Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/7103—Interference-related aspects the interference being multiple access interference
  • H04B1/7107—Subtractive interference cancellation
  • H04B1/71075—Parallel interference cancellation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/711—Interference-related aspects the interference being multi-path interference
  • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/7103—Interference-related aspects the interference being multiple access interference
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/711—Interference-related aspects the interference being multi-path interference
  • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
  • H04B1/712—Weighting of fingers for combining, e.g. amplitude control or phase rotation using an inner loop
• • 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
  • H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
• • 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
  • H04B7/08—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
  • H04B7/0837—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station using pre-detection combining
  • H04B7/0842—Weighted combining
  • H04B7/0845—Weighted combining per branch equalization, e.g. by an FIR-filter or RAKE receiver per antenna branch
• • 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
  • H04B7/08—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
  • H04B7/0837—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station using pre-detection combining
  • H04B7/0842—Weighted combining
  • H04B7/0848—Joint weighting
  • H04B7/0857—Joint weighting using maximum ratio combining techniques, e.g. signal-to- interference ratio [SIR], received signal strenght indication [RSS]
• • 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
  • H04B7/08—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
  • H04B7/0891—Space-time diversity

Definitions

• the present invention relates generally to wireless digital receivers, and more particularly to a wireless DS-CDMA communication network having multiple concurrent transmitters .
• multiple access interference is one of the major factors that limits the performance of the network.
• MAI multiple access interference
• many digital receivers have been proposed.
• prior art digital receivers are generally characterized by a fairly high computational complexity.
• the major cause of the computational complexity lies on the block-based processing in the receivers, i.e., the receivers detect the signal based on a block of received samples .
• Multi-user receivers also require a great deal of additional information which typically includes a code sequence, relative timing, carrier phase, and sometimes the instantaneous received signal strength for each mobile transmitter or transceiver, e.g., a cellular telephone.
• Channel fading is another cause of performance degradation in wireless communication networks.
• the increased mobility of receivers in wireless communication networks often results in fast fading and the resulting Doppler spread substantially degrades the receiver performance.
• MMSE minimum mean square error
• the present invention provides an adaptive receiver for detecting multiple user signals in a DS-CDMA network.
• the receiver includes multiple antennas.
• a time-frequency rake receiver is connected to each of the antenna.
• An interference cancellers is connected to an output of each rake receiver, and a diversity combiner, connected to outputs of each interference canceller, determines decision symbols corresponding to input signals received at the antennas.
• the antennas are spaced about three to five times the wavelength of the baseband signals.
• Figure 1 is a block diagram of an adaptive DS-CDMA multi-user receiver according to the invention
• Figure 2 is a block diagram of a time-frequency receiver of the receiver of Figure 1;
• Figure 3 is a block diagram of an adaptive filter based interference canceller of the receiver of Figure 1 ;
• Figure 4 is a block diagram of a compensator circuit used by the receiver of Figure 1;
• FIG. 5 is a block diagram of details of the compensator circuit of Figure 4.
• Our invention provides an adaptive multi-user receiver for detecting digital symbols in a direct sequence - code division multiple access (DS-CDMA) network.
• Our receiver exploits space-time-frequency diversities to mitigate the effects of channel fading and multiple-access interference.
• the operation of the adaptive noise IC is based on a minimum mean square error (MMSE) criterion.
• MMSE minimum mean square error
• Receiver Overview Figure 1 shows an adaptive DS-CDMA multi-user receiver 100 according to our invention.
• the receiver concurrently detects baseband signals from multiple transmitters, e.g., cellular telephones.
• the receiver 100 includes M antennas 101.
• Each antenna 101 is widely spaced such that each concurrently received baseband signal r L - r M 102 can be considered independent from any signal received at another antenna.
• the distance between the antennas 101 is about three to five times the wavelength of the received signal.
• Each antenna 101 is connected to a time-frequency rake (T-F Rake) receiver 200, see Figure 2 for details.
• Each down-sampled signal u i(j is filtered for interference cancellation and channel equalization by a MMSE adaptive filter based interference canceller (IC) 300, see Figure 3.
• IC MMSE adaptive filter based interference canceller
• the IC 300 uses a training signal 105 during an initialization stage to establish weightings for coefficients of equalizer taps of the interference canceller.
• the MMSE based IC 300 outputs two signals, an error signal E i(j 308 and a contributing symbol C iti 309, for data decision by a combiner 110.
• the combiner makes a decision symbol d 109 by maximizing the ratio for the combined contributing symbols 309 from the ICS 300 .
• our receiver 100 can detect the signal on a bit duration basis, instead on a block basis.
• Our receiver is less complex than a conventional block-processing based receiver of the prior art.
• our receiver combats fast fading channels by using the frequency diversity feature. This is a major cause for the degraded performance of conventional MMSE receivers. Therefore, our receiver is particularly suited for base stations in cellular telephone networks .
• FIG. 2 shows the T-F rake receiver 200 in greater detail.
• the T-F rake receiver combines time diversity and frequency diversity to combat multipath-fading channels.
• the received baseband signal ri 102 is modulated to different Doppler frequencies using multiple frequency shifts 210 to generate frequency diversity.
• the number of the frequency shifts depends on the channel conditions .
• a typical number of the frequency shift is one to two.
• the T-F rake receiver 200 provides 3dB improvement in the signal-to-noise-ratio at BER of 10 "4 see, U.S Patent Application Sn.09/487.095" Software-Based Digital Receiver Adaptable to Multiple Multiplexing Schemes," filed by Horng at al. on January 19, 2000.
• FIG. 3 shows the IC 300 in greater detail.
• the real part (Re(*)) 301 of the down sampled signal it i 104 is applied to a corresponding adaptive filter 310,
• the adaptive filter 310 updates its tap-weights w i(j every symbol time T b 103 according to a least mean square (LMS) process 320.
• the sign 302 of the adaptive filter output 303 is taken as a reference signal.
• the reference signal is subtracted 330 by the adaptive filter output to form the error signal e i(j 308.
• the error signal is used as the input for the adaptive process 320 to update the coefficients of the adaptive filter 310.
• the tap-weight vector v ⁇ i ⁇ i is updated as follow
• w i 0 +1) w. ( )+ ⁇ e itJ ( ⁇ ) ⁇ Re(u : j (n)) , (1)
• ⁇ is the step size of the LMS process 320.
• the training signal 105 is used as the reference signal instead of the sign of the adaptive filter output, the contributing symbol C i(j 309.
• the training signal 105 is a predetermined pseudo random (PN) sequence generated by using a polynomial.
• the receiver knows the sequence.
• a transmitter e.g. a cellular telephone, transmits the training signal periodically.
• Different PN offset or different PN sequence can be chosen for different user phones to help resolve interference between users.
• equalizers of the receiver use a locally stored version of the PN training sequence to compare with received training sequence. The difference of the two is used to update the coefficients of the adaptive equalizers. As the equalizers converge over the training stage, the decision error will gradually reduce. This ensures that the coefficients of the adaptive filter are optimal for channel equalization at the end of the training stage.
• All contributing symbols C lfj 309 are combined in an optimal way to form the decision symbol d 109.
• a maximum ratio combiner 110 to perform combining operation, i.e., the contributing symbols C i;j s 309 are combined with different weights according to the error signal E i(j 308.
• the decision symbol d 109 is defined as
• the diversity combiner selects the branch which instantaneously has the highest signal-to-noise ratio (SNR) or smallest error signal E it i 308.
• SNR signal-to-noise ratio
• the present receiver estimates the frequency offset by identifying the location of the possible contributing symbols C 1#j with the smallest error signal ⁇ i(j 308, i.e., j A , is the index for the jth branch in ith antenna element.
• the frequency offset ⁇ f at ith antenna is determined
• ⁇ is the frequency shift 210 in Figure 2.
• f 2 is equal to ( 2 ⁇ ) .
• the frequency offset compensation can be done either at/by the receiver (base station) or at/by the mobile transmitter (cell phone). Here we consider these two cases.
• Case 1 A mobile transceiver (cell phone) does not have the T-F rake receiver. In this case, the compensation is done during the downlink transmission from the base station, because the mobile transceiver cannot estimate the frequency offset using Eq. (4) .
• a compensator 500 determines the frequency offset ⁇ fi and makes transmission carrier frequency
• Figure 5 shows the compensator 500 in greater detail. The frequency offset at ith antenna, ⁇ f if is first determined
• Case 2 A mobile transceiver has the T-F Rake receiver.
• the compensation is done at the mobile user side because the mobile receiver has the capability to estimate the frequency offset, and access to the network by the mobile receivers is random-
• the mobile transmitter can make compensation in the uplink transmission.

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

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• 2001
  • 2001-08-20 CN CN 01802489 patent/CN1419749A/zh active Pending
  • 2001-08-20 WO PCT/JP2001/007118 patent/WO2002017508A2/en not_active Application Discontinuation
  • 2001-08-20 EP EP01956970A patent/EP1230743A2/de not_active Withdrawn
  • 2001-08-20 JP JP2002522083A patent/JP2004507925A/ja not_active Withdrawn

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