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Training-Aided Broadband Wireless Networks

Final Activity Report Summary - TABWIN (Training-Aided Broadband Wireless Networks)

The currently conceptual communication systems beyond the 3G systems address the future needs of a universal high-speed wireless network. The major drives for the 4G systems are higher user mobility and data rates. In order to meet the needs of future high-performance applications like multi-media, full-motion video and wireless teleconferencing, we need a network technology that extends 3G capacity by an order of magnitude. 4G systems will accommodate the spectrally more efficient communication systems that cannot be retrofitted into 3G infrastructure.

The underlying challenges in mobile and wireless systems are the unavoidable interference in wireless environment, and the time variance of the system. The interference can be due to other users sharing the same communication medium, or it can be self-interference, caused by the multiple echoes of a transmitted signal that is received, each reflected and scattered by the objects in the medium. In general, high data rates give rise to frequency selective propagation whereas mobility induced Doppler shifts introduce time selectivity in wireless links. Mitigating these effects has been a major concern over the last decade. These fading channels are challenging to mitigate, but once acquired, they offer joint multipath and Doppler diversity gains. The determination of the channel distortion, or channel parameter estimation, is typically achieved based on a known pilot or training signal inserted in the transmitted signal. The use of training simplifies the challenging task of receiver design for unknown channels. We propose the study of trade-offs in such systems in order to achieve better bandwidth utilisation for future high speed mobile wireless systems.

An emerging trend in the duplex mode of operation of mobile services is time-division duplexing (TDD) as opposed to the more broadly used frequency-division duplexing (FDD). The TDD mode of code-division multiple access (CDMA) has already have been adopted in two 3G standards: time-division / code-division multiple access (TD-CDMA) as part of the 3GPP standards, and time-division / synchronous code-division multiple access (TD-SCDMA) in China. TDD-CDMA has also been considered as a path for 3.5G / 4G systems, and it is likely that TDD will be chosen as the main duplex mode of operation for 4G systems. One of the advantages of using TDD over FDD is its hardware simplicity. This is because only one set of electronics is required at both mobile and base stations for both forward and reverse link transmissions. This can be significant in the low-power, low-size end of the mobile communications market, simplifying hardware requirements for the handsets.

A second advantage is the better bandwidth utilisation in unpaired continuous bands, since using the FDD mode necessitates a large frequency guard-band, and therefore inefficient use of the spectrum. A third advantage is the flexibility of capacity allocation in the uplink and downlink. By selecting an appropriate ratio of time slots for uplink and downlink, an optimal allocation of resources can be made according to the type of traffic and customers. FDD-based systems cannot provide such flexibility. A fourth and important advantage of TDD over FDD is that since the same frequency band is used in both directions, the channel characteristics are reciprocal for uplink and downlink.

This fact can be used to implement important techniques in an open-loop fashion: transmit antenna diversity combining, multipath diversity combining, power control, and pre-filtering to reduce multi-access interference. These techniques result in enhanced system throughput and simplified receiver structures.