The carrier modulation block will not be simulated in

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be used as a model for all different carrier frequencies, which is an advantage. The carrier modulation block will not be simulated in Matlab, but is essential in a real system and hence it is necessary to understand its operation. 4.2 Channel Several different channels are possible, the simplest one being the AWGN chan- nel in Figure 15. This channel simply adds white Gaussian noise to the signal. Another common channel model is a multipath channel, shown for the two-path case in Figure 16. This channel causes intersymbol interference in the received signal as well, which will deteriorate the performance. 15
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From transmitter To receiver White noise Figure 15: An AWGN channel. From transmitter To receiver White noise Delay h 0 h 1 Figure 16: A simple two-path channel. 4.3 Receiver The receiver is the most complex part in the system and is depicted in Fig- ure 17. It consists of two main parts, a continuous-time front end, usually implemented with analog components, and a discrete-time baseband processing part, commonly implemented in a digital signal processor and/or ASICs. The front end consists of bandpass filtering (BP), down-conversion and sampling to form a discrete-time signal. The baseband part consists of matched filtering (MF), phase estimation, synchronization, sampling, phase correction, and de- cision. Only the baseband part is usually simulated, although the front-end is crucial for the performance and a thorough understanding of its properties is required. 4.3.1 Bandpass Filtering The first block in the receiver is a bandpass filter with a center frequency equal to the carrier frequency f c and a bandwidth matching the bandwidth of the transmitted signal (as given by the pulse shape used). The purpose of the bandpass filter is to remove out-of-band noise. Choosing the bandwidth of the bandpass filter requires some care. If the bandwidth is chosen too large, more noise energy than necessary will pass on to the subsequent stages. If it is too narrow, the desired signal will be distorted. The bandwidth of the transmitted signal can be defined in several different ways, which must be taken into account when choosing the filter bandwidth in the receiver. Furthermore, the phase characteristic of the bandpass filter must be such that the phase is not severely distorted as it carries the desired information. The bandpass filter is part of the front end and will not be included in the simulation. 4.3.2 Down Conversion and Sampling The down conversion block simply down-converts the received signal to the baseband, resulting in a complex-valued baseband signal. Down-conversion is 16
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Front End Baseband Processing BP Down conv. Phase est. Phase est. MF Extract training Sync. Phase corr. Dec. Figure 17: A possible receiver for the QPSK system. a simple operation, just multiplying the input signal with the local oscillator signal, followed by low-pass filtering to remove terms of frequency 2 f c and fulfill the sampling theorem when sampling and converting into a discrete-time signal.
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  • Fall '15
  • prasad

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