Coded Fluid Antenna Multiple Access Over Fast Fading Channels

Pilot-symbol aided coherent M-ary PSK modems in digital cellular mobile radio systems are analyzed theoretically. The error-floors caused by the Doppler spread in a fast fading channel are removed in both flat and selective fading channels. However, the error-floors caused by the delay spread are lower-bounded by those that exist in the ideal coherent detection. The systems are modeled as frequency-selective fast Rayleigh fading channels, corrupted by co-channel interference (CCI) and additive white Gaussian noise (AWGN). In the proposed scheme, pilot symbols are inserted periodically to monitor the channel characteristics. The fading processes experienced by the pilot symbols are used to estimate those suffered by the data symbols using interpolation or filtering. The estimated fade characteristics are used to compensate the random phase variation caused by the Doppler spread, so that the signals can be demodulated coherently. The theoretical performances of the fade compensated coherent modems are evaluated. The results show that the fade compensated coherent demodulation with the least redundancy achieves the same performance as the ideal differential detection in a fading channel. The performance approaches that of the ideal coherent demodulation as more redundancy is allowed. The pilot-symbol-insertion (PSI) scheme is also applicable to M-ary QAM modems and Rician channels. The residual frequency offset can also be compensated by the PSI technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

This paper investigate the properties of space-time bit-interleaved coded modulation (ST-BICM) systems in fast Rayleigh fading channels. We first show that ST-BICM with QPSK signaling in fast fading channels possesses the uniform distance property, which makes performance analysis tractable. We also derive the probability distribution of the squared Euclidean distance between space-time symbols assuming uniform bit-interleaving. Based on the distribution, we show that the diversity order for each codeword pair becomes maximized as the frame length becomes sufficiently long. We analyze the performance of ST-BICM in fast fading channels by deriving an FER upper bound. The derived bound turns out very accurate, requiring only the distance spectrum of the binary channel codes of ST-BICM. Numerical results demonstrate that the bound is tight enough to render an accurate estimate of performance of ST-BICM systems.

This paper proposes a wavelet based receiver structure for frequency-flat time-varying Rayleigh channels, consisting of a receiver front-end followed by a Maximum A-Posteriori (MAP) detector. Discretization of the received continuous time signal using filter banks is an essential stage in the front-end part, where the Fast Haar Transform (FHT) is used to reduce complexity. Analysis of our receiver over slow-fading channels shows that it is optimal for certain modulation schemes. By comparison with literature, it is shown that over such channels our receiver can achieve optimal performance for Time-Orthogonal modulation. Computed and Monte-Carlo simulated performance results over fast time-varying Rayleigh fading channels show that with Minimum Shift Keying (MSK), our receiver using four basis functions (filters) lowers the error floor by more than one order of magnitude with respect to other techniques of comparable complexity. Orthogonal Frequency Shift Keying (FSK) can achieve the same performance as Time-Orthogonal modulation for the slow-fading case, but suffers some degradation over fast-fading channels where it exhibits an error floor. Compared to MSK, however, Orthogonal FSK provides better performance.

Time-selectivity in fast fading channels has been one of big obstacles to attain high efficiency and diversity gain for space-times codes. Orthogonal-frequency-division-multiplexing (OFDM) combined with space-time trellis codes has been introduced to combat effect of frequency-selectivity. Subsequently, space-time block codes, space-frequency codes and concatenation with diverse error-correcting codes were investigated. However, for multiple-input multiple-out (MIMO) OFDM systems, to design high-efficiency robust space-time-frequency codes becomes one of big problems. In this paper, we presented pairwise error probability analysis of space-frequency-time block codes, and then concluded one general design criterion for space-time-frequency block codes by utilizing the frequency-correlation feature of sub-channel in mobile OFDM systems. Then, we investigated performance of space-frequency block codes for MIMO-OFDM systems with 4 transmit antennas. And comparison between space-frequency block codes and space-time block codes is performed with different number of transmit antenna. Link-level simulations on COSSAP® verified analytical results. The results show that SFBC is superior to STBC in fast fading channel. And power-delay profile of fading channel, i.e., channel order and delay spread, must be included into design consideration on space-time-frequency codes.

As the first class of constructive channel codes achieving the symmetric capacity of the binary-input discrete memoryless channels, polar codes have been extended to the fading channel. But most constructions of polar codes in fading channel are only suitable for the Rayleigh fading and involve complex iterative-calculation. In this paper, we establish a systematic framework in term of the polar spectrum to analyze and construct polar codes in various fast fading channels, such as Rician, Rayleigh and Nakagami channels. In these three fading channels, by using the polar spectrum, we derive the upper bound of the error probability of the polarized channel and that of block error rate of polar codes. The analysis based on the polar spectrum explicitly reveals the relationship between the diversity order and the codeword weight. Furthermore, we propose two construction metrics, named logarithmic upper-bound weight (LUW) and minimum-weight LUW (MLUW) respectively, to design the polar codes in fast fading channels. These two constructions are simple with a linear complexity and explicit for the practical implementation. In addition, for various fading channels, they can construct polar codes with similar or better performance over those based on traditional methods.

A modified pseudonoise (PN) code tracking loop is proposed for direct-sequence spread-spectrum communications in the fast fading channel and strong interference environment. First, to formulate the problem we analyze PN code autocorrelation character in the fast fading channel and strong interference. Then the modified PN code tracking model using limited-length curve fitting algorithm is developed to improve the tracking performance. Analytical expressions of the curve fitting algorithm are derived and then confirmed by means of extensive computer simulation results. In addition, utilizing a DS/BPSK experimental system, several simulation results for PN code tracking performance are also presented in this paper. Very attractive behaviors obtained using the proposed technique are verified.

Irregular LDPC codes can achieve better error rate performance than regular LDPC codes. However, irregular LDPC codes have higher error floors than regular LDPC codes. The Ordered Statistic Decoding (OSD) algorithm achieves approximate Maximum Likelihood (ML) decoding. ML decoding is effective to lower error floors. However, the OSD estimates satisfy the parity check equation of the LDPC code even the estimates are wrong. Hybrid decoder combining LLR-BP decoding algorithm and the OSD algorithm cannot also lower error floors, because wrong estimates also satisfy the LDPC parity check equation. We proposed the concatenated code constructed with an inner irregular LDPC code and an outer Cyclic Redundancy Check (CRC). Owing to CRC, we can detect wrong codewords from OSD estimates. Our CRC-LDPC code with hybrid decoder can lower error floors in an AWGN channel. In wireless communications, we cannot neglect the effects of the channel. The OSD algorithm needs the ordering of each bit based on the reliability. The Channel State Information (CSI) is used for deciding reliability of each bit. In this paper, we evaluate the Block Error Rate (BLER) of the CRC-LDPC code with hybrid decoder in a fast fading channel with perfect and imperfect CSIs where 'imperfect CSI' means that the distribution of channel and those statistical average of the fading amplitudes are known at the receiver. By computer simulation, we show that the CRC-LDPC code with hybrid decoder can lower error floors than the conventional LDPC code with hybrid decoder in the fast fading channel with perfect and imperfect CSIs. We also show that combining error detection with the OSD algorithm is effective not only for lowering the error floor but also for reducing computational complexity of the OSD algorithm.

In this paper, we use the minimum mean square error (MMSE) criterion to design the scaled and offset belief propagation (BP)-based decoding algorithms for short low-density parity-check (LDPC) codes on the fast Rayleigh fading channel. Based on the MMSE criterion, the theoretical formulas and numerical calculations on the optimum factors for these two improved BP-based algorithms are provided. The simulation results for several short (Chen et al., 2005 and Gallager, 1962) regular LDPC codes on the fast Rayleigh fading channel demonstrate that the scaled and offset BP-based algorithms with our proposed factors can achieve the performance better than that of the BP algorithm.

This paper aims at comparing the performance of numerous space-time trellis coded modulations (STTCM) over slow and fast Rayleigh fading channels in order to identify the most efficient ones. After a brief presentation of the STTCM concept, we propose to recall the criteria used to construct the codes studied herein, depending on the channel properties and the number of transmit and receive antennas. Then, we introduce our new representation of the STTCM as an extension of the one currently used to describe regular trellis codes, namely the polynomial representation. Simulation results and especially new performance relative to BER scale are then given in order to compare the behavior of Tarokh's, Baro's, Blum's, Chen's and Firmanto's codes over slow and fast Rayleigh fading channels. From these comparisons, it follows that the codes constructed with the trace criterion, namely Chen's codes, outperform all the other tested codes over both channels.

We develop an approach for using an antenna array for tracking fast Rayleigh flat fading channels and suppressing cochannel interference. A general autoregressive (AR) filter is used to model fast flat fading channels, making it possible to characterize the temporal variation of the channels and evaluate its effect on the receiver structure and performance. The optimal array receiver structure that minimizes the probability of error for BPSK signals is derived, which includes a Kalman filter to predict fading channels. A simple expression for the probability of error is also derived for the optimal receiver. In particular, we analyze the case with an AR-1 shaping filter and identical independent fading channels. An irreducible probability of error is shown to exist due to the prediction error of multiple channels. Another interesting observation from the study is that the diversity gain with m antenna elements in the presence of k interferences is usually greater than (m-k), i.e. m antennas with k interferences always perform better than (m-k) antennas without interference. Simulations are carried out to verify the theoretical analysis.

This paper analyzes the performance of a system consisting of a central base transmitter transmitting to a group of mobiles with on-off orthogonal modulation over a fast fading channel. Because the channel is changing across the symbol interval, inter-code interference appears at the output of each user's matched filter. The cumulative distribution function at one user's matched filter output is derived. From this distribution, a metric is introduced to evaluate which pairs of codes lead to the most severe inter-code interference for a particular fading spectrum.

In this paper we study the problem of active eavesdropping in fast fading channels. The active eavesdropper is a more powerful adversary than the classical eavesdropper. It can choose between two functional modes: eavesdropping the transmission between the legitimate parties (Ex mode), and jamming it (Jx mode) - the active eavesdropper cannot function in full duplex mode. In this paper we only consider the best-case scenario, when the transmitter knows the eavesdropper's strategy in advance - and hence can adaptively choose an encoding strategy. We show that, even under these most optimistic assumptions, an active eavesdropper can induce moderate to severe degradation of the achievable secrecy rate.

In this paper we consider a time synchronization problem for a minimum shift keying (MSK) signal received over a time-selective, fast fading channel. We calculate the Cramer Rao bound (CRB) for the case when the transmitted data bits are a-priori known and examine the effects of parameters such as signal-to-noise ratio (SNR) and temporal correlation of the channel on the synchronization performance. In addition, we derive an explicit high SNR approximation to the CRB. Lastly, we use the bound to study how the choice of transmitted bits (the training sequence) influences synchronization performance thus enabling us to identify the best and worst case bit sequences.

Super‐orthogonal space–time trellis codes (SOSTTC) are full rate, full diversity space–time codes with high coding gains for quasi‐static fading channels. In this paper, this design approach is extended to fast fading channels and new SOSTTC are proposed for BPSK and QPSK modulations based on the design criteria valid for this type of channels. The new full rate codes have 4‐ and 16‐state trellises for BPSK and QPSK cases, respectively, to avoid parallel transitions which restrict the error performance. The frame error performances are evaluated through computer simulations and it is shown that the proposed codes have superior performance in the fast fading case compared to their counterparts previously given in the literature for fast and quasi‐static fading channels. Copyright © 2007 John Wiley &amp; Sons, Ltd.

This paper proposes a new adaptive modulation scheme, which is robust to a fast time-variation of Rayleigh fading. Adaptive modulation, which chooses an appropriate modulation format according to the instantaneous carrier-to-noise power ratio (CNR) of the fading channel, is an efficient technique capable of improving the transmission performance in a multipath fading channel. However, in a fast fading channel environment, the performance improvement will drastically be reduced. When CNR varies during the transmission burst, the transmission often fails since the modulation format chosen at the beginning of the burst to obtain the required bit error rate (BER) does not satisfy the required BER at the end of the burst. In order to overcome this problem, we propose a new adaptive modulation scheme, which employs the array antenna based Doppler spread compensator. Computer simulation results show the proposed adaptive modulation scheme drastically improves the packet error rate and throughput performance in fast fading environment.