Study Of An Equalization Algorithm In UWB Systems

IntroductionWith the development of modern communication technology and the demand of its applications, the requirement for ability of short range high speed data transmission is higher and higher. Because of its numerous advantages, Ultra-wideband (UWB) technology becomes a kind of solution of indoor high speed data transmission, and it is the research focus of current wireless communications.Because the data rate in UWB communication systems is at least 100Mbps, which presents that symbol duration is less than 10ns. Even in the line-of-sight (LOS) of the channel, in order to achieve such a high transmission rate, the maximum multi-path delay expansion is larger than symbol duration, and is bound to cause inter-symbol interference (ISI). In addition, UWB signal transmission is affected by large-scale path loss, shadow effect, small-scale multi-path fading and other factor. As a result, the signal received in receiver is seriously distorted. Usually, we consider that the impact of UWB pulse fine time resolution can greatly improve the multi-path fading phenomenon, and its extreme wide bandwidth of the access road expands the capacity, however, it is not so ideal. In the case of low speed, the low duty circle of pulse signal makes the impulse radio avoid the multi-path interference to a certain extent. But with the increase of data speed, the duty circle of pulse increases. Then in the dense multi-path channel, the probability of pulse overlapping increases sharply and causes serious interference.As everyone knows, the channel equalization techniques are very effective method to resolve multi-path interference, and Rake receiver also reduces multi-path interference to a certain extent. The focus of this paper is the study of the performance of a joint Rake and channel equalization receiver. In the paper, by a brief introduction of the Rake receiver, followed by a brief introduction of the performance analysis of a joint Rake with MMSE equalizer receiver, an algorithm of a joint Rake and MBER equalizer receiver is proposed and the performance analysis for the algorithm is given.1. Rake ReceiverRake receiver for DS-CDMA usually provides system diversity gain, captures energy, eliminates ISI, and improves the receiver performance. Because ultra-wideband communication systems use very short transmission pulse sequence, the signal received contains a large number of multi-path signal components. In order to make good use of these multi-path components, improve performance of the receiver, the majority of ultra-wideband communication system uses Rake receiver.However, one channel has N multi-path components requires N-tap Rake receiver to get all energy. In an UWB environment with dense multi-path components, the multi-path components increase with the increasing of bandwidth. Even in the sparse environment, such as the standard channel model of IEEE 802.15.3a, which need 80 taps to collect 80 percent of energy, while typical high-speed UWB indoor channel contains 60-200 multi-path components. In order to collect the energy of such a dense multi-path components, Rake fingers increase linearly, and the receiver becomes much more complex in structure, and brings to UWB system a lot of overhead.2. Rake-MMSEAt high SNR, it is mainly the ISI that affects the system performance, whereas at low SNR, the system noise is a major contribution in system degradation. Simulation results show that at low to medium SNR, the receiver with more rake fingers outperforms the one that has more equalizer taps but fewer rake fingers. With the increase of SNR, the performance of perfect channel estimation and non-perfect channel estimation becomes more and more similar. This is that the channel estimation error variance in proportion with the noise variance. Increasing fingers can improve channel estimation error caused by attenuation.3. Rake-MBERBecause MMSE equalizer complexity is low, it is used for reducing ISI. However, for the digital communication systems, BER is the true performance indicator, so MMSE equalizer is not optimal. According to MBER algorithm and the clue of UWB channel equalization algorithm, we proposed a joint Rake and MBER equalizer receiver. The simulation results show that in the bit rate at 200Mbps to 2Gbps range, Rake-MBER algorithm outperforms Rake-MMSE algorithm. With the rate increasing, the performance of both algorithms is becoming lower and lower. In addition, from the simulation results, we can also find that the DS-UWB systems only used Rake receiver with 5 fingers is difficult to meet the data communications requirements of the bit error rate, however, together with the equalization this requirement can be met.