Scheduling Gain in Spatial Diversity Systems: Asymptotic Analysis

A key feature in the design of any MAC protocol is the throughput it can provide. In wireless networks, the channel of a user is not fixed but varies randomly. Thus, in order to maximize the throughput of the MAC protocol at any given time, only users with large channel gains should be allowed to transmit. In this paper, a compressive sensing based opportunistic protocol for exploiting multiuser diversity in wireless networks is proposed. This protocol is based on the traditional protocol of R-ALOHA which allows users to compete for channel access before reserving the channel to the best user. We use compressive sensing to find the best user, and show that the proposed protocol requires less time for reservation and so it outperforms other schemes proposed in the literature. Also, as the proposed scheme requires less reservation time, it can be seen as an enhancement for R-ALOHA schemes in fast fading environment.

Energy-efficient opportunistic scheduling schemes in wireless networks

Cyber-Physical System (CPS) is envisioned to tightly integrate the cyber-world of computation, communication, and control with the physical world. CPS is typically designed as a networked system of interacting sensors, actuators, and embedded computing devices to monitor and control the physical world. Thus, one of the essential building blocks of such a system is a highly efficient networking infrastructure. In this paper, we aims to develop an efficient wireless networking technology which can be utilized in CPS. More specifically, we develop a cross-layer optimization model based on the Network Utility Maximization (NUM) framework and its distributed solution for wireless multihop multicast networks exploiting multi-user diversity. It is known that the capacity of a wireless network can be increased by exploiting different channel conditions at different users, i.e., multi-user diversity; however, it is yet to be determined how much performance gain can be achieved by exploiting multi-user diversity in wireless multihop multicast networks. To address this problem, we extend the NUM framework and derive a new optimization problem including the benefits of multi-user diversity for multicasting scenarios in wireless multihop networks under a probabilistic media access control (MAC). In our problem, multi-user diversity is achieved via opportunistic scheduling. Then, we propose a distributed approximation algorithm for the problem. Our numerical results confirm that the benefit of multi-user diversity is prominent in a wireless multihop network with multicast flows.

In this paper, the outage performance of multiuser multiple-input single-output (MISO) systems exploiting joint spatial and multiuser diversities is investigated for Rayleigh fading channels with outdated feedback. First, we derive closed-form exact outage probabilities for the joint diversity schemes that combine user scheduling with different spatial diversity techniques, including: 1) transmit maximum-ratio combining (TMRC); 2) transmit antenna selection (TAS); and 3) orthogonal space-time block coding (OSTBC). Then the asymptotic outage probabilities are analyzed to gain more insights into the effect of feedback delay. It is observed that with outdated feedback, the asymptotic diversity order of the multiuser OSTBC (M-OSTBC) scheme is equal to the number of transmit antennas at the base station, while that of the multiuser TMRC (M-TMRC) and the multiuser TAS (M-TAS) schemes reduce to one. Further by comparing the asymptotic outage probabilities, it is found that the M-TMRC scheme outperforms the M-TAS scheme, and the M-OSTBC scheme can perform best in the outage regime of practical interest when the feedback delay is large. Theoretical analysis is verified by simulation results.

Bit error rate analysis of the scheduled TAS with MRC for CR-MIMO systems

Opportunistic medium access (i.e., multiuser diversity) and MIMO techniques (i.e., multiple-antenna techniques) are two effective ways to achieve a substantial throughput gain in a multiuser wireless system. In this paper, we propose a medium access control (MAC) protocol with opportunistic medium access and multiuser MIMO techniques (MAC-OMA/MM) in Multi-channel Multi-radioWireless Local Area Networks (WLANs) to explore the utility of the joint design of these two techniques for the challenging MAC design. Specially, in addition to utilizing multiple channels simultaneously and multiple radio transceivers dynamically, multiuser spatial multiplexing and multiuser diversity are employed in each frequency channel to improve system performance. The key ideas of MAC-OMA/MM can be summarized as follows. By utilizing ATIM (Ad-hoc Traffic Indication Message) windows as in IEEE 802.11 power saving mechanism (PSM) under the distributed coordinate function (DCF) mode, user selection and channel negotiation are conducted between the AP and users via ATIM messages on a common channel. Multiuser diversity are employed to opportunistically schedule among multiple candidate users to optimize data transmission. During data exchange, on each frequency channel, the AP can transmit data to two distinct users simultaneously in the downlink with the help of multiuser spatial multiplexing, and two users can concurrently send data to the AP by uplinkdownlink duality in the uplink, which creates an extra dimension in spatial domain to further leverage the effect of multiuser diversity and multi-channel gains. Another contribution of this paper is to provide an analytical model to characterize the impact of our protocol on the system throughput and energy efficiency performance. Extensive simulations have been conducted and the results demonstrate that our protocol outperforms existing multi-channel MAC protocols with only minimal additional overhead and minor enhancements to the IEEE 802.11 PSM.

The paper shows how both spatial and multi-user (MU) diversity can be jointly exploited in a cellular system. By combining space-time block-coding with transmit antenna selection (AS), system performance in terms of channel capacity and for a low to moderate number of users can be substantially improved with respect to the single-input single-output (SISO) case. However, a SISO configuration with AS outperforms both configurations for the whole range of active users. In terms of throughput, however, part or all of the advantage exhibited by the SISO and SISO-AS configurations is lost in favor of those featuring STBC schemes. Closed-form expressions are derived analytically for the SNR densities along with an approximate lower bound for system throughput.

MIMO can enhance the performance of wireless system greatly. However, the limited battery life and terminal size of the UE in a cellular system put a constraint on the performance enhancement from MIMO. Especially when the transmitter has more antenna than the receiver in downlink, the conventional MIMO schemes can only exploit partial spatial multiplexing gain and diversity gain. For different users' independent locations lead to independent spatial fading one another, adaptive spatial and temporal scheduling can be developed to exploit the spatial multiuser diversity and achieve the tradeoff between multiplexing gain and spatial diversity gain available. In this paper, the enhanced MIMO schemes with greedy multiuser diversity are compared when Node B has more antenna than UE, e.g. zero forcing beamforming (ZFB), dual space time block coding (DSTBC), singular value decomposition (SVD) and vertical Bell labs layered spatial time code plus antenna selection (VBLAST-AS). From the simulation results, the ZFB has achieved the best spectrum efficiency, and the gain exceeds VBLATS-AS and DSTBC more than 50%

In this paper, we investigate the performance of bit-interleaved coded multiple beamforming (BICMB). We provide interleaver design criteria such that BICMB achieves full spatial multiplexing of min( N, M) and full spatial diversity of NM with N transmit and M receive antennas over quasi-static Rayleigh flat fading channels. If the channel is frequency selective, then BICMB is combined with orthogonal frequency division multiplexing (OFDM) (BICMB-OFDM) in order to combat ISI caused by the frequency-selective channels. BICMB-OFDM achieves full spatial multiplexing of min(N, M), while maintaining full spatial and frequency diversity of NML for an NtimesM system over L-tap frequency-selective channels when an appropriate convolutional code is used. Both systems analyzed in this paper assume perfect channel state information both at the transmitter and the receiver. Simulation results show that, when the perfect channel state information assumption is satisfied, BICMB and BICMB-OFDM provide substantial performance or complexity gains when compared to other spatial multiplexing and diversity systems.

This paper presents a novel strategy for free space optical (FSO) communication system that combines hybrid spatial-time diversity and optical code division multiple access (OCDMA). The authors demonstrate a 10 Gb/s hybrid spatial-time diversity FSO-CDMA experimental system for the first time. They investigate the bit error rate (BER) and physical-layer security under weak and strong turbulence conditions. The experimental results demonstrate that the proposed scheme can improve the performance of the FSO-CDMA system. In the hybrid spatial-time diversity FSO-CDMA system, the BER of the eavesdropper (Eve) exceeds the hard-decision forward-error-correction threshold of 3.8E-3. It is also worth noting that Eve can intercept and recover data without a matched decoder in a spatial diversity FSO-CDMA system. As a result, the proposed scheme can simultaneously enhance reliability and physical-layer security compared to time or spatial diversity systems.

In this paper, we for the first time consider the space-time network coding (STNC) to propose a novel cooperative data dissemination strategy in the time-division multiple access (TDMA) manner for the vehicle network, to overcome the shortcomings of the traditional network coding (NC) techniques with carrier sense multiple access/collision avoidance (CSMA/CA) protocol, i.e, high signal detection complexity and channel collision problem. An optimal relay selection strategy in the procedure of cooperative data dissemination is proposed to maximize the average system capacity for the data dissemination. In a addition, a suboptimal relay selection strategy is put forward to reduce the computation complexity and thus makes the developed data dissemination strategy more practical. Compared with the non-cooperative data dissemination, the proposed cooperative data dissemination via STNC has better performance in terms of the average system capacity, the average user equipment (UE) capacity, and the average data dissemination delay.

In this paper, we derived closed-form expressions for the SINR distributions in OFDMA cellular system using fixed relays. First of all, the intercell interference at the BS and the RS are characterized and it is found that the intercell interference power is successfully modeled as the average of it. From the interference power modeling, the pdfs of SINRs for the four uplink cases are analyzed. The analytical results are verified from computer simulations. The analytical results can be applied to evaluate various aspect of OFDMA cellular systems using fixed relays such as the average system capacity, the average system capacity for a specific region, the outage probability for a given QOS and so on.

This chapter implements detection processes for spatial multiplexing MIMO systems. It compares the performances of MIMO system with various detection methods. The chapter investigates the performances of MIMO system with different system parameters. MIMO systems may be broadly classified into two categories: spatial diversity (SD) systems and spatial multiplexing (SM) systems. SM MIMO systems exploit the multiple transmit and receive antennas to transmit multiple data streams simultaneously in the same frequency band to increase the spectral efficiency. Some of the existing literature defines only SM MIMO systems as MIMO systems. The user-defined MATLAB function zf_bpsk( ) implements zero forcing (ZF) detection for BPSK MIMO systems. It takes the received signal vector r and channel matrix H as its inputs. The chapter is designed to help teach and understand communication systems using a classroom-tested, active learning approach.

We develop and analyze low-complexity cooperative diversity protocols that combat fading induced by multipath propagation in wireless networks. The underlying techniques exploit space diversity available through cooperating terminals' relaying signals for one another. We outline several strategies employed by the cooperating radios, including fixed relaying schemes such as amplify-and-forward and decode-and-forward, selection relaying schemes that adapt based upon channel measurements between the cooperating terminals, and incremental relaying schemes that adapt based upon limited feedback from the destination terminal. We develop performance characterizations in terms of outage events and associated outage probabilities, which measure robustness of the transmissions to fading, focusing on the high signal-to-noise ratio (SNR) regime. Except for fixed decode-and-forward, all of our cooperative diversity protocols are efficient in the sense that they achieve full diversity (i.e., second-order diversity in the case of two terminals), and, moreover, are close to optimum (within 1.5 dB) in certain regimes. Thus, using distributed antennas, we can provide the powerful benefits of space diversity without need for physical arrays, though at a loss of spectral efficiency due to half-duplex operation and possibly at the cost of additional receive hardware. Applicable to any wireless setting, including cellular or ad hoc networks-wherever space constraints preclude the use of physical arrays-the performance characterizations reveal that large power or energy savings result from the use of these protocols.

Antenna systems based on polarization diversity can be significantly smaller and easier to deploy than conventional vertically polarized horizontal space diversity systems. As such there is great interest in the substitution of polarization diversity for space diversity. This chapter compares and evaluates the efficacy of polarization diversity relative to the classic vertically polarized 20-wavelength, two-antenna space diversity configuration. It was observed that bottom line performance with a randomly oriented handheld unit was almost identical for polarization and space diversity systems. For a vertical mobile antenna the bottom line performance was approximately 3 dB worse for the polarization diversity system relative to the horizontal space diversity system with vertical polarization. Significant polarization discrimination, which is one slant favored over the other, was observed at close ranges (less than 1 km) when there is a nearly clear line of sight between mobile and base. Significant depolarization was observed at longer ranges and when the mobile is in the clutter.