6.1 Rake Receiver Architecture used in WCDMA
6.1.1 Block Diagram of Conventional Rake Receiver
6.1.1 Conventional Rake Receiver used in WCDMA
6.1.2 Issues with Conventional Receiver Architecture
The main advantage of Rake Receiver is that it improves the SNR (or Eb / No). Naturally, this improvement is observed in larger environments with many multipaths than in environments without obstruction. In simplified form: we have a better signal than we would have without using Rake Receiver!
This specific architecture is used in WCDMA system to improve the SNR by reducing the Multipath Interference Effect. In WCDMA systems SNR improvement is the only major constraint.
In outdoor environment the delay between multipath components is usually large and, if chip rate is properly selected then low correlation properties of CDMA spreading sequence can assure that multipath components will appear nearly uncorrelated with each other. However the Rake Receiver in WCDMA has been found to perform poorly in indoor environments which are to be expected since multipath delay spreads in indoor channel (approximately 100 ns) are much smaller than chip duration 800 ns. In such cases, this architecture will not work since multipath components are irresolvable.
Figure 6.1.2 Multipath components with delay grater than 1.5 chips
Figure 6.1.2 Multipath components with delay less than 1.5 chips
6.1.3 Why this architecture can not be used in GNSS Receiver?
In GNSS Receiver Pseudo range error along with the SNR improvement are the major constraints. Conventional Rake Receiver can not resolve the issues related to multipath components that lie within 1.5 chip duration. In GNSS receiver one has to resolve each multipath component in order to reduce pseudo range error. Hence this architecture can not be used in GNSS receiver. For that we have used Modified Architecture in GNSS Receiver, which is explained in upcoming sections of this report.
6.2 Pseudo range Measurement
The definition of pseudo range to SVi, where i is the PRN number is as follows:
Where:
c = speed of light = 299,792,458 (m/s)
TR (n) = Time when signal is received at receiver (Seconds)
TTi(n) = Time when signal is transmitted from satellite (seconds)
Pseudo range error is originates due to relative motion between the satellite & GNSS ReceiverWhen the GPS receiver schedules a set of measurements, it doesthis based on its own internal clock, which contains a bias error with respect to trueGPS time. Eventually the navigation process learns this bias error as a byproduct ofthe GPS navigation solution. The SV transmit time also contains a bias error withrespect to true GPS time, although the control segment ensures that this is maintainedat less than 1 ms. This correction is transmitted to the receiver by SVi as clockcorrection parameters via the navigation message. However, neither of these correctionsis included in the pseudorange measurement of (5.27). These corrections andothers are determined and applied by the navigation process.
6.3 Modified Architecture used in GNSS Receiver
6.3.1 General concept of Modified Architecture
Figure 6.3.1 General concept of Modified Architecture
A serial multipath interference cancellation method and the corresponding RAKE receiver are proposed. Theoretical analysis and computer simulations are carried out, and comparison with conventional RAKE receiver is performed It is shown that RAKE receiver performance can be improved using this method with simple structure and easy implementation.
In a mobile communication system, the received signalconsists of many replicas of the transmitted signal. Thesereplicas arrive at the receiver at different time instants.This causes multipath fading in the received signal, andmultipath interference to the symbol detector. Turin proposed severalantimultipath techniques, such as post detectionintegrator (PDI), RAKE, and so on. As an efficientsolution, the RAKE receiver of a direct sequence codedivisionmultiple-access system resolvesseveral replicas from the received signal, and utilizes themto obtain diversity gain. In each finger demodulator of aconventional coherent RAKE receiver, multipathinterference was often treated as white noise and waspartially filtered out during channel estimation .In this paper, a serial multipath interferencecancellation scheme is proposed. The purpose ofusing pilot is to improve the accuracy of channelestimation.
6.3.2 Modified Architecture
Figure 6.3.2 Block Diagram of Modified Rake Architecture
Modification - 1
In the modified approach the signal received at the receiver is spreaded using the local code and from this the local code corresponding to the weakest multipath is subtracted and outcome of this subtractor is used as the input to the next correlator and in this way at the last finger we will have only the signal corresponding to the strongest multipath.
This approach will try to extract information from the signals which are at the output of the correlators and in this way we will be able to track only the strongest multipath.
This Rake receiver structure is similar to the structure which is implemented in mobile phones & enablesa coherent superposition of the incoming signals. The incoming signals are multiplied by the conjugate version of their channel co-efficient. After the first path is evaluated by the multiplication with complex conjugate of channel co-efficient & by dispreading. At this point we completely know the portion of the first path which can be subtracted from the incoming signal.This procedure continues until no path with significant power is remained. The despreaded signals are now summed up & used for further procedure. The described procedure results in both the smaller mean pseudo range error & reduction in variance of this error.
6.3.3 Simulation results for different multipath profiles for Modified Architecture
Signal= (α *LOS + β * Multipath 1 + γ * Multipath 2)LOS=Line of Sight Signal, MRC= Maximal Ratio Combined Signal
WEIGHTAGE / PERFORMANCE / REMARKS
α =1, β =0, γ =0 / SNR -33 dB / No multipath, only LOS
α =0.5, β =0.3, γ =0.2 / SNR -31 dB (LOS)
SNR -32 dB (MRC) / LOS performance degradation due to multipath
α =0.35, β =0.41, γ =0.24 / SNR -27dB (LOS)
SNR -30dB (MRC) / MRC has better performance than LOS when LOS is degraded due to obstructions & multipath signals are stronger than LOS
α =0.21, β =0.5, γ =0.29 / SNR -21dB (LOS)
SNR -28dB (MRC)
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