Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
  • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
  • H04L1/0618—Space-time coding

Definitions

• This invention relates to radio communication systems employing antenna arrays and has particular application to multi-carrier communications systems such as those employing code division multiple access (CDMA) techniques.
• CDMA code division multiple access
• CDMA employs spread spectrum signaling whereby individual users in the communications network use the same RF carrier frequency, but are separated by the use of individual spreading codes. Hence, multiple communications channels are allocated using a plurality of spreading codes within the portion of radio spectrum, each code being uniquely assigned to a receiving station.
• one carrier frequency is allocated for communication on the downlink and another carrier frequency is allocated for communication on the uplink. This is known as FDD (frequency division duplex).
• TDD time division duplex
• Some systems which utilise knowledge of the propagation characteristics of the communication channel to adjust a characteristic of the transmission in order to achieve a more efficient use of the available resources. Most of these systems use feedback of information on the state of the link. Some systems which operate in TDD mode, use a calculated estimate of the propagation channel characteristics in one direction for adjusting a characteristic of the transmission in the other direction. Thus an estimate of the characteristics of the downlink propagation channel from received uplink measurements can be derived.
• the transmitting and receiving equipment may be fitted with single antenna or with an array of antenna elements.
• Antenna arrays can provide improved performance relative to a single antenna by providing a better antenna pattern for a coverage area.
• receive and transmit diversity schemes have been developed to optimise the receive and transmit path of communications systems employing antenna arrays.
• receive and transmit diversity schemes have been developed to optimise the receive and transmit path of communications systems employing antenna arrays.
• by varying the weight of the signals detected by each of the individual antennas in the array it is possible to vary the antenna pattern to better detect signals from a particular direction or to arrange for non- destructive combination of multi-path signals.
• These techniques adjust the weights of the antenna array signals to maximise the receive path gain by measuring the output of a receiver.
• Transmit diversity schemes can significantly improve the downlink performance of the so-called third generation systems or UMTS (universal mobile telecommunications system) which employ CDMA techniques.
• CDMA digital versatile code Division Multiple Access
• diversity techniques have been employed in order to improve the bit error rate (BER) performance of such systems.
• BER bit error rate
• TDD time duplex mode
• this can lead to a significant increase in complexity of channel estimation at the mobile station.
• the UMTS TDD mode uses TD-CDMA as multiple access technology.
• the receiving equipment uses interference cancellation techniques or joint detection to decode the communications signals.
• the information-bearing signal is impressed upon a much more rapidly varying signal known as a spreading sequence.
• the increased frequency bandwidth of the spread signal provides immunity against narrowband interferers and spectral nulls in the frequency response of the propagation medium (frequency selective fading).
• the use of direct-sequence spreading allows a receiver to extract the information contained within a plurality, K, of independently spread user signals which simultaneously occupy the same frequency band.
• Each of the Kuser signals consists of logical data bits (+1) which are mapped on to corresponding (possibly multi-valued) modulated symbols.
• the modulated symbols may be generated either in the form of a continuous stream or blocks of N symbols. Linear modulation schemes (such as M-ary quadrate amplitude modulation) are most commonly adopted in order to simplify receiver design.
• the Kcode sequences are chosen to be mutually orthogonal in order to facilitate separation of the K user signals at the receiver.
• a propagation channel represents the composite effects of the transmit filters, air-interface and receive filters and can be identified by a filter with an impulse response of length W chips.
• ISI Intersymbol interference
• GSM Global System for Mobile Communications
• MAI Multiple-access interference
• Kco ⁇ e sequences are designed to be mutually orthogonal at the transmitter.
• propagation through the composite propagation channel implies that the codes are no longer orthogonal by the stage at which they arrive at the receiver.
• symbols of the K spread signals interfere with one another, each acting as a source of noise towards others.
• the spreading codes are essentially mutually orthogonal at the receiver.
• the receiver can consist of a bank of K correlators (or matched filters) each matched to one of the K spreading signals.
• the output of the A" 1 matched filter then consists of noisy estimates of the modulated symbols carried by the A" 1 user signal.
• the plurality of received signals are not mutually orthogonal.
• the output of the A 111 matched filter contains interference from the remaining K-1 user signals.
• the combined operation of matched filtering and MAI-ISI elimination is referred to as joint-detection. Joint-detection schemes attempt to eliminate MAI and ISI by suppressing the mutual interference present amongst all the transmitted symbols carried by the K ser signals. In a typical joint detection receiver, the receiver is provided with all the codes and is therefore able to decode all the channels within a given time slot.
• One known method of estimating the unknown transmitted symbol sequence in a joint-detection receiver is zero-forcing block-linear equalisation. Normally, the joint-detection equations can be solved using propagation channel estimation and the knowledge of the spreading codes used.
• This invention seeks to provide an open loop transmit diversity scheme, applicable to a CDMA communication system using short spreading sequences (e.g. UMTS TDD mode), without incurring a substantial complexity increase to the joint detection process.
• short spreading sequences e.g. UMTS TDD mode
• a further aim is to provide a transmit diversity scheme which can be applied to a TD-CDMA communication system.
• the present invention consists of a transmitter including at least first and second antennas for transmitting data blocks of symbols in a spread- spectrum communication system, including means for generating a complex conjugate of each data block, and being adapted to transmit simultaneously a first data block via the first antenna and a complex conjugate of a second data block via the second antenna.
• the invention also provides a receiver, to be described here below, for decoding the data blocks transmitted in accordance with the invention.
• the invention provides a benefit over an alternative transmit diversity system known to the inventors in which consecutive symbols are transmitted simultaneously with their complex conjugates.
• this known case if it is assumed that a multi-path propagation channel is present, there will be symbol overlap at the receiving end with the amount of overlap being dependent on the length of the impulse response of the propagation channel.
• this symbol overlap has to be accounted for in the joint detection receiver. This means that in the receiving chain each joint detection will have to estimate the transmitted symbols and their conjugates.
• joint detection requires the inversion of a matrix, the number of symbols to be detected is doubled and so the size of the joint detection matrix is doubled also. Therefore, the complexity of the matrix inversion is multiplied by 8.
• the joint detection implemented at the receiver will only need to estimate the same amount of symbols as in the case of a single transmit antenna. Therefore, open loop transmit diversity gains can be obtained without a receiver complexity penalty because the size of the joint detection matrix does not have to be increased.
• Figure 1 is a block diagram of a known transceiver chain without incorporation of transmit diversity
• FIG. 2 is a block diagram of a transceiver chain in accordance with the invention and including space time transmit diversity.
• a typical TD-CDMA burst structure which is used to transmit information in each time-slot of a given frame is designated 1 in Figure 1.
• Data to be transmitted by a single antenna 2 is split into two blocks D1 and D2 which are separated by a mid-amble training sequence M.
• the mid-amble is used at a receiver 3 to estimate the propagation channel 4.
• the burst received at the single antenna 5 is decoded by two joint detection modules 6 and 7 with a first joint detection module 6 outputting an estimate of the data comprising D1 from the first part of the received burst and the joint detection module 7 outputting an estimate of the data comprising D2 from the second part of the received burst.
• Figure 2 shows a TD-CDMA mode transceiver chain incorporating space time transmit diversity and operating in accordance with the invention.
• a transmitter 8 is provided with two antennas 9 and 10 for transmitting two distinct bursts 11 and 12 simultaneously over two respective propagation channels 13 and 14.
• Each burst 11 , 12 is provided with a respective mid-amble sequence M1 and M2 for the purposes of estimating the two channels 13 and 14 at a receiver 15 in accordance with known methods.
• the data blocks to be transmitted are each split in the transmitter into two sub-blocks D11 and D12 and D21 and D22 to form the first burst 11.
• the transmitter 8 is also adapted to generate the complex conjugates of each of these data sub-blocks to form the second burst 12 where this burst 12 comprises the sub-blocks - D12 * , D11 * , M2, -D22 * , D21 * , where the minus sign indicates the binary complement and the asterisk denotes the complex conjugate.
• the transmitter 8 transmits in the first quarter of the frame comprising a burst (not counting the mid-amble sequence) and via the two antennas 9 and 10, sub-blocks D11 and -D12 * .
• the second quarter it transmits data sub-blocks D12 and D11 * .
• the third quarter it transmits data sub-blocks D21 and -D22 * .
• the fourth quarter it transmits data sub-blocks D22 and D21 * .
• a single antenna 16 receives the signals transmitted by both transmit antennas 9 and 10.
• the burst received 17 contains mixed information from both the antennas 9 and 10.
• Burst 17 comprises sub-blocks R1 1 , R12, a midamble and sub-blocks R21 and R22.
• Each received sub-block contains mixed information contained within the corresponding sub-blocks transmitted in parallel by the transmitter 8 (via both antennas 9, 10) and filtered by the two propagation channels 13, 14.
• a first joint detection module 18 processes data received during first and second quarters of the frame and from inputs comprising R11 and R12 * , it estimates data blocks D11 and D12.
• a second joint detection module 19 processes data received during the third and fourth quarters of the frame and from inputs comprising R21 and R22 * it estimates data blocks D21 and D22. Both joint detection modules 18 and 19 can be operated in accordance with known techniques.
• the data blocks D1 and D2 of Figure 1 are divided into more than two sub-blocks.
• the performance of the invention is not degraded in this case provided that the sub-block length remains substantially longer than the length of the propagation channel.
• Joint detection is not the only implementation of the receiver 15 and other options will be evident to those skilled in the art.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)
• Radio Transmission System (AREA)

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• 1999
  • 1999-07-13 EP EP99401757A patent/EP1069707A1/de not_active Withdrawn
• 2000
  • 2000-07-05 AU AU64329/00A patent/AU6432900A/en not_active Abandoned
  • 2000-07-05 CA CA002379122A patent/CA2379122A1/en not_active Abandoned
  • 2000-07-05 WO PCT/EP2000/006446 patent/WO2001005060A1/en not_active Application Discontinuation
  • 2000-07-05 EP EP00951359A patent/EP1198904A1/de not_active Withdrawn
  • 2000-07-05 CN CN 00810157 patent/CN1360765A/zh active Pending

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