Ergodic capacity and outage probability are two important performance measures of communication systems in fading channels. The goal of this work is to develop a simple method for tightly estimating the ergodic capacity and outage probability of a Rayleigh channel with and without antenna combining for constellation constrained capacity. Researchers have determined methods for calculating the ergodic Shannon capacity for Rayleigh channels for single and multiple receive antenna systems. However, in practical communications systems, the input signal is constrained to a discrete signalling set such as finite-size quadrature amplitude modulation constellations. Under these conditions the ergodic constellation constrained capacity is a more accurate measure. Relatively loose bounds for the ergodic constellation constrained capacity of a Rayleigh channel have been exposited in the literature for a single antenna system. This paper details a method that provides a uniform expression for accurately estimating the ergodic capacity, both Shannon and constellation constrained, of Rayleigh channels with and without antenna combining. Expressions are derived using this method for both the noise limited and interference limited cases. These expressions facilitate straightforward computation of outage probability as well. To date, no other method for calculating the ergodic constellation constrained capacity of an interference limited Rayleigh channel or estimating the outage probabilities for the constellation constrained capacity of a noise limited or interference limited Rayleigh channel has been published.

This manuscript presents an iterative detector based on a generalized Schnorr-Euchner enumeration for the case of imperfect channel state information. The detector determines the most contributive N∈{1,2} terms in each of the two log-sum-exponents. The objective function of the tree search is modified to account for (a) the channel estimation error variance and (b) the input a priories. Due to (a), the decomposition of the objective into the form used in conventional tree search is not possible. Moreover, both (a) and (b) invalidate the use of the progressive zigzag order. Because of this, a generalized Schnorr-Euchner enumeration is proposed where explicit sorting is done. Unlike the conventional tree search, the node weights are not computed in an additive way where the weight of a child node is the sum of the weights of the parent and the branch connecting them. The advantage of the proposed detectors compared to iterative channel estimation and decoding is that the repetition of the channel estimation and the QR decomposition is not needed. Also since they are based on the Schnorr-Euchner enumeration, they do not require an initial radius estimation and if channel estimation errors are small enough to be ignored, the search step becomes division- and square-root-free.

This paper focuses on studying the fundamental performance limits and linear dispersion code design for the MIMO-ARQ slow fading channel. The optimal average rate of well-known HARQ protocols is analyzed. The optimal design of space-time coding for the MIMO-ARQ channel is discussed. Information-theoretic measures are used to optimize the rate assignment and derive the optimum design criterion, which is then used to evaluate the optimality of existing space-time codes. A different design criterion, which is obtained from the error probability analysis of space-time coded MIMO-HARQ, is presented. Examples are studied to reveal the gain of ARQ feedback in space-time coded MIMO systems.

The most common physical layer models for network simulations are the bit error rate and the SNR threshold model. In time-varying channels such as those experienced in vehicular networks, these models are assumed valid as long as the packet duration is less than the coherence time of the channel. The coherence time is a statistical measure of the channel invariance. It is independent of the signal parameters or the receiver structure. While it is convenient to decouple the system performance from the channel, this paper shows that this simplification may lead to inaccurate performance assessments and erroneous conclusions. This paper suggests a new metric, the normalized empirical coherence time (NETC), based on results from an extensive simulation campaign of a typical IEEE 802.11p system. The NETC delineates the minimum time (as a percentage of signal duration) over which the system achieves some performance threshold. The metric is explicitly a function of modulation, packet duration, and the traditional coherence time. This new metric could be used in place of the traditional coherence time as a constraint on the packet duration necessary to assume channel variation has negligible impact on performance.

Location aware applications benefit from accurate position information. The position uncertainty can be reduced through cooperative localization in distributed systems with accurate point-to-point ranging. Received signal strength (RSS) measurement enables low cost, low complexity distance estimation. The distance estimate suffers from two sources of error: RSS estimation error and propagation model error. This paper explores the contribution of each type of error to the overall point-to-point distance error. Prior research focused on the model error which establishes a floor on the distance error. However, the oft ignored RSS estimation error significantly degrades the distance estimation accuracy as the distance increases. By including the affect of RSS estimation error, practical estimation algorithms can be compared. Two proposed RSS estimation algorithms are compared in terms of their impact on distance error.

This paper presents and compares two iterative coded modulation techniques for deep-space optical communications using pulse-position modulation (PPM). The first code, denoted by SCPPM, consists of the serial concatenation of an outer convolutional code, an interleaver, a bit accumulator, and PPM. The second code, denoted by LDPC-PPM, consists of the serial concatenation of an LDPC code and PPM. We employ Extrinsic Information Transfer (EXIT) charts for their analysis and design. Under conditions typical of a communications link from Mars to Earth, SCPPM is 1 dB away from capacity, while LDPC-PPM is 1.4 dB away from capacity, at a Bit Error Rate (BER) of approximately 10-5. However, LDPC-PPM lends itself naturally to low latency parallel processing in contrast to SCPPM.

The multistrata space-time codes proposed by Wachsmann et al. were intended then to fix the drawback of orthogonal space-time block codes which is the relatively low spectral efficiency. However, these codes lack two things: 1) their performance is worse than codes designed using number theory like the Golden code. 2) The advantages of their structure were never exploited in reducing the detection complexity. In this paper, it is shown how the multistrata structure can potentially reduce the detection complexity of the sphere decoder in the uncoded case. The reduction in complexity is achieved in both the preprocessing and the search steps of the sphere decoder. Spacetime block codes having such a multistrata structure were found via optimization. Simulation results show that the obtained code has sufficiently high minimum determinant with a very small performance penalty compared to the best known 2 x 2 spacetime block code in literature, the Golden code. Also a simple means of generating bit log-likelihood ratios in a bit-interleaved coded modulation scenario is presented.

Two kinds of iterative sphere detectors based on modifications of the Schnorr-Euchner enumeration are presented. The first is capable of finding "good" candidate lattice points relatively soon. Its complexity and performance are controlled by an adaptive radius backlash update strategy. The second systematically searches for the most contributive term in each of the two log-sum-exponents. Both kinds require modifications to the original Schnorr-Euchner enumeration. The objective function must account for the input a priories and explicit sorting of the branches stemming from the tree nodes must be done because the simple zigzag order in this case is not valid. The impact of the a priories on the log-likelihood ratio clipping is also addressed. The advantages of the new detectors compared to the Fincke-Pohst-based ones are: i) No initial radius is required, ii) the search step is division- and square-root-free and iii) the average computational complexity is lower.

Channel estimation of coded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) in fast time-varying channels are considered. Maintaining high performance with manageable complexity relies on iterative soft-in soft-out equalization and decoding. Conventional frequency domain channel estimation methods have an irreducible error floor due to unaccounted intercarrier interference (ICI). This paper proposes an efficient and high performance time domain pilot symbol assisted modulation (PSAM) system for channel estimation. Simulation results are reported for the iterative receiver with application to the mobile worldwide interoperability for microwave access (WiMAX).

Two efficient iterative sphere detectors based on the Schnorr-Euchner enumeration are the topic of this manuscript. The first one is capable of finding dasiagoodpsila candidate lattice points soon enough and its efficiency/performance are controlled by an adaptive radius update strategy. The second is a max-log detector. Because of the Schnorr-Euchner nature of the search, estimation of the initial sphere radius is not required and unlike the iterative sphere detector of Vikalo et al., the search step does not require any division or square root operations. In the new sphere detectors, the search step must account for the input a priories and explicit sorting of the branches stemming from the tree nodes must be done.

Orthogonal frequency division multiplexing (OFDM) system suffers extra performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. Realizing that ICI channels are intrinsically ISI channels, this paper proposes a soft-in soft-out (SISO) maximum a posteriori (MAP) equalizer by extending Ungerboeck's maximum likelihood sequence estimator (MLSE) formulation to ICI channels. The SISO MAP equalizer employs BCJR algorithm and computes the bit log-likelihood ratios (LLR) for the entire received sequence by efficiently constructing a trellis that takes into account of the ICI channel structure. A reduced state (RS) formulation of the SISO MAP equalizer which provides good performance/complexity tradeoff is also described. Utilizing the fact that ICI energy is clustered in adjacent subcarriers, frequency domain equalization is made localized. This paper further proposes two computational efficient linear minimum mean square error (LMMSE) based equalization methods: recursive q-tap SIC-LMMSE equalizer and recursive Sliding-Window (SW) SIC-LMMSE equalizer respectively. Simulations results demonstrate that the iterative SISO RS-MAP equalizer achieves the performance of no ICI with normalized Doppler frequency fdTsup to 20.46% in realistic mobile WiMAX environment.

The problem of efficient communication over a scalar quasi-static fading channel is considered. The single-layer transmission (SLT) and multi-layer transmission (MLT) schemes do not require any knowledge of the channel state information (CSI) at the transmitter, but their performance is also limited. It is shown that using Hybrid-ARQ (HARQ) can significantly improve the average rate performance, provided that the rate assignment between different ARQ rounds is carefully chosen. The average rate performance of several HARQ schemes is optimized and compared. In addition, optimal power allocation among retransmissions is derived and shown to further increase the average rate. This power allocation gain is remarkable at low signal-to-noise ratio (SNR), but becomes negligible at high SNR. Comparison of two different types of limited feedback, sequential feedback (ARQ) and one-shot feedback (quantized CSI), is made from several perspectives. Although the optimization problem is formed with respect to the average rate, simulation results give a comprehensive comparison under different metrics, including average rate, outage probability, and the combination of both. Substantial performance improvement is observed with even one ARQ retransmission in all simulations. More importantly, this gain appears to be robust with respect to the fading distributions.

Different error protection capabilities associated with different bit positions in a constellation are utilized to provide unequal error protection (UEP) with multilevel coding (MLC) on AWGN and fast/slow fading channels. Unlike previous work on this problem, the design tool in this paper is the symmetric information rate (SIR), which is well suited to the use of modern component codes. The constellation design problem is addressed from two perspectives: bits-to-symbol mapping, and positions of constellation points. Design guidelines of MLC for UEP are presented, and examples are shown to demonstrate the validity of the proposed method. It is further shown that the proposed design is especially desirable for communication over a slow-fading wireless channel, as it allows for a gradual performance degradation with the decreasing receive SNR. Throughput advantages are shown to support this claim.

The problem of efficient communication over a quasi-static wireless fading channel is considered in this paper. The disadvantages of two well-known schemes, single-layer transmission (SLT) and multi-layer transmission (MLT), are pointed out. A new scheme named aggressive transmission with ARQ (AT-ARQ) is developed and optimized to provide better performance, at the expense of requiring ARQ feedback. Optimal power allocation among (re)transmissions is derived. A comprehensive performance comparison of the three schemes, under different performance metrics such as throughput, outage probability, and the combination of two, is reported via numerical simulations. Substantial performance improvement is observed with even 1-bit ARQ feedback in Rayleigh fading.

The observation that different bit positions in a constellation typically have different error protection capabilities is utilized to design unequal error protection (UEP) with multilevel coding (MLC) in AWGN and fading channels. Both parallel independent decoding (PID) and multi-stage decoding (MSD) are considered. The design tool in this work is the symmetric information rate (SIR), which is well suited to the use of capacity-approaching component codes. This paper first formulates the UEP design as a utility maximization problem, and then considers some optimal UEP designs, including mapping, non-uniform constellation, bits grouping, and decoding order in MSD. Several exemplary utility functions are studied, corresponding to different application scenarios. The UEP design is especially beneficial in a slow fading channel, as it allows for a gradual performance degradation with the decreasing receive SNR. Average rate advantage is shown to quantify this gain.

Orthogonal frequency division multiplexing (OFDM) systems suffer performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. In order to perform coherent detection and estimation, channel state information (CSI) is critical. Conventional frequency domain channel estimation (CE) methods have an irreducible error floor at high normalized Doppler frequency fdTs, since ICI corrupts the orthogonality among subcarriers. Considering that the fast time-varying channel is also a source of temporal diversity, CE ought to take place in the pre-FFT time domain. With soft a priori information about the data symbols becomes available, this paper proposes a turbo channel estimator (TCE) structure which provides a way to consistently improve the bit error rate (BER). The complexity of TCE is further reduced by completely avoiding matrix inversion. Simulation results demonstrate that the PSAM system with TCE achieves no ICI with normalized Doppler frequency fdTsup to 20.46% with realistic mobile WiMAX channel environment.

Orthogonal frequency division multiplexing (OFDM) systems suffer performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. In order to perform coherent detection and estimation, channel state information (CSI) is indispensable. Being able to accurately and effectively acquire CSI in the fast time-varying channel is critical. Conventional frequency domain channel estimation (CE) methods have an irreducible error floor at high normalized Doppler frequency fdTs, since ICI corrupts the orthogonality among subcarriers. Considering that the fast time-varying channel is also a source of temporal diversity, CE ought to take place in the pre-FFT time domain. Realizing channel variations in time are often smooth, this paper proposes a time domain channel estimator using pilot symbol assisted modulation (PSAM) with complexity O(N), where N is the FFT size. Simulation results demonstrate that a PSAM system with channel estimation provides excellent trade-off between performance, complexity and spectral efficiency.

The MIMO-OFDM beamforming design problem is addressed from a system level standpoint. A beamforming method is proposed which helps improve the receiver channel estimation performance without degrading any benefit of a conventional beamformer. To that end, the smoothed singular value decomposition (SSVD) algorithm is first developed to get "close" effective channels after beamforming for two adjacent subcarriers. Based on the SSVD algorithm, the frequency smoothed beamformer (FSB) design is then derived, in which smooth effective channels across all subcarriers are generated and thus the receiver can apply interpolation and smoothing to improve the channel estimation performance. The close singular value problem is discussed. Statistical characteristics of the effective channel are analyzed, which is used to design channel estimation for the beamformed channel. Simulation results show that the FSB design is efficient in the IEEE 802.11n setting.

This paper proposes a new peak-to-average power ratio (PAPR) reduction scheme in multiple transmit antenna environments. Instead of applying individual PAPR reduction on each antenna, a joint PAPR reduction technique is proposed. By applying a relevant unitary rotation over the transmit antennas, overall PAPR of the multiple transmit antenna system is reduced. This scheme does not require any side information to decode the signal in the receiver, enabling throughput-lossless PAPR reduction. Furthermore, there is no increase in the complexity of the receiver.

Iterative turbo processing between detection and decoding shows near-capacity performance on a multiple-antenna system. Combining iterative processing with optimum front-end detection is particularly challenging because the front-end maximum a posteriori (MAP) algorithm has a computational complexity that is exponential. Sub-optimum detector such as the soft interference cancellation linear minimum mean square error (SIC-LMMSE) detector with near front-end MAP performance has been proposed in the literature. The asymptotic computational complexity of SIC-LMMSE is O(nt2nr+ ntnr3+ ntMc2Mc) per detection-decoding cycle where ntis number of transmit antenna, nr is number of receive antenna, and Mc is modulation size. A lower complexity detector is the hard interference cancellation LMMSE (HIC-LMMSE) detector. HIC-LMMSE has asymptotic complexity of O(nt2nr+ ntMc2Mc) but suffers extra performance degradation. In this paper, two front-end detection algorithms are introduced that not only achieve asymptotic computational complexity of O(nt2nr+ ntnr2[Gamma (beta) + 1] + ntMc2Mc) where Gamma(beta) is a function with discrete output {-1, 2, 3, ...,nt} and O(ntMc2Mc) respectively. Simulation results demonstrate that the proposed low complexity detection algorithms offer exactly same performance as their full complexity counterpart in an iterative receiver while being computational more efficient.

This paper proposes improved space-time multiple TCM (ST-MTCM) codes via a systematic expansion of space- time block code (STBC) for two transmit antennas. Starting from the orthogonal STBC, the STBC set is expanded into super- orthogonal STBC, and further expanded up to spatial multiplexing in the case of BPSK and QPSK constellations. Exploiting the expanded set of STBCs as the number of states increase, improved full diversity ST-MTCM codes can be designed by increasing the coding gain and Euclidean distance of the pairwise errors.

Orthogonal frequency division multiplexing (OFDM) system suffers extra performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. Using the fact that ICI energy is clustered in adjacent subcarriers, frequency domain equalization is made localized. Conventional localized q-tap frequency domain equalizer still bears a formidable computational complexity of O(K2q), where K is the number of subcarriers. By further restricting the observation window size, sliding-window (SW) type of equalizer is proposed in the literature with complexity O(Kq3). This paper proposes a recursive SW LMMSE based equalizer which further reduces the computational complexity to O(Kq2) and more importantly without compromising performance.

A geometric decoding technique for finite lattices was presented by Seethaler et al. for flat-fading multiple-input multiple-output channels in the case of linear modulations with constant amplitude (PSK). In this paper, geometric decoding is extended to square PAM and QAM constellations. Geometric decoding is suboptimal but has the advantage that the search is done in the vicinity of a X-dimensional affine set. Therefore, a very small subset of the whole lattice is considered in decoding. This makes the worst case complexity very low. For a MIMO channel with Lt transmit antennas and M-QAM constellation, the complexity is about O ((radicMLt)K). Simulation results show that geometric decoding performs very close to maximum likelihood in many practical scenarios.

Traditional constellations are uniformally spaced. By giving up uniform spacing, constellations can be designed to have larger joint (i.e. overall) capacity or parallel decoding capacity. In this paper non-uniformally spaced (i.e. 'geometrically' shaped) constellations are designed to maximize either of these quantities. By way of numerical capacity computations we show that except in special cases, there are no universally optimal geometrically shaped constellations across all code rates, and that the optimization of a constellation has to target a specific code rate. Unlike joint capacity, optimizing for parallel decoding capacity is label dependent. For PAM and PSK constellations, we found the maximum parallel decoding capacity to be achieved using gray (not necessarily binary reflective gray) labels. However, for PAM constellations, not all gray labels can yield the highest parallel decoding capacity. Besides the conventional use of a (log2(M) -1) /log2(M) code rate with an M-point constellation for bandwidth efficient communications, the optimized constellations could offer further non-trivial gains at lower code rates (unlike traditional constellations). An optimized constellation is used with a state-of-the-art LDPC code and simulation results are presented. This paper also draws a distinction between probabilistic shaping and geometric shaping and in fact proves under broad conditions, that any gain in capacity which can be found via probabilistic shaping can also be achieved or exceeded solely through geometric shaping.

The multistrata space-time codes were proposed by Wachsmann et al. to increase the spectral efficiency of orthogonal space-time block codes. However, these codes lack two things: 1) their performance is worse than codes designed using number theory like the Golden code. 2) The advantages of their structure were never exploited in reducing the detection complexity. In this paper, it is shown how the multistrata structure can potentially reduce the detection complexity of the sphere decoder in the uncoded case and a simple means of generating bit log-likelihood ratios in a bit-interleaved coded modulation scenario is presented. Simulation results show that the obtained code has a very small performance penalty compared to the Golden code in the uncoded case and outperforms it in bit-interleaved coded modulation.

Orthogonal frequency division multiplexing (OFDM) system suffers extra performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. Using the fact that ICI energy is clustered in adjacent subcarriers, frequency domain equalization is made localized. A computational efficient recursive q-tap SIC-LMMSE equalizer is derived. Comparing to conventional method which requires matrix inversion using O(32 ldr q2K2), where K is number of subcarriers, the proposed recursive equalizer only uses O(20 ldr qK2) but without sacrificing performance.

This paper investigates the properties of several non- Jakes Doppler spectra proposed for the vehicle-to-vehicle non- isotropic scattering environment of the open highway. Expressions for the ideal model correlations, level-crossing rates, and average fade durations are derived. The classic Clarke channel model is modified to generate fading signals corresponding to the non-isotropic scattering profiles. The simulation results match well with the theoretical ideal channel.

The authors develop a generalized layered orthogonal lattice detector (G-LORD) for the golden code. G-LORD includes the previously developed LORD algorithm and exhaustive search ML detection as special cases, and is proved to achieve the perfect balance between error performance and complexity. Meanwhile, G-LORD is suitable for parallel implementations, and has a deterministic complexity as opposed to the sphere decoder approach. Thanks to the special structure of the golden code, G-LORD can implement the optimal ordering with very low complexity. Most importantly, G-LORD can efficiently generate bit soft-output metric when golden code is concatenated with outer channel code. The development of G-LORD boosts the practical application of the golden code.

Recently a soft-output multiple-input multiple-output (MIMO) detector has been proposed, namely LORD (layered orthogonal lattice detector), able to achieve maximum-likelihood (ML) performance (in the max-log sense) for two transmit antennas, and near-optimal for greater than two, in MIMO-OFDM bit interleaved coded modulation (BICM) systems. This paper presents a near-optimal layer ordering technique applicable to the hard-output version of the algorithm, essential to achieve near-ML performance up to high-dimensional MIMO systems. The Euclidean distance metrics are minimized over a constant number of transmit sequences, a property desirable to maintain constant throughput and not guaranteed by the most known sphere decoder. Differently from previous art works, attention is also paid to the optimization of the implementation complexity. Finally, it is also shown how the above mentioned ordering technique should be modified for the soft-output version of the algorithm, significantly improving the performance of MIMO-OFDM BICM systems. Simulations in the realistic context of next generation WLANs (802.11n) confirm this statement.

This paper is a study of high-performance soft-output demodulation for slow or moderate frequency-selective and flat Rayleigh fading using an autoregressive (AR) channel model. For channel taps modeled as AR processes, the discrete-time-equivalent channel model is derived for a matched filter (matched to the transmit pulse) and symbol rate-sampled receiver front end. The optimum symbol-by-symbol demodulator is then derived and shown to consist of a joint data and Kalman filter (KF) channel estimator. Additionally, a symbol-by-symbol demodulator with an extended KF is proposed that jointly identifies and tracks the channel and the unknown parameters in AR channel models. A simulation study shows that the proposed algorithms offer significant advantages in performance or complexity compared to several previously proposed algorithms. The algorithms do not exhibit a significant error floor, provide soft-output metrics needed for interleaved coded modulation, provide high performance with a blind initialization, are capable of blind operation with fast acquisition though compatible with pilot-symbol-assisted modulation, and are robust to parameter mismatch.

The singular value decomposition (SVD) is a important tool for MIMO radio communications. However, independent computation of the SVD for each matrix in a MIMO channel sample path places an ordering condition on the singular values that results in singular value sample paths whose evolution is not consistent with the structure of the underlying random process. This problem is addressed as follows: (i) a generic SVD with relaxed identification conditions is proposed, (ii) an optimization problem is formulated for computing the SVD's of two adjacent matrices in the sample path with the objective of maximizing the correlation between the two matrices' singular vectors, and (iii) an efficient algorithm is given for untangling the singular value sample paths. The algorithm produces a unique solution conditioned on the seed matrix's SVD. The algorithm's effectiveness is demonstrate on spatially correlated and spatially white MIMO channnels. A primary application of the algorithm is in closed loop MIMO communications and MIMO channel estimation in general.

This paper takes a system view of the MIMO-OFDM beamformer design problem. We present a design method that helps improve the receiver channel estimation performance without damaging the optimality of beamforming. We design a so-called smoothed singular value decomposition (SSVD) algorithm to get smooth equivalent channels after beamforming, and thus the receiver can apply interpolation and smoothing to improve channel estimation. We then present some analysis of the statistical characteristics of the equivalent channel, which will be useful in designing the channel estimation. Simulation results are shown to support our algorithm.

Iterative turbo processing between detection and decoding shows near-capacity performance on a multiple-antenna system. Combining iterative processing with optimum front-end detection is particularly challenging because the front-end maximum a posteriori (MAP) algorithm has a computational complexity that is exponential in the throughput. Sub-optimum detector such as the soft interference cancellation linear minimum mean square error (SIC-LMMSE) detector with near front-end MAP performance has been proposed. The asymptotic computational complexity of SIC-LMMSE remains O(nt2nr+ntnr3+ntMc2Mc) per detection-decoding cycle where nt is number of transmit antenna, nr is number of receive antenna, and Mc is modulation size. A lower complexity detector is the hard interference cancellation LMMSE (HIC-LMMSE) detector. HIC-LMMSE has asymptotic complexity of O(nt2nr+ntMc2Mc)but suffers exta performance degradation. In this paper, we introduce a frontend detection algorithm that achieves asymptotic computational complexity of O(ntMc2Mc). Simulation results demonstrate that the proposed low complexity detection algorithm offers exactly same performance as their full complexity counterpart in an iterative receiver while being computational more efficient.

This paper presents a soft-output lattice detector algorithm for multiple-input multiple-output (MIMO) communications called layered orthogonal lattice detector (LORD). LORD adopts a new lattice formulation and relies on a channel orthogonalization process. Building on the optimality of LORD for two transmit antennas this paper is a generalization of the algorithm to any number of transmit antennas. The most interesting aspect of LORD is that for two transmit antennas max-log bit soft-output information can be simply generated and for greater than two antennas approximate max-log detection is achieved with reasonable complexity. LORD can be implemented in a parallel fashion, as desirable for VLSI. Extensive simulation results in different scenarios of interest for next generation WLANs (IEEE 802.11n) are reported. The simulations show that LORD is able to achieve very high signal-to-noise ratio (SNR) gains compared to current practical soft-output MIMO detectors.

Iterative turbo processing between detection and decoding shows near-capacity performance on a multiple-antenna system. Combining iterative processing with optimum front-end detection is particularly challenging because the front-end maximum a posteriori (MAP) algorithm has a computational complexity that is exponential in the throughput. Sub-optimum detector such as the soft interference cancellation linear minimum mean square error (SIC-LMMSE) detector with near front-end MAP performance has been proposed. The asymptotic computational complexity of SIC-LMMSE remains O(nE2tnr+ ntnE3r+ ntMc2Mc) per detection-decoding cycle where ntis number of transmit antenna, nris number of receive antenna, and mc is modulation size. A lower complexity detector is the hard interference cancellation LMMSE (HIC-LMMSE) detector. HIC-LMMSE has asymptotic complexity of O(nE2tnr+ ntMc2Mc) but suffers extra performance degradation. In this paper, we introduce a low complexity front-end detection algorithm that not only achieves asymptotic computational complexity of O(nE2tnr+ ntnE3r[Gamma (beta)] + ntMc2Mc) where [Gamma (beta) is a function with discrete output {-1,2,3, ...,nt}. Simulation results demonstrate that the proposed low complexity detection algorithm offers exactly same performance as its full complexity counterpart in an iterative receiver while being computational more efficient

In this correspondence, a new bound on the performance of multiuser receivers in fading channels is presented. The bound is based on a novel criterion for decomposability of error sequences. A constructive method to find the set of indecomposable error sequences under the new criterion is presented. Although the presentation is in the context of a narrow-band multiuser system, the improved bounds can be applied directly to code-division multiple-access (CDMA) multiuser systems over flat-fading channels. A modified bound is also developed for multipath-fading channels. Several examples demonstrate the improved tightness of the new bound, relative to previous results

In this paper, we study the effect of the scattering environment on the capacity of a single carrier linear modulation used in a multipath, multiple antenna channel. The work incorporates detailed 3D geometric channel models to assess the effect of spatial correlation. The analysis techniques, based on the pioneering work of Hirt and Massey (1988), provide a method to evaluate outage capacity. Channels that are derivatives of the ETSI standard hilly terrain and typical urban are considered in detail. The results quantify the loss of capacity produced by higher spatial correlation. When comparing signals of different bandwidth, wideband systems produce more diversity due to better temporal resolution. Surprisingly, narrowband systems gain some diversity back by being less susceptible to loss of diversity from spatial correlation

This paper proposes a new peak-to-average power ratio (PAPR) reduction scheme in multiple transmit antenna environments. By applying a unitary rotation, overall PAPR of the multiple transmits antenna system is reduced. This scheme doesn't require any side information to decode the signal in the receiver, enabling throughput-lossless PAPR reduction. Furthermore, there is no increase in the complexity of the receiver.

This paper provides details about experiments in realistic, urban, and frequency flat channels with space-time coding that specifically examines the impact of the number of receive antennas and the design criteria for code selection on the performance. Also the performance characteristics are examined of the coded modulations in the presence of finite size array geometries. This paper gives some insight into which of the theories are most useful in realistic deployments

In this paper, a new technique to design improved high-rate space-time (ST) codes is proposed based on the concept of concatenated ST block code (STBC) and outer trellis-coded modulation (M-TCM) encoder constructions. Unlike the conventional rate-lossy STBC-MTCM schemes, the proposed designs produce higher rate ST codes by expanding the codebook of the inner orthogonal STBC. The classic set partitioning concept is adopted to realize the STBC-MTCM designs with large coding gains. The proposed expanded STBC-MTCM designs for the two-, three-, and four-transmitter cases are illustrated. Simulation results show the proposed STBC-MTCM designs significantly outperform the traditional ST-TCM schemes. Furthermore, decoding complexity of the proposed scheme is low because signal orthogonality is exploited to ease data decoding.

In this work, we analyze the pairwise error probability (PEP) performance of space-time codes in the presence of transmit fading correlation. Unlike receive correlation, the effect of transmit correlation depends on the specific space-time code used. We show that at low SNR, depending on the code, transmit correlation can either improve or degrade the PEP performance. By analyzing the worst case scenario for asymptotically high or low SNR, we show that, to guarantee robust performance for arbitrary transmit correlation, the minimum eigenvalue of the codeword pair difference matrix should be maximized among all codeword pairs. Numerical and simulation results are given to corroborate the theoretical analysis.

A two-dimensional space-time pilot-symbol assisted demodulation system is proposed in this paper. Since complex scattering environments can produce nontrivial space-time correlation characteristics, this paper proposes coherent demodulation by estimating the fading channel distortion using pilot observations across both space and time. The optimum interpolation filter takes in multiple signals from receiving antennas and exploiting the space-time channel correlation to produce multiple channel-estimate outputs corresponding to every antenna. The optimal channel-estimation filter, its mean-square error performance, and the pairwise symbol-error probability are derived for realistic scattering models to characterize the system performance. A design criterion is proposed for the pilot-symbol spacing and the filter length. Since the filter is a function of the statistical description of the channel, we investigate if this statistical description and the resulting optimal filters can be estimated simply from the pilot observations.

We derive the exact pairwise error probability (PEP) for space-time coding over quasi-static Rayleigh fading channels in the presence of spatial fading correlation. We show that receive correlation always degrades the PEP for all signal-to-noise ratios (SNRs). We quantify the effect of receive correlation by employing the notion of "majorization". We show that the stronger the receive correlation, the worse the PEP for all SNRs. We show that at low SNR, depending on the codes, transmit correlation can either improve or degrade the PEP performance. We show that to guarantee robust performance for arbitrary transmit correlation, the minimum eigenvalue of the codeword pair difference matrix should be maximized among all codeword pairs.

How to calculate the symmetric information rate (SIR) for continuous phase channels is given. This problem has traditionally been viewed highly complicated due to nonlinearity of the modulation. Through the simulated SIR it can be observed that using the continuous phase modulation (CPM) in a multiple antenna system (MIMO) will likely produce both high performance and high throughput. As the optimum CPM MIMO demodulation has a significant complexity, the BLAST architecture is applied and simulation results are provided.

This paper addresses the space-time code design for Rayleigh-fading channels using continuous phase modulation (CPM). General code construction is desirable due to the nonlinearity and inherent memory in CPM signals which make hand design or computer search computationally impractical. Several sufficient conditions for full rank are identified for CPM signaling schemes and for linear representations of CPM. Simulation results verify the resulting performance.

In this letter, we present a simple generalization of the maximum ratio combining principle for space-time coded systems. This result leads to a maximum-likelihood decoder implementation that does not depend on the number of receive antennas and avoids the loss in performance incurred in the decoders proposed by Tarokh and Lo (1998) and Biglieri et al. The insights offered by this decoding rule allow for a simple and elegant proof for the space-time code design criterion in systems with large number of receive antennas. We further present an upper bound on probability of error that captures the dependence of space-time code design on the number of receive antennas. Finally, we present a computationally efficient approach for constructing space-time trellis codes that exhibit satisfactory performance in systems with variable number of receive antennas.

In this correspondence, we propose an algorithm for computing the distance spectrum of a space-time trellis code achieving maximal diversity gain in quasi-static fading channels. We further present a state reduction technique for trellis codes that can reduce the complexity of the distance spectrum computation. We provide numerical results supporting the empirical evidence that a truncated union bound obtained from the distance spectrum provides an accurate characterization of the relative performance ordering of different space-time trellis codes and, therefore, it offers a tool for better space-time trellis code design.

The authors introduced an algebraic design framework for space-time coding in flat-fading channels . We extend this framework to design algebraic codes for multiple-input multiple-output (MIMO) frequency-selective fading channels. The proposed codes strive to optimally exploit both the spatial and frequency diversity available in the channel. We consider two design approaches: The first uses space-time coding and maximum likelihood decoding to exploit the multi-path nature of the channel at the expense of increased receiver complexity. Within this time domain framework, we also propose a serially concatenated coding construction which is shown to offer a performance gain with a reasonable complexity iterative receiver in some scenarios. The second approach utilizes the orthogonal frequency division multiplexing technique to transform the MIMO multipath channel into a MIMO flat block fading channel. The algebraic framework is then used to construct space-frequency codes (SFC) that optimally exploit the diversity available in the resulting flat block fading channel. Finally, the two approaches are compared in terms of decoder complexity, maximum achievable diversity advantage, and simulated frame error rate performance in certain representative scenarios.

We discuss the interrelationship between the spatial and the temporal correlation of the frequency-flat Rayleigh fading channels, and propose a joint design of the mobile station (MS) antenna separation (AS), base station (BS) antenna separation, and the channel interleaving depth (ID) for achieving the spatial, temporal, and space-time channel independence. The proposed design is based on a model generalized from the Jakes/Clarke (1968, 1974) ring-of-scatterer fading model. A space-time channel correlation function of two BS and two MS antennas is derived and examined to produce the AS and ID design rule. Analytical binary phase shift keying (BPSK) bit error probability with perfect channel state information is used to compare the performance.

In this paper, we propose a new technique for designing an improved high-rate space-time code. The proposed code construction is based on a typical concatenation of a space-time block code and an outer TCM/MTCM encoder. However, unlike the existing STB-MTCM schemes which are rate-lossy, the proposed technique produces higher-rate space-time codes by expanding the cardinality of the available orthogonal space-time signal points before concatenating with an outer M-TCM encoder. The set partitioning concept is then employed to realize large coding gains. Here, we present design examples for the 2-transmitter and 3-transmitter cases. The new space-time codes considerably outperform the existing designs. Furthermore, signal orthogonality is exploited to keep decoding complexity of the proposed construction relatively low.

Space-time coding has shown great promise for digital transmission in wireless communication links, especially when the channel response is known at the receiver. Combined with continuous phase modulation (CPM), better bandwidth and power efficiency can be obtained. But without the perfect channel state information, the high performance of space-time CPM system can be greatly degraded due to the channel estimation errors. In this paper we present an adaptive soft algorithm which performs joint channel estimation and data detection for space-time CPM system. Properly designed pilot symbols are inserted at the very beginning to give a good initial estimate of the channels. This soft demodulator is further applied to the interleaved space-time CPM system to yield better performance with moderate complexity. Simulation results show that the receiver can often achieve near-coherent performance in quasi-static fading as well as in relatively fast fading.

This paper makes a case for continuous phase modulation (CPM) as an option for space-time modem implementations. Using CPM can reduce the cost of individual transmitters while the use of multiple transmitters linearly increases the potential data rate. This paper show the performance of several proposed systems and compares the symmetric information rates of linear and continuous phase modulations.

In this paper, we propose a new technique for constructing improved high-rate space-time codes. The proposed code construction is based on a concatenation of an orthogonal spacetime block code and an outer M-TCM encoder. However, unlike the existing STB-MTCM schemes which are rate-lossy, the proposed construction yields higher-rate space-time codes by expanding the cardinality of the orthogonal space-time block code before concatenating with an outer M-TCM encoder. An advantage of the proposed construction is that the standard techniques for designing a good TCM code, such as the classic set partitioning concept, can be adopted to realize the STB-MTCM designs with large coding gains. We present several design examples of improved full-rate space-time codes for a system with 2 transmit antennas. Simulation results show that the new space-time codes considerably outperform the existing ST-TCM designs. For example, the new 4-state 2-bits/symbol QPSK space-time code performs even better than the original 32-state design, while performance of the new 32-state QPSK code is only 1.5 dB away from the outage probability limit. Moreover, decoding complexity of the proposed M-TCM construction is made reasonably low by exploiting signal orthogonality.

A three-dimensional model is proposed for simulating the wireless channel. The model allows simulation of the effects of realistic channels on space-time processing and coding systems. The model is characterized by a set of scattering centers with a given mean angle of arrival (departure), angle spread (azimuth and elevation), and propagation delay. Additionally, the model accommodates either transmitter or receiver motion. Spatial characteristics of the simulated channels match previously obtained analytical results. We use the model to generate standardized temporal and spatial channel conditions from Global System for Mobile communication and third-generation standards. We demonstrate the need for such a spatial description by contrasting our results to classic methods of generating these standardized channel conditions.

Space-time coding has been studied extensively as a powerful error correction coding for systems with multiple transmit antennas. An important design goal is to maximize the level of space diversity that a code can achieve. Toward this goal, the only systematic algebraic coding theory so far is binary rank theory by Hammons and El Gamal (see ibid. vol. 46, p.524-42, 2000) for binary phase-shift keying (BPSK) modulated codes defined over binary field and quaternary phase-shift keying (QPSK) modulated codes defined over modulo four finite ring. To design codes with higher bandwidth efficiency, we develop an algebraic rank theory to ensure full space diversity for 2/sup 2k/ quadrature and amplitude modulated (QAM) codes for any positive integer k. The theory provides the most general sufficient condition of full space diversity so far. It includes the BPSK binary rank theory as a special case. Since the condition is over the same domain that a code is defined, the full space diversity code design is greatly simplified. The usefulness of the theory is illustrated in examples, such as analyses of existing codes, constructions of new space-time codes with better performance, including the full diversity space-time turbo codes.

This article provides a new formulation for the pairwise error probability for any coherently demodulated system in arbitrarily correlated Rayleigh fading. The novelty of the result is that the error probability expression can be described as a function of the eigenvalues of a "signal"-only matrix. We also provide the relationship between the pole location of the characteristic function and the resulting error probability. This result allows us to approximate and bound the desired probability. A new simple bound on the pairwise error probability is derived that is better than the standard Chernoff bound and asymptotically tight with the signal-to-noise ratio (SNR) to the true probability.

Abstract It is well known that the performance of TCP deteriorates in a mobile wireless environment. This is due to the fact that although the majority of packet losses are results of transmission errors over the wireless links, TCP senders still take packet loss as an indication of congestion, and adjust their congestion windows according to the additive increase and multiplicative decrease (AIMD) algorithm. As a result, the throughput attained by TCP connections in the wireless environment is much less than it should be. The key problem that leads to the performance degradation is that TCP senders are unable to distinguish whether packet loss is a result of congestion in the wireline network or transmission errors on the wireless links. In this paper, we propose a light‐weight approach, called syndrome, to improving TCP performance in mobile wireless environments. In syndrome, the BS simply counts, for each TCP connection, the number of packets that it relays to the destination host so far, and attaches this number in the TCP header. Based on the combination of the TCP sequence number and the BS‐attached number and a solid theoretical base, the destination host will be able to tell where (on the wireline or wireless networks) packet loss (if any) occurs, and notify TCP senders (via explicit loss notification, ELN) to take appropriate actions. If packet loss is a result of transmission errors on the wireless link, the sender does not have to reduce its congestion window. Syndrome is grounded on a rigorous, analytic foundation, does not require the base station to buffer packets or keep an enormous amount of states, and can be easily incorporated into the current protocol stack as a software patch. Through simulation studies in ns‐2 (UCB, LBNL, VINT network simulator, http://www‐mash.cs.berkeley.edu/ns/ ), we also show that syndrome significantly improves the TCP performance in wireless environments and the performance gain is comparable to the heavy‐weight SNOOP approach (either with local retransmission or with ELN) that requires the base station to buffer, in the worst case, a window worth of packets or states. Copyright © 2001 John Wiley & Sons, Ltd.

A new bound on the performance of multiuser receivers in fading channels is presented. The bound is based on a novel criterion for decomposability of error sequences. Although the bound development is in the context of a narrowband multiuser system, the improved bounds can be applied directly to CDMA multiuser systems over a fading channels. Several examples demonstrate the tightness of the new bound, relative to previous results from the literature.

The potential for extraordinary improvements in channel capacity promised by space-time modem technology has necessitated a greater understanding of space-time channel characteristics. A model that coheres the essential temporal and spatial character of wireless channels would benefit space-time code design but is as yet unformulated. This study concerns itself with the development and execution of field tests to more fully characterize narrowband wireless channels in multiple-antenna systems for land mobile radio systems. Results of these tests suggest an important relationship between channel independence and the relative geometric orientation of the local scattering corridor and transmit antenna array. An explanation involving the available scattering angle at the receiver is suggested as the mechanism for this relationship.

This paper studies the space time code design for single carrier transmission over frequency selective fading channels. The design criteria are derived first and then we apply the algebraic /spl Sigma//sub 0/-rank theory to show how to design codes to take advantage of space and frequency diversity simultaneously. Finally, example codes are shown to achieve desired level of diversity by simulation results.

A three dimensional model is proposed for simulating the wireless channel. The model allows simulation of the effects of realistic channels on space-time processing and coding systems. The model is characterized by a set of scattering centers with a given mean angle of arrival (departure), angle spread (azimuth and elevation) and propagation delay. Additionally the model accommodates either transmitter or receiver motion. Several examples are considered and the simulation characteristics match the previously obtained analytical model results.

Soft output detection algorithms are useful in systems where soft-decision metrics are important, e.g., systems with interleaved coded modulation. Receive diversity techniques can provide significant performance improvement for fading channels encountered in mobile communication systems. A soft-output receive diversity combining algorithm for the demodulation of continuous phase modulated (CPM) signals over space-time correlated frequency flat Rayleigh fading channels in the absence of perfect channel state information (CSI) is developed. The derived soft output algorithm is used in the iterative detection of interleaved TC-CPM. The performance of the algorithm is extensively characterized by means of Monte-Carlo simulations.

In this paper, we document our design of a MAC protocol, called OSU-MAC, subject to the physical layer characteristics and constraints of a narrow-band wireless modem testbed currently being built at the Ohio State University. The narrow-band wireless modem testbed is expected to support both real-time (bus location tracking) and non-real-time (regular) data applications. A number of techniques are proposed to support QoS imposed by the real-time applications, to deal with the asymmetry on the forward and reverse channels and the half-duplex transmission constraint imposed by the physical layer, and to enhance the error control capability of OSU-MAC. We also present simulation results to demonstrate the key functional characteristics of OSU-MAC.

This paper proposes a class of full space diversity full rate space-time turbo codes. Both parallel concatenated and serially concatenated codes are designed. A rank theory proposed by the authors earlier is employed to check the full space diversity of the codes. The simulations show that the space-time turbo codes can take full advantage of space diversity and time diversity if they are available in the channels. We also study the robustness of performance of both turbo codes and trellis codes in space-time correlated fading channels.

This paper proposes a class of full space diversity QPSK space-time codes based on parallel concatenated convolutional (turbo) codes. A rank criterion of full space diversity is used in the design. Compared with space-time trellis codes, the simulations show it has robust performance at both a quasi-static fading channel and a time varying fading channel.

Space-time trellis code designs mainly focus on maximizing the diversity and coding gains. However, the performance of a space-time code (frame or bit error) is also a function of its "distance spectrum". In this work expurgated union bounds for frame and bit error probability using the distance spectrum are derived. An efficient distance spectrum computation method which uses a reduced error state diagram is proposed. Distance spectrum results for the best recently proposed codes are presented.

Recursive constituent codes are required for parallel concatenated turbo-codes. Some conditions for recursiveness of non-linear constituent codes are given.

Symbol-by-symbol detection algorithms are useful in systems in which soft-decision metrics are important, e.g., systems with interleaved coded modulation. A soft-output algorithm for the detection of continuous phase modulated (CPM) signals transmitted over frequency flat, Rayleigh fading channels is developed. Since the optimum detector is computationally too complex for any practical implementation, some suboptimal detectors which give near optimal performance are proposed. Some theoretical approximations for the performance of the interleaved coded system are given. The performance of the soft-output algorithms is also extensively characterized by means of Monte-Carlo simulations.

This paper extends the traditional Clarke/Jakes (1968, 1974) model for a frequency flat fading process in a land mobile radio system to facilitate the examination of coherent space-time demodulation systems. The work develops a space-time correlation function using a ring of scatterers model around the mobile unit. The resulting correlation function permits the investigation of a variety of issues concerning base station configurations in space-time systems. The interrelationship of the fading process between the space and the time domain is explored. A detailed example regarding the effects of antenna separation in a receiver diversity system is considered. A set of design rules for interleaving depth and antenna separation in a space-time modem is presented and quantified.

Pilot symbol assisted modulation (PSAM) is known to provide good performance in estimating the multiplicative distortion of fading channels. The 2-D space-time PSAM is derived and analyzed in this paper by applying the multiple-antenna reception at the base station. The interrelationship between temporal and spatial correlations is shown to provide better channel information than using the temporal correlation solely. The average mean square error (MSE) per antenna and the binary phase shift keying (BPSK) bit error probability (BEP) are used to characterize the performance analytically and draw comparisons with the traditional 1-D PSAM. When 5 antennas are used, the 2-D space-time PSAM has better average performance over the l-D PSAM for about 4 dB of MSE or 1 dB of BPSK BEP.

This paper provides a new formulation for the pair-wise error probability for any coherently demodulated system in flat Rayleigh fading. The novelty of the result is that the resulting error rate expression is a polynomial function of the eigenvalues of a 'signal' matrix. This view also enables a simple new asymptotically tight bound on the pair-wise error probability. Examples of single and multiple transmit antenna systems are considered.

An automatic repeat request (ARQ) scheme using rate compatible trellis coded modulation, pilot-symbol aided channel estimation, soft output packet combining, and a retransmission request strategy supporting a variable quality of service within a packet is proposed. Simulation results show that this type II hybrid ARQ has about 3-5 dB SNR gain over its type I ARQ counterpart.

Two modified decision feedback equalization (DFE) structures are presented for the efficient equalization of long sparse channels with strong precursor, such as those encountered in high-speed communications over multipath channels with large delay spread. Unlike the conventional DFE, these structures allow the channel's sparseness to be exploited by simple tap allocation, before the sparseness is degraded by feedforward filtering. Both structures yield large reductions in complexity while maintaining performance comparable to the conventional DPE, hence overcoming a key computational bottleneck when equalizers are implemented in hardware for speed. Fast channel estimate-based algorithms for computing the modified DFE coefficients are derived. Simulation results are presented for data rates and channel profiles of the type considered for the proposed North American high definition television (HDTV) terrestrial broadcast mode.

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.

We propose a novel approach to efficiently estimate multipath channel parameters, which is particularly useful in sparse multipath channels. Conventional methods do not fully exploit the inherent structure present in the combined channel response; the excess number of parameters to be estimated by conventional methods makes the identification difficult. By utilizing a priori knowledge of the transmission data pulse, the channel identification problem is transformed into the mode estimation problem. Then, the parameters directly related to the multipath propagation are extracted in the the modal analysis framework, and hence, the number of estimation parameters are significantly reduced. Finally, the multipath channel parameters are obtained by inverse-transforming the mode parameters. Simulation results show significant improvement in the normalized mean square error over existing approaches.

Decision feedback equalization (DFE) structures have been proposed for the efficient equalization of wireless channels with long postcursor response, which is a bottleneck problem for high speed communications over multipath channels with large delay spread. These structures are equivalent to the conventional DFE, but remove postcursor intersymbol interference (ISI) prior to feedforward filtering. We investigate the relationship between these structures and fast equalizer coefficient computation. Based on this relationship, we obtain a fast algorithm for computing optimal DFE settings which has significantly lower complexity than other known approaches for these high speed wireless channels. An example is given for data rates and channel profiles of the type considered for the proposed North American high definition television (HDTV) terrestrial broadcast mode.

In applications requiring soft-decision metrics, e.g., systems with interleaved coded modulation, symbol-by-symbol detection is preferred to sequence detection. This paper develops the optimum soft-output algorithm (OSA) for the demodulation of continuous phase modulated (CPM) signals. Since this optimum detector is computationally complex, a class of suboptimum symbol-by-symbol detectors, called the reduced state soft-output algorithm (RS-SOA) is developed. By varying certain parameters, the RS-SOA offers an effective tradeoff between performance and complexity, as measured by arithmetic operation count and the number of integrators required. Additionally, some simple complexity reduction schemes can be used in conjunction with the OSA and the RS-SOA. The properties of the OSA and the RS-SOA, and the various complexity reduction schemes are explored in an extensive simulation study.

Pilot-symbol-assisted modulation (PSAM) is a method to reduce the effects of fading in mobile communications by periodically inserting known symbols in the data stream. The receiver uses these pilot symbols to derive its amplitude and phase reference. One aspect of this procedure which has not received much attention in the literature is the method used by the receiver to locate the pilot symbols. This paper uses optimum frame synchronization techniques to develop two synchronizers for PSAM systems; one is based on a standard maximum likelihood (ML) estimation formulation, and the other is a sequential testing algorithm. Both methods use a simple quadratic correlation filter with an energy correction factor. Simulation results and a theoretical analysis are presented.

Pilot symbol assisted modulation (PSAM) is a practical technique in mobile digital communications since it can provide high performance in fading with large constellations and it has a simple implementation. Maintaining high performance with PSAM requires an accurate estimate of the transmitted carrier frequency. This paper examines model-based frequency estimation for mobile digital communications with PSAM. Maximum-likelihood estimators (MLE), which include the model for the fading, are derived and compared with those in an additive white Gaussian noise (AWGN) channel, and performance hounds are computed. Reduced complexity frequency estimators based on the MLE are derived and the performance is quantified by Monte-Carlo simulation.

This paper introduces a new family of deconvolution filters for digital communications subject to severe intersymbol interference. These fixed lag smoothing filters for known channel demodulation are called Bayesian filters. Bayesian filters are derived using a new approach to suboptimal recursive minimum mean square error estimation for non-Gaussian processes. The family of Bayesian filters interpolates between the optimum fixed lag linear filter (i.e., the Kalman filter) and the optimum fixed lag symbol-by-symbol demodulator in both performance and complexity. The complexity of the Bayesian filter is exponential in a parameter, typically chosen smaller than the channel length and the filter lag. Hence, the Bayesian filter decouples the channel length and the filter lag from the exponential complexity in these parameters found in many other high performance demodulation algorithms. Simulations characterize the performance and compare the Bayesian filter to both optimal and reduced complexity demodulation algorithms.

This paper introduces extended Bayesian filters (EBFs), a new family of blind deconvolution filters for digital communications. The blind deconvolution problem is formulated as a nonlinear and non-Gaussian fixed-lag minimum mean square error filtering problem, and the EBF is derived as a suboptimal recursive estimator. The model-based setting makes extensive use of the transmitted symbol and noise distributions. A key feature of the EBF is that the filter lag can be chosen to be larger than the channel length, while the complexity is exponential in a parameter which is typically chosen to be smaller than both the channel length and the filter lag. Extensive simulations characterizing the performance of EBFs in severe intersymbol interference channels are presented. The fast convergence and robust equalization of the EBFs are demonstrated for uncoded linearly modulated signals [e.g., differentially encoded quaternary phase shift keying (QPSK)] transmitted over unknown channels. Comparisons are made to other blind symbol-by-symbol demodulation algorithms. The results show that the EBF provides much better performance (at increased complexity) compared to the constant modulus algorithm and the extended Kalman filter, and achieves a better performance-complexity trade-off than other Bayesian demodulation algorithms. The simulations also show that the EBF is applicable with large constellations and shaped modulations.

There has been great interest in reduced complexity suboptimal MAP symbol-by-symbol estimation for digital communications. We propose a new suboptimal estimator suitable for both known and unknown channels. In the known channel case, the MAP estimator is simplified using a form of conditional decision feedback, resulting in a family of Bayesian conditional decision feedback estimators (BCDFEs); in the unknown channel case, recursive channel estimation is combined with the BCDFE. The BCDFEs are indexed by two parameters: a "chip" length and an estimation lag. These algorithms can be used with estimation lags greater than the equivalent channel length and have a complexity exponential in the chip length but only linear in the estimation lags. The BCDFEs are derived from simple assumptions in a model-based setting that takes into account discrete signalling and channel noise. Extensive simulations characterize the performance of the BCDFE and BCDPE for uncoded linear modulations over both known and unknown (nonminimum phase) channels with severe ISI. The results clearly demonstrate the significant advantages of the proposed BCDFE over the BCDFE in achieving a desirable performance/complexity tradeoff. Also, a simple adaptive complexity reduction scheme can be combined with the BCDFE resulting in further substantial reductions in complexity, especially for large constellations. Using this scheme, we demonstrate the feasibility of blind 16QAM demodulation with 10/sup -4/ bit error probability at E/sub b//N/sub 0//spl ap/ 18.5 dB on a channel with a deep spectral null.>

This paper presents a generalized nonlinear (Markov) analysis technique that is used to evaluate the statistical performance of uniformly sampled digital phase-locked loop (DPLL) demodulators. This paper characterizes the first-order, decision-directed DPLL based demodulator as a Markov chain. Traditional analytical techniques are used to evaluate the steady-state statistical performance. Numerical results for the steady-state density function are derived for BPSK, QPSK, and 8PSK. The resulting steady-state bit error probabilities for these modulations are also calculated. Traditional Markov analytical techniques (absorbing boundaries) permit a numerical evaluation of the transient characteristics of the DPLL. The numerical work focuses on loops for both unmodulated and BPSK, QPSK, and 8PSK modulated input signals. The transient characteristics are shown to be a function of the loop bandwidth but converge to those predicted by a diffusion analysis as the loop bandwidth decreases. The susceptibility of small bandwidth loops to hangup is shown to be the largest reason for the acquisition performance difference. The cycle slipping characterization for MPSK modulated signals is also a function of the loop bandwidth and each increase (doubling) of the modulation alphabet size reduces the slipping performance by approximately 6 dB.>

The optimal MAP detection algorithm for demodulation of M-QAM signaling in the frequency flat Rayleigh fading channel is proposed. A recursive structure is derived which allows for symbol-by-symbol metrics to be computed and data decisions to be made in an efficient manner. The complexity problem inherent to optimal detection schemes is addressed and approximations to optimal detection are considered which reduce complexity to practical levels yet still provide near-optimal performance levels. Reduced complexity algorithms are obtained through the use of decision feedback and thresholding which allow for higher order modulations such as 16-QAM to be implemented. Performance characterization of the algorithms presented are obtained via Monte Carlo simulation and results show that near-optimal bit error probability levels can be achieved. Extensions of the algorithms to include diversity combining techniques are also developed and shown to provide improved performance. Some theoretical derivations for optimal detection of 16-QAM in fading with diversity are also presented.>

This paper applies the Bayesian conditional decision feedback estimator (BCDFE) to rapidly fading frequency selective channels. The BCDFE is a model-based deconvolution algorithm which jointly estimates the transmitted data and channel parameters. The BCDFE applies joint symbol-by-symbol MAP demodulation and channel estimation to equalize channels, and consequently provides robust performance in rapidly fading channels. We provide a brief derivation of the BCDFE and characterize the performance on the land mobile radio channel. We assess the BCDFE's principle design characteristics and the resulting performance in both transient and steady-state operation. The effects of delay spread, Doppler spread, and co-channel interference on the BEP performance are also presented. The BCDFE demonstrates many of the desirable characteristics of an equalizer for mobile radio.>

Presents a bit error probability analysis of a digital phase-locked loop based demodulator, of differentially encoded BPSK and QPSK modulations. Differential decoding is a method of resolving a phase ambiguity, typical of fully modulated signals, that uses two consecutive demodulated symbols to estimate the information symbols. The effects of a noisy phase reference on demodulator performance are well documented for uncoded modulations (single symbol demodulation). The paper investigates performance for phase reference time variations between the two symbols. The time varying reference investigated is produced by a digital phase-locked loop. The noisy phase reference has negligible additional effect on the bit error probability for differentially encoded BPSK and QPSK.>

The author proposes a new frequency estimator for a single complex sinusoid in complex white Gaussian noise. The estimator is applicable to problems in communications requiring high speed, recursive frequency estimation. The estimator is computationally efficient yet obtains near optimum performance at moderate signal-to-noise ratios.>

The nonlinear digital carrier synchronization structure is a coherent demodulation technique that has many advantages for mobile communications. The nonlinear digital synchronizer is investigated in the presence of time varying fading for BPSK, QPSK, and pi /4-QPSK modulations. This paper addresses the design of optimal unconstrained smoothing filters for the nonlinear synchronizer structure. An approximate bit error probability analysis demonstrates that performance is quite near ideal and that no error floor exists in time varying fading.>

Stein's (1964) method can be extended to the analysis of the bit error probability (BEP) of quadrature phase shift keying (QPSK), staggered QPSK, and minimum shift keying (MSK) communication systems. The resultant noisy reference BEP waterfall curves are presented. The numerical advantages of this technique and some practical results are discussed. In a parallel manner, the BEP for quadrature amplitude modulation (QAM) communication systems can be analyzed. This technique is numerically more intensive but is used to generate noisy reference BEP waterfall curves for 16-QAM and 64-QAM modulations. Unfortunately, few carrier synchronizers produce a complex Gaussian reference signal, but pragmatically many reference signals can be accurately approximated by a complex Gaussian at moderate to high SNR. Actual BEP performance and the approximate results are compared.>

A decision-directed, digitally implemented carrier synchronizer for channels with both frequency and phase uncertainty is presented. This combined algorithm and its derivatives are analyzed with respect to the achievable carrier acquisition time and the resulting bit error probability. For certain data rates (approximately 50 MSps or less), this algorithm can be implemented using CMOS gate array technology. As examples BPSK and QPSK modulation formats are studied herein and compared to their differentially coherent counterparts of DPSK and DQPSK.>

The nonlinear (nondecision-aided) digital phase estimator provides rapid phase estimation of an M-ary phase-shift-keyed modulated carrier. This carrier synchronization technique exhibits an anomaly, dubbed equivocation, which can degrade performance. A statistical description of equivocation, a jump in the reference phase trajectory of approximately=2 pi /M rad in a carrier synchronization technique, for the simple case of negligible frequency offset and additive white Gaussian noise is derived. Equivocation is shown to significantly degrade the bit error probability performance even in this benign channel. Since the algorithm as proposed is unsuitable for general use, a modified nonlinear (nondecision-aided) equivocation-free phase estimator is presented. The results of a simulation of this modified architecture developed to demonstrate the improved performance are presented.>