Performance of RAKE receiver over different UWB channel

In this paper, simplified RAKE receiver structures is analysed and its performance in UWB channels is studied. This paper also contains simulation and comparison of different types of RAKE receivers – ARAKE receiver, SRAKE receiver, and PRAKE receiver. The improvement in the bitrate for the specific distance between transmitter and receiver is shown here. The finger selection is one of the important tasks for the RAKE receiver performance. The selection of the number of fingers is shown with simulation results. I. RAKE RECEIVER BLOCK DIAGRAM A RAKE receiver is used to collect multipath components at receiver. The concept of ‘garden rake’ which is used for collecting leaves inspired the RAKE receiver structure. These multipath components are generated due to reflection, scattering and diffraction from various obstacles in the transmission path [1]. When signals arrive at the receiver they are affected with different time delay. The RAKE receiver possesses the characteristic of capturing the delayed signal at different correlators. Output of these correlators is combined to achieve the signal with strong SNR [4]. If the output of RAKE receiver is based on only one correlator, it may give inferior result in terms of BER, as the multipath components possessed by that correlator could have been affected by fading. So a RAKE receiver has multiple correlators. Usually all the correlators will not receive the faded signal at the same time. If the output from one correlator is corrupted by fading, the others may not be corrupted, and through 1098 Twinkle V. Doshi weighing process corrupted signal can be avoided. Figure 1 Block diagram of RAKE receiver [6] RAKE receiver usually consists of correlators, code generator, channel estimator, phase rotator, delay equalizer and combiner. Matched filters, shown in the figure 1, are useful to measure the impulse response of multipath channel. This response generates the channel peaks and gives timing which is useful for successful despreading. Depending on the speed of mobile station and the transmission these peaks are measured and monitored. The number of available RAKE fingers depends on the channel profile and the chip rate. If the chip rate is higher than one, receiver can receive more resolvable paths. But higher chip rate require wider bandwidth. To catch all the energy from the channel more RAKE fingers are needed. Losses and practical implementation problems also increase with higher number of fingers. II. TEMPORAL DIVERSITY AND THE RAKE RECEIVER Transmitted signals are always affected with the reflection, refraction and diffraction from the several obstacles in the propagation path. The net combining effect could be constructive or destructive. The destructive effect reduces the signal power. This phenomenon is known as fading. Temporal diversity is used to improve the signal quality in fading channels. The multipath affected received signal r(t) consists of the superimposition of several attenuated, delayed, and eventually distorted replicas of a transmitted waveform sm(t). When propagation fluctuations within an observation time T>>Tb and path dependent distortion can be neglected, r(t) can be expressed as follows [2]: ) ( ) ( ) ( t n t s a t r j m j j     where n(t) is the AWGN at the receiver input. Performance of RAKE receiver over different UWB channel 1099 Equation can be rewritten for IR transmissions on the basis of the statistical channel model [2].