Adaptive interference cancellation between basestations in a TDD based system

Abstract

ABSTRACT

There is a considerable and growing interest in broadband system for providing high capacity communications and broadcasting, through for example high speed Internet.

Because of the large bandwidth available in the radio spectrum above 20GHz, millimeter fixed radio systems are particularly suitable for providing high communication and broadcasting services with the same user premises equipment.

In this thesis, a millimeter fixed radio systems based on TDD duplexing scheme was discussed. The main motivation for using TDD is its ability to dynamically adjust to varying traffic patterns, which of course is impossible with FDD. Traffic simulations patterns have also shown that capacity gains can be achieved for busty traffic and for traffic where the up-to-downlink ratio cannot be predicted.

Although TDD thus gives the flexibility of dynamic capacity allocation making it more spectrum efficient than FDD, but since the same frequency is used for both the uplink and downlink, several new interference situation arises which are not present in an FDD system. Handling interference in TDD thus provides a daunting challenge to the system engineer.

Base-stations to base-station interference have a very high probability of occurring in a TDD system. This can either be within the same operator system or between different operator’s systems. However, the consequences can be devastating when this thus occurs.

It was proposed in this thesis that in other to deal the a situation where we have base-station to base-station interference, an extra antenna and some signal processing hard-ware could be employed.

The extra antenna is pointed at the main source of interference measuring this and sending the measurement through an adaptive filter for the purpose of interference cancellation. The adaptive algorithm employed in our proposed system setup is the Least Mean Square algorithm (LMS). The system works by instructing its entire user to stop transmitting for say 1% of the TDD frame length, at this interval, the system measures the channel, the co-efficient of the adaptive filter are locked and the system allows it user terminals to continue transmitting. This is repeated over each TDD frame. It was found that considerable interference suppression is achieved through this system setup. Up to 6.5dB interference suppression in some cases depending on the interference-to- noise ratio in both the main antenna path and the extra antenna path.

The thesis is divided into three parts. Part one deals with the general introduction to broadband fixed wireless access and the available broadband technologies. Part two deals with topics such as TDD, adaptive filters and principles of adaptive noise canceller. Part three deals with the simulations and results.

This thesis is written within the BROADWAN project (www.broadwan.org). This is a EU funded integrated project under the Information Society Technologies (IST) priority of the sixth framework program (FP6).