There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Wireless Communications

Transmit diversity generally requires more than one antenna at the transmitter. However, many wireless devices are limited by size or hardware complexity to one antenna. Recently, a new class of methods called cooperative communication has been proposed that enables single-antenna mobiles in a multi-user environment to share their antennas and generate a virtual multiple-antenna transmitter that allows them to achieve transmit diversity. This article presents an overview of the developments in this burgeoning field.

Cooperative communication in wireless networks

Coding strategies that exploit node cooperation are developed for relay networks. Two basic schemes are studied: the relays decode-and-forward the source message to the destination, or they compress-and-forward their channel outputs to the destination. The decode-and-forward scheme is a variant of multihopping, but in addition to having the relays successively decode the message, the transmitters cooperate and each receiver uses several or all of its past channel output blocks to decode. For the compress-and-forward scheme, the relays take advantage of the statistical dependence between their channel outputs and the destination's channel output. The strategies are applied to wireless channels, and it is shown that decode-and-forward achieves the ergodic capacity with phase fading if phase information is available only locally, and if the relays are near the source node. The ergodic capacity coincides with the rate of a distributed antenna array with full cooperation even though the transmitting antennas are not colocated. The capacity results generalize broadly, including to multiantenna transmission with Rayleigh fading, single-bounce fading, certain quasi-static fading problems, cases where partial channel knowledge is available at the transmitters, and cases where local user cooperation is permitted. The results further extend to multisource and multidestination networks such as multiaccess and broadcast relay channels.

Cooperative strategies and capacity theorems for relay networks

Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

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Survey on Free Space Optical Communication: A Communication Theory Perspective

When mobiles cannot support multiple antennas due to size or other constraints, conventional space-time coding cannot be used to provide uplink transmit diversity. To address this limitation, the concept of cooperation diversity has been introduced, where mobiles achieve uplink transmit diversity by relaying each other's messages. A particularly powerful variation of this principle is coded cooperation. Instead of a simple repetition relay, coded cooperation partitions the codewords of each mobile and transmits portions of each codeword through independent fading channels. This paper presents two extensions to the coded cooperation framework. First, we increase the diversity of coded cooperation in the fast-fading scenario via ideas borrowed from space-time codes. We calculate bounds for the bit- and block-error rates to demonstrate the resulting gains. Second, since cooperative coding contains two code components, it is natural to apply turbo codes to this framework. We investigate the application of turbo codes in coded cooperation and demonstrate the resulting gains via error bounds and simulations.

/pdf/coded-cooperation-in-wireless-communications-space-time-1s3hh8r840.pdf

Coded cooperation in wireless communications: space-time transmission and iterative decoding

This correspondence studies linear receivers for multiple-input-multiple-output (MIMO) channels under frequency-nonselective (flat) quasi-static Rayleigh fading. The outage probability and diversity gain of minimum mean square error (MMSE) and zero forcing (ZF) receivers are investigated. Assuming M. transmit and JV receive antennas, the ZF receiver always has diversity N - M + 1, unlike the MMSE receiver which may exhibit a rate-dependent behavior. Under separate spatial encoding, where the parallel data streams are not jointly encoded, MMSE is no better than ZF in terms of diversity. Under joint spatial encoding, the MMSE receiver achieves diversity MN at low spectral efficiencies but has diversity only M - N + 1 at high spectral efficiencies. These results are established via simulations and an outline for the corresponding analysis is presented.

/pdf/outage-and-diversity-of-linear-receivers-in-flat-fading-mimo-499dvc01ct.pdf

Outage and Diversity of Linear Receivers in Flat-Fading MIMO Channels

Antenna spacing and the properties of a scattering environment can create correlation between channel coefficients. Temporal correlation between fading coefficients may also be present, because one may be unable or unwilling to fully interleave the channel symbols. This paper presents a comprehensive analysis of multiple-input multiple-output systems under correlated fading. We calculate pairwise-error-probability (PEP) expressions under quasi-static fading, fast fading, block fading, as well as arbitrarily temporally correlated fading, under Rayleigh and Rician conditions. We use the PEP expressions to calculate union bounds on the performance of trellis space-time codes, super orthogonal space-time codes, linear-dispersion codes, and diagonal algebraic space-time codes.

/pdf/analysis-of-space-time-coding-in-correlated-fading-channels-26j9zbo311.pdf

Analysis of space-time coding in correlated fading channels

Techniques are provided for a multi-user reservation sequence used when transmitting a multi-user transmission in a wireless network. At a wireless access point device configured to operate in a wireless network, a multi-user request-to-send packet is transmitted as part of a multi-user reservation sequence prior to a multi-user transmission from the wireless access point device to a plurality of wireless client devices that are the intended recipients for the multi-user transmission. The multi-user request-to-send packet comprises duration information and address information configured to address at least some of the plurality of wireless client devices. Clear-to-send packets transmitted from two or more of the plurality of wireless client devices in response to the multi-user request-to-send packet are received at the wireless access point device. Each clear-to-send packet comprises identical content. In some scenarios, the multi-user request-to-send packet is preceded by a single-user request-to-send packet directed to a designated wireless client device.

Ahmadreza Hedayat

Papers

Cooperative communication in wireless networks

Coded cooperation in wireless communications: space-time transmission and iterative decoding

Outage and Diversity of Linear Receivers in Flat-Fading MIMO Channels

Analysis of space-time coding in correlated fading channels

Medium Reservation Techniques for Multi-User Transmissions