Reduced-complexity Receiver Research Articles

Reduced-Complexity Receiver Structures for Space–Time Bit-Interleaved Coded Modulation Systems

Transmission efficiency in radio channels can be considerably improved by using multiple transmit and receive antennas and employing a family of schemes called space-time (ST) coding. Both extended range and/or improved bandwidth efficiency can be achieved, compared with a radio link with a single transmit and receive antenna. Bit-interleaved coded modulation schemes give diversity gains on fading channels with higher order modulation constellations combined with conventional binary convolutional codes also for the case of a single transmit and receive antenna radio link. In this paper, we study a family of flexible bandwidth-efficient ST coding schemes which combine these two ideas in a narrowband flat-fading channel and single-carrier modems. We address receiver complexity for the case of a large number of transmit antennas and higher order modulation constellations. Especially, we focus on practical configurations, where the number of transmit antennas is greater than that of receive antennas. Simplified receivers using tentative decisions are proposed and evaluated by means of simulations. Tradeoffs between complexity reduction and performance loss are presented. We emphasize systems that are of particular interest in applications where the number of transmit antennas exceeds the number of receive antennas. A system with four transmit antennas with an eight-fold complexity reduction and a performance loss of about 1 dB is demonstrated

Towards multi-mode terminals

The increasing demand for wireless based services continues to grow with a multiplicity of standards developing. This is driving a need to develop integrated mobile terminals which have multi-mode capabilities, i.e., multiple communication systems which can coexist. The attention of this paper is focused on the coexistence of multiple communication systems via a simple hardware based structure with the algorithms and architectures developed within the framework of multi-mode terminals. The term multi-mode in this paper is limited to a set of standards and to a receiver that is expected to handle different systems with a low hardware complexity. Dedicated hardware is shared and reused by different systems. Existing specifications DAB, DVB-T, HIPERLAN-2 and UMTS are considered as examples in this paper, although the methodology could be applied to other systems. The systems considered can be classified into two categories: multi-carrier systems and single-carrier systems. This paper considers multi-mode terminals from both the algorithm and the architecture viewpoint, reducing receiver complexity by taking advantage of the commonalities among different specifications and receiver requirements. As an example, a joint channel estimation, chip-level frequency domain equalization and parallel interference cancellation structure for WCDMA is presented

Optimum and reduced complexity multiuser detectors for asynchronous CPM signaling

Maximum likelihood detector algorithms are developed for the matrix of transmitted symbols in a multiuser system in which the received signal is the sum of K cochannel continuous phase modulated (CPM) signals and additive white Gaussian noise. We illustrate that the maximum likelihood matrix detector, which provides optimum detector performance, consists of K sets of front-end matched filters followed by a Viterbi algorithm. We also derive two reduced complexity receivers, demonstrating through simulation that they perform within 1-2 dB of the optimal while substantially reducing complexity. The paper demonstrates how performance can be traded off against complexity, giving particular attention to cochannel Gaussian minimum shift keyed (GMSK) signals

Reduced-Complexity Receiver Structures for Space–Time Bit-Interleaved Coded Modulation Systems

Reduced-Complexity Receiver Structures for Space–Time Bit-Interleaved Coded Modulation Systems

An Iterative Receiver for Uplink of Coded MIMO DS-CDMA System Employing Layered Space-Time Transmission

We consider a coded multiple-input multiple-output (MIMO) DS-CDMA system using layered space-time transmission in multipath wireless channels, where space-time signals from multiple antennas of multiple users propagate through rich scattering multipath fading. We propose a receiver employing iterative joint detection and decoding with a reduced-complexity detector using linear minimum mean squared error filtering with a priori information and parallel soft-input soft-output (SISO) decoders. Computer simulation results show that the proposed receiver for multi-user MIMO transmission provides high-spectral efficiency and performance approaching to single-user bound. Furthermore, the reduced-complexity receiver outperforms an iterative soft decision-directed maximal ratio combining (DD-MRC) receiver, RAKE receiver as well as a conventional non-iterative receiver.

Trellis-Based Iterative Adaptive Blind Sequence Estimation for Uncoded/Coded Systems with Differential Precoding

We propose iterative, adaptive trellis-based blind sequence estimators, which can be interpreted as reduced-complexity receivers derived from the joint ML data/channel estimation problem. The number of states in the trellis is considered as a design parameter, providing a trade-off between performance and complexity. For symmetrical signal constellations, differential encoding or generalizations thereof are necessary to combat the phase ambiguity. At the receiver, the structure of the super-trellis (representing differential encoding and intersymbol interference) is explicitly exploited rather than doing differential decoding just for resolving the problem of phase ambiguity. In uncoded systems, it is shown that the data sequence can only be determined up to an unknown shift index. This shift ambiguity can be resolved by taking an outer channel encoder into account. The average magnitude of the soft outputs from the corresponding channel decoder is exploited to identify the shift index. For frequency-hopping systems over fading channels, a double serially concatenated scheme is proposed, where the inner code is applied to combat the shift ambiguity and the outer code provides time diversity in conjunction with an interburst interleaver.

Reduced complexity receivers for layered space-time CPM

Layered space-time (LST) transmissions employing continuous phase modulations (CPM) are well motivated for both bandwidth- and power-limited multiantenna communications. However, one of the major challenges for LST-CPM is the high complexity it incurs with maximum likelihood (ML) detection. In this paper, we develop reduced complexity LST-CPM receivers. First, we consider single antenna systems. Specifically, we study a reduced complexity Viterbi receiver for binary CPM. Based on this design, we introduce differential encoding for a class of CPM signals and analyze its performance gain both theoretically and with simulations. Second, we focus on multiantenna LST systems with minimum shift-keying (MSK)-type modulations. With group nulling-canceling (NC) and low-complexity linear equalization, we convert a coded multiuser detection problem into an uncoded one with small equalization loss. We also find that the combination of sphere decoding with hard-decision iterative processing is effective in boosting performance with a controllable complexity increase. Both analytical and simulated performance confirm that the novel LST-MSK receiver exhibits markedly improved performance relative to conventional NC detectors with moderate complexity increase.

On Walsh Filtering Method for Decoding of CPM Signals

Walsh filtering has been used as a method to reduce receiver complexity in several coding and modulation systems, especially in continuous phase modulation systems. In this paper, we show that its lowpass filtering ability is poor and alias components arising from an adjacent channel can significantly degrade the maximum-likelihood decoding. Instead, a lowpass filtering method is more robust against adjacent channel interference and thus gives less decoding errors than the Walsh filtering method.

Design of Reduced-Rank MMSE Multiuser Detectors Using Random Matrix Methods

Reduced-rank minimum mean-squared error (MMSE) multiuser detectors using asymptotic weights have been shown to reduce receiver complexity while maintaining good performance in long-sequence code-division multiple-access (CDMA) systems. In this paper, we consider the design of reduced-rank MMSE receivers in a general framework which includes fading, single and multiantenna receivers, as well as direct-sequence CDMA (DS-CDMA) and multicarrier CDMA (both uplink and downlink). In all these cases, random matrix results are used to obtain explicit expressions for the asymptotic eigenvalue moments of the interference autocorrelation matrix and for the asymptotic weights used in the reduced-rank receiver.

A RAKE-Based Iterative Receiver for Space-Time Block-Coded Multipath CDMA

A turbo multiuser receiver is proposed for space-time block and channel-coded code division multiple access (CDMA) systems in multipath channels. The proposed receiver consists of a first stage that performs detection, space-time decoding, and multipath combining followed by a second stage that performs the channel decoding. A reduced complexity receiver suitable for systems with large numbers of transmitter antennas is obtained by performing the space-time decoding along each resolvable multipath component and then diversity combining the set of space-time decoded outputs. By exchanging the soft information between the first and second stages, the receiver performance is improved via iteration. Simulation results show that while in some cases a noniterative space-time coded system may have inferior performance compared with a system without space-time coding in a multipath channel, proposed iterative schemes significantly outperform systems without space-time coding, even with only two iterations. Furthermore, the performance loss in the reduced-complexity receiver due to decoupling of interference suppression, space-time decoding, and multipath combining is very small for error rates of practical interest.

Reference-based dual switch and stay diversity systems over correlated Nakagami fading channels

We provide new generic and exact analytical results for the performance of nonideal reference-based dual predetection switch and stay diversity systems in receiving M-ary digitally modulated signals in the presence of additive white Gaussian noise and correlated slow and nonselective Nakagami-m fading channels. Pilot-tone-aided, pilot-symbol-aided, and differential detection (DD) reference-based systems are considered. The impact of symbol alphabet cardinality, normalized distance between antennas, fading severity, and normalized Doppler frequency on the performance of these systems is analyzed. Optimum switching threshold and optimum pilot-to-signal power ratio as a function of channel fading characteristics, normalized distance between antennas, and modulation type are determined. Furthermore, some fixed switching strategies - minimum cost strategy, fixed average strategy, and midpoint strategy - that allow one to obtain diversity gain with a reduced complexity receiver are considered.

A new reduced-complexity algorithm for multiple-symbol differential detection

Although multiple-symbol differential detection (MSDD) provides much better error performance than conventional differential detection, its complexity is much higher. We propose a reduced-complexity algorithm for MSDD. Our simulations show that the performance of the reduced complexity receiver is nearly identical to that of the original receiver, but with typical parameters, the number of phase sequences searched can be reduced by a factor of sixteen for QPSK to over four thousand for 16-PSK thereby leading to significant receiver complexity reduction.

Layered space-time multiuser detection over wireless uplink systems

In this paper, we investigate the use of layered space-time (also known as the vertical Bell Laboratories layered space-time (V-BLAST) scheme) for multiuser detection in fading channels. The multiple transmit antennas in V-BLAST are treated as individual mobile station transmitters, while the base station consists of multiple receive antennas. In the proposed system, users are organized in groups and allocated a unique spreading code within the same group. Using these codes, we are able to separate the different groups, and layered space-time algorithm is then invoked to further remove the remaining interference between users. A decorrelator-type receiver-based layered space-time detection is proposed for both complex and real constellations. For the latter case, we derive our receiver after evaluating and comparing the performance of two decorrelators based on the V-BLAST scheme. It is demonstrated that a significant performance improvement and increase in system capacity is obtained with very low spreading factors. Further results are also introduced by considering reduced complexity receivers based on serial layered space-time group multiuser detection, and parallel layered space-time group multiuser detection.

Optimum diversity detection over fading dispersive channels with non-Gaussian noise

We consider the problem of M-ary signal detection over a single-input-multiple-output (SIMO) channel affected by frequency-dispersive Rayleigh-distributed fading and corrupted by additive non-Gaussian noise, modeled as a spherically invariant random process. We derive both the optimum detection structure and a suboptimal, reduced-complexity receiver, based on the low-energy-coherence approach. Interestingly, both detection structures are canonical, i.e., they are independent of the actual noise statistics. We also carry out a performance analysis of both receivers, with reference to the case that the channel is affected by a frequency-selective fading and for a binary frequency-shift-keying signaling format. The results obtained through both a Chernoff-bounding technique and Monte Carlo simulations reveal that the adoption of diversity also represents a suitable means to restore performance in the presence of dispersive fading and impulsive non-Gaussian noise. Interestingly, it is also shown that the suboptimal receiver incurs a limited loss with respect to the optimum (unrealizable) receiving structure.

Multiuser detectors with decision feedback for asynchronous spread-spectrum multiple access in multipath fading channels

This paper extends earlier work of the authors to the multipath fading channel. For a normalized Doppler spread of f/sub D/T=0.005 we show that a reduced state sequence estimation reduced-complexity receiver with K/sub s/=2 users per state and a processing gain of N/sub s/=127 performs only 0.5 dB worse than the single-user receiver for a bit-error rate of 10/sup -4/.

Multiple-symbol detection for orthogonal modulation in CDMA system

We investigate in this paper the performance of orthogonal modulations with noncoherent multiple-symbol detection (MSD). It is shown that for uncoded systems, the asymptotic error performance of the MSD is almost as good as that of the ideal coherent detector. The application of MSD to the CDMA IS-95 reverse link, which represents a coded system, is also considered. The simulation results show that the MSD receiver is about 0.5 dB better than the conventional symbol-by-symbol detector in both the additive white Gaussian noise and Rayleigh fading channels. Finally, reduced-complexity MSD receivers with little performance degradation are also presented in this paper.

DPSK versus pilot-aided PSK MAP equalization for fast-fading channels

This letter compares pilot-aided phase-shift keyed (PSK) and differential PSK (DPSK) modulation when using new maximum a posteriori joint channel estimation and equalization receivers with frequency-selective fast-fading channels. We conclude that pilot-aided PSK has superior bit-error rate performance in this case. However, at low signal-to-noise ratio, performance is similar, and DPSK is competitive due to reduced receiver complexity.

Decomposition of fading dispersive channels-effects of mismatch on the performance of MLSE

Decompositions of fading dispersive channels find utility in channel modeling and in the estimation of channel state information in a receiver. This letter considers decompositions based on the f-power series and Karhunen-Loeve expansions and investigates their relative ability to model selected channels. Development of such models should enable the design of reduced complexity receiver structures.

Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels

Transmitter diversity wireless communication systems over Rayleigh fading channels using pilot symbol assisted modulation (PSAM) are studied. Unlike conventional transmitter diversity systems with PSAM that estimate the superimposed fading process, we are able to estimate each individual fading process corresponding to the multiple transmitters by using appropriately designed pilot symbol sequences. With such sequences, special coded modulation schemes can then be designed to access the diversity provided by the multiple transmitters without having to use an interleaver or expand the signal bandwidth. The code matrix notion is introduced for the coded modulation scheme, and its design criteria are also established. In addition to the reduction in receiver complexity, simulation results are compared to, and shown to be superior to, that of an intentional frequency offset system over a wide range of system parameters.

Multi-user performance of direct-sequence CDMA using combined binary PPM/orthogonal modulation

A new modulation format is proposed for cellular code-division multiple-access (CDMA) communications where binary pulse position modulation (PPM) is embedded in the chip waveform and combined with orthogonal modulation using Walsh/Hadamard codes. Compared to the conventional CDMA using orthogonal codes, this scheme allows reduction in receiver complexity by lowering the modulation level for the second-stage orthogonal modulation. The staggered (half-chip) quadrature direct-sequence signaling is adopted to uniformly distribute the transmit power and allow noncoherent detection at the receiver because carrier phase tracking is not feasible because of the binary PPM, suitable for the reverse link in cellular networks. Statistics of inter-user interferences are characterized, and then derive the symbol error probability for the proposed M-ary modulation format. It is shown that the advantage in view of receiver complexity can be achieved without deteriorating the multi-user performance in terms of the number of users affordable at a specified error rate.