A perspective on multiaccess channels

The information theoretic approach and the collision resolution approach to multiaccess channels are reviewed in terms of the underlying communication problems that both are modeling. Some perspective on the strengths and weakness of these approaches is given, and the need of a more combined approach focused on coding and decoding techniques is argued.     

Type based estimation over multiaccess channels

We study the problem of communicating sensor readings over a Gaussian multiaccess channel. We focus on the scenario that each sensor observes a single random variable and transmits it using certain signaling in a shared channel. The objective is the design of channel waveforms (i.e., the signal constellation) to facilitate the estimation of field parameters from the channel output. We propose a communication scheme in which sensors transmit according to the type of their observations-type-based multiple access (TBMA)-and show that the TBMA is asymptotically optimal in the limit of large number of sensors if the sensor channel-gains are identical. In particular, we show that TBMA together with a variant of the maximum-likelihood estimator achieves the Cramer-Rao bound asymptotically. We then extend the asymptotic analysis of TBMA to fading channels and compare the performance of TBMA with other orthogonal allocation methods such as time-division multiple access.     

Evaluation of capture effect in LEO satellite channel

A sophisticated evaluation model dedicated to non-coherent frequency shift keying (NC-FSK) demodulation is proposed, by which the capture effect in the low Earth orbit (LEO) satellite channel can be examined and the resulting improvement in the throughput of slotted ALOHA protocols described     

A spread-spectrum multiaccess system with cochannel interferencecancellation for multipath fading channels

The authors propose and analyze a direct-sequence spread-spectrum multiaccess (DS/SSMA) receiver that employs a cascade of cochannel interference (CCl) cancellers for communication over multipath fading channels. The receiver first coherently demodulates and despreads the received signal to produce correlator outputs and initial data estimates. Based on these estimates, the cancellation scheme essentially creates replicas of the contributions of the CCl embedded in the correlator outputs and removes them for a second improved hard data decision. By repeating this operation over and over, a cascade of CCl cancellers is derived. Through theoretical analysis and simulation, the authors investigate the canceller's bit error rate (BER) performance in both the absence and presence of errors in the amplitude and phase estimates of each user's received signal. Numerical results show the considerably large improvement in performance that can be attained by the cancellation scheme, even under partially degraded estimates     

Minimum duration outages in Rayleigh fading channels

Minimum duration outages have been introduced for lognormal shadow fading where the durations of signal fades were considered in evaluating outages. We develop and analyze such minimum duration outages for channels susceptible to Rayleigh fading. The results show that under typical Doppler frequencies the outages due to Rayleigh fading are more likely to cause frame or packet errors rather than call dropping due to the short time scales in effect     

The tone sense multiaccess protocols with partial collisiondetections (TSMA/PCD) for packet satellite communications

The tone sense multiaccess with partial collision detection (TSMA/PCD) protocol is particularly suitable for a packet satellite system serving an area with a dense population of earth stations. By incorporating a narrowband ground radio channel for broadcasting busy ones, the earth stations are able to avoid packet collisions by sensing for the absence of busy tones before transmitting packets. Partial collision detection capability can also be achieved. Single-tone TSMA/PCD gives 97% of the carrier-sense multiaccess with collision detection (CSMA/CD) throughput when N=10 tones are used, while for multitone and slot-by-slot announcement TSMA/PCD protocols only N=8 and N=2, respectively, are sufficient to drive the system to the CSMA/CD performance     

Capacity of correlated MIMO Rayleigh channels

Abstract-This paper presents some exact results on the capacity of multiple-input-multiple-output (MIMO) channels subject to correlated Rayleigh fading when perfect channel state information (CSI) is known at the receiver. The authors focus on the semicorrelated scenario in which correlation exists either at the transmitter or at the receiver., They consider two cases: 1) the transmitter does not have any CSI and as such allocates power equally among transmitter antennas and 2) the transmitter only knows the statistical distribution of the channel. The first case derives the moment generating function (MGF) of the mutual information (MI) and then deduces from this MGF the mean MI. The authors also study the cumulative distribution function (CDF) of the MI, which can serve as an upper bound to the outage probability under the capacity versus outage formulation when the channel is nonergodic. The second case studies the capacity achieved by optimum power-loading and beamforming schemes based on covariance feedback. Numerical results illustrate that the full capacity of MIMO systems can be preserved even for relatively high values of correlation coefficients.     

FHMA receiver over Rayleigh fading channels

A novel FH/MFSK receiver is proposed which utilises the side information of interfering signals for asynchronous frequency hopping multiple-access (FHMA) systems in the presence of Rayleigh fading. It is shown that the novel receiver performs much better than the conventional receiver for a wide range of signal to noise ratios.     

Shannon capacity of STBC in Rayleigh fading channels

Space-time block coding (STBC) is a powerful transmit diversity scheme for multiple antenna systems. A closed-form expression for the Shannon capacity of MIMO systems is derived using STBC under independent and quasi-static flat Rayleigh fading channels. The Shannon capacity of STBC over such channels is expressed as a finite sum of functions that are easy to evaluate, thereby avoiding the need for numerical integration or Monte Carlo simulation.     

The throughput of some wireless multiaccess systems

We compute the throughput of some multiaccess wireless systems for delay-tolerant data communications, characterized by an infinite population of uncoordinated users accessing a common channel. The channel is affected by block fading, and the channel state is perfectly known to the receiver but unknown to the transmitters. To cope with multiaccess interference (MAI) and fading, the users employ retransmission of erroneously received packets. We consider unspread and randomly spread (code-division multiple-access (CDMA)) systems with decentralized (single-user) decoding and a system where the receiver employs joint multiuser decoding. The following conclusions can be drawn from our analysis: (a) unspread systems with packet retransmission outperforms CDMA systems with conventional detection, but are outperformed by CDMA with linear minimum mean-square error (MMSE) detection. (b) For all systems based on single-user decoding (SUD), there exists a threshold value of (E/sub b//N/sub o/) below which the throughput is maximized by an infinite number of users per dimension transmitting at vanishing rate, and above which the throughput is maximized by a finite average number of users per dimension transmitting at nonvanishing rate. Moreover, as (E/sub b//N/sub o/) increases, the optimal average number of users per dimension tends to one. In this sense, we say that the optimized systems "self-orthogonalize." (c) For the system based on joint multiuser decoding, a simple slotted ALOHA strategy is able to recover the throughput penalty due to fading in the limit for high (E/sub b//N/sub o/), while an incremental redundancy (INR) strategy recovers the fading penalty for any (E/sub b//N/sub o/).     

Multi-branch DFE for Rayleigh fading multipath channels

Multipath Rayleigh fading channels which have very fast time variation can cause loss of tracking in equalisers, from which recovery without retraining is unlikely. A new multi-branch decision feedback equaliser is suggested to improve bit error rate performance while maintaining very low complexity. Simulation results demonstrate its suitability     

Finite-state Markov model for Rayleigh fading channels

We form a finite-state Markov channel model to represent Rayleigh fading channels. We develop and analyze a methodology to partition the received signal-to-noise ratio (SNR) into a finite number of states according to the time duration of each state. Each state corresponds to a different channel quality indicated by the bit-error rate (BER). The number of states and SNR partitions are determined by the fading speed of the channel. Computer simulations are performed to verify the accuracy of the model     

GQR models for multipath Rayleigh fading channels

In this paper, reduced complexity statistical models for the representation of wide sense stationary-uncorrelated scattering doubly selective fading channels are developed. Their derivation is based on the evaluation of Fourier integrals by means of Gaussian quadrature rules. The accuracy and the complexity of the proposed models is assessed, and is then compared to that provided by other modeling techniques available in the literature     

Irreducible error rate in aeronautical satellite channels

The author examines the irreducible error rate in aeronautical satellite systems. It is shown that the presence of a delay in the multipath component of a Rician channel increases the irreducible error rate of the receiver     

Combined equalization for uplink MC-CDMA in Rayleigh fading channels

In this letter, we propose the concept of combined equalization for uplink multicarrier code-division multiple access (MC-CDMA) and perform a theoretical analysis which shows that better single-user bounds than the classical matched-filter bounds are achieved with this new concept. Moreover, we illustrate how to properly design an uplink MC-CDMA transmitter and receiver for combined equalization, and show by Monte Carlo simulations that the improved single-user bounds are closely approached, even in the case of a fully loaded system.     

A trellis-coded DS-CDMA system in correlated Rayleigh fading channels

We present the trellis-coded code-division multiple-access (TC-CDMA) system based on a multisequence signaling, called orthogonal plane sequence modulation (OPSM), in correlated Rayleigh fading channels and derive its pairwise error probability with different degrees of channel state information. Numerical results show that the OPSM-based TC-CDMA system outperforms conventional convolutionally coded CDMA or TC-CDMA systems.     

Frequency-hopped multiple-access communication with nonorthogonalBFSK in Rayleigh fading channels

Nonorthogonal binary frequency-shift keying is considered in frequency-hopped multiple-access communication systems with Reed-Solomon coding. The effect of tone spacing on the average number of successfully transmitted information bits per unit time per unit bandwidth (called normalized throughput) is examined in Rayleigh fading channels. The tradeoff among tone spacing, code rate, and number of frequency slots in maximizing the normalized throughput is examined, keeping the total bandwidth fixed. The optimal tone spacing, code rate, and number of frequency slots in terms of the number of users and E¯_b/N₀ is also discussed. The throughput gain attained by using the optimal tone spacing becomes more significant as the number of users is increased     

Exact analysis for downlink MC-CDMA in Rayleigh fading channels

In this letter, downlink multicarrier code-division multiple access (MC-CDMA) systems with maximal ratio combining (MRC) in frequency-selective Rayleigh fading channels is considered. An exact bit error rate (BER) is provided based on the characteristic function (CF) and residue calculation method. The BER performance is evaluated in the time domain instead of the frequency domain. This method does not require any assumption regarding the statistical or spectral behavior of multiple access interference (MAI).     

Multiuser receivers for CDMA systems in Rayleigh fading channels

Multiuser demodulation in relatively fast fading channels is analyzed. The optimal maximum likelihood sequence detection (MLSD) receiver is derived and a general suboptimal receiver to approximate the MLSD is proposed. The performance of the parallel interference cancellation (PIC) and decorrelating receivers is compared. The PIC receiver is demonstrated to achieve better performance in known channels than the decorrelating receiver, but it is observed to be more sensitive to the channel coefficient estimation errors than the decorrelator. At high channel loads the PIC receiver suffers from bit error rate (BER) saturation, whereas the decorrelating receiver does not. The performance of data-aided (DA) and decision-directed (DD) multiuser channel estimation is also compared. DA channel estimation is shown to be more robust than DD channel estimation, which may suffer from BER saturation caused by hangups at high signal-to-noise ratios (SNRs). The impact of channel estimation filter impulse response on the BER is studied by comparing optimal and suboptimal channel estimation filters. The implementation complexity of the decorrelating and PIC receivers is compared in terms of required floating point operations and clock cycles in a practical communication scenario. It is observed that the PIC receiver is only moderately more complex to implement than the conventional matched filter bank receiver, whereas the decorrelating receiver is significantly more complex     

Computer simulation of Rayleigh fading channels in MC-CDMA systems

For Multi-Carrier-Code Division Multiple Access (MC-CDMA) systems,it is usually as- sumed that the fading of the subcarriers is frequency non-selective and independent of each other. This paper shows that the two assumptions are incompatible.In fact,the MC-CDMA signals at each subcarrier undergo fading that are highly correlated.Based on this observation,this paper develops a simulation algorithm for Rayleigh fading channels via frequency-domain correlation function,which incorporates the Doppler effect simultaneously.Numerical examples are presented to demonstrate the effectiveness of the new algorithm,with the conclusion that the independence assumption of subcar- rier fading overrates the system performance.