Design of capacity-optimal high-rank line-of-sight MIMO channels

This paper describes a technique for realizing a high-rank channel matrix in a line-of-sight (LOS) multiple-input multiple-output (MIMO) transmission scenario. This is beneficial for systems which are unable to make use of the originally derived MIMO gain given by independent and identically distributed (i.i.d.) flat Rayleigh fading channels. Typical applications are fixed wireless access (FWA) and radio relay systems. The technique is based on optimization of antenna placement in uniform linear arrays with respect to mutual information (MI) for pure LOS channels. Both the case where the channel is only known at the receiver and the case where the channel is known at the transmitter and receiver are treated. By introducing a new and more general 3-D geometrical model than that applied in earlier works, additional insight into the optimal design parameters is gained. We also perform a novel analysis of the sensitivity of the optimal design parameters, and derive analytical expressions for the eigenvalues in the pure LOS channel case, which are valid also when allowing for non-optimal design. Furthermore, we investigate the approximations introduced in the derivations, in order to reveal when the results are applicable, which turns out to be for most practical situations. The LOS transmission matrix is used in a Ricean fading channel model which incorporates spatial correlation between the non-LOS components, and performance is evaluated with respect to the average MI and the MI cumulative distribution function. Our results show that even with some deviation from the optimal design, the LOS MIMO scenario outperforms the i.i.d. Rayleigh scenario in terms of MI

Rate-Optimal Multiuser Scheduling with Reduced Feedback Load and Analysis of Delay Effects

In this paper we propose a multiuser scheduling algorithm that has the maximum average system spectral efficiency, but obtains a significant reduction in feedback load compared to full feedback by using a feedback threshold. An expression for the threshold value that minimizes the feedback load is found. Novel closed-form expressions are also found for the system spectral efficiency when using M-ary quadrature amplitude modulation. Finally, we analyze the impact of scheduling delay and outdated channel estimates. 1

Modeling and analysis of a 40 GHz MIMO system for fixed wireless access

The throughput of a possible future fixed wireless access multiple-input multiple-output (MIMO) system operating at high frequencies is investigated. We extend our previous theoretical work on MIMO for line-of-sight (LOS) channels to show that a considerable gain in throughput is achieved compared to single-input single-output transmission for a practical system, which among other things is subject to antenna array size constraints. In our investigation we apply a propagation model applicable for high frequencies, where often LOS is required for sufficient coverage, and where weather phenomena like rain have considerable impact on the quality of the radio link. A state of the art transmission scheme is utilized with eigenmode transmission, low density parity check coded modulation, power control, and bit loading. The performance is evaluated both with respect to throughput and Shannon capacity

Feedback Protocols for Increased Multiuser Diversity Gain in Cellular ALOHA Networks : A Comparative Study

Multiuser diversity (MUD) underlies much of the recent work on scheduling design in wireless networks. This form of diversity can for example be exploited by opportunistically scheduling the mobile user with the best channel quality. In cellular networks exploiting MUD, the base station collects channel state information (CSI) from the mobile users. The process of obtaining CSI will be performed within a guard time, and the length of this guard time will depend on the feedback protocol implemented. In this context, it has already been shown that by applying multiple carrier-to-noise ratio thresholds, the number of mobile users giving feedback can be significantly decreased. However, it has not been evaluated how the algorithm in can be implemented in protocols for real-life networks. In this paper we analyze feedback protocols for reducing the guard time and resolving the feedback contention problem in a cellular, slotted ALOHA-based network. We propose three new feedback protocols based on the algorithm in and we develop closed-form expressions for the guard time duration and the system spectral efficiency of these protocols. We also compare the three new protocols with the Splitting algorithm proposed by Qin and Berry and a new and modified version of this algorithm. Plots show that the spectral efficiency in an IEEE 802.11 network can increase significantly for a high number of users when the Modified Splitting algorithm is used