Designing the next generation of WLAN terminals
The major challenge facing the future wireless communication systems is to provide high data rate access to multiple mobile users, while ensuring end-to-end quality of service (QoS). Multiple-Input and Multiple-Output (MIMO) wireless technology seems to provide for the increased spectral efficiency required through spatial multiplexing as well as improved link reliability due to antenna diversity. The principle requirement for the practical implementation of MIMO systems is to be able to transmit data from a set of physically separated antennas and receive signals in a like manner. In particular, the individual identity of signals transmitted and received at each antenna must be preserved, as well as possible, from baseband through to the antenna and vice versa. The European project FLOWS focused on the radio-frequency (RF) components of MIMO systems that translate the received messages from the multiple antenna array to baseband. Subject to several effects, the RF system may be the source of possible degradation in the performance of the overall MIMO system. More specifically, the effect of the RF system's impairments consisting of non-linearities and imbalances present in both the transmitter and receiver was evaluated for different architectures. Building on existing approaches for multiplexing together signals from several antennas in time or frequency, researchers at the Phillips Research Laboratories in Surrey designed a new architecture for MIMO receivers. The received signal at each antenna was given a unique identity by the application of an orthogonal code and these were then combined into one, prior to passing them through a single radio receiver. Moreover, whether a reduction in the replication of RF elements is possible and what impact this would have on performance was investigated and confirmed by means of extensive modelling work. The code-multiplexed receiver was proven to offer a promising solution for reducing parallelism and functional replication in the MIMO RF system, while ensuring high BER (bit error rate) performance. This method of combining several signals into a single carrier by means of Walsh functions has already been used to differentiate users and services in Code Division Multiple Access (CDMA) mobile telephony.
