Networking together hundreds or thousands of cheap microsensor nodes allows users to accurately monitor a remote environment by intelligently combining the data from the individual nodes. These networks require robust wireless communication protocols that are energy efficient and provide low latency. We develop and analyze low-energy adaptive clustering hierarchy (LEACH), a protocol architecture for microsensor networks that combines the ideas of energy-efficient cluster-based routing and media access together with application-specific data aggregation to achieve good performance in terms of system lifetime, latency, and application-perceived quality. LEACH includes a new, distributed cluster formation technique that enables self-organization of large numbers of nodes, algorithms for adapting clusters and rotating cluster head positions to evenly distribute the energy load among all the nodes, and techniques to enable distributed signal processing to save communication resources. Our results show that LEACH can improve system lifetime by an order of magnitude compared with general-purpose multihop approaches.

/pdf/an-application-specific-protocol-architecture-for-wireless-1s7y9fiv40.pdf

An application-specific protocol architecture for wireless microsensor networks

Wireless Communications

We introduce space-time block coding, a new paradigm for communication over Rayleigh fading channels using multiple transmit antennas. Data is encoded using a space-time block code and the encoded data is split into n streams which are simultaneously transmitted using n transmit antennas. The received signal at each receive antenna is a linear superposition of the n transmitted signals perturbed by noise. Maximum-likelihood decoding is achieved in a simple way through decoupling of the signals transmitted from different antennas rather than joint detection. This uses the orthogonal structure of the space-time block code and gives a maximum-likelihood decoding algorithm which is based only on linear processing at the receiver. Space-time block codes are designed to achieve the maximum diversity order for a given number of transmit and receive antennas subject to the constraint of having a simple decoding algorithm. The classical mathematical framework of orthogonal designs is applied to construct space-time block codes. It is shown that space-time block codes constructed in this way only exist for few sporadic values of n. Subsequently, a generalization of orthogonal designs is shown to provide space-time block codes for both real and complex constellations for any number of transmit antennas. These codes achieve the maximum possible transmission rate for any number of transmit antennas using any arbitrary real constellation such as PAM. For an arbitrary complex constellation such as PSK and QAM, space-time block codes are designed that achieve 1/2 of the maximum possible transmission rate for any number of transmit antennas. For the specific cases of two, three, and four transmit antennas, space-time block codes are designed that achieve, respectively, all, 3/4, and 3/4 of maximum possible transmission rate using arbitrary complex constellations. The best tradeoff between the decoding delay and the number of transmit antennas is also computed and it is shown that many of the codes presented here are optimal in this sense as well.

/pdf/space-time-block-codes-from-orthogonal-designs-1oj68nrilr.pdf

Space-time block codes from orthogonal designs

A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval. Time-division duplex operation combined with reverse-link pilots enables the base station to estimate the reciprocal forward- and reverse-link channels. The conjugate-transpose of the channel estimates are used as a linear precoder and combiner respectively on the forward and reverse links. Propagation, unknown to both terminals and base station, comprises fast fading, log-normal shadow fading, and geometric attenuation. In the limit of an infinite number of antennas a complete multi-cellular analysis, which accounts for inter-cellular interference and the overhead and errors associated with channel-state information, yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve. In particular the effects of uncorrelated noise and fast fading vanish, throughput and the number of terminals are independent of the size of the cells, spectral efficiency is independent of bandwidth, and the required transmitted energy per bit vanishes. The only remaining impairment is inter-cellular interference caused by re-use of the pilot sequences in other cells (pilot contamination) which does not vanish with unlimited number of antennas.

Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas

Convergence of Probability Measures. By P. Billingsley. Chichester, Sussex, Wiley, 1968. xii, 253 p. 9 1/4“. 117s.

Convergence of Probability Measures

We propose a novel transmit diversity scheme for the downlink of communication systems that employ four transmit antennas. The scheme can be seen as a simplified transmission architecture admitting an entire family of space-time codes. It can be also seen as an extension of previously proposed techniques (such as those presented in Alamouti (1998), Tarokh et al. (1998), and Hochwald et al. (2001)) to the case of four transmit antennas with complex input data symbols. Our technique has a number of appealing features, namely, it enables a significant portion of the open-loop channel capacity, it requires simple receiver processing (involving typically 2/spl times/2 matrix operations in conjunction with single-user or 2-user decoding) and it admits single-user encoding in an overlay fashion.

A space-time coding approach for systems employing four transmit antennas

The Shannon limit governs the ultimate bit rate at which a digital wireless communications system may communicate data. A digital wireless receiver may be enhanced such that it approaches the Shannon limit by decomposing a n-dimensional system into n-one dimensional systems of equal capacity.

Wireless communications system having a layered space-time architecture employing multi-element antennas

In a MIMO system the signals transmitted from the various antennas are processed so as to improve the ability of the receiver to extract them from the received signal even in the face of some correlation. More specifically the number of bit streams that is transmitted simultaneously is adjusted, e.g., reduced, depending on the level of correlation, while multiple versions of each bit stream, variously weighted, are transmitted simultaneously. The variously weighted versions are combined to produced one combined weighted signal. The receiver processes the received signals in the same manner as it would have had all the signals reaching the receive antennas been uncorrelated. The weight vectors may be determined by the forward channel transmitter using the channel properties of the forward link which are made known to the transmitter of the forward link by being transmitted from the receiver of the forward link by the transmitter of the reverse link or the weight vectors may be determined by the forward channel transmitter using the channel properties of the forward link and the determined weight vectors are made known to the transmitter of the forward link by being transmitted from the receiver of the forward link by the transmitter of the reverse link. The channel properties used to determine the weight vectors may include the channel response from the transmitter to the receiver and the covariance matrix of noise and interference measured at the receiver.

Space-time processing for multiple-input, multiple-output, wireless systems

We describe experiments and simulation of second-order polarization mode dispersion (PMD) components in optical fibers with emphasis on polarization-dependent chromatic dispersion (PCD). Excellent agreement is found in comparisons of experimental, simulated, and theoretical probability densities. To our knowledge, these are the first such comparisons for the second-order PMD magnitude and the PCD.

Probability densities of second-order polarization mode dispersion including polarization dependent chromatic fiber dispersion

A local fiber-optic communications network capable of supporting tens of thousands of simultaneous users, each requiring on the order of 10-Mb/s continuous data rate, is proposed. The system uses coherent optical techniques to fully utilize the vast bandwidth offered by single-mode optical fibers (tens of thousands of GHz) a spread-spectrum technique is used to circumvent the problem caused by the instabilities of present-day semiconductor lasers. These include difficulty in reliably setting a laser's frequency with an accuracy better than several hundred GHz, and phase noise in the laser output, which would otherwise result in excessive amounts of interference among the various users. The method proposed is a variant of code-division multiple access (CDMA) that is called random-carrier (RC) CDMA, since the modulated carriers can be assumed to be completely randomly placed in the available optical band. >

Gerard J. Foschini

Papers

A space-time coding approach for systems employing four transmit antennas

Wireless communications system having a layered space-time architecture employing multi-element antennas

Space-time processing for multiple-input, multiple-output, wireless systems

Probability densities of second-order polarization mode dispersion including polarization dependent chromatic fiber dispersion

Using spread-spectrum in a high-capacity fiber-optic local network