and one or more receivers, the receiver must capture the timing of the signal transmitted by the transmitter and synchronize it with it to extract information from the received signal. In a wireless communication system, the timing of the signal transmitted from the base station is generally referred to as system timing.

In a cellular wireless communication system using Orthogonal Frequency Division Multiplexing (OFDM), synchronization to the timing of a signal is a Fast Fourier Transform (FFT) used by the receiver of the signal to extract information from the signal. This allows for precise positioning of the window.

In any cellular wireless communication system having multiple base stations (BTSs) and multiple mobile communication devices, a synchronization process must be frequently performed between the BTS and mobile communication devices for the system to operate. Hereinafter, mobile communication devices will be simply referred to as user equipment (UE).

In addition, each BTS defines a geographical transmission area, commonly referred to as a cell, in which case a UE that is substantially close to a particular BTS will access the wireless communication system. Selecting a BTS for a particular UE to access a cellular wireless communication system is called cell selection. In order to optimize the reception of the BTS signal, the UE must switch to identify the highest quality signal among the signals received from the different BTSs and tune its receiver to the best BTS at a given time. Thus, due to the mobility of the UE, the synchronization process must be performed frequently to allow seamless handoff from one BTS to another as the UE changes location.

In modern cellular wireless communication systems, high speed system access and cell selection are essential functions for proper mobile UE operation. The purpose of fast acquisition is to synchronize the UE to the desired BTS. Cell selection and re-selection measure and synchronize signal power (including interference) between adjacent BTSs and select and switch BTSs with the highest signal quality, i.e., maximum carrier-to-interference (C / I) ratio. Is performed by the UE.

Existing solutions for accessing wireless communication systems using OFDM are designed for wireless local area network (LAN) systems for high-speed packet access in single input-single output (SISO) configurations. However, wireless LANs cannot handle UE mobility requiring seamless BTS handoff. On the other hand, some cellular systems, such as 3G UMTS, can perform cell selection, BTS identification, and BTS C / I ratio measurements.

Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing (MIMO-OFDM) is a novel spectral-efficient technique used to transmit high-speed data over a wireless channel with fast fading at both frequency and time. In the case of a high speed downlink packet data transmission system, the design of the physical layer packet structure is important.

OFDM technology is being adopted by DAB, DVB-T and IEEE 802.11 standards. DAB and DVB-T are used for audio and video terrestrial broadcasting. In these systems, the signal is transmitted in a continuous data stream. Since fast packet access is not critical, a preamble is unnecessary. DAB and DVB-T also apply to single frequency networks. In this case, all transmitters transmit the same signal as simulcast. Interference from neighboring transmitters can be treated as an active echo that can be handled by proper design of the prefix. IEEE 802.11 is a wireless LAN standard. This is a packet-based OFDM transmission system. The preamble header is introduced in this standard.

Synchronization in a MIMO-OFDM system where each transmitter and each receiver has multiple antennas is much more difficult. In addition to the complexity of these tasks, the fast synchronization process must be very reliable even at very low C / I ratios to allow high success rates for the entire cell. In addition, high mobility results in high Doppler spreading, which makes reliable synchronization more difficult.

In a MIMO-OFDM system, synchronization may be performed in two steps. First, frame synchronization (also referred to as coarse synchronization) is performed to determine a coarse position range for the start position of the first OFDM symbol in the frame. Second, timing synchronization (also known as fine synchronization) is performed to determine the correct FFT window position so that demodulation in the frequency domain can be performed correctly.

Typically, fine synchronization is implemented in the time domain. This is accomplished by inserting a known pilot training sequence into the time domain for the receiver to calculate cross correlation in the selected time slot.

For example, as shown in FIGS. 1A and 1B, the OFDM frame structure of the IEEE 802.11 standard uses several short OFDM symbols that are repeated, generally indicated by 5, which is the frame for the selected sub-carriers. Arranged as several headers in the time domain at the beginning of, followed by a training OFDM symbol 207 for fine synchronization. The header 5 is used for frame (ie schematic) synchronization. The training OFDM symbol 207 is used to correctly position the FFT window so that demodulation in the frequency domain can be performed correctly. The training OFDM symbol 207 is followed by a TPS OFDM symbol 205 and a data OFDM symbol 30.

Transmission parameter signaling (TPS) OFDM symbol 205, shown more clearly in the frequency domain, is transmitted at a frequency corresponding to the adaptive coding and modulation period. The training OFDM symbol, the TPS OFDM symbol, and the data OFDM symbol use all sub-carriers. In the 802.11 system described above, it is transmitted only through the fourth sub-carrier, which is repeated for coarse synchronization. This design is only suitable for simple SISO OFDM systems with only one transmit antenna. In the case of a MIMO-OFDM system, the design of the preamble is more complicated because there are multiple transmit antennas. In addition, for mobile communications, the design of an efficient preamble is much more difficult due to the multi-cell environment, requirements for initial access when no BTS information is available, BTS switching and soft handoff.

The existing method of cell acquisition and synchronization process utilizes a three-step synchronization approach applied by the UMTS WCDMA system, which requires a relatively long access time. Although fine synchronization can be performed in the time domain, the performance of this approach is limited under conditions where the C / I ratio is very low due to self-interference of the MIMO channels. Increasing the length of the correlation can improve fine synchronization performance in the time domain, but at the expense of increased overhead and processing complexity. Existing designs are based on time sequence training sequence correlation for one transmit antenna and one receive antenna system. However, a direct extension of this time domain synchronization approach results in performance loss, especially in low C / I ratio applications. The cause of this loss of performance is self-interference between the MIMO channels which is difficult to reduce in the time domain.

One broad aspect of the present invention provides each antenna for transmitting a header OFDM symbol only through a separate set of sub-carriers to a MIMO-OFDM transmitter adapted to transmit a header symbol format, in which case the subs of the header OFDM symbol The carriers are divided into discrete sets of sub-carriers for each of the plurality of antennas.

In some embodiments, there are N antennas, each of which is assigned a different set of sub-carriers separated into N sub-carriers.

In some embodiments, the header symbol includes a dedicated pilot channel on the multiplexed dedicated pilot channel sub-carriers and a common synchronization channel on the common synchronization channel sub-carriers for each of the plurality of antennas.

In some embodiments, the header OFDM symbol also includes multiplexed broadcasting sub-carriers for each of the plurality of antennas.

In some embodiments, the transmitter is also applied to transmit a preamble with a prefix, followed by two identical OFDM symbols with the header OFDM symbol format. In some embodiments, the prefix is a periodic extension of two identical OFDM symbols.

In some embodiments, the pilot channel has a complex sequence mapped to BTS specific that allows for efficient BTS identification.

In some embodiments, a common synchronization channel is designed for fast and accurate initial acquisition.

In some embodiments, a common synchronization channel is used for coarse synchronization and fine synchronization, and a pilot channel is used for fine synchronization.

In some embodiments, a common synchronization channel is used to transmit a complex sequence that is different for each transmit antenna for one transmitter but common for individual transmit antennas for different transmitters in a communication network.

In some embodiments, the transmitter also applies to transmitting OFDM frames starting with the preamble and having distributed pilots throughout the rest.

In some embodiments, during the preamble, for each of the N transmit antennas, dedicated pilot channel sub-carriers are transmitted, common synchronization channel sub-carriers are transmitted, and broadcasting channel sub-carriers are transmitted.

In some embodiments, the sub-carriers of the preamble OFDM symbols are configured as a repetitive sequence of [dedicated pilot channels for each of N transmit antennas, common synchronization channel sub-carriers for each of N transmit antennas] arranged in a predetermined order. .

In some embodiments, one or more dedicated pilot channel sub-carriers for each of the N transmit antennas, one or more common synchronization channel sub-carriers for each of the N transmit antennas, with sub-carriers of the preamble OFDM symbols arranged in a predetermined order. , One or more broadcast channel sub-carriers].

A broader alternative aspect of the present invention provides each antenna for transmitting the header OFDM symbol only through a separate set of sub-carriers to a MIMO-OFDM receiver adapted to receive the header symbol format, in which case the subs of the header OFDM symbol The carriers are divided into discrete sets of sub-carriers for each of the plurality of antennas.

In some embodiments, the receiver is applied to receive from N transmit antennas having a different set of sub-carriers separated into N sub-carriers assigned to each of the plurality of receive antennas.

In some embodiments, the receiver is also applied to perform fine synchronization based on common synchronization channel sub-carriers and / or dedicated pilot channel sub-carriers.

Another broad aspect of the present invention provides a receiver that is applied to transmit a packet data frame structure. The packet data frame structure has a superframe having a length corresponding to the synchronization period of the network; A superframe includes a plurality of radio frames; Each radio frame includes a plurality of transmission parameter signaling (TPS) frames corresponding to adaptive encoding and modulation periods; Each TPS frame includes a plurality of slots corresponding to an air interface slot size; Each slot includes a plurality of OFDM symbols, in which case the first two symbols of the first slot of the first TPS frame for each OFDM frame are used as the header of the OFDM symbol.

In some embodiments, the header OFDM symbol has a header OFDM symbol format in which the sub-carriers of the header OFDM symbol are divided into discrete sets of sub-carriers for each of the plurality of antennas, where each antenna is sub- The header OFDM symbol is transmitted only on a separate set of carriers.

In some embodiments, the header OFDM symbol includes multiplexed pilot channel sub-carriers and common synchronization channel sub-carriers for each of the plurality of antennas.

In some embodiments, the header OFDM symbol also includes multiplexed broadcasting channel sub-carriers for each of the plurality of antennas.

In some embodiments, the transmitter is also adapted to transmit in a plurality of different modes by transmitting different numbers of OFDM symbols per slot with changed slot intervals and no frame structure changes on the slots.

In some embodiments, a mode with an increased number of OFDM symbols per slot is realized by shortening the OFDM symbol interval and shortening the FFT size but not changing the sampling frequency.

In some embodiments, the receiver also transmits to a separate set of users for each TPS frame and also applies to each TPS frame to signal users that must demodulate the entire TPS frame.

Another broad aspect of the present invention provides a method for performing synchronization in an OFDM receiver. The method includes sampling, at each of the one or more receive antennas, a received signal to generate a separate set of time domain samples; Determining at least one coarse synchronization location; On each of one or more receive antennas:

a) for each of the plurality of candidate fine synchronization positions for the one or more coarse synchronization positions:

Iii) for each receive antenna, positioning the FFT window at the candidate fine synchronization position and changing time domain samples to a separate set of frequency domain components by the FFT;

Ii) for each of the one or more transmit antennas, extracting an individual receive training sequence corresponding to the transmit antenna from sets of frequency domain components;

Iii) for each transmit antenna, calculating a correlation between each respective receive training sequence and a known separate transmit training sequence;

Iii) combining the correlation values for one or more transmit antennas to produce an overall correlation result for each candidate synchronization location;

b) determining a fine synchronization position from the plurality of correlation values;

Combining the fine synchronization positions from the one or more receiving antennas into an overall fine synchronization position.

In some embodiments, coarse synchronization locations are determined for each receive antenna and used to determine individual fine synchronization locations.

In some embodiments, a coarse synchronization position is determined for each receive antenna and the first of the positions is used to determine the fine synchronization position for all receive antennas.

In some embodiments, by finding the correlation peak between time domain samples over two OFDM symbol intervals, a coarse synchronization location is determined in the time domain for one or more receive antennas.

In some embodiments, the method is applied to an OFDM receiver having two or more antennas, and combining the fine synchronization positions from the one or more receiving antennas into an overall fine synchronization position selects the first of the fine synchronization positions. It includes.

In some embodiments, sampling the received signal to generate a set of time domain samples is performed for three or more OFDM symbol intervals; Determining at least one coarse synchronization location includes:

a) each correlation value is immediately accompanied by a first set of time domain samples and a first period received during a first period with one OFDM symbol period and time domain samples received during a second period with an OFDM symbol period Calculating, for each of the plurality of start times for the first period, a plurality of correlation values, the correlation values calculated between the second set of values;

b) identifying a coarse synchronization position such that it is the maximum of the plurality of correlation values, thereby finding a correlation peak between time domain samples received in two OFDM symbol intervals to identify the coarse synchronization position. Performing coarse synchronization in the region.

In some embodiments, combining the correlation values for one or more transmit antennas to produce an overall correlation result for each candidate synchronization position may multiply the correlation values for one or more transmit antennas for each candidate synchronization position together. Steps.

In some embodiments, the method is applied to one transmit antenna and one receive antenna system.

In some embodiments, the training sequence is received on common synchronization channel sub-carriers.

In some embodiments, the training sequence is received during the OFDM frame preamble.

In some embodiments, the training sequence is received on dedicated pilot channel sub-carriers.

In some embodiments, the training sequence is received during the OFDM frame preamble.

Another broad aspect of the invention includes one or more receive antennas; For each of the one or more receive antennas, receive circuitry adapted to sample the received signal to produce a separate set of time domain samples; A coarse synchronizer applied to determine one or more coarse synchronization locations; At least one FFT, at least one correlator, and at least one combiner, each of at least one receive antenna,

a) for each of the plurality of candidate fine synchronization positions with respect to the one or more coarse synchronization positions:

Iii) for each receive antenna, locate the FFT window at the candidate fine synchronization position and change the time domain sample to a separate set of frequency domain components by the FFT;

Ii) for each of the one or more transmit antennas, extract an individual receive training sequence corresponding to the transmit antenna from sets of frequency domain components;

Iii) for each transmit antenna, calculate a correlation between each respective receive training sequence and a known separate transmit training sequence;

Iii) combine the correlation values for one or more transmit antennas to produce an overall correlation result for each candidate synchronization location;

b) a fine synchronizer adapted to determine a fine synchronization position from the plurality of correlation values,

An OFDM receiver is also provided that is adapted to combine the fine synchronization positions from one or more receive antennas into one global fine synchronization position.

In some embodiments, the receiver has two or more receive antennas and is adapted to combine the fine sync positions from one or more receive antennas into one global fine sync position by selecting the first of the fine sync positions.

In some embodiments, the receiver combines the correlation values for one or more transmit antennas by multiplying the correlation values for one or more transmit antennas with each candidate synchronization position, resulting in an overall correlation result for each candidate synchronization position. Applies to generate

In some embodiments, a receiver is applied to receive a training sequence on common synchronization channel sub-carriers.

In some embodiments, a receiver is applied to receive a training sequence on dedicated pilot channel sub-carriers.

Another broad aspect of the present invention provides a method of performing fine synchronization. The method comprises at each of one or more receive antennas receiving an OFDM symbol comprising a separate receive frequency domain training sequence for each of the one or more transmit antennas; Performing fine synchronization in the frequency domain by finding a maximum correlation between known frequency domain training sequences and received frequency domain training sequences.

Another broad aspect of the present invention provides a method for transmitting a signal that enables fine synchronization. The method includes transmitting, from each of the one or more transmit antennas, an OFDM symbol comprising a separate frequency domain training sequence.

In some embodiments, different frequency domain training sequences are transmitted by each transmit antenna, but the same frequency domain training sequence is transmitted by corresponding antennas of other transmitters.

Another broad aspect of the present invention provides a method for performing cell selection in an OFDM receiver. The method includes sampling, at each of the one or more receive antennas, a received signal to generate a separate set of time domain samples; Determining at least one coarse synchronization location; On each of one or more receive antennas:

a) performing a frequency domain correlation between one or more received common synchronization sequences extracted from the common synchronization channel sub-carriers of the received signal and corresponding common synchronization sequences of the respective plurality of transmit antennas to identify a plurality of candidate correlation peaks. step;

b) selecting M strongest correlation peaks for further processing;

c) at each correlation peak, reconverting the time domain samples into frequency domain components and processing pilot channel sub-carriers containing transmitter specific information to identify the transmitter associated with each correlation peak. ;

d) determining a C / I or similar value for each transmitter so identified;

Selecting a transmitter determined to have the best C / I for any of the one or more receive antennas.

In some embodiments, a plurality of candidate correlation peaks may be performed by performing frequency domain correlation between one or more received common synchronization sequences extracted from common synchronization channel sub-carriers of a received signal and corresponding common synchronization sequences of individual plurality of transmit antennas. The steps to identify are:

a) for each of the plurality of candidate fine synchronization positions relating to one of the one or more coarse synchronization positions:

Iii) for each receive antenna, positioning the FFT window at the candidate fine synchronization position and changing the time domain samples into a separate set of frequency domain components by the FFT;

Ii) for each of the one or more common synchronization sequences transmitted by the transmit antennas of each of the one or more transmitters, extracting an individual receive training sequence corresponding to the transmit antennas from sets of frequency domain components;

Iii) for each of the one or more common synchronization sequences, calculating a correlation value between each respective received common synchronization sequence and a known individual common synchronization sequence;

Iii) combining the correlation values to produce an overall correlation result for each candidate synchronization location;

b) determining, from the correlation values, one or more peak values that are partial maximum values of the correlation values.

In some embodiments, the method includes re-changing the time domain samples into frequency domain components based on the fine synchronization position of the selected transmitter and performing additional fine synchronization based on a dedicated pilot channel for that transmitter. It includes more.

In some embodiments, the method allows subcarriers of a header symbol to be divided into discrete sets of subcarriers for each of a plurality of antennas, each antenna transmitting header symbols only through a separate set of sub-carriers, The symbols include multiplexed pilot channel sub-carriers and common synchronization channel sub-carriers for each of the plurality of antennas, wherein a frame in which the contents of the pilot channel sub-carriers are repeated and the contents of the synchronization channel sub-carriers are repeated is 2 Starting with the same header OFDM symbol, the common synchronization channel sub-carriers convey a complex sequence that is different for the individual antennas of one base station and common for a plurality of base stations, and the contents of the dedicated pilot channel sub-carriers At least locally unique header symbol format for Which it is applied to a MIMO-OFDM frame format.

In some embodiments, the method differs from the currently selected transmitter, at the end of the time period, for averaging the C / I or similar value over the time period for each transmitter so identified for transmitter switching. If so, further comprising performing transmitter switching with the transmitter having the highest average C / I or similar value.

2A, an OFDM packet frame structure provided as an embodiment of the present invention is shown. The transmit OFDM symbol streams consist of these frames. Each frame consists of three main components: preamble 300, scattered pilot 302, and traffic data symbol 304. By inserting the preamble, the UE can perform the following basic operations: fast BTS access, BTS identification and C / I ratio measurement, frame and timing synchronization, frequency and sampling clock offset estimation and initialization channel estimation. For maximum spectral efficiency and radio capacity, it is important to design the frame preamble with minimal overhead.

2b, there is shown a frame hierarchy for MIMO-OFDM constructed in accordance with the present invention, where the OFDM superframe 500 (two shown) is located at the highest level. The interval of the superframe is determined by the network synchronization period (for example, 1 second). The superframe consists of several 10ms radio frames 502, also called OFDM frames. There are 100 10 ms OFDM frames 502 in the ls superframe 500.

In order to support adaptive coding modulation (ACM), a fast signaling channel (TPS channel-transmission parameter signaling) is introduced. Each OFDM frame 502 is subdivided into TPS frames 504, in which there are five 2 ms TPS frames for each 10 ms radio frame 502. In some embodiments, the frame length used for the TPS is equal to the duration of the ACM unit. Each TPS frame also includes signaling information that allows each user to determine whether the current TPS frame contains data for them. The TPS frame may contain data for multiple users.

TPS frame 504 may be further subdivided into several slots 506, each consisting of several OFDM symbols. In the example shown, each TPS frame 504 is subdivided into three slots 506. The interval of slot 506 depends on the air interface slot size. The minimum transmission unit is one OFDM symbol 508, 510. One OFDM symbol interval is determined by the transmission environment characteristics, for example maximum channel delay, system-sampling clock and maximum Doppler. In the example shown, there are four OFDM symbols 508 and 510 per slot 506.

To reduce the overhead incurred by inserting guard intervals between OFDM symbols, design different OFDM symbol modes, e.g., 0.5k mode and 1k mode, each with different symbol intervals and different prefixes. can do. For simplicity of the system, the sampling frequency is preserved unchanged during the mode switching. These different modes are described in more detail below.

The frame structure of FIG. 2B presents an example of a frame hierarchy that is compatible with the UMTS radio-interface. At the OFDM symbol level, there are two different OFDM symbol types. This includes a preamble OFDM symbol 508 and a regular data symbol 510.

Referring to FIG. 4, which is a representation of a time domain, each OFDM frame is a preamble composed of several identical header OFDM symbols 603, 605 preceded by a prefix 607, which is a cyclic extension of the header OFDM symbol. To start. An iterative structure is used to aid synchronization. By performing correlation between adjacent OFDM symbols until two identical symbols are identified, the start of an OFDM frame can be found. As an example, 1056 samples per OFDM symbol may be used. For the preamble, during prefix 607, the last 64 samples of the header OFDM symbol are transmitted. There is no prefix for the second header OFDM symbol. The header is inserted periodically, in the example of FIG. 2B this insertion occurs every 10 ms, i.e. every time the OFDM frame starts.

Referring again to FIG. 2B, in the case of a non-header OFDM symbol, i.e., a general OFDM symbol 510, all OFDM symbols preferably have a prefix. In the "1K" mode, there are 32 prefix samples and 1024 actual samples representing the FFT size for a total of 1056 samples per symbol. In 1 / 2K mode, for a total of 528 samples / symbols, there are 16 sample prefixes and 512 samples per symbol (representing the FFT size). Preferably, using the frame structure of FIG. 2B, these different modes can be supported without changing the sampling frequency. In the 1 / 2K mode, there are twice as many OFDM symbols 510 per slot 506. For a particular mode selected at any moment, the prefix size must be greater than the maximum channel delay. In 1 / K mode, more OFDM symbols are sent on fewer sub-carriers. Since the symbol interval is short, this is more robust to high Doppler. Also, the larger the spacing between sub-carriers, the better the tolerance for Doppler. Thus, there is a unified frame structure that accepts different FFT sizes but has the same sampling rate at the receiver. It is preferable to use the same preamble for different modes.

OFDM is a parallel transmission technique. The useful full band is divided into many sub-carriers, with each sub-carrier being modulated independently. According to one embodiment of the invention, during the header, not all sub-carriers are used on all transmit antennas to separate transmissions of different antennas having a plurality of antennas. Rather, the sub-carriers are split between the antennas. 3, an example of such a thing is demonstrated. The sub-carrier frequencies included in the OFDM symbol are each represented by circles. In the above example, it is assumed that two transmit antennas exist in the MIMO system. 3 shows an OFDM symbol with various sub-carriers arranged along the frequency axis 400, the contents of all the sub-carriers at a given instant in time, as indicated along the time axis 402. Represents a symbol of. In this case, the first two OFDM symbols 408 and 410 are used for dedicated pilot channel information and the remaining symbols (only two are shown; 412 and 414 are used for normal OFDM symbols). The dedicated pilot channel information transmitted on the first two OFDM symbols 408, 410 alternates between being transmitted by the first antenna by the sub-carrier and being transmitted by the second antenna. This is represented by the first sub-carrier 404 transmitting dedicated pilot channel information for the first transmitter and the sub-carrier 406 transmitting dedicated pilot channel information for the second sub-carrier, the pattern being the remaining sub-carriers. Is repeated for each. The remaining OFDM symbols 412 and 414 contain the information transmitted by both antennas. Alternatively, it can be seen that other arrangements can be used. In addition, if there are more than two transmit antennas, the pilot channel information will be alternated in a predetermined pattern between all transmit antennas by the sub-carriers.

In another embodiment, the common synchronization channel and the dedicated pilot channel are frequency multiplexed with respect to the header symbol. To transmit individual dedicated pilot channels and a common synchronization channel, a separate set of non-overlapping sub-carriers is assigned to each antenna.

In yet another embodiment, the common synchronization channel, the dedicated pilot channel, and the broadcast channel are frequency multiplexed with respect to the header symbol. Under this configuration, the useful whole sub-carriers of the header symbol are divided into three groups. These three groups are mapped onto a common synchronization channel, a dedicated pilot channel and a broadcasting channel, respectively.

An example of mapping of different channels in a MIMO-OFDM system with two transmitter diversity is shown in FIG. 5. In this example, four OFDM symbols 712, 714, 716, 718 are shown, of which two 712, 714 are header symbols. During header symbols 712 and 714, every second sub-carriers are used for the first antenna and the remaining sub-carriers are used for the second antenna. This is easily generalized for a larger number of antennas. In the above example, it is assumed that two transmit antennas exist in the MIMO system. Starting at the first sub-carrier 700, every sixth sub-carriers are for the first transmitter dedicated pilot channel sub-carrier. Starting at the second sub-carrier 702, every sixth sub-carriers are for a second transmitter dedicated pilot channel sub-carrier. Every sixth sub-carriers starting at the third sub-carrier 704 are for the first transmitter common synchronization channel sub-carrier. Starting at the fourth sub-carrier 706, every sixth sub-carriers are for a second transmitter common synchronization channel sub-carrier. Every sixth sub-carriers starting at the fifth sub-carrier 708 are for a broadcasting channel sub-carrier for the first antenna, and every sixth sub-carriers starting at the sixth sub-carrier 710. For broadcasting channel sub-carrier for the second antenna.

The common synchronization channel is a universal channel for initial access. It can also be used for synchronization and spare channel estimation. When transmitter diversity is applied, different transmitters share common synchronization sub-carriers. In such cases, the common synchronization channel is divided between different transmitters. A common complex sequence known by all terminals is used to modulate sub-carriers reserved for the common synchronization channel. The same common synchronization sequence is transmitted by all base stations in the system. If there is a plurality of transmit antennas in which each transmit antenna can transmit a unique synchronization sequence, there may be more than one such synchronization sequence. Using the synchronization sequence, the mobile station can find the initial synchronization location for further BTS identification by finding the correlation peak between the received synchronization sequence and the known known synchronization sequence.

Dedicated pilot channels are used for BTS / cell identification and support C / I measurements for cell selection, cell switching, and handoff. A unique complex sequence, for example a PN code, is assigned to each BTS and used to modulate dedicated pilot sub-carriers. For a plurality of transmit antennas, different unique sequences are transmitted by each antenna. Unlike the case of the common synchronization channel, different base stations transmit using different pilot sequences. Access point identification and initial interference measurements can be performed due to the quasi-orthogonality of the PN code assigned to different BTSs. Dedicated pilot channels can also be used to assist in synchronization processing.

As mentioned above, in order to fully utilize the sub-carriers of the header OFDM symbol, several sub-carriers are preferably used as the broadcasting channel. In the example of FIG. 5, two out of every six sub-carriers are used for this purpose. Broadcast channels can carry important system information. The space time transmit diversity (STTD) scheme cannot be used for a broadcasting channel (or any sub-carrier of a header OFDM symbol) because it destroys the repetitive structure of the header OFDM symbol required by the synchronization algorithm. However, transmitting broadcast information on the same sub-carrier in all transmitters may cause destructive interference between transmitters. To solve this problem, the broadcasting channel is split between different transmitters so that the sub-carriers (mapped for the broadcasting channel) provide transmit antennas, such as two transmit antennas. Can be assigned in turn. Power boosting may be used to further enhance the broadcasting channel.

Broadcasting information from different BTSs may be different. In some embodiments, the broadcasting information is protected so that users near the cell boundary can correctly receive the broadcasting information even in the presence of strong interference. Short PN codes can be used to spread the broadcasting information. Adjacent BTSs are assigned different codes. Insertion of a broadcasting channel reduces preamble overhead and increases spectral efficiency.

Broadcast channels are used to transmit information unique to a particular base station. One broadcast message may be transmitted on the combined broadcast channel carriers for the two antennas. By designing the preamble header symbol to have a pilot channel, a synchronization channel, and a broadcasting channel, the preamble header overhead is reduced. The common synchronization channel is designed for fast and accurate initial acquisition. A dedicated pilot channel with symbols mapped to BTS-specific allows for efficient BTS identification. The combined common synchronization channel and pilot channel are all used for MIMO channel estimation. The use of a combined common synchronization channel and a dedicated pilot channel also enables high accuracy synchronization. Frequency domain training symbols are robust to timing errors and multipath environments. The preamble design allows for flexibility of the UE (user equipment) to enable more efficient algorithms.

Specific disturbances of sub-carriers between dedicated pilot channels in one embodiment, between dedicated pilot channels and common synchronization channels in another embodiment, and between common synchronization channels and broadcast channels in another embodiment are only specific examples. These may be assigned in any suitable way.

6, a conceptual schematic diagram of a MIMO-OFDM transmitter 10 is shown. The first sample set 201 of four OFDM symbols is shown to be transmitted from a first transmit antenna 21 and the second sample set 203 of four OFDM symbols to be transmitted from a second transmit antenna 23. . In general, an OFDM transmitter has N _ant transmit antennas, in which case N _ant is a design parameter. In the MIMO-OFDM transmitter 10, data from the demultiplexer 23 is transmitted to the first OFDM component 24 connected to the transmit antenna 21 or the second OFDM component 24 connected to the transmit antenna 23. ) Is sent to one of them. These components organize data on sub-carriers, each at a different orthogonal frequency, into an OFDM symbol and an OFDM frame. Each OFDM component 24, 26 has a separate header inserter 29 for inserting header OFDM symbols. Sample sets 201 and 203 of OFDM symbols represent the first four OFDM symbols of an OFDM frame transmitted from transmit antennas 21 and 23, respectively, where each row of a data symbol or pilot symbol is an OFDM symbol. . The first OFDM symbol 13 and the second OFDM symbol 14 (same as the first) represent two header OFDM symbols unique to the OFDM frame transmitted by the first transmit antenna. Similarly, the third OFDM symbol 17 and the fourth OFDM symbol (same as the third) represent two header OFDM symbols unique to the OFDM frame transmitted by the second transmit antenna. The four OFDM symbols 15, 16, 19 and 20 are typically non-identical OFDM symbols consisting of a plurality of data symbols, with at least one data symbol generally indicated 11 on each OFDM sub-carrier. The entire OFDM frame typically has more data symbols. In addition, OFDM symbols 201 are transmitted simultaneously with the OFDM symbols 203 and at the same timing.

In the above example, two identical header OFDM symbols have dedicated pilot channel sub-carriers 12 and common synchronization channel sub-carriers 9. There may be broadcast channel sub-carriers, not shown. Dedicated pilot channel sub-carriers are used for C / I ratio measurement and BTS identification and fine synchronization, as described below; These can also be used for initial channel estimation. Common synchronization channel sub-carriers 9 are used for coarse synchronization and fine synchronization, initial access, and initial channel estimation.

In the example shown, during two header OFDM symbols, the first of every four consecutive sub-carriers is used to carry the dedicated pilot channel symbols transmitted by the transmit antenna 21. Similarly, the second every four consecutive sub-carriers are used to carry the dedicated pilot channel symbols transmitted by the transmit antenna 23.

The dedicated pilot channel symbol transmitted on pilot channel sub-carriers 12, 25 is defined by a base station / sector specific PN sequence. One set of symbols from the complex pseudo-random PN sequence unique to the base station is mapped onto dedicated pilot channel sub-carrier positions of the header OFDM symbols.

The third for every four consecutive sub-carriers in the two header symbols is used to convey the common synchronization channel symbols transmitted by the transmit antenna 21. Likewise, the fourth of every four consecutive sub-carriers is used to convey common synchronization channel symbols transmitted by the transmit antenna 23.

Common synchronization channel symbols transmitted on common synchronization sub-carriers 9 and 27 are defined by a complex pseudo-random PN sequence unique to each transmit antenna 21 and 23. A set of symbols from this complex pseudo-random PN sequence are mapped onto common synchronization channel sub-carriers of header OFDM symbols. That is, the common synchronization channel symbols of each frame transmitted through each transmit antenna are unique for that transmit antenna but use the same PN code for the corresponding transmit antenna of other base stations. In this example, PN _SYNC ^{1 is associated} with transmit antenna 21 and PN _SYNC ² is associated with transmit antenna 23. However, similar antennas in different transmitters in the entire communication network will use the same PN code. For example, the common synchronization channel symbols for the first transmit antenna 21 on all transmitters in the network will use one PN code (PN _SYNC ⁽¹⁾ ) and the second transmit antenna 23 on all transmitters in the network. Common synchronization channel symbols will use one different PN code (PN _SYNC ⁽²⁾ ).

Referring to FIG. 7A, as will be described later, a block diagram of a MIMO-OFDM receiver function in which coarse synchronization is performed based on two repeated OFDM header symbols transmitted from each transmit antenna is shown. An OFDM receiver includes a first receive antenna 734 and a second receive antenna 735 (although more generally there are multiple (N) receive antennas). The first receive antenna 734 receives the first received signal at the RF receiver 736. The first received signal is a combination of two signals transmitted by the two transmit antennas 21 and 23 of FIG. 6, where each of the two signals is in a separate channel between the separate transmit antenna and the first receive antenna 734. To be changed. The second receive antenna 735 receives the second received signal at the RF receiver 739. The second received signal is a combination of two signals transmitted by the two transmit antennas 21 and 23, where each of the two signals is changed by a separate channel between the separate transmit antenna and the second receive antenna 735. Will be. The four channels (between each of the two transmit antennas and each of the two receive antennas) can change over time and frequency and are generally different from each other.

To determine the approximate range for the location of the start position of the first header symbol, coarse synchronization for the first receive antenna 734 is performed by the coarse synchronizer 737 on discrete time samples of the received signal. do. Similar processes are performed for the second antenna 735 by the coarse synchronizer 741. By using repeated header symbols for the OFDM transmitter, coarse synchronization is facilitated. The coarse synchronizer 737 performs a correlation measurement on time domain signal samples of successive OFDM symbols. The time domain signal sample that yields the best correlation measure is the _coarse synchronization position (n _coarse ). The _coarse synchronization position n _coarse is then used as the position for placing the FFT window within the FFT function used for fine synchronization.

Initially, coarse synchronizer 737 starts coarse synchronization processing of the time domain. A running buffer (not shown) is used to buffer the discrete time samples of the received signal over three consecutive OFDM symbol periods, and between samples collected during two consecutive OFDM symbol intervals Compute the self-correlation (γ _t (n))

Here, x (n) is time-domain samples of the received signal, and N _header is the number of samples taken over one OFDM symbol interval.

In some embodiments, a moving correlator is applied to the real-time implementation to save computational power.

In one embodiment, the γ _t (n) value is computed sequentially for n = 1 (up to n = Nheader) until the correlation value exceeds the threshold, after which the maximum value search is enabled. The calculation of the correlation continues until the correlation result is again below the threshold and the maximum value search process continues. The sample position corresponding to the maximum correlation value is the coarse synchronization position.

The threshold is typically calculated from the average autocorrelation within one frame. Alternatively, another way to find the maximum is to determine the partial maximum for each OFDM symbol, for example, over an OFDM frame that may be 60 symbols in length. The global maximum is then taken as the largest of the partial maximums. This process is performed in both schematic synchronizers. When fine synchronization proceeds jointly, the overall coarse synchronization position can be taken as any combination of two synchronization values, preferably taking the faster of the two coarse synchronization positions thus determined. Alternatively, each fine synchronizer (described below) may start from an individual coarse synchronization position. Referring to FIG. 7B, a block diagram of the MIMO-OFDM fine synchronization function is shown. In one embodiment, the fine synchronization function is suitable for performing fine synchronization based on two repeated OFDM header symbols transmitted by each transmit antenna using a common synchronization channel and / or a dedicated pilot channel as described above. More generally, the fine synchronization function may perform fine synchronization for OFDM frames in which some known training sequences are embedded. Also, the input to the fine synchronization process is a coarse synchronization position. This coarse synchronization location may be determined using the method described above or using any other method as appropriate. If the common synchronizer of FIG. 7A is used, the same components as those of FIG. 7A are similarly numbered and shared in the actual implementation. The function of FIG. 7B is repeated for each of one or more receive antennas.

After performing a fine synchronization process for each of the one or more receive antennas, an overall synchronization position is taken based on the combination of fine synchronization positions. For review, once the coarse synchronizer determines the _coarse synchronization position (s) n _coarse , and each fine synchronizer performs an FFT on the signal samples on both sides of the coarse synchronization position, Generate spanned frequency domain components. Each fine synchronizer searches for frequency domain components to locate the FFT window correctly. The exact location of the FFT window is necessary to perform OFDM demodulation in the frequency domain. The fine synchronizer performs an accurate measurement of the FFT window by performing a correlation measurement between the known PN codes (PN _SYNC ⁽¹⁾ and PN _SYNC ⁽²⁾ ) and the frequency components within the search window defined for _coarse synchronization position (n _coarse ). Catches. Correlation measurements performed by each fine synchronizer are performed in the frequency domain, and a set of correlation measurements is performed for each of the known PN codes (PN _SYNC ⁽¹⁾ and PN _SYNC ⁽²⁾ ), that is, for each transmit antenna ( 21 and 23 (or for as many as one or more transmit antennas present).

Each fine synchronizer selects N _symbol signal samples starting from the initial signal samples in the search window, where N _symbol is the number of signal samples in the OFDM symbol. For each transmit antenna, each fine synchronizer determines a correlation measure between frequency domain signal samples and the PN code corresponding to the transmit antenna.

More specifically, the fine synchronization search is performed near n _coarse . Assuming that the search window is 2N + 1, the search range is from (n _coarse -N) to (n _coarse + N). n _start (i) = n _coarse + Ni (where i = 0, ..., 2N) to indicate the sample index in the fine search window. Fine synchronization starts from i = 0. Take N _symbol samples from n _start (0), then remove the prefix and perform FFT. The received OFDM symbol in the frequency domain may be recorded as follows.

Where N _prefix is the number of prefix samples and N _FFT is the FFT size.

Since the common synchronization channels are divided between different transmitters of the MIMO OFDM system, from R, the complex data R ^{(j, k)} _SYNC carried by the common synchronization channel of the different transmitters is extracted. More generally, complex data corresponding to the transmitted training sequence is extracted. The correlation between R ^{(j, k)} _SYNC and PN ^{* (j)} _SYNC is given by

Where j = 1, 2, ..., N _Tx denotes a transmitter, k = 1, 2, ..., N _Rx denotes a receiver, and PN ^(j) _SYNC denotes the jth The common SYNC PN code for the transmitter and N _SYNC is the size of the common PN code.

Then, the starting point index n _start is shifted by 1 (n _start (1) = n _start (0) -1), and other N _symbol samples are processed as described above. In order to have new frequency domain data R ^{(j, k)} _SYNC (m, i), the FFT must be performed again. To reduce the computational complexity, the following iteration can be used.

Where NFFT is the FFT size. After extracting R ^{(j, k)} _SYNC (m, i), a new correlation value is calculated. The process continues until n _start leaves the fine search window.

For each receive antenna, the individual fine synchronization position by finding n _start (i) corresponding to the maximum value for the products of correlation results from different antennas over i = 0, ......, 2N. You can find From a mathematical point of view, for the kth receive antenna, the individual fine synchronization positions can be selected according to the following equation.

To reduce the likelihood of a false alarm, criteria can be set. For example, if the following conditions are met, fine synchronization can be considered as realized.

Here, N _threshold is a factor determined by a preset fine search window size. Preferably, the overall fine synchronization position is taken as the first of the fine synchronization positions determined for the different receive antennas.

The fine synchronization process for one receive antenna is schematically illustrated in FIG. 7B. At the output of the first receiver 736, blocks D0 738 through D2N 742 specify the arrangement of FFT blocks 744,... 748 for various candidate fine synchronization positions (all 2N + 1). Indicates. FFT blocks 744,..., 748 calculate the FFT for each of a set of individual samples. Each FFT output is fed to the correlator block for each transmit antenna. If there are two transmit antennas, then there will be two such correlator blocks per FFT output. For example, the output of the FFT 744 is supplied to the first correlator block 745 for the first transmit antenna and also to the second correlator block 755 for the second transmit antenna. If the training sequence (common synchronization sequence or pilot channel sequence in the above examples) is transmitted using the interval between these sub-carriers, the entire FFT need not be completed to recover the training sequence component. The correlator block 745 for the first antenna multiplies, by a multiplier 747, the recovered training sequence symbol position of the FFT output by a known training sequence for the first transmit antenna, and these multiplication values It is added by the adder 751. The correlator 755 performs this same calculation for the known training sequence for the second transmit antenna and the training sequence position for the second transmit antenna. This is done for all possible different shifts in each transmit antenna at the first receiver. The correlation result for the different transmit antennas for each possible shift is multiplied together in multiplier 753. Shifts that generate maximum values for these multiplication values are selected to be a fine synchronization position for a particular receiver. The same process is involved for any other receive antennas, and the overall fine synchronization position is preferably taken as the first of the calculated fine synchronization positions.

If there is a drift or loss in the synchronization position, timing synchronization can track every frame. For example, in systems using the preamble described above, each time the preamble arrives at the receiver, a two-step synchronization process is repeated using the same method for coarse synchronization and fine synchronization. In this case, a smaller search window N may be used based on the assumption that the deviation of the synchronization position will be near the current place. After acquisition, a dedicated pilot channel code assigned to modulate a dedicated pilot channel for a different BTS can be used for the correlator, or a common synchronization sequence or any other training sequence.

One embodiment of the present invention has been described with respect to a MIMO-OFDM transmitter having one or more transmit antennas. The method of performing synchronization in an OFDM receiver may also be applied to a signal received from an OFDM transmitter having one transmit antenna as long as a known training sequence is inserted into a frame by the OFDM transmitter.

Lastly, embodiments of the present invention have described one transmitter having a plurality of antennas and one receiver having a plurality of antennas. As such, the inventive concept can be extended to include a multi-cellular environment with many MIMO-OFDM transmitters and many MIMO-OFDM receivers.

[Access in a Multi-Cellular Environment]

System access in a multi-cellular environment presents a new problem of cell selection because there are many transmitters transmitting the same common pilot symbol. In another embodiment of the present invention, the aforementioned transmission header is used by the receiver to perform system access and cell selection.

During initial acquisition, the UE initiates operation by performing coarse synchronization. This can be done using the methods described above or any other method. After one frame interval, the coarse synchronization position is determined. Then, a fine synchronization search algorithm is performed based on the common synchronization channel. Since the data carried by the common synchronization channel is the same for all BTSs, several fingers (peaks) can be observed in the multi-cell environment and in the multi-path fading propagation channel. These fingers generally correspond to different BTSs and / or different paths. Referring to FIG. 8, an example of fine synchronization raw output (for a common synchronization channel) calculated in a multi-cellular environment as a function of sample index is shown. In this example, there are five major fingers 400, 402, 404, 406 and 408. M strongest fingers are selected and their corresponding positions are found, where M is a system design parameter. These positions are used as candidates for final synchronization and also used as positions at which BTS identification is performed.

Since the BTS transmits the same common synchronization sequence, the result of FIG. 8 does not allow BTS identification. At each candidate synchronization location, the correlation value of all possible complex sequences (dedicated pilot PN sequence) assigned to a different BTS and received dedicated pilot channel sub-carriers is calculated to scan for the presence of all possible neighboring BTSs. In the case of a plurality of transmit antennas, this correlation is preferably done based on a dedicated pilot PN sequence combined for all dedicated pilot sub-carriers to produce one correlation result for each index. 9 shows an example of a relationship between a BTS scanning result and a checking point (a candidate synchronization position). BTS identification is realized by detecting a PN code corresponding to the maximum correlation value at each candidate synchronization position. C / I can be calculated based on all correlation results at each test location. In the initial acquisition phase, cell selection is determined by selecting the BTS with the highest C / I ratio. In this example, two BTSs, a first BTS (BTS1) and a second BTS (BTS2), are identified. For multi-antenna receiver diversity, the final decision of cell selection should be based on a comparison of the highest C / I obtained by different receiver antennas in the receiver.

In order to obtain the final synchronization position, fine synchronization is performed again, using a dedicated complex sequence found through dedicated pilot channel and BTS identification. A smaller search window near the fine sync location is used. Final synchronization results from different receivers are compared. The position corresponding to the earliest sample in time is used as the final synchronization position. This step is to reduce the likelihood that a weak path (multi-path) will be selected due to short-term fading. To reduce the likelihood of a failure alert, a threshold is set. This threshold may be the ratio of finger strength to the final synchronization position and the correlation average in the search window.

In a typical data processing step, the fine synchronization and BTS identification steps are repeated every frame when a new preamble is received, but a small set of candidate PN codes is applied to the BTS scan. After BTS identification, the BTS candidate list may be generated through strong interference search. This list is updated periodically, for example every 10 ms, providing information about BTS switching and soft handoff. Certain criteria may be set to trigger BTS switching and soft handoff. To average the impact from fading, the decision for BTS switching and soft handoff may be based on observations over a period of time. This criterion may be a comparison of the maximum correlation value expressed in C with the strongest I. Also, after cell switching and soft handoff, synchronization may be adjusted by the last step in the initial access. The overall cell selection and reselection method is shown in FIG.

In a first step 600, coarse synchronization is performed, for example, based on the preamble header in the time domain. This involves finding a rough boundary between each frame by finding two identical symbols. This correlates samples across adjacent symbol intervals until a peak is found. Step 600 depends on the preamble for the frame starting with two adjacent identical symbols.

Next, during step 602, at the coarse synchronization correlation peak, the FFT is calculated and switching to processing for the common synchronization channel in the frequency domain is made. The center of the search window is at [synchronization position +/- some number of samples]. In step 604, the M strongest correlation peaks are selected. At this time, it is not known which BTS each peak value is associated with. BTS identification has not yet been determined.

Next, in step 606, for each correlation peak, the FFT is recalculated and the correlation associated with the fine synchronization process is calculated using a dedicated pilot channel containing the base station specific complex sequence. This is followed immediately by step 608 where correlation with the BTS identification complex sequence is performed to allow identification for the associated base station. In step 610, the C / I ratio is calculated for each BTS so identified. BTS selection and BTS switching are performed in step 612 based on these C / I ratios. As mentioned above, BTS switching is performed based on these C / I ratios averaged over some time period.

Finally, for access, according to step 614, the FFT is calculated and fine synchronization is applied to the dedicated pilot channel of the BTS with the highest C / I ratio.

BTS initial synchronization was performed for the common synchronization channel. The BTS specific sequence is embedded in the frequency domain and the BTS identification processing is performed in the frequency domain while enabling the elimination of MIMO-OFDM interchannel interference. BTS power estimation is performed based on the pilot channel for each MIMO-OFDM BTS. BTS selection is performed based on the C / I ratio measurement.

The result is an improvement in the synchronization and identification of the serving BTS in a strong multi-path channel and high interference environment, by a joint of BTS synchronization and cell selection. Channel estimation may be performed based on a combination of a common synchronization channel and a dedicated pilot channel. C / I estimation provides a basis for cell switching and soft handoff.

In the above example, access is performed based on the synchronization channel and the pilot channel embedded in the preamble described above. More generally, access may be performed on channels that are embedded in a suitable manner within the OFDM symbol stream.

The foregoing are only examples of the application of the principles of the invention. Those skilled in the art can implement other configurations and methods without departing from the spirit and scope of the invention.

With reference to the accompanying drawings will be described in detail preferred embodiments of the present invention.

Is a frame structure in the time domain of the IEEE 802.11 standard.

1B is a frame structure in the frequency domain of FIG. 1A;

2A is a packet data frame structure provided as an embodiment of the present invention.

2B is a packet frame hierarchy provided as an embodiment of the present invention.

3 is a preferred header structure provided as an embodiment of the present invention.

4 is a preamble header structure in the time domain provided as an embodiment of the present invention.

5 is a preamble header structure in the frequency domain provided as an embodiment of the present invention.

6 is a conceptual schematic diagram of a MIMO-OFDM transmitter provided as an embodiment of the present invention.

7A is a block diagram of a MIMO-OFDM coarse synchronization function.

7B is a block diagram of a MIMO-OFDM fine synchronization function.

8 is a plot of a signature sequence correlation output for pilot channels showing several candidate synchronization locations.

9 is a plot of BTS identification simulation.

10 is a flowchart of a cell selection and reselection method in MIMO-OFDM, provided as an embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

10: MIMO-OFDM Transmitter

21: first transmitting antenna

23: second transmit antenna

23: Demultiplexer

24: first OFDM component

26: second OFDM component

29: header inserter

300: preamble

302: distributed pilot

304: traffic data symbol

500: superframe

502: OFDM frame

504: TPS frame

506: slot

508, 510: OFDM symbol

Claims (2)

A method of transmitting a signal that enables fine synchronization, Transmitting signals including OFDM symbols comprising respective frequency domain training sequences from each of the at least one transmit antenna Signal transmission method comprising a. The method of claim 1, A different frequency domain training sequence is transmitted by each transmit antenna, but the same frequency domain training sequence is transmitted by a corresponding antenna of other transmitters.

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