Radio Receiver Design .page404

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                        Spread Spectrum Receivers                                                         389


                        bandpass characteristics (bandwidth, rolloff, passband ripple, phase shift, etc.)
                        of the first IF and the bandpass filter in the local reference are well matched, which
                        helps to reduce losses in the synchronization process in the receiver.
                               The synchronization process in the receiver depends on a high degree of
                        correlation between the first IF output signal and the local reference. The output
                        signal from the second mixer is the recovered, information-modulated, despread
                        signal. That is, it is no longer a spread spectrum signal, and if the correlation
                        between the IF signal and the local reference has been properly carried out, it
                        possesses latent process gain that can be used to advantage by the receiver.
                               3. The second mixer’s output signal bandwidth, when its input signals are
                        synchronized, has a bandwidth that is a function of the information bandwidth,
                        and not the code rate, because the code is no longer present. Therefore, the second
                        IF filters its input signal to the two-sided, information-modulated signal band-
                        width. It is here that the process gain is realized, due to the trading of code-
                        modulated RF bandwidth and information-modulated second IF bandwidth, and
                        the process gain that is available is

                                      RF bandwidth
                              10log                    (in dB)
                                        data rate

                        which, in receivers such as those used in the GPS system (see chapter 14) can be as
                        high as 50 dB.
                              4. Demodulation in a direct sequence receiver is usually performed by
                        some form of double-sideband, suppressed carrier demodulator, such as a Costas
                        loop or squaring loop. It is unusual for a direct sequence receiver’s demodulator to
                        encounter more than biphase or quadriphase modulated signals.
                              5 . AGC in a direct sequence receiver is common but is often very difficult
                        to implement, because such receivers must operate under widely varying inter-
                        ference conditions. This usually means that the AGC loop must have independent
                        controls when desired signals dominate or when interference dominates the input.
                        An offset is included in the AGC loop control to the preamplifier to accomplish
                        “delayed” AGC and avoid desensitizing the receiver at small signal levels.
                              Direct sequence receivers that are intended to work in interference-laden
                        environments must avoid limiting, on both desired signals and interference.
                        Otherwise, any interference that is present can cause the signal to be suppressed,
                        further enhancing the interference. Therefore, a direct sequence receiver has no
                        choice but to either possess the ability to handle extremely large interfering or
                        signal-plus-interfering levels linearly or provide automatic gain control that pre-
                        vents the receiver from limiting.
                              6. Synchronization in a direct sequence receiver involves synchronizing
                        the receiver’s code generator with the incoming code and then tracking the
                        incoming code rate as it varies due to frequency inaccuracy or Doppler shifts. This




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