RAKE Receiver

Abstract – The goal for the third generation of mobile communications system is to integrate a wide variety of communication services such as high speed data, video and multimedia traffic as well as voice signals. Under the Universal Mobile Telecommunication System (UMTS) environment the Third Generation (3G) has many advantages such as highly efficient spectrum utilisation and variable user data rates. In this paper, we present the bit error rate (BER) performance analysis of Rake Receiver under UMTS environment with BPSK modulation technique and the convolutional coding at the transmitter and viterbi decoding at the receiver side. The Standard Gaussian Approximation (SGA) is used to evaluate the performance of Rake Receiver over a frequency selective Rayleigh fading channel. The data is modulated, encoded, spread and transmitted through a frequency selective Rayleigh fading channel. The transmitted signal is corrupted by multiple access interference, and is further corrupted by AWGN at the receiver. In the receiver, dispreading, decoded and demodulated. Rake Receiver, directive antenna are employed to improve the system performance. We examined the BER performance of Rake Receiver with, varying the number of users, spreading factor, Rake fingers, Interfering Cells, and the value of directivity of antenna at base Station. From the results we have seen that the BER performance of Rake Receiver is affected by varying these parameters and gives useful results.

improvement in throughput and greater coverage for high data rate service on the other [4]. At the same time for maximal ratio combining (MRC) Rake receiver, the paths with highest signal-t o-noise ratio(SNR) are selected, which is an optimal scheme in the absence of interfering users and inter-symbol interference(ISI). For a minimum mean error (MMSE) Rake receiver, the conventional finger selection algorithm is to choose the paths with highest signal-to-interference-plus –noise ratios (SINR) [5]. Multipath fading in wireless communication is modelled by several distribution methods at the same time the gain of each channel is made proportional to signal level and inversely proportional to the noise level [6].

Compared to other approaches, CORDIC is a clear winner when a hardware multiplier is unavailable, e.g. in a microcontroller, or when you want to save the gates required to implement one, e.g. in an FPGA. On the other hand, when a hardware multiplier is available, e.g. in a DSP microprocessor, table-lookup methods and good old- fashioned power series are generally faster than CORDIC. The CORDIC algorithm is used to compute the sine and cosine values which are required to calculate the In-phase and Quadrature phase components of the received signals of the RAKE receiver [3] .

In wireless communication, fading is one of the prob- lems that cause the signal fluctuation so it may cause degradation of signal level at the receiver. CDMA as one of the wireless communication types also experiences fading. Although downlink channels in CDMA are tran- smitted with codes that are orthogonal to one another; that is, they are encoded for minimal mutual interference, multipath propagation causes the downlink signal to be “smeared” in time, destroying some of this orthogonality [1]. Another problem that CDMA suffers is signal inter- ference from other users because all users in CDMA system use the same frequency. One of the solutions to mitigate fading is diversity. In a fading environment, the principal means for a direct-sequence (DS-CDMA) sys- tem to obtain the benefits of diversity combining is by using rake receiver—by coherently combining resolvable fading. Diversity can improve the quality of the received signal at the receiver. To combat other users’ interfer- ence, power control is used to improve the CDMA sys- tem performance because power control can minimize the interference between users. Spread-spectrum signals inherently exhibit frequency diversity. Due to the in-

In this paper, we describe the design and implementation of WCDMA Rake receiver using VHDL. 3G generation mobile communication is advanced and emerging technology in the field of wireless communication. Third generation Communication has advanced facilities like multimedia accessing, internet service and higher capacity of data rates [1]. The Rake receiver involves descrambling, dispreading, channel estimation and fading cancellation [2]. The main principle behind Rake receiver is that they exploit multipath propagation by receiving the multipath components of the transmitted signal separately and combining their energies. The Rake receiver is used to tackle the problems of time dispersion (echoes) caused by multipath propagation in mobile communication [3]. RAKE receiver consists of four fingers and all the energies of four fingers are combined in a single block. Which is time delayed version of original signal and combining is done in such a way to improve signal to noise ratio and reduces the multipath delay [1]. The above paper solves the above mention problems we have proposed architecture of WCDMA Rake receiver which is implemented in VHDL. The synthesis done by Xillling and simulation is done by modelSim6.3f.

Another approach to mitigating the effects of fading is in- volving diversity techniques, such as space, frequency or time diversity [11, Ch 5]. Receiver antenna diversity using var- ious combining methods [11, Ch 5] and transmitter antenna diversity employing space-time codes [12], [13] belong to the family of space-diversity schemes. By contrast, Rake- receiver [14] based Direct-Sequence (DS) Code Division Multi- ple Access (CDMA) [15] and Multi Carrier CDMA [16], [17], [18] can be classified as frequency-diversity assisted techniques. Since the above two approaches are independent of each other, both Rake-receiver and antenna diversity aided schemes can be combined with AQAM. Hence, the aim of this contribu- tion is to analyse the performance of a combined frequency- and space-diversity assisted AQAM system. Here, we em- ployed constant-power AQAM, justified by the arguments of [4] regarding the transmit power requirements, since a variable-power scheme would result in increased co-channel interference. We note furthermore that the additional throughput gain due to applying a variable-power, rather than a constant-power scheme is small [10]. Based on the performance analysis of our combined AQAM system, we study the upper-bound performance of Rake receiver as- sisted AQAM employing antenna diversity.

Abstract - Currently, a global third generation cellular system based on CDMA is being developed with a wider bandwidth than existing second generation system. The RAKE Receiver instead of completely eliminating multipath uses techniques to effectively utilize multipath to produce the desired signal. This paper proposes interference mitigation by CDMA RAKE receiver with Walsh- Hadamard Sequence for DS- CDMA systems. In CDMA RAKE receiver multiple correlator are used to dispread the multipath signals and before making a bit decision alignment and combination of those signal are necessary. Proposing aim of the study are- 1)Design and Implementation of CDMA RAKE Receiver 2) Implementation of Walsh-Hadamard sequence 3) Receiver structure is that it requires multiple correlators and to achieve better Bit Error Ratio(BER).

Abstract – CDMA is interference limited system hence the capacity of the system is limited by interference as number of users’ increases. This problem occurs when many mobile users share the same channel. CDMA takes the advantage of multipath fading .This system is resistant to multipath fading & also use the delayed multipath components to improve the performance of the system. This can be done using RAKE receiver in which multipath propagation delays of the transmitted signals are considered and combines the information obtained from several resolvable multipath components to form a stronger version of the signal using Rake receiver.. To overcome the problem of fading in channel Rake receiver can be used against mismatch & modeling errors which enhance its interference cancellation capability. This receiver consists of three stages. In the first stage, with LMS algorithm, which reduces BER i.e. there is reduction in interference. The matched filter (MF) can use for more reduction of the IPI and MAI in each RAKE finger in the second stage. In the third stage, the output signals from the matched filters can be combined according to the conventional maximal ratio combining (MRC) principle and then are fed into the decision circuit of the desired user. By measuring BER we can conclude that using rake receiver &LMS algorithm BER of the system for AWGN & Rayleigh Fading channelcan beminimized at lower SNR.

. Such coordinated channels expand the Signal- to-Interference-in addition to Noise Ratio (SINR) at its yield if the impedance in addition to clamor is white. This might be roughly the case if client subordinate scrambling (an occasional spreading) is utilized[9]. This is one choice for the uplink in third era frameworks. Be that as it may, this isn't the situation in the synchronous downlink with cell- subordinate scrambling, orthogonal codes and straight beneficiaries for the downlink, displaying a restricted or no many-sided quality increment concerning the RAKE recipient. The direct beneficiaries in this class have an indistinguishable structure from a RAKE collector, however the channel coordinated channel gets supplanted by an equalizer channel that is intended to augment the SINR at the yield of the recipient. The multifaceted nature of the equalizer channel is variable and can be taken to be as low as in the RAKE beneficiary (same structure as the channel coordinated channel), while its adjustment ensures enhanced execution as for the RAKE recipient. A simple correlate receiver will get rid of all intra cell interference (and optimize output SNR in the presence of white noise). This is true, at least, in the absence of delay spread, due to multipath propagation. With multipath propagation, the contributions from the different paths can be combined in a maximum ratio combining fashion with a RAKE receiver, to maximize SNR. However, the combined operation of the multipath channel and the RAKE receiver destroys the orthogonality of the intra cell user codes, leading to intra cell interference at the RAKE output[10]. In we presented another receiver approach which is based on channel equalization. This approach was introduced independently.

One of the applications of the RAKE receiver is in wideband code division multiple access systems (WCDMA). In a WCDMA system all the users transmit in the same band concurrently and each transmitted bit is spread by the transmitter by orthogonal variable spreading factor (OVSF) and scrambling code. The length of the scrambling code is known as the spreading factor and larger spreading factors give a better resistance against interference. Furthermore, at the receiver, the RAKE receiver de-spreads the received multipath signal by multiplying it by the same spreading sequence. The code generator gives the spreading sequence which is employed by the RAKE receiver in de-spreading and correlation operations. The RAKE receiver has multiple fingers to correlate different delayed signals that are received from different paths and combines the results to produce one output signal [4].

This paper develops a performance of DS- CDMA wireless communication system over a multipath fading channel is examined. The analysis is related to DS-CDMA with Rake Receiver. BER is a strategic parameter that is used in evaluating systems that transfer digital data from one place to another. Rake receiver having many fingers also called sub-receiver. In which received signal get multiplied with time delayedforms of locally generated code sequence.To takings advantage of the SNR and minimize the BER the CDMA Rake receiver is used. Using rake receiver we have better signal than without rake receiver. It is used in CDMA as effectual multipath signal reception, where several receptors are able to reconstruct the same signal with different time codes, amplitude & phase. A Fading Channel is well-known as communications channel. In this paper different fading systems also briefly explained. BER simulation is done using MATLAB as software tool.

The basic idea of RAKE receiver was first given by price and green. In outdoor, the delay between multipath components is usually large and, if the chip rate is properly selected, the low autocorrelation property of a CDMA spreading sequence can assure that multipath components will appear nearly uncorrelated with each other. However the RAKE receiver in IS-95 CDMA has been found to perform poorly in indoor environments, which is to be expected since the multipath delay spread in indoor channel (=100ns) are much smaller than IS_95 chip duration (=800ns). In such cases, a RAKE will not work since multipath is unresolvable, and Rayleigh flat fading typically occurs within a single chip period.

Abstract— In this paper we have shown the performance analysis of DS-CDMA wireless communication system with and without Rake Receiver for multiple input using MATLAB simulator. It could satisfy the next generation requirement of multiple input environments. Here we use a simulator which simulates the multiple input waveform signals in the form of channels or fingers through wireless towards the Rake Receiver which will follow through the DS-CDMA technique due to its performance. As we know the CDMA is an excellent technique to analyze the cellular systems. The simulator will give the tremendous functional ideas about the different values of the design option. Here, the backbone of this system is a Matlab Simulator. As nowadays this becomes the iconic tradition to do the analysis. The effective method of system modeling is used to speed up the simulations. With the help of simulator we get the variations in the system parameters due to its different inputs. The transmitted data and the received data can be analyzed in the form of signal waveforms and channels. We can also analyze the input data with Rake and without Rake for different attenuation factor with Bit error rate.

in which the receive signal is multiplied by time-shifted versions of a locally generated code sequence. The intention is to separate signals such that each finger only sees signals coming in over a single (resolvable) path. The spreading code is chosen to have a very small autocorrelation value for any nonzero time offset. This avoids crosstalk between fingers. In practice, the situation is less ideal. It is not the full periodic autocorrelation that determines the crosstalk between signals in different fingers, but rather two partial correlations with contributions from two consecutive bits or symbols. It has been attempted to find sequences that have satisfactory partial correlation values, but the crosstalk due to partial (non-periodic) correlations remains substantially more difficult to reduce than the effects of periodic correlations the rake receiver is designed to optimally detect a DS-CDMA signal transmitted over a dispersive multipath channel. It is an extension of the concept of the matched filter [2, 9].

Abstract. Signals transmitted over a multi-path propagation channel exhibit Inter-Path Interference (IPI) and fading. The receiver has to employ measures to mitigate these effects or it will incur severe performance degradation. A classic ap- proach in Code Division Multiple Access (CDMA) commu- nications is the Rake receiver. Alternatively, a Linear Mini- mum Mean Square Error (LMMSE) equalizer can further im- prove performance. This paper compares performance and computational complexity of these two algorithms. A hy- brid solution will be introduced in order to decrease compu- tational complexity while retaining most of the interference suppression capabilities of the LMMSE equalizer.

UWB has increased over the past several years, devel- opers of UWB systems began pressuring the FCC to approve UWB for commercial use. In Ultra Wideband (UWB) multi-user environments, the existence of se- vere multi-user access interference (MAI) can cause significant performance degradations. In this paper, we propose an adaptive least bit error rate (LBER)- Rake receiver for synchronous CDMA-UWB systems. The combining weights of the Rake fingers are adapted based on the LBER criterion for MAI mitigation. I study the receiver performance by varying the number of fingers under different number of users. Performance analysis has been done by generating the conditional probability density function of the receiver output. Results show that the proposed receiver can suppress MAI more effectively and leads to an increase in the number of supported users.

In a mobile radio channel reflected waves arrive with small relative time delays, self interference occur. Direct Sequence (DS) Spread Spectrum is often claimed to have particular properties that makes it less vulnerable to multipath reception. In particular, the rake receiver architecture allows an optimal combining of energy received over paths with different [6]. It avoids wave cancellation (fades).If delayed paths arrive with phase differences and appropriately weighs signals coming in with different signal-to-noise ratios [2].

In Fig.6 the number of multipaths L =8; the number of Users K =3; Spreading Factor N c =32; Antenna Directivity D a = 5dB; Number of Interfering Cells M c =4; n umber of Rake Fingers M =3; Code Rate r =1/2; Constraint length C L =3. From this at BER value of 10 − 4 there is 8 dB improvements in BER Performance of communication system with using MIMO Rake Receiver in comparison with Generalized Rake Receiver with 3 taps (i.e. with 3 resolvable paths).

The goal for the next generation of mobile communications system is to seamlessly integrate a wide variety of communication services such as high speed data, video and multimedia traffic as well as voice signals. The technology needed to tackle the challenges to make these services available is popularly known as the Third Generation (3G) Cellular Systems. The received signal at the WCDMA Receiver Antenna subsystem is the sum of attenuated and delayed versions of the transmitted signals due to the so-called multi path propagation introduced by the channel. At the receiver side, a RAKE receiver is implemented to resolve and compensate for such effect. This project is mainly focus on the WCDMA concept and Rake receiver. In this project, bit error rate simulation was carried out to study different modulation techniques and the contribution of rake receiver in WCDMA system for the overall system performance. Next, evaluation on the bit error rate for rake receiver at different conditions such as varying number of rake fingers,

practice, when the transmitted signal passes through mo- bile radio channel, duplicates of the transmitted signal are generated by reflection, refraction, and diffraction, and the signal power is distributed in multipath. In CDMA system, the transmitted signal bandwidth is much larger than the coherent bandwidth of the channel, in which case the channel is frequency selective [4,8,9]. For the frequency-selective channels, the received signals are multiple copies of the transmitted signals with different channel delays and fading, combining the multipath components as multipath diversity. Thus, if one of the multipath components is attenuated by fading, some oth- ers may not be and the receiver could use unfaded com- ponents to make the decision. The idea behind the rake reception technique is that the signals propagating through different multipath are received in individual fingers of the rake receiver and the outputs from these fingers are then coherently combined to provide the input signal for the symbol decision. The received signal is chip matched and sampled at the chip rate. Figure 1, illustrates the structure of a typical rake receiver in which