Pulse or digital communications – Synchronizers
Reexamination Certificate
1998-06-08
2003-11-25
Pham, Chi (Department: 2631)
Pulse or digital communications
Synchronizers
Reexamination Certificate
active
06654432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to communication systems, and more particularly to an apparatus and method for achieving synchronization in digital receivers used in communication systems.
2. Related Art
In synchronous digital transmission, information is conveyed by uniformly spaced pulses and the function of any receiver is to isolate these pulses as accurately as possible. However, due to the noisy nature of the transmission channel, the received signal has undergone changes during transmission and an estimation of certain reference parameters is necessary prior to data detection. Estimation theory proposes various techniques for estimating these parameters depending on what is known of their characteristics. One such example technique is called maximum likelihood (ML). A maximum likelihood estimation assumes the parameters are deterministic or, at most, slowly varying over the time interval of interest. The term deterministic implies the parameters are unknown but of a constant value and are, therefore, not changing over time. These unknown parameters can cover factors such as the optimum sampling location, the start of a packet (or a frame marker for continuous data streams) or the phase offset introduced in the channel or induced by instabilities between the transmitter and receiver oscillators. It is widely recognized that maximum likelihood estimation techniques offer a systematic and conceptually simple guide to the solution of synchronization problems. Maximum likelihood offers two significant advantages: it leads to appropriate circuit configuration and provides near optimum or optimum performance depending on the known channel conditions (J. G. Proakis, “Digital Communications,” Third Edition, McGraw-Hill Publishers, pp. 333-336, 1995).
Generally, if the transmitter does not generate a pilot synchronization signal, the receiver must derive symbol timing from the received signal. The term symbol is used in this context to refer to transmitted signals that are phase modulated with discrete phase relationships, where each assigned phase relationship is a symbol that is subject to detection at the receiver. Both the transmitter and receiver employ separate clocks which drift relative to each other, and any symbol synchronization technique must be able to track such drift. Therefore choosing the proper sampling instants for reliable data detection is critical, and failure to sample at the correct instants leads to Inter-Symbol Interference (ISI), which can be especially severe in sharply bandlimited signals (M. H. Meyers and L. E. Franks, “Joint Carrier Phase and Symbol Timing Recovery for PAM Systems,” IEEE Transactions on Communications, COM-28(8):1121-1129, 1977). The term ISI refers to two or more symbols that are superimposed upon each other; phase detection of each symbol, thus, becomes extremely difficult. Incorrect sampling implies the receiver is inadvertently sampling the signal where the influence of the previous data symbol is still present (J. G. Proakis, “Digital Communications,” Third Edition, McGraw-Hill Publishers, pp. 536-537, 1995).
In a receiver, the signal following demodulation is first passed through a matched filter and sampled. The optimum sampling times correspond to the maximum eye opening and are located approximately at the peaks of the signal pulses. The term “eye opening” refers to the amplitude variations of the signal at the output of the pulse-shaping filter. An eye is formed by superimposing the output of the pulse shaping filter for each symbol upon the other until the central portion takes on the shape of an eye as illustrated in
FIGS. 10
a
and
10
b
. Note that at high signal to noise conditions, the “eye” is open, whereas at low signal to noise conditions, the “eye” is closed.
Among synchronization systems, a distinction is made between feedforward and feedback systems. A feedback system uses the signal available at the system output to update future parameter estimates. Feedforward systems process the received signal to generate the desired estimate without explicit use of the system output. A particular form of feedback system is data directed. Data directed techniques make explicit use of prior data decisions to estimate the current estimate of the unknown parameter, which is then used to update the data decisions (J. G. Proakis, “Digital Communications,” Third Edition, McGraw-Hill Publishers, pp. 333-336, 1995). Whether the design approach is feedforward or feedback, both techniques are related to the maximum likelihood parameter estimation. Both feedforward and feedback techniques are being used in the current technology. However, it should be noted that there are advantages and disadvantages associated with both approaches. The disadvantages of feedback techniques are well documented in the literature (U. Mengali and N. D'Andrea, “Synchronization Techniques for Digital Receivers,” Plenum Press Publishers, p. 398, 1998).
There are two alternatives to receiver design namely, coherent demodulation and non-coherent demodulation. Coherent demodulation is used when optimum error performance is essential. This implies that the baseband data signal is derived making use of a local reference with the same frequency and phase as the incoming carrier. This requires accurate frequency and phase measurements insofar as phase errors introduce crosstalk between the in-phase and quadrature channels of the receiver and degrade the detection process. The extraction of the phase occurs in a process termed phase estimation. Furthermore, frequency estimation is necessary when the local receiver oscillator and the received signal frequency differ in frequency and phase by a sufficient amount such that phase recovery is not sufficient to ensure reliable data detection. Phase recovery algorithms have a limited pre-tracking ability, and when the phase offset exceeds the tracking ability of the phase recovery circuitry, frequency estimation becomes necessary. Depending on the phase/frequency offset present, frequency estimation algorithm have a much wider tracking range than their phase tracking counterparts. In fact frequency estimation is generally done first and followed by phase estimation.
An alternative receiver design approach is to use non-coherent demodulation techniques, such as differential demodulation where the phase difference between one data symbol and the next is assumed constant. In applications where simplicity and robustness of implementation are more important than achieving optimum performance, differentially coherent and non-coherent demodulation are attractive alternatives to coherent demodulation.
The effect of a poorly designed carrier loop increases the dispersion of the received symbols about their nominal values, bringing the received points considerably closer to the decision boundaries and decreasing the error margin. Of course, large phase perturbations can cause errors without any noise. Similarly, timing phase errors will cause the receiver to sample away from the maximum eye opening and reduce the margin for error. In traditional analog implemented receivers, synchronization is typically performed using an error tracking synchronizer whereby a feedback loop constantly adjusts the phase of a local clock to minimize the error between the estimated and the optimum sampling instant. Flexibility in the design of the synchronization unit in a receiver has increased in recent times with the advent of increasingly powerful silicon chips. This has led to the adoption of open loop (otherwise known as feedforward) estimation techniques for synchronization purposes.
Digital synchronization methods recover timing, phase and frequency estimates by operating only on signal samples taken at a suitable rate.
FIGS. 11
a
and
11
b
illustrate the concept of sampling a signal.
FIG. 11
a
illustrates that when oversampling occurs at a rate of four samples per symbol, the information available with regard to the received signal is much greater than that in
FIG. 11
b
. This
Lakkis Ismail
O'Shea Deirdre
Safavi Saeid
Tayebi Masood K.
Burd Kevin M
Knobbe Martens Olson & Bear LLP
Pham Chi
Wireless Facilities, Inc.
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