Performance of Optimum Combining in a Poisson Field of Interferers and Rayleigh Fading Channels

This paper studies the performance of antenna array processing in distributed multiple access networks without power control. The interference is represented as a Poisson point process. Desired and interfering signals are subject to both path-loss fading (with an exponent greater than 2) and to independent Rayleigh fading. Using these assumptions, we derive the exact closed form expression for the cumulative distribution function of the output signal-to-interference-plus-noise ratio when optimum combining is applied. This results in a pertinent measure of the network performance in terms of the outage probability, which in turn provides insights into the network capacity gain that could be achieved with antenna array processing. We present and discuss examples of applications, as well as some numerical results.Comment: Submitted to IEEE Trans. on Wireless Communication (Jan. 2009

Adaptive space-time processing for digital mobile radio communication systems

The performance of digital mobile radio communication systems is primarily limited by cochannel interference and multipath fading. Antenna arrays, with optimum combining (OC), have been shown to combat multipath fading of the desired signal and are capable of reducing the power of interfering signals at the receiver through spatial filtering. With OC, the signals received by several antenna elements are weighted and combined to maximize the output signal-to-interference-plus-noise ratio (SLNR). We derive new closed-form expressions for (1) the probability density function (PDF) of the SINR at the output of the optimum combiner, (2) the average probability of bit error rate (BER) and its upper bound, and (3) the outage probability in a Rayleigh fading environment with multiple cochannel interferers. The study covers both the case when the number of antenna elements exceeds the number of interferers and vice versa. We consider independent fading at each antenna element, as well as the effect of fading correlation. The analysis is also extended to processing using maximal ratio combining (MRC). The performance of the optimum combiner is compared to that of the maximal ratio combiner and results show that OC performs significantly better than MRC. We investigate the performance of OC in a microcellular environment where the desired signal and the cochannel interference can have different statistical characteristics. The desired signal is assumed to have Rician statistics implying that a dominant multipath reflection or a line-of-sight (LOS) propagation exists within-cell transmission. Interfering signals from cochannel cells are assumed to be subject to Rayleigh fading due to the absence of LOS propagation. This is the so called Rician/Rayleigh model. We also study OC for a special case of the Rician/Rayleigh model, the Nonfading/Rayleigh model. We derive expressions for the PDF of the SJNR, the BER and the outage probability for both Rician/Rayleigh and Nonfading/Rayleigh models. Similar expressions are derived with MRC. Another area in which space-time processing may provide significant benefits is when wideband signals (such as code division multiple access (CDMA) signals) are overlaid on existing narrowband user signals. The conventional approach of rejecting narrowband interference in direct-sequence (DS) CDMA systems has been to sample the received signal at the chip interval, and to exploit the high correlation between the interference samples prior to spread spectrum demodulation. A different approach is space-time processing. We study two space-time receiver architectures, referred to as cascade and joint, respectively, and evaluate the performance of a DS-CDMA signal overlaying a narrowband signal for personal communication systems (PCS). We define aild evaluate the asymptotic efficiency of each configuration. We develop new closed-form expressions for the PDF of the SINR at the array output, the BER and its upper bound, for both cascade and joint configurations. We also analyze the performance of this system in the presence of multiple access interference (MAJ)

The Effect of Macrodiversity on the Performance of Maximal Ratio Combining in Flat Rayleigh Fading

The performance of maximal ratio combining (MRC) in Rayleigh channels with co-channel interference (CCI) is well-known for receive arrays which are co-located. Recent work in network MIMO, edge-excited cells and base station collaboration is increasing interest in macrodiversity systems. Hence, in this paper we consider the effect of macrodiversity on MRC performance in Rayleigh fading channels with CCI. We consider the uncoded symbol error rate (SER) as our performance measure of interest and investigate how different macrodiversity power profiles affect SER performance. This is the first analytical work in this area. We derive approximate and exact symbol error rate results for M-QAM/BPSK modulations and use the analysis to provide a simple power metric. Numerical results, verified by simulations, are used in conjunction with the analysis to gain insight into the effects of the link powers on performance.Comment: 10 pages, 5 figures; IEEE Transaction of Communication, 2012 Corrected typo

A combined channel-modified adaptive array MMSE canceller and viterbi equalizer

In this thesis, a very simple scheme is proposed which couples a maximum-likelihood sequence estimator (MLSE) with a X-element canceller. The method makes use of the MLSE\u27s channel estimator to modify the locally generated training sequence used to calculate the antenna array weights. This method will increase the array\u27s degree of freedom for interference cancellation by allowing the dispersive, desired signal to pass through the array undisturbed. Temporal equalization of the desired signal is then accomplished using maximum-likelihood sequence estimation. The T-spaced channel estimator coefficients and the array weights are obtained simultaneously using the minimum mean square error criteria. The result is a X-element receiver structure capable of canceling X- 1 in-band interferences without compromising temporal equalization

Performance Analysis of Dual-User Macrodiversity MIMO Systems with Linear Receivers in Flat Rayleigh Fading

The performance of linear receivers in the presence of co-channel interference in Rayleigh channels is a fundamental problem in wireless communications. Performance evaluation for these systems is well-known for receive arrays where the antennas are close enough to experience equal average SNRs from a source. In contrast, almost no analytical results are available for macrodiversity systems where both the sources and receive antennas are widely separated. Here, receive antennas experience unequal average SNRs from a source and a single receive antenna receives a different average SNR from each source. Although this is an extremely difficult problem, progress is possible for the two-user scenario. In this paper, we derive closed form results for the probability density function (pdf) and cumulative distribution function (cdf) of the output signal to interference plus noise ratio (SINR) and signal to noise ratio (SNR) of minimum mean squared error (MMSE) and zero forcing (ZF) receivers in independent Rayleigh channels with arbitrary numbers of receive antennas. The results are verified by Monte Carlo simulations and high SNR approximations are also derived. The results enable further system analysis such as the evaluation of outage probability, bit error rate (BER) and capacity.Comment: 24 pages, 7 figures; IEEE Transaction of Wireless Communication 2012 Corrected typo

Performance evaluation for communication systems with receive diversity and interference

Optimum combining (OC) is a well-known coherent detection technique used to combat fading and suppress cochannel interference. In this dissertation, expressions are developed to evaluate the error probability of OC for systems with multiple interferers and multiple receiving branches. Three approaches are taken to derive the expressions. The first one starts from the decision metrics of OC. It facilitates obtaining closed-form expressions for binary phase-shift keying modulation. The second approach utilizes the moment generating function of the output signal to interference plus noise ratio (SINR) and results in expressions for symbol and bit error probability for multiple phaseshift keying modulation. The third method uses the probability density function of the output SINR and arrives at expressions of symbol error probability for systems where the interferers may have unequal power levels. Throughout the derivation, it is assumed that the channels are independent Rayleigh fading channels. With these expressions, evaluating the error probability of OC is fast, easy and accurate. Two noncoherent detection schemes based on the multiple symbol differential detection (MSDD) technique are also developed for systems with multiple interferers and multiple receiving branches. The first MSDD scheme is developed for systems where the channel gain of the desired signal is unknown to the receiver, but the covariance matrix of the interference plus noise is known. The maximum-likelihood decision statistic is derived for the detector. The performance of MSDD is demonstrated by analysis and simulation. A sub-optimum decision feedback algorithm is presented to reduce the computation complexity of the MSDD decision statistic. This suboptimum algorithm achieves performance that is very close to that of the optimum algorithm. It can be shown that with an increasing observation interval, the performance of this kind of MSDD approaches that of OC with differential encoding. The second MSDD scheme is developed for the case in which the only required channel information is the channel gain of the interference. It is shown that when the interference power level is high, this MSDD technique can achieve good performance

Exploiting Spatial Interference Alignment and Opportunistic Scheduling in the Downlink of Interference Limited Systems

In this paper we analyze the performance of single stream and multi-stream spatial multiplexing (SM) systems employing opportunistic scheduling in the presence of interference. In the proposed downlink framework, every active user reports the post-processing signal-to-interference-plus-noise-power-ratio (post-SINR) or the receiver specific mutual information (MI) to its own transmitter using a feedback channel. The combination of scheduling and multi-antenna receiver processing leads to substantial interference suppression gain. Specifically, we show that opportunistic scheduling exploits spatial interference alignment (SIA) property inherent to a multi-user system for effective interference mitigation. We obtain bounds for the outage probability and the sum outage capacity for single stream and multi stream SM employing real or complex encoding for a symmetric interference channel model. The techniques considered in this paper are optimal in different operating regimes. We show that the sum outage capacity can be maximized by reducing the SM rate to a value less than the maximum allowed value. The optimum SM rate depends on the number of interferers and the number of available active users. In particular, we show that the generalized multi-user SM (MU SM) method employing real-valued encoding provides a performance that is either comparable, or significantly higher than that of MU SM employing complex encoding. A combination of analysis and simulation is used to describe the trade-off between the multiplexing rate and sum outage capacity for different antenna configurations