Direct-sequence Code-division Multiple-access Downlink Research Articles

Performance of relay-aided downlink DS-CDMA system using transmitter preprocessing based on feedback information

In this contribution, we investigate the performance of relay-aided downlink (DL) direct sequence code division multiple access (DS-CDMA) system with multi-user transmitter preprocessing (MUTP) based on vector quantized channel impulse responses (VQ-CIRs). Specifically, the CIRs are estimated with the aid of training-sequence based estimation technique at each of the relays. These estimated CIRs are then VQ and the magnitudes and phases are fed back to the base station (BS) through feedback channels, that conflict noise and fading. At the BS, the CIRs recovered using a linear detector are then exploited to formulate the preprocessing matrix to mitigate the DL multi-user interference at the relays. Our study shows that the attainable bit-error-rate (BER) degrades when noise and fading contaminated VQ-CIRs are invoked to realize the preprocessing matrix. Nevertheless, the resultant BER performance of the MUTP based on VQ-CIRs acquired via ideal feedback remains close to that achieved with perfect CIRs assumption.

DS-CDMA downlink systems with fading channel employing the generalized receiver

In this paper, we investigate a generalized receiver (GR) constructed on the basis of the generalized approach to signal processing (GASP) in noise employing by direct-sequence code-division multiple access (DS-CDMA) downlink wireless communication system with multipath fading. Transmitted signaling technique is based on using the orthogonal unified complex Hadamard transform spreading sequences. The use of GR allows us to maintain the orthogonality between users and reduce the multipath fading effect and interference from other users. A general multipath-fading model is assumed. Bit-error rate (BER) performance of system is evaluated by means of the signal-to-interference-plus-noise ratio (SINR) at the output of GR employed by DS-CDMA downlink wireless communication system. Using the orthogonal unified complex Hadamard transform spreading sequences as the transmitted signaling technique, we obtain that SINR at the GR output is independent of the phase offsets between different paths. If the Walsh-Hadamard (WH) spreading sequences are used as the transmitted signaling technique, the SINR at the output of GR employed by the same system is a function of squared cosine of path phase offsets. As a result, the BER performance of the last DS-CDMA downlink wireless communication system is worse in comparison with that of the first system. Comparative analysis between the BER performance of DS-CDMA downlink wireless communication systems employing both the GR and the Rake receiver, which consists of a bank of correlation receivers, with each individual receiver correlating with a different arriving multipath component, shows us a superiority of the first system over the second one both at high and low SINRs.

English

The transmitter-based pre-rake diversity combining technique reduces the complexity, size and cost of the mobile unit (MU), while achieving the same inter symbol interference (ISI) mitigation effects of rake receiver for direct sequence-code division multiple access (DS-CDMA) systems. The technique is based on preprocessing of transmitted signal relying on knowledge of the channel state information (CSI) before transmission. In most of the previous works, this a priori information is either assumed or estimated for the uplink and the same is applied to the downlink in time division duplex (TDD) systems due to channel reciprocity. In this paper, a method for channel prediction to evaluate the pre-rake system using binary phaseshift keying (BPSK) modulation in frequency-division duplex (FDD) through analytical and computer simulations for DS-CDMA downlink has been proposed. The performance of the system was also evaluated under ideal and predicted channel conditions using different spreading codes. The findings will have widespread applications in defence communication equipment. Defence Science Journal, 2010, 60(3), pp.282-289 , DOI:http://dx.doi.org/10.14429/dsj.60.355

Transmitter-Preprocessing-Assisted Cooperative Downlink Transmission in DS-CDMA Systems Experiencing Propagation Path Loss and Nakagami-$m$ Fading

In this paper, we propose and investigate a relay-diversity transmission scheme for the direct-sequence code-division multiple-access (DS-CDMA) downlink, where each (destination) mobile terminal (MT) is aided by a cluster of relays to achieve the relay diversity. In the considered system, downlink multiuser interference (MUI) is suppressed with the aid of transmitter preprocessing operated at the base station (BS). Two transmitter preprocessing schemes are considered, which are operated in the principles of transmitter zero forcing (TZF) and transmitter minimum mean square error (TMMSE). At the MTs, signals received from the BS and relays are combined based on the principles of maximal ratio combining (MRC) or maximum signal-to-interference-plus-noise ratio (MSINR). In this paper, the bit-error-rate (BER) performance of the relay-assisted DS-CDMA downlink is investigated when the communication channels are assumed to experience both propagation path loss and generalized Nakagami- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> fading. Our study and simulation results show that the transmitter preprocessing can help to achieve the relay diversity by efficiently mitigating the MUI present at the relays and MTs. Furthermore, in our proposed relay-diversity scheme, the relays only require low-complexity signal processing to forward information to their served MTs.

Performance of DS-CDMA downlink using transmitter preprocessing and relay diversity over Nakagami-m fading channels

In this letter we propose and investigate a cooperative downlink transmission scheme for direct-sequence code-division multiple-access (DS-CDMA) systems, in order to achieve relay diversity. Transmitter zero-forcing (TZF)-based preprocessing is employed at the base-station (BS) of our proposed scheme for suppressing the downlink multiuser interference (MUI). At the mobile terminals (MTs) the signals are combined based on the maximal ratio combining (MRC) principles. As a benefit of TZF, the BS decontaminates the relayed signals from MUI without requiring any information from the relays. Furthermore, only low-complexity signal processing is necessary at the relays for forwarding information to the destination MTs (DMTs), while achieving the relay diversity by only exploiting the knowledge of the DMTs' spreading sequences.

Multiple-Access Interference Plus Noise-Constrained Least Mean Square (MNCLMS) Algorithm for CDMA Systems

Since multiuser code-division multiple-access (CDMA) communications systems suffer significantly from multiple-access interference (MAI) and from classical white Gaussian noise, it is therefore necessary to consider their impact on the performance of these systems. It is well known that the learning speed of any adaptive filtering algorithm is increased by adding a constraint to it. In this paper, a constrained least-mean-square (LMS) algorithm, which incorporates the knowledge of the number of users, spreading sequence length, and additive noise variance, is developed subject to the new combined constraint comprising the MAI and noise variance for a synchronous downlink direct-sequence CDMA system. The novelty of this constraint resides in the fact that the MAI variance was never used as a constraint. In our approach, a Robbins-Monro algorithm is used to minimize the conventional mean-square-error criterion subject to the variance of the new constraint (MAI plus noise). This constrained optimization technique results in an (MAI plus noise)-constrained LMS (MNCLMS) algorithm. The MNCLMS algorithm is a type of variable step-size LMS algorithm where the step-size rule arises naturally from the constraints on MAI and noise variance. Convergence and tracking analysis of the proposed algorithm are carried out in the presence of MAI. Finally, a number of simulations are conducted to compare the performance of the MNCLMS algorithm with other adaptive algorithms.

Transmitter-based processing for down-link DS/CDMA systems with multiple transmit antennas

In cellular wireless communication systems, there have been various receiver-based techniques for performance improvement. However, it may be desirable to use transmitter- based techniques to improve the down-link capacity, since the implementation complexity is less critical at a base station (BS) than at a mobile station (MS). This paper presents a transmitter- based processing for the down-link direct-sequence code-division multiple-access (DS/CDMA) systems with multiple transmit antennas. We propose a combined pre-rake/pre-decorrelating approach. This approach combines the advantage of pre-rake scheme, to achieve diversity gain and average received signal-to- noise ratio (SNR) gain, with that of pre-decorrelating scheme, to suppress multiple access interference (MAI) and multipath interference (MPI). Furthermore, to make the total transmit power the same as that without pre-rake/pre-decorrelating processing, two power normalization methods are presented. Simulation results show that the proposed schemes significantly outperform the conventional transmitter-based techniques. The effects of the number of users and the block size on the bit error rate (BER) performance are also investigated.

Analysis of Serial-Search-Based Code Acquisition in the Multiple-Transmit/Multiple-Receive-Antenna-Aided DS-CDMA Downlink

In this paper, we investigate the serial-search-based initial code-acquisition performance of direct-sequence code division multiple access (DS-CDMA) employing multiple transmit/multiple receive antennas when communicating over uncorrelated Rayleigh channels. We characterize the associated performance trends as a function of the number of antennas. It is demonstrated that, in contrast to our expectation, the achievable correct-detection probability degrades in our typical target operational range as the number of transmit antennas is increased. When maintaining a given total transmit power, our findings suggest that increasing the number of transmit antennas results in the combination of the low-energy noise-contaminated signals of the transmit antennas, which ultimately increases the mean acquisition time (MAT). However, it is extremely undesirable to increase the MAT when the system is capable of attaining its target bit-error-ratio performance at reduced signal-power levels, as a benefit of employing multiple transmit antennas.

Frequency-Domain Interchip Interference Cancelation for DS-CDMA Downlink Transmission

The use of frequency-domain equalization (FDE) based on minimum-mean-square-error criterion can significantly improve the bit-error-rate (BER) performance of orthogonal multicode direct-sequence code-division multiple access downlink signal transmission in a frequency-selective fading channel. However, the presence of residual interchip interference (ICI) after FDE produces the orthogonality distortion among the spreading codes, and the BER performance degrades as the number of multiplex order increases. In this paper, we propose a frequency-domain ICI cancellation scheme, in which the residual ICI replica in the frequency domain is generated and subtracted from each frequency component of the received signal after FDE. Three types of ICI cancelation scheme are presented, and the effectiveness of the proposed ICI cancelation scheme is confirmed by computer simulation

Performance of DS-CDMA Downlink Systems With Orthogonal UCHT Complex Sequences

This letter investigates a transmitted signaling technique using orthogonal unified complex Hadamard transform (UCHT) spreading sequences and the coherent RAKE receiver in direct-sequence code-division multiple-access (DS-CDMA) downlinks to maintain the orthogonality between users and reduce the effect of multipath fading and interference from other users. A general multipath-fading channel model is assumed. System performance is evaluated by means of signal-to-interference-plus-noise ratio (SINR) at the RAKE receiver. It is shown that the SINR of the system employing UCHT complex sequences is independent of the phase offsets between different paths, while the SINR of the system using Walsh-Hadamard (WH) sequences is related to the squared cosine of path phase offsets. As a result, the bit-error ratio performance of the DS-CDMA downlink system employing UCHT complex sequences is better than that of the system with WH sequences at high SINRs

Convolutional Spreading CDMA and Comparison With the DS-CDMA With RAKE Receiver

For synchronous downlink direct-sequence code-division multiple access (DS-CDMA), we introduce a cyclic-prefix (CP)-based convolutional spreading CDMA (CS-CDMA/CP) scheme employing zero correlation zone (ZCZ) codes, which gives a class of multiuser interference (MUI)-free CDMA schemes, including Suehiro's CDMA and block spreading (BS) CDMA as special cases. We show that CS-CDMA/CP employing appropriately selected binary and ternary ZCZ codes have good user capacity, peak-to-average power ratio (PAPR), and transmission power loss, compared with previously proposed BS-CDMA schemes. By simulation, we show the effect of PAPR to its bit-error rate (BER) performance when nonlinear amplifiers are used, and also compare its BER performance with the conventional DS-CDMA scheme employing a RAKE receiver

DS-CDMA downlink system capacity enhancement through interference suppression

In the downlink of direct-sequence code-division multiple-access (DS-CDMA) cellular systems, system capacity is limited by multiuser interference. This interference can be suppressed at the receiver by interference whitening approaches such as chip equalization and generalized RAKE (G-RAKE) reception. In this paper, we show how these advanced receivers can increase system capacity. First, the G-RAKE receiver formulation is extended to include soft handoff and other-cell interference. Then, voice capacity is evaluated, including the effects of fast fading, interference correlation, power control, orthogonality properties of own-cell signals, and soft handoff. Numerical results for the wideband CDMA system show that when the channel is dispersive, advanced receivers improve capacity by 30-34%

SINR for DS-CDMA with random spreading

For wireless multipath fading channels, the performance of a direct-sequence code-division multiple-access (DS-CDMA) system is degraded by the interferences, including the multipath interference (MPI) and the multiuser interference (MUI). For the downlink DS-CDMA with random spreading, we derive the variances of the interferences and obtain the signal-to-interference-plus-noise ratio (SINR). The simple and elegant expressions for the variances of the interferences provide useful insights, while the corresponding SINR representation is helpful in evaluating the bit-error-rate (BER) performance that takes the impact of the interferences into consideration. In order to demonstrate our approach, an example regarding the performance analysis of the two-input–single-output (2ISO) space–time block coded DS-CDMA with RAKE reception is also provided.

Adaptive chip-rate equalization of downlink multirate wideband CDMA

We consider a downlink direct sequence-code division multiple access (DS-CDMA) system in which multirate user signals are transmitted via synchronous orthogonal short codes overlaid with a common scrambling sequence. The transmitted signal is subjected to significant time- and frequency-selective multipath fading. In response to this scenario, a novel two-mode receiver is proposed that accomplishes chip-rate adaptive equalization aided by filtering and/or cancellation of multiaccess interference (MAI). In the acquisition mode, a code-multiplexed pilot is used to adapt the equalizer from cold start or loss-of-lock. The use of MAI filtering results in a third-order least mean squares (LMS) algorithm, which has significant advantages over standard (i.e., first-order) LMS in nonstationary environments. In the tracking mode, decision-direction facilitates MAI-cancellation in the equalizer update, which enhances performance. The receiver monitors pilot decision quality as a means of switching between the two modes. The performance of the adaptive receiver is studied through analysis and simulation.

Asymptotically Fair Transmission Scheduling Over Fading Channels

The problem of scheduling delay insensitive data over a direct sequence code-division multiple-access downlink fading channel is considered. This work considers the case where only one user at a time is allowed to transmit. As the channels can be considered to vary asynchronously among the users, multiuser diversity gains can be obtained by exploiting channel adaptive scheduling. For this, schedulers of different adaptation rate are suggested and compared. In particular, the authors analyze a simple fast scheduler that schedules a user to transmit when its channel is good relative to its mean. This scheduler is resource fair in the sense it can provide the users with the same asymptotic channel access. They show that the achievable scheduling gain compared to round robin is equal to the gain of a selection diversity scheme. The scheduling gain is determined, both on closed-form and by numerical integration, for several scenarios, including multicellular systems and the effect of time delayed channel estimates. The results show that the scheduling gain is larger for channels with low average quality.

Data throughput enhancement by frequency selectivity suppression in downlink DS-CDMA beamforming systems

A novel multi-transmit and multi-receive (MTMR) adaptive array antenna (AAA) system combined with analogue-type transmit power control (TPC) to suppress channel frequency selectivity is proposed. This technique increases throughput with adaptive modulation and coding (AMC) employed at the downlink of direct-sequence code division multiple access (DS-CDMA) systems. The proposed scheme employs beam directivity control using a path manipulation technique. Its superiority over the conventional AAA systems is demonstrated by computer simulation.

Blind carrier frequency offset estimation for noncircular constellation-based transmissions

We address the problem of nondata aided frequency offset estimation for noncircular transmissions over frequency-selective channels in a linear precoder-based communications context. Linear precoding is representative of a downlink direct-sequence code division multiple access (DS-CDMA) system or of an orthogonal frequency division multiplexing (OFDM) system. We observe that twice the frequency offset is a cyclic frequency of the received signal. We thus introduce an estimator relying on the maximization of the empirical cyclocorrelations. We analyze its asymptotic behavior and obtain a closed-form expression for the asymptotic covariance. This enables us to design relevant system parameters. Numerical illustrations are provided and confirm our theoretical assertions. Comparisons with existing methods are also reported.

Space-time block coding for single-carrier block transmission DS-CDMA downlink

The combination of space-time block coding (STBC) and direct-sequence code-division multiple access (DS-CDMA) has the potential to increase the performance of multiple users in a cellular network. However, if not carefully designed, the resulting transmission scheme suffers from increased multiuser interference (MUI), which dramatically deteriorates the performance. To tackle this MUI problem in the downlink, we combine two specific DS-CDMA and STBC techniques, namely single-carrier block transmission (SCBT) DS-CDMA and time-reversal STBC. The resulting transmission scheme allows for deterministic maximum-likelihood (ML) user separation through low-complexity code-matched filtering, as well as deterministic ML transmit stream separation through linear processing. Moreover, it can achieve maximum diversity gains of N/sub T/N/sub R/(L+1) for every user in the system, irrespective of the system load, where N/sub T/ is the number of transmit antennas, N/sub R/ the number of receive antennas, and L the order of the underlying multipath channels. In addition, it turns out that a low-complexity linear receiver based on frequency-domain equalization comes close to extracting the full diversity in reduced, as well as full load settings. In this perspective, we also develop two (recursive) least squares methods for direct equalizer design. Simulation results demonstrate the outstanding performance of the proposed transceiver compared to competing alternatives.