CDMA reverse link spatial combining gains: optimal vs. MRC in a faded voice-data system having a single dominant high data user

The reverse link (mobile to base station) average data rate carrying capacity of a CDMA communications system is analyzed comparing different antenna array spatial combining algorithms: optimum combining (OC) versus maximal ratio combining (MRC) in a multiuser system employing multi-rate traffic (combined voice and data users) over Rayleigh fading channels. Typical CDMA power and interference relationships for an embedded multi-cell CDMA reverse link are discussed in the context of spatial OC algorithms and related to the total reverse link data throughput. Other-cell interference and chosen system rise-over-thermal metrics are found to limit the possible gain increase using OC vs. MRC.

The BER expressions for optimum combining (OC) and maximal ratio combining (MRC) in the presence of one co-channel interferer are derived in this paper for a dual antenna TDMA communication system with correlated channel diversity. Numerical results demonstrate the impact of the different correlation coefficients in combating fading and reducing CCI. It is shown that the BERs of OC and MRC are insensitive to the channel correlation of the interfering signal. However, the BERs of OC and MRC are degraded significantly when the channel correlation of the desired signal approaches unit.

The benefits of employing antenna array processing for the interference limited UMTS system is the increase in the number of simultaneous users and the reduction in the transmitted uplink power. The joint power control and antenna processing offers maximum achievable capacity with lower transmission powers for the given target SINR (signal-to-interference+noise ratio). The analysis about possible achievements using antenna array processing, varying parameters such as the number of users in a microcell, processing gain and antenna array size for the given target SINR, has been done using a deterministic ray tracing model developed for urban microcells and considering two antenna processing cases, OC (optimum combining) and MRC (maximum ratio combining). The paper shows that the ratio between the target value and the processing gain is the most dominant factor for the antenna array performance in the power controlled environment, and that optimum transmitted powers obtained using OC are the least sensitive to the increase in the number of users, as compared to MRC and simple decorrelation on a single antenna element.

This work presents the approximate symbol error rate (SER) of M-ary phase shift keying (MPSK) for optimum combining (OC) with multiple co-channel interferers in a flat Rayleigh fading channel. The closed-form expression for the ordered mean eigenvalues of interference-plus-noise covariance matrix is derived for the first-order approximation to avoid Monte Carlo simulation and multiple numerical integrals in the cases of arbitrary numbers of interferers and antenna elements. We also derive the closed-form expression for approximate SER of MPSK for OC in terms of the average SER of MPSK for maximal ratio combining (MRC). From the simple evaluation of ordered mean eigenvalues, the first-order approximation gives a simple and accurate way to analyze the performance of OC in the presence of the arbitrary number of interferers.

Postdetection diversity reception weights and combines all the detector outputs before symbol decision to combat the effects of multipath fading. A theoretical analysis of a postdetection optimal diversity combiner that can minimize the symbol error probability for differential phase shift keying (DPSK) differential detection in the presence of multiplicative Rayleigh fading, and co-channel interference (CCI) is presented. The effect of unequal average powers among diversity branches is taken into account. It is shown that the postdetection maximal-ratio combiner (MRC) described previously by the author is not optimal unless all branches have the same average power. It is also found that the combiner optimized for the effect of CCI (fading induced random FM noise) should weight each branch detector output in inverse proportion to the average CCI power (desired signal power). Assuming two-branch diversity, calculated BER (bit-error-rate) performance of pi /4-shift QDPSK due to AWGN, CCI, and random FM is presented. In addition, the BER due to multipath channel delay spread (which is not treated in the theoretical analysis) is also computed to find the optimal combiner.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

To improve signal-to-interference ratio (SIR) and make better use of file diversity provided by random caching, we consider the maximal ratio combining (MRC) receiver at each user in a large-scale cache- enabled single-input multi-output (SIMO) network. First, by utilizing tools from stochastic geometry, we derive a tractable expression and closed-form upper and lower bounds for the successful transmission probability in the general SIR threshold regime. We also derive a closed-form expression for the asymptotic outage probability in the low SIR threshold regime. The analytical results reveal that the successful transmission probability increases with the number of receive antennas M at each user in the general SIR threshold regime, the coefficient of the asymptotic outage probability decreases with M and the order gain of the asymptotic outage probability does not depend on M. Then, we consider the successful transmission probability maximization. In the general SIR threshold regime, we consider the maximization of the tight upper bound on the successful transmission probability by optimizing the caching distribution, which is a non-convex problem. We obtain a stationary point, by solving an equivalent difference of convex (DC) programming problem using concave-convex procedure (CCCP). We also obtain a closed-form asymptotically optimal solution in the low SIR threshold regime. The optimization results indicate that files of higher popularity get more storage resources, and the caching distribution becomes more flat when M is larger.

Random unitary beamforming (RUB) is an attractive low-complexity transmission scheme for multiuser MIMO broadcast channel. When there are a moderate number of users in the system, the sum rate performance of this signaling scheme becomes mainly interference-limited over high signal-to-noise ratio (SNR) region. In this paper, we study the performance of RUB-based multiuser MIMO systems when optimum combining is employed at mulitple-antenna mobile receivers to suppress multiuser interference. We adopt a geometric approach to derive the statistics of the output signal-to-interference ratio (SIR) of the optimum combiner, based on which a sum rate upper bound is obtained over high SNR region. We show through selected numerical examples that the proposed RUB with optimal combining achieves much better sum rate performance while maintaining low feedback load.

A Monte Carlo simulation was conducted to assess the performance of Multi-Carrier Code Division Multiple Access (MC-CDMA) in a sectored cell, in an indoor environment. In each sector, terminal schedule packet transmission using slotted p-persistent Inhibit Sense Multiple Access, while the packets themselves are transmitted using MC-CDMA. The simulated bit error rate performance of MC-CDMA with combining strategies maximal ratio combining (MRC) and equal gain combining (EGC) used in the frequency domain are presented. A comparison is made between analytically determined bit error probability for a single user in an additive white Gaussian noise channel using binary phase shift keying (BPSK), and simulated bit error rate for a single user using MC-CDMA with MRC and EGC. The determination of the number of sectors in a cell is also given. MC-CDMA with diversity combining outperforms BPSK in a non-fading AWGN channel. Further, as the user-number increases, EGC performs better than MRC if these combining strategies are used with MC-CDMA. Finally, the results show the basis for determining the number of sectors in a cell. When using slotted p-persistent ISMA, it is envisaged that only a single user will transmit at a time in a sector.

This paper examines the impact of channel estimation errors on the distribution of the signal-to-interference ratio (SIR) at the output of the optimum combiner for space diversity reception in the uplink of wireless systems with multiple interferers. We generalize the results of prior work obtained for Rayleigh fading channels to a Rician/Rayleigh fading environment where the desired user is subject to Rician fading while the interferers experience Rayleigh fading. Using multivariate statistical analysis, an exact closed-form expression is derived for the probability density function (PDF) of the output SIR. The PDF expression is a useful tool to derive the analytical expressions of some other important measures of performance such as the outage probability and the average bit error probability. The theoretical result is verified by Monte Carlo simulations

An optimal maximal ratio combiner (MRC) based on the expectation-maximization (EM) algorithm is devel- oped for noisy constant envelope signals transmitted over a Rayleigh fading channel. Instead of using a transmitted pilot signal with the data to estimate the combiner gains, the EM algorithm is used to perform this estimation. In the developed MRC, estimation of the transmitted data sequence is performed also by the EM algorithm. Estimation using the EM algo- rithm provides an iterative solution to the maximum likelihood (ML) approach. Therefore, the resulting receiver is opti- mum and does not suffer from the difficulties resulted from direct application of the ML procedure. One of these difficul- ties is the computational complexity which depends exponentially on the data sequence length. Introducing an iterative structure in the developed MRC achieves a linear computational complexity and enables efficient data extraction by the Viterbi algorithm when trellis coding is used.

We considered call admission control (CAC) algorithms both for the reverse and forward link with multiple types of services requiring different SIRs (signal to interference ratio) and transmission rates. In the forward link, a call is admitted if the requested power is available at the base station, whereas in reverse link, a call is admitted if the interference level is below a certain revel that guarantees the communication quality. First, the optimal power ratio that characterizes the effect of different service types is considered. Then a simple CAC algorithm is proposed and evaluated through simulation. With these CAC algorithm, we compare the capacity of forward link and reverse link for commonly used SIR equations.

In this paper, we present mathematical frameworks for error performance analysis in interference limited networks such as cellular networks. Due to the increasing irregularity in the spatial deployment of nodes in the emerging heterogeneous cellular networks (HCNs), we employ stochastic geometry approach by abstracting the node locations as a homogeneous Poisson point process (PPP). First, we characterize the average error probability of an intended communication link with a given transmitter-receiver separation, which is subject to interference from these Poisson distributed nodes. More specifically, we develop uniform approximation (UA), which is highly accurate over the whole range of signal-to-interference ratio (SIR) and hence, serve as an alternative to existing complex analytical results. Error probability UAs for both single-antenna and maximal ratio combining (MRC) receivers are derived in this paper. Next, we evaluate the average error probability of any typical user in the network, which is served by the node providing the maximum received power. Mellin-transform based method is proposed in this case, which often yield closed-form solution. An example of BPSK modulation is given in the paper.

To improve signal-to-interference ratio (SIR) and make better use of file diversity provided by random caching, we consider two types of linear receivers, i.e., maximal ratio combining (MRC) receiver and partial zero forcing (PZF) receiver, at users in a large-scale cache-enabled single-input multi-output network. First, for each receiver, by utilizing tools from stochastic geometry, we derive a tractable expression and a tight upper bound for the successful transmission probability (STP). In the case of the MRC receiver, we also derive a closed-form expression for the asymptotic outage probability in the low SIR threshold regime. Then, for each receiver, we maximize the STP. In the case of the MRC receiver, we consider the maximization of the tight upper bound on the STP by optimizing the caching probabilities, which is a non-convex problem. We obtain a stationary point, by solving an equivalent difference of convex programming problem using concave-convex procedure. We also obtain a closed-form asymptotically optimal solution in the low SIR threshold regime. In the case of the PZF receiver, we consider the maximization of the tight upper bound on the STP by optimizing the caching probabilities and the degrees of freedom allocation (for boosting signal power and canceling interference), which is a mixed discrete-continuous problem. Based on structural properties, we obtain a low-complexity near optimal solution by using an alternating optimization approach. The analysis and optimization results reveal the impact of antenna resource at users on random caching.

Multi-carrier code division multiple access (MC-CDMA) scheme with base station antenna arrays has the potential of being an appropriate candidate for the next generation mobile and wireless personal communication systems due to its high system capacity. In this paper, the reverse link performance of a MC-CDMA system along with base station antenna arrays employing three combining schemes: equal gain combining (EGC), maximal ratio combining (MRC) and minimum mean square error combining (MMSEC) in Rayleigh fading channel environments is analyzed and compared. Simulation results and analysis results are also presented and compared. It is shown that MRC can improve system performance significantly over EGC in the given channel while MMSEC just gives a little performance improvement over MRC.

Distributed power-control algorithms for systems with hard signal-to-interference ratio (SIR) constraints may diverge when infeasibility arises. We present a power-control framework called utility-based power control (UBPC) by reformulating the problem using a softened SIR requirement (utility) and adding a penalty on power consumption (cost). Under this framework, the goal is to maximize the net utility, defined as utility minus cost. Although UBPC is still noncooperative and distributed in nature, some degree of cooperation emerges: a user will automatically decrease its target SIR (and may even turn off transmission) when it senses that traffic congestion is building up. This framework enables us to improve system convergence and to satisfy heterogeneous service requirements (such as delay and bit error rate) for integrated networks with both voice users and data users. Fairness, adaptiveness, and a high degree of flexibility can be achieved by properly tuning parameters in UBPC.