Downlink transmission of broadband OFCDM systems-part III: turbo-coded

Orthogonal frequency and code-division multiplexing (OFCDM) systems have been introduced for high-speed data transmission in future wireless mobile communications. In this paper, a hybrid multicode interference cancellation (MCI) and minimum mean-square error (MMSE) detection scheme is presented for the turbo-coded OFCDM systems. Channel estimation based on a code-multiplexed pilot channel is employed. The weights of the hybrid detection are derived theoretically and an effective method to generate the weights in practical applications is proposed. By means of computer simulation, the effects of system parameters on the performance are studied extensively. It is shown that the hybrid detection outperforms pure MMSE detection in various channel conditions, especially for high-level modulation schemes. To carry out interference regeneration for the hybrid detection, the conventional turbo decoding algorithm which only decodes systematic bits should be extended to decode parity bits as well. Moreover, two iterations in turbo decoding are sufficient to provide good performance for the system with the multistage hybrid detection. Finally, given time-domain spreading factor N/sub T/, the system performance improves with frequency-domain spreading factor N/sub F/. However, the frequency diversity gain is saturated when N/sub F/ is large (i.e., N/sub F//spl ges/16).     

Adaptive control of surviving symbol replica candidates in QRM-MLD for OFDM MIMO multiplexing

This paper proposes adaptive control of the number of surviving symbol replica candidates, S/sub m/ (m denotes the stage index), based on the minimum accumulated branch metric of each stage in maximum-likelihood detection employing QR decomposition and the M-algorithm (QRM-MLD) in orthogonal frequency-division multiplexing with multiple-input-multiple-output (MIMO) multiplexing. In the proposed algorithm, S/sub m/ at the mth stage (1/spl les/m/spl les/N/sub t/, N/sub t/ is the number of transmission antenna branches) is independently controlled using the threshold value calculated from the minimum accumulated branch metric at that stage and the estimated noise power. We compared the computational complexity of QRM-MLD employing the proposed algorithm with that of conventional methods at the same average packet error rate assuming the information bit rate of 1.048 Gb/s in a 100-MHz channel bandwidth (i.e., frequency efficiency of approximately 10 bit/s/Hz) using 16QAM modulation and turbo coding with the coding rate of 8/9 in 4-by-4 MIMO multiplexing. Computer simulation results show that the average computational complexity of the branch metrics, i.e., squared Euclidian distances, of the proposed adaptive independent S/sub m/ control method is decreased to approximately 38% that of the conventional adaptive common S/sub m/ control and to approximately 30% that of the fixed S/sub m/ method (S/sub m/=M=16), assuming fair conditions such that the maximum number of surviving symbol replicas at each stage is set to M/spl circ/=16.     

Experiments on fast cell search algorithm for intercellasynchronous W-CDMA mobile radio

Laboratory and field experiment results for a two-step cell search algorithm using scramble code masking for intercell asynchronous W-CDMA mobile radio are presented. The scramble code is masked at different time positions during each scramble period in the forward link common control channel (CCH) for scramble code timing detection at the mobile receiver. 16 scramble codes are used. The cell search time achievable with the authors' recently proposed three-step cell search algorithm is estimated from the experimental results; cell search can be accomplished within ~960 ms at a probability of 93% in the case of 512 scramble codes and 16 scramble code groups     

Experimental evaluation of combined effect of coherent RAKEcombining and SIR-based fast transmit power control for reverse link ofDS-CDMA mobile radio

The combined effect of coherent RAKE combining using the weighted multislot averaging (WMSA) channel estimation filter and closed-loop fast transmit power control (TPC) in the 4.096 Mchip/s direct sequence code division multiple access (DS-CDMA) mobile radio reverse link is experimentally evaluated. The WMSA channel estimation filter utilizes periodically transmitted pilot symbols (four pilot symbols are time-multiplexed in each 40-symbol time slot). Its observation period is extended to 2-K slots in order to improve the accuracy of the channel estimation. The fast TPC is based on the measurement of signal-to-interference plus background noise ratio (SIR) using pilot symbols. Laboratory experiments show that the use of the K=2 WMSA channel estimation filter reduces the required E_b/I₀ at the average BER of 10^-3 by approximately 0.5 dB compared to use of the linear interpolation filter, and that the required E_b/I₀ is minimized when the SIR measurement interval is M=10 symbols (one slot TPC delay). It was also clarified that SIR-based TPC works satisfactorily when two users with different information data rates, i.e., SF, independently employ fast TPC. Field experimental results obtained in an area nearby Tokyo showed that the average BER of 10^-3 is achieved at the target E_b/I₀ per antenna of approximately 2.5 dB by using four-finger branch RAKE and two-branch antenna diversity. Although the target E_b/I₀ to achieve same BER, when there is one interfering user with a fourfold greater transmit power than that of the desired user that independently employs fast TPC, is almost the same as that in the single-user case, the mobile transmit power is increased by 1.0-2.0 dB due to the increased MAI. These results indicate that the combination of coherent RAKE combining and fast TPC works well in practical multipath fading channels     

Experiments on pilot symbol-assisted coherent multistageinterference canceller for DS-CDMA mobile radio

A three-stage coherent multistage interference canceller (COMSIC) employing pilot symbol-assisted (PSA) channel estimation for replica generation of multiple access interference (MAI) is implemented and its performance in the presence of frequency selective multipath fading is experimentally evaluated by a multipath fading simulator. A fast transmission power control (TPC) method suitable for COMSIC is also proposed, in which the signal-to-interference plus background noise power ratio (SIR) at the matched filter (MF) based RAKE receiver is measured to achieve a short TPC delay and the target signal-to-interference ratio value is compensated by an outer loop so that the measured block error rate (BLER) is equal to the prescribed target value. The experimental results show that as expected the COMSIC satisfactorily reduces the MAI even when the number of active users is equal to the spreading factor in a multipath fading environment, and thus, improves the bit error rate (BER) performance in a multiuser environment. The results also show that the proposed fast TPC method with a two-slot delay associated with COMSIC works satisfactorily and the combination of COMSIC and fast TPC significantly decreases the transmission power of a mobile station (required transmission power of a mobile station with COMSIC at the average BER of 10^-3 is decreased by approximately 2.0 (3.0) dB compared with the MF-based RAKE receiver with (without) antenna diversity reception). This extends the cell coverage, battery life, and increases the system capacity in the reverse link     

Tree-structured generation of orthogonal spreading codes withdifferent lengths for forward link of DS-CDMA mobile radio

Tree-structured generation of orthogonal spreading codes with different lengths is presented for orthogonal multiplexing of forward-link code-channels of different data rates in direct sequence code division multiple access DS-CDMA mobile radio. The bit error rate performance under a multi-user environment suffering multipath Rayleigh fading is evaluated by computer simulation     

Decision feedback differential phase detection of M-ary DPSKsignals

Multiple-symbol differential phase detection (DFDPD) based on decision feedback of past detected symbols is presented for M-ary DPSK modulation. Adopting a Gaussian phase noise assumption, we obtain the a posteriori joint probability density function (PDF) of the outputs of L DPD defectors of orders of 1 to L symbols and derive a DF-DPD algorithm which is based on feeding back the L-1 past detected symbols and minimizing the sum of phase errors of L DPD detectors. A practical implementation of the DF-DPD receiver is presented that uses a single conventional (one-symbol) DPD detector. The bit error rate (BER) performance in an additive white Gaussian noise (AWGN) channel is analyzed taking into account decision error propagation. Performance improvements are evaluated by computer simulations in AWGN and Rayleigh fading channels     

Experiments on real-time 1-Gb/s packet transmission using MLD-based signal detection in MIMO-OFDM broadband radio access

This paper presents experimental results on real-time packet transmission of greater than 1 Gb/s using 4-by-4 multiple-input-multiple-output (MIMO) multiplexing and maximum-likelihood detection (MLD)-based signal detection with a decreased level of computational complexity in orthogonal frequency-division multiplexing (OFDM) radio access. We apply our previous algorithm called adaptive selection of surviving symbol replica candidates (ASESS) based on the maximum reliability in MLD employing QR decomposition and the M-algorithm (QRM-MLD) to reduce the extremely high level of computational complexity in the conventional MLD. The experimental results using multipath fading simulators are in good agreement with the computer simulation results. The loss in the required received signal energy per bit-to-background noise power spectrum density ratio (E/sub b//N/sub 0/) is suppressed to within approximately 1-2 dB. Therefore, through experiments, we demonstrate that the QRM-MLD employing ASESS is very beneficial in reducing the influence of hardware implementation loss, as well as in decreasing the required received E/sub b//N/sub 0/. We further show that the extremely high-speed real-time packet transmission of greater than 1 Gb/s in a 100-MHz channel bandwidth (i.e., 10 bit/s/Hz) is achieved at the average received E/sub b//N/sub 0/ per receiver antenna of approximately 12 dB using 16QAM modulation and turbo coding with the coding rate of 8/9 in 4-by-4 MIMO multiplexing.     

Multicarrier 16QAM transmission with diversity reception

A 96 multicarrier 16QAM transmitter and a diversity receiver for 3.072 Mbit/s data transmission are described. Pilot symbol aided (PSA) coherent detection is applied. A laboratory experiment demonstrates that an irreducible bit error rate (BER) of 10^-3 can be achieved at the RMS delay spread τ_rms=6.3 μs under frequency selective Rayleigh fading when two branch maximal ratio combining (MRC) diversity is used     

Multipath interference canceller for high-speed packet transmissionwith adaptive modulation and coding scheme in W-CDMA forward link

This paper proposes a multipath interference canceller (MPIC) associated with orthogonal code-multiplexing that achieves much higher peak throughput than 2 Mbit/s with adaptive data modulation for high-speed packet transmission in the wideband direct sequence-code division multiple access (W-CDMA) forward link, and evaluates its throughput performance by computer simulation. The simulation results elucidate that sufficient multipath interference (MPI) suppression is achieved by a four-stage MPIC with 6-12 orthogonal code-multiplexing using one iterative channel estimation with pilot and decision feedback data symbols and further that the interference rejection weight control according to the number of observed multipaths is effective in improving the throughput. It is also demonstrated that MPIC exhibits a superior MPI suppression effect to a chip equalizer in the lower received signal energy per bit-to-background noise spectrum density (E_b/N₀) channel around 0-3 dB owing to the successive channel estimation at each stage. We show that the maximum peak throughput using MPIC is approximately 2.1 fold that without MPIC in a two-path and three-path Rayleigh fading channel and that the peak throughput of 8.0 Mbit/s is achieved using 64 QAM data modulation in a two-path fading channel within a 5 MHz bandwidth. Furthermore, the required average E_b/N₀ for satisfying the same throughput with MPIC is decreased by more than 2.0 dB