Ning Kong

Comparison of diversity combining techniques for Rayleigh-fading channels

Several methods of diversity combining for a Rayleigh-faded channel are evaluated and compared. The methods considered are, for coherent reception, maximal ratio combining (MRC), selection combining (SC), and a generalization of SC, whereby the two (three) signals with the two (three) largest amplitudes are coherently combined. We will call this method second (third) order SC, and denote it SC2 (SC3). Similar techniques are also investigated for noncoherent reception, with equal gain combining (EGC) replacing MRC, and noncoherent versions of SC2 and SC3. Numerical results indicate that SC2 and SC3 significantly enhances the bit-error rate (BER) performance relative to that achievable with SC, and under certain conditions approaches the performance achieved by MRC or EGG. The performance enhancement of SC2 and SC3 is especially noticable for noncoherent reception, where EGC is seen to provide the best performance only for low BER values. In fact, when the BER is 10/sup -3/ or greater, SC2 and SC3 performed comparably to EGG, and in some cases performed better than EGC.

Average SNR of a generalized diversity selection combining scheme

The average signal-to-noise ratio (SNR) of a generalized selection combining scheme, in which the m diversity branches (m/spl les/L, where L is the total number of diversity branches available) with the largest instantaneous SNRs are selected and coherently combined, is derived. A Rayleigh fading channel is assumed, and a simple closed-form expression for the SNR is found which is upper bounded by the average SNR of maximal ratio combining, and lower bounded by average SNR of conventional selection combining.

Error probability of multicell CDMA over frequency selective fading channels with power control error

It is well known that power control is critical in CDMA mobile systems. In this paper, the error probability of a multicell CDMA system, operating with imperfect power control over a frequency selective multipath fading channel, is derived. Asymptotic expressions (for high signal-to-interference-plus-noise ratios) of the error probability show that the use of diversity combining exacerbates the effect of the power control error (PCE), since the power control is identical on each diversity channel; also, the error probability increases exponentially with PCE, while (asymptotically) increasing linearly with the number of users and decreasing inverse linearly with the processing gain. In addition to the asymptotic results mentioned above, both Chernoff bounds on, and approximations to, the error probability are found. Numerical results reveal the tightness of the Chernoff bounds, and show that the approximations are actually much tighter than the bounds, at least for PCE standard deviations of 1.5 dB or less.

SNR of generalized diversity selection combining with nonidentical Rayleigh fading statistics

A closed-form expression for the average signal-to-noise-ratio (SNR) of generalized diversity selection combining, using the m largest (in instantaneous SNR) diversity signals, for arbitrary m, assuming that the Rayleigh fading statistics on each diversity branch are identically, independently distributed (i.i.d.), already exists in the literature. In this paper, a similar closed-form expression, but for nonidentically distributed statistics, is derived, This expression specializes to known results, such as the average SNRs of maximal-ratio combining with either i.i.d. or non-i.i.d. diversity statistics and that of conventional selection combining (CSC) with i.i.d. diversity statistics. In addition, it provides, for the first time, a simple closed-form solution to the combined SNR of CSC with non-i.i.d. diversity statistics. Further, the closed-form solution for the SNR of selecting the m, largest diversity signals has negligible computational complexity.

Asymptotic Performance of Digital Communication Over Fading Channels

This paper presents a simple way to obtain the asymptotic performance of various digital communications systems operating over fading channels. The resulting expression for performance contains only the inverse average SNR raised to the systems diversity order, by averaging the conditional error probability (conditioned on the fading) with the dominant term in the fading density function. The proposed approach often renders simple closed-form asymptotic error probabilities (ASEPs), even when the corresponding closed-form actual error probabilities (ACEPs) are too difficult to find, such as those in generalized selection combining, denoted by GSC (m,L), where the strongest m branches from L (1lesmlesL) total branches are selected and combined Using the proposed approach, the paper derives simple closed-form ASEPs for GSC (m,L) which show its diversity order and SNR gaps among different m, over both iid and non-iid Rayleigh fading channels for both MPSK and QAM. It is found that the SNR gaps of GSC (m,L) are not a function of modulation type or orders. In addition, the paper also presents simple closed-form ASEPs for a Nakagami-m fading channel with both MPSK and QAM in conjunction with maximal ratio combining (MRC).

Performance of multicell CDMA with power control error

It has been found that power control is a critical factor in the performance of a CDMA mobile system. The error probability of a multicell mobile CDMA system operating with imperfect power control over a frequency selective multipath fading channel is derived. A simple asymptotic expression for the error probability is found when the signal to interference and Gaussian noise ratio is large. In addition, an upper bound on the error probability is obtained by using Jensens inequality for a frequency nonselective fading channel. It is found analytically that the error probability increases exponentially with the power control error, and linearly with other factors such as the number of users or the processing gain. It is also found that diversity techniques increase the error probability due to the power control error, because the power control error is identical for each diversity channel.

Simple closed-form asymptotic symbol error rate of selection combining and its power loss compared to the maximal ratio combining over Nakagami m fading channels

This paper derives closed-form asymptotic symbol error rates (ASERs) for MPSK and MQAM signals with selection combining (SC) over Nakagami-m fading channels. It also presents a closed-form expression for the power loss of SC compared to maximal ratio combining (MRC) over a Nakagami- m fading channel. It is found that this power gap is a function of only the number (L) of independent Nakagami-m channels and the fading parameter m. For a fixed m, the gap monotonically increases with L. For a fixed L, the gap monotonically increases with m and approaches 10 log10 L dB (the asymptotic gap between MRC and SC in an AWGN channel) as m ¿ ¿. In addition, numerical results show the accuracy of the ASERs obtained from closed-form expressions for both MRC and SC with MPSK and MQAM signals for large SNRs. They also show that the SNR gap is identical to that predicted by the analysis for arbitrary L and m.

Asymptotic performance comparison between dominant eigenmode transmission and Tx/ selection-Rx/MRC for MIMO diversity gain over a Rayleigh fading channel

In this paper, asymptotic symbol error rates (ASERs), of dominant eigenmode transmission (DET) for MIMO (multiple inputs and multiple outputs) diversity gain with small diversity order (the product of the number of Tx antennas and the number of Rx antennas) and Tx/selection (SC)-Rx/MRC with an arbitrary diversity order over a Rayleigh fading channel are derived. The SNR gaps among DET, Tx/SC-Rx/MRC and that with a full diversity gain of the same order are also derived. It is found that for a 2x2 MIMO, DET performs better over Tx/SC-Rx/MRC by 1.18dB and for 3x2 MIMO DET performs better by 1dB. It is also found that a 3x2 Tx/SC-Rx/MRC outperforms a 2x3 Tx/SC-Rx/MRC by 1dB. Simulation results match those in [1] and theoretic derivations in this paper.

Asymptotic Symbol Error Rate for Selection Combining on Nakagami-m Fading Channels

This paper derives closed-form asymptotic symbol error rates (ASERs) for MPSK and MQAM signals with selection combining (SC) over Nakagami-m fading channels. It also presents a closed-form expression for the power loss of SC compared to maximal ratio combining (MRC) over a Nakagami-m fading channel. It is found that this power gap is a function of only the number (X) of independent Nakagami-m channels and the fading parameter m. For a fixed m, the gap monotonically increases with L. For a fixed L, the gap monotonically increases with m and approaches 10log10 L dn (the asymptotic gap between MRC and SC in an AWGN channel) as m rarr infin . In addition, numerical results show the accuracy of the ASERs obtained from closed-form expressions for both MRC and SC with MPSK and MQAM signals for large SNRs. They also show that the SNR gap is identical to that predicted by the analysis for arbitrary L and m.

Groupwise generalized selection diversity combining and its applications in wireless communication

The paper introduces a new concept of groupwise generalized selection combining (GGSC), denoted by GGSC(n, m, L) , in which there are n groups of generalized selection combining GSC(m, L) where the largest m (in SNR) elements are selected from L independent elements. The paper proves that GGSC(n, m, L) has the same diversity order (DO) as that of MRC with nL elements, denoted by MRC(nL) and GSC(nm, nL) which selects the same number of branches globally from the same total of nL elements. The paper quantifies the performance differences among GGSC(n, m, L) , GSC(nm, nL) and MRC(nL) . The paper shows the SNR loss of GGSC(n, m, L) relative to GSC(nm, nL) is less than 2.59dB for arbitrary m, n and L. For diversity orders of practical interest (≪12), the loss is less than 0.6dB