Transmit diversity schemes with multiple streams obtain reliable transmission but suffer from interference when decoding multiple streams simultaneously. In this paper, we present closed-loop space-time block codes for multiple streams with limited channel information in order to ease suppressing interference at a receiving station. The proposed scheme includes constellation rotation in Alamouti coding pairs by feedback angle information. The sub-optimum solution for feedback angle is found to maximize the output signal-to-noise ratio (SNR). Simulation results show the proposed scheme with only a few bits feedback achieves nearly full diversity gain with reduced interference.

In this paper, new space time/frequency block coding (STBC/SFBC) scheme with angle feedback for 4-transmit antennas is proposed. This scheme achieves the full diversity gain with the full code rate by rotating constellation symbols in STBC/SFBC codewords using the quasi-orthogonal structure. With multiple receive antennas, this scheme can also be applied with additional receive antenna diversity. We show the channel characteristics can be much improved with as small as 1-bit feedback overhead. Simulation shows that the proposed scheme with 1-bit feedback achieves near-optimum performance.

A subspace beamforming method is presented that decomposes a MIMO channel into multiple pairs of subchannels. The pairing is done based on singular values such that similar channel capacity is obtained between different subchannel pairs. This new capacity balancing concept is key to achieving high performance with low complexity. We apply the subspace idea to geometric mean decomposition (GMD) and maximum likelihood (ML) detection. The proposed subspace GMD scheme requires only two layers of detection/decoding, regardless of the total number of subchannels, thus alleviating the latency issue associated with conventional GMD. We also show how the subspace concept makes the optimization of ML beamforming and ML detection itself feasible for any K times K MIMO system. Simulation results show that subspace beamforming performs nearly as well as optimum GMD performance, and to within only a few dB of the Shannon bound.

This paper describes a layered space-time (LST) approach for multiple-input multiple-output (MIMO) channels where list decisions of each data substream are used to subtract interference when detecting the next data substream. We study this approach in the context of the IEEE 802.11 standard using orthogonal frequency-division multiplexing (OFDM) and bit-interleaved coded modulation of each subcarrier. We show that ideal list LST with increased list size can potentially approach the MIMO Shannon bound, but performance of real implementation based on a reduced-complexity demapper is limited by competing Euclidean distances common in any reduced-state maximum likelihood detection.

By deriving a generalized Shannon capacity formula for multiple-input, multiple-output Rayleigh fading channels, and by suggesting a layered space-time architecture concept that attains a tight lower bound on the capacity achievable, Foschini (see Wireless Personal Communications, vol.6, no.3, p.311-35, 1998) has shown a potential enormous increase in the information capacity of a wireless system employing multiple-element antenna arrays at both the transmitter and receiver. This paper includes two important contributions. First, we show that Foschini's lower bound is, in fact, the Shannon bound when the output signal-to-noise ratio (SNR) of the space-time processing in each layer is represented by the corresponding "matched filter" bound. This proves the optimality of the layered space-time concept. Second, we present an embodiment of this concept for a coded system operating at a low average SNR and in the presence of possible intersymbol interference. This embodiment utilizes the already advanced space-time filtering, coding and turbo processing techniques to provide yet a practical solution to the processing needed. Performance results are provided for quasistatic Rayleigh fading channels with no channel estimation errors. We see for the first time that the Shannon capacity for wireless communications can be both increased by N times (where N is the number of the antenna elements at the transmitter and receiver) and achieved within about 3 dB in average SNR, about 2 dB of which is a loss due to the practical coding scheme we assume-the layered space-time processing itself is nearly information-lossless!.

Enhanced Data Rates for Global Evolution (EDGE) is currently being standardized as an evolution of GSM in Europe and of IS-136 in the United States as an air interface for high speed data services for third generation mobile systems. In this paper, we study space-time processing for EDGE to provide interference suppression. We consider the use of two receive antennas and propose a joint equalization and diversity receiver. This receiver uses feedforward filters on each diversity branch to perform minimum mean-square error cochannel interference suppression, while leaving the intersymbol interference to be mitigated by the subsequent equalizer. The equalizer is a delayed decision feedback sequence estimator, consisting of a reduced-state Viterbi processor and a feedback filter. The equalizer provides soft output to the channel decoder after deinterleaving. We describe a novel weight generation algorithm and present simulation results on the link performance of EDGE with interference suppression. These results show a significant improvement in the signal-to-interference ratio (SIR) performance due to both diversity (against fading) and interference suppression. At a 10% block error rate, the proposed receiver provides a 20 dB improvement in SIR for both the typical urban and hilly terrain profiles.

By deriving a generalized Shannon capacity formula for multiple-input, multiple-output Rayleigh fading channels, and by suggesting a layered space-time architecture concept that attains a tight lower bound on the capacity achievable. Foschini (see Wireless Pers. Commun., vol.6, no.3, p.311-35, 1998) has shown a potential enormous increase in the information capacity of a wireless system employing multiple-element antenna arrays at both the transmitter and receiver. The layered space-time architecture allows signal processing complexity to grow linearly, rather than exponentially, with the promised capacity increase. This paper includes two important contributions. First, we show that Foschini's lower bound is, in fact, the Shannon bound when the output signal-to-noise ratio (SNR) of the space-time processing in each layer is represented by the corresponding "matched filter" bound. This proves the optimality of the layered space-time concept. Second, we present an embodiment of this concept for a coded system operating at a low average SNR and in the presence of possible intersymbol interference. This embodiment utilizes the already advanced space-time filtering, coding and turbo processing techniques to provide yet a practical solution to the processing needed. Performance results are provided for quasi-static Rayleigh fading channels with no channel estimation errors. We see for the first time that the Shannon capacity for wireless communications can be both increased by N times (where N is the number of the antenna elements at the transmitter and receiver) and achieved within about 3 dB in average SNR about 2 dB of which is a loss due to the practical coding scheme we assume-the layered space-time processing itself is nearly information-lossless.

Ariyavisitakul et al. (IEEE Trans. Commun., vol.47, p.1073-83, 1999 July) provided a unified analysis of optimum space-time processors based on the following two analytical diversity receiver models: 1) a general model-with a linear filter on each diversity branch, and 2) a "matched filter" model-with a bank of matched filters on each branch, followed by common filter. Closed-form results were given for each receiver model, in terms of the minimum mean-square-error or maximum signal-to-noise ratio solutions for different types of equalizers, including a linear equalizer (LE), a decision-feedback equalizer (DFE), and a maximum-likelihood sequence estimator (MLSE). Although we implied that the two receiver models lead to the same optimum solutions, this requires some proof that is not directly obtainable from the results we presented. We therefore provide such a proof in the present paper for the completeness of the overall unified analysis.

We consider optimum space-time equalizers with unknown dispersive interference, consisting of a linear equalizer that both spatially and temporally whitens the interference and noise, followed by a decision-feedback equalizer (DFE) or maximum-likelihood sequence estimator (MLSE). We first present a unified analysis of the optimum space-time equalizer, and then show that, for typical fading channels with a given signal-to-noise ratio (SNR), near-optimum performance can be achieved with a finite-length equalizer. Expressions are given for the required filter span as a function of the dispersion length, number of cochannel interferers, number of antennas, and SNR, which are useful in the design of practical, near-optimum space-time equalizers.

Transmitter diversity is an effective technique to improve wireless communication performance. In this paper, we investigate transmitter diversity using space-time coding for orthogonal frequency division multiplexing (OFDM) systems in high-speed wireless data applications. We develop channel parameter estimation approaches, which are crucial for the decoding of the space-time codes, and we derive the MSE bounds of the estimators. The overall receiver performance using such a transmitter diversity scheme is demonstrated by extensive computer simulations. For an OFDM system with two transmitter antennas and two receiver antennas with transmission efficiency as high as 1.475 bits/s/Hz, the required signal-to-noise ratio is only about 7 dB for a 1% bit error rate and 9 dB for a 10% word error rate assuming channels with two-ray, typical urban, and hilly terrain delay profiles, and a 40-Hz Doppler frequency. In summary, with the proposed channel estimator, combining OPDM with transmitter diversity using space-time coding is a promising technique for highly efficient data transmission over mobile wireless channels.

We consider optimum space-time equalizers with unknown dispersive interference, consisting of a linear equalizer that both spatially and temporally whitens the interference and noise, followed by a decision-feedback equalizer or maximum-likelihood sequence estimator. We first present a unified analysis of the optimum space-time equalizer, and then show that, for typical fading channels with a given signal-to-noise ratio (SNR), near-optimum performance can be achieved with a finite-length equalizer. Expressions are given for the required filter span as a function of the dispersion length, number of cochannel interferers, number of antennas, and SNR, which are useful in the design of practical near-optimum space-time equalizers.

This paper introduces a new approach for joint convolutional coding and decision feedback equalization (DPE). To minimize error propagation, the DFE uses a combination of soft decisions and delayed tentative decisions to cancel intersymbol interference (ISI). Soft decisions are obtained by passing the DFE output through a (soft) nonlinear device. This simple method is shown to perform almost as well as an optimum soft feedback approach on wireless channels with diversity. Tentative decisions from the Viterbi decoder are used to cancel ISI due to multipath with large delays, thus remedying the increasing effect of error propagation in channels with large delay spreads. We consider the use of this soft/delayed feedback DFE (S/D-DFE) technique in broadband wireless channels (with delay spreads up to several tens of the symbol period) typical in high-bitrate mobile data applications. Simulation results indicate that the proposed joint coding and S/D-DFE technique performs to within 1-2 dB [in required signal-to-noise ratio (SNR)] of an ideal coded DFE without error propagation. When combined with antenna diversity and a reduced-complexity DFE concept with adaptive feedforward tap assignment, it provides high packet throughput against Rayleigh fading, severe delay spreads, and high Doppler rates.

This paper presents an adaptive tap assignment technique for improving the performance and robustness of a reduced-complexity decision-feedback equalizer (DFE) for broadband wireless mobile communications. The spacing of individual feedforward taps of the DFE is made variable such that, when the channel consists of sparsely-distributed multipath with a large delay spread (e.g., "hilly terrain" delay profiles), the feedforward taps can be sparsely assigned to increase the equalizer span without increasing the total number of the feedforward taps. We provide an analysis of the optimum tap assignment problem and propose a practical channel-estimation-based tap assignment algorithm which can be executed prior to equalizer training. Simulation results are obtained for various outdoor delay profiles with dispersions ranging up to several tens of symbol periods and while using only a small number of feedforward taps (e.g., 5 taps), compared to a conventional contiguous-tap DFE, the proposed tap-selectable DFE is found to give relatively stable performance (measured in terms of required SNR) over all profiles, and also to provide improved robustness to fast fading.

We present specifications for transmission of code-division-multiple-access (CDMA) signals over an optical link, and demonstrate that unisolated, uncooled Fabry-Perot lasers can meet these requirements. Since full testing using a CDMA base station is not practical, a simple test using two tones to simulate the RF channel has been developed and used.

This paper presents reduced-complexity equalization techniques for broadband wireless communications, both outdoors (fixed or mobile wireless asynchronous transfer mode (ATM) networks) and indoors [high-speed local-area networks (LANs)]. The two basic equalization techniques investigated are decision-feedback equalization (FE) and delayed decision-feedback sequence estimation (DDFSE). We consider the use of these techniques in highly dispersive channels, where the impulse response can last up to 100 symbol periods. The challenge is in minimizing the complexity as well as providing fast equalizer start-up for transmissions of short packets. We propose two techniques which, taken together, provide an answer to this challenge. One is an open-loop timing recovery approach (for both DFE and DDFSE) which can be executed prior to equalization; the other is a modified DFE structure for precanceling postcursors without requiring training of the feedback filter. Simulation results are presented to demonstrate the feasibility of the proposed techniques for both indoor and outdoor multipath channel models. The proposed open-loop timing recovery technique plays a crucial role in maximizing the performance of DFE and DDFSE with short feedforward spans (the feedforward section of DDFSE is a Viterbi sequence estimator). A feedforward span of only five is quite sufficient for channels with symbol rate-delay spread products approaching 100. The modified DFE structure speeds up the training process for these channels by 10-20 times, compared to the conventional structure without postcursor precancellation. The proposed techniques offer the possibility of practical equalization for broadband wireless systems.

This paper presents an adaptive tap assignment technique for improving the performance of a previously reported reduced-complexity decision-feedback equalizer (DFE) for broadband band wireless systems. The spacings of individual feedforward taps of the DFE are made selectable so that, when the channel consists of a sparsely distributed multipath with a large delay spread (e.g. "hilly terrain" (HT) delay profiles), the equalizer span can be increased without increasing the total number of taps. We propose simple tap selection algorithms and show that they provide: (1) performance gains over a contiguous-tap approach in various outdoor delay profiles and (2) improved robustness against fast fading.

Broadband analog transport facilities using fiber or fiber/coax cable can play a significant role in the evolution of the network infrastructure for personal communications services (PCSs). Low-power PCS systems require a dense grid of radio ports to provide connectivity to the telephone network. Analog transport has a number of important advantages over digital transmission facilities, including the flexibility to support a variety of air interface formats, shared infrastructure cost with other services such as video distribution, and centralized call processing allowing the use of low cost and simple radio ports. A simulcast technique can be used in such systems to permit low rates of handoff (no handoff within each simulcast area) and sharing of hardware resources among multiple radio ports. This paper provides a detailed model and a simulation analysis of the cochannel interference and noise performance as well as the resource sharing benefit of a simulcast PCS system. Several potential PCS air interfaces are considered, including time division multiple access (TDMA) and code division multiple access (CDMA) techniques. Our investigation shows that the impact of multiple antenna noise in a simulcast system is offset by the improved signal-to-interference (SIR) ratio brought about by distributed antennas. Even with distributed antennas, multiple antenna noise places a limit on the maximum number of radio ports that can be assigned to each simulcast group. This limit, however, is shown to have little impact on the achievable resource sharing benefit of simulcasting (i.e., grouping beyond this limit has diminishing returns). A saving of 40% to 60%, in terms of the required central hardware resources, is typical for both TDMA and CDMA systems in suburban environments.

This paper discusses the performance of a slow-frequency-hopped time-division multiple access (SFH-TDMA) technique, which can be considered as a "high-tier" extension of a low-complexity TDMA architecture optimized for low-power pedestrian applications. A hard-limited receiver is employed and a novel weighting factor for diversity combining is developed to achieve an effect similar to that of a maximal ratio combiner. Numerical performance results for frequency-selective fading channels over a range of fading rates were obtained through computer simulations.

For pt.I see ibid., vol 41, p.1626-34 (1993). Power control is essential in the use of direct-sequence code division multiple-access (CDMA) techniques. Early system-level performance analyses of a CDMA approach to wireless mobile and personal communications have assumed the ability of power control to equalize the absolute signal powers of CDMA users received at each base station. The present paper studies a more practical, although analytically more complicated, uplink power control technique that uses measurements of the received signal-to-interference ratio (SIR) instead. A combination of discrete-event link simulation and analysis of the obtained SIR statistics is used to explore the previously little-known behavior of a CDMA system using SIR-based power control and to obtain performance estimates for such a system under various operating assumptions. The overall results indicate that power control based on SIR has the potential for somewhat higher system performance than power control based on absolute signal strength assumed in the early analyses.>

Power control is essential in the use of direct-sequence code-division multiple-access (CDMA) techniques over fading radio channels. The paper investigates a feedback power control approach that allows power commands to be updated at a higher rate than the rate of multipath fading. The signal and interference statistics as received at the base stations after power control are obtained for a simulated CDMA system which includes multiple base stations with diversity receivers and a large number of power-controlled users continuously moving at various speeds. The authors show that often-used analyses based on perfect average power control lead to optimistic capacity results (by about 25%) because interference is underestimated by as much as 1 dB.>

A combined burst synchronization and error detection scheme is studied for a time-division multiple access (TDMA) portable radio system. Analysis, computer simulation, and experimental results of the false acceptance rate of the system using the combined scheme are presented. It is found that for a (161,147) code used in this study, which is capable of correcting +or-6-b sync slippage, the error detection capability of the code is degraded by 3 b on a slow Rayleigh fading channel using coherent QPSK demodulation with differential decoding. However, lower implementation complexity and higher efficiency are obtained than with separate burst synchronization and error detection processes.>

Describes a low-complexity equalization technique for improving the reliability of portable radio links in the presence of multipath time delay spread. A technique that operates on hard-limited received signals, with only the phase information available, is presented. Suboptimum receivers based on a maximum likelihood estimation criterion are discussed, including a sequence estimator and a decision feedback phase detector. A low-complexity adaptive phase equalizer structure using decision-directed phase tracking is proposed. The equalizer does not require the multiplication operations required in most conventional equalization algorithms. The author also proposes a receiver that includes a time-reversal structure and a joint estimator for optimum timing recovery and equalizer training. The time-reversal structure plays a crucial role in maximizing the compensation capability of the phase equalizer. The combined use of phase equalization and diversity reception is also considered.>

This work describes the use of a receiver with a time-reversal structure for low-complexity decision feedback equalization of slowly fading dispersive indoor radio channels. Time-reversal is done by storing each block of received signal samples in a buffer and reversing the sequential order of the signal samples in time prior to equalization. As a result, the equivalent channel impulse response as seen by the equalizer is a time-reverse of the actual channel impulse response. Selective time-reversal operation, therefore, allows a decision feedback equalizer (DFE) with a small number of forward filter taps to perform equally well for both minimum-phase and maximum-phase channel characteristics. The author evaluates the theoretical performance bounds for such a receiver and quantifies the possible performance improvement for discrete multipath channels with Rayleigh fading statistics. Two extreme cases of DFE examples are considered: an infinite-length DFE; and a DFE with a single forward filter tap. Optimum burst and symbol timing recovery is addressed and several practical schemes are suggested. Simulation results are presented. The combined use of equalization and diversity reception is considered.>

This paper describes a new modulation technique called DSK (double phase-shift keying) which shows an interesting antimultipath feature over mobile frequency-selective fading environments. The effect of this technique is shown for various multipath channels by detection waveform and theoretical bit error rate (BER) analyses. A result obtained from a laboratory test is also presented. In the latter part of the paper, a generalized form of DSK is described and extensive investigations are made so as to clarify the anti-multipath effect of DSK in view of the diversity concept and to further explore the possibility of a narrowband scheme. A major result shows that this technique can give a kind of diversity effect which is implicit in the arrival of signals via multiple paths with different delays.