Non-data-aided feedforward timing synchronization method

Details Non-data-aided feedforward timing synchronization method Non-data-aided feedforward timing synchronization method

1998-11-05

2002-08-06

Chin, Stephen (Department: 2634)

Pulse or digital communications

Receivers

Angle modulation

C375S355000, C375S371000, C370S516000

Reexamination Certificate

active

06430235

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to communication systems, and more particularly to an apparatus for achieving synchronization in a receiver.
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 undergoes changes during transmission and therefore, a complete estimation of certain reference parameters is necessary prior to data detection. The unknown parameters may cover such factors as the optimum sampling or the phase offset introduced in the channel or induced by the instabilities of the transmitter and receiver oscillators.
Estimation theory proposes various techniques for the estimation of these parameters; one such technique is Maximum Likelihood. However, ad-hoc techniques which are either totally unrelated to the optimum estimation derived from Maximum Likelihood or are at most loosely related, can also give excellent performance.
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. Both the transmitter and the receiver employ separate clocks which drift relative to each other, and any symbol synchronization technique must be able to track such drift.
Choosing the proper sampling instants is critical for reliable data detection. Failure to sample at the correct instants leads to Inter-Symbol Interference (ISI), which can be especially severe in sharply bandlimited signals. The term ISI refers to two or more symbols that are superimposed upon each other which makes phase detection of each symbol extremely difficult. Incorrect sampling implies the receiver is inadvertently sampling 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 conventional digital receiver, the signal following demodulation is first passed through an anti-aliasing filter, which is used to limit the bandwidth of the received signal, and is subsequently sampled asynchronously. Asynchronous sampling implies there is no control over the instant at which the sampling of the continuous time signal occurs.
FIGS. 1-
a
and
1
-
b
illustrate the concept of oversampling a continuous signal at four and two samples per symbol, respectively. The optimum sampling instants correspond to the maximum eye opening and are located approximately at the peaks of the signal pulses.
FIG. 1-
a
shows that oversampling at four samples per symbol provides more information about the received signal than oversampling at two samples per symbol (
FIG. 1-
b
).
The term “eye opening” refers to the amplitude variations of the signal at the output of the pulse-shaping filter. An “eye” pattern 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”. This is illustrated in
FIGS. 2-
a
and
2
-
b
for a BPSK (Binary Phase Shift Keying) modulation scheme for high and low signal to noise conditions. 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 techniques, 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.
In an error tracking feedback loop, the timing estimator constantly adjusts the phase of a local clock oscillator to minimize the phase error between the estimated and the optimum.sampling instant as illustrated in FIG.
3
. In a feedforward-timing loop, as illustrated in
FIG. 4
, the incoming signal is sampled asynchronously and applied to a timing estimator. This timing estimate is subsequently fed to an interpolator to estimate the received signal at the instant of the estimated timing offset phase. Interpolation estimates the signal value at the optimum sampling instant using the timing phase from the timing phase estimation unit (H. Meyr, M. Moeneclaey and S. A. Fechtel, “
Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,
” John Wiley Publishers, Chapter 9, pp. 505-532, 1998). Redundant samples are removed using a decimator. Both feedforward and feedback techniques are currently in use; however, it should be noted that there are advantages and disadvantages associated with both approaches.
Problems with feedback techniques include the length of the acquisition time, the high probability of hangup and cycle slips associated with phase locked loop (PLL) based structures, especially in the presence of channel fading. Fading occurs when signal components arriving via different propagation paths add destructively. Hangup occurs when the initial phase error of the estimator is close to an unstable equilibrium point, which can result in an extremely long acquisition time (i.e., a long time for the loop to adjust to the correct phase); in fact, the loop may never recover. Hangup is very serious as it can even occur in perfect channel conditions (H. Meyr, M. Moeneclaey and S. A. Fechtel, “
Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,
” John Wiley Publishers, pp. 94-97, 1998). Cycle slips are very harmful to the reliability of the receiver's decisions, because a cycle slip corresponds to the repetition or omission of a channel symbol (H. Meyr, M. Moeneclaey and S. A. Fechtel, “
Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,
” John Wiley Publishers, pp. 385-399, 1998).
These problems can be circumvented through the use of feedforward estimation. The advantages of feedforward estimation are that acquisition time is solely dependent on loop bandwidth and is not influenced by channel conditions. In addition, hangup does not occur and implementation costs are lower, as feedforward designs are more suited to VLSI (Very Large Scale Integration) implementation. Flexibility in the design of the synchronization unit has increased with the advent of increasingly powerful silicon chips.
However, feedforward techniques in the literature generally require a higher oversampling ratio than is prevalent in sampled feedback estimators (F. M. Gardner, “
A BPSK/QPSK Timing Error Detector for Sampled Receivers,
” IEEE Transactions on Communications, COM-44, pp. 399-406, March 1996), which generally require an oversampling rate of one or two samples per symbol for reliable operation. In feedforward designs, the oversampling rate is generally four or more samples per symbol (H. Meyr, M. Moeneclaey and S. A. Fechtel, “
Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing,
” John Wiley Publishers, pp. 289-295, 1998). This in turn is in contrast to analog feedback methods, which require a continuous waveform (J. G. Proakis, “
Digital Communications,
” Third Edition, McGraw-Hill Publishers, pp. 358-365, 1995). Digital synchronization methods recover timing by operating only on samples taken at a suitable rate. Digital implementation of an estimator has enormous appeal in communications technology and influences the design of all modem receivers.
There are two distinct stages involved in timing estimation: first, the estimation of the timing phase offset and second, the use of this estimate in the interpolationldecimation process. The estimated sampling instant within a symbol is the timing phase. The configuration of the feedforward timing estimator loop is very different from that of a feedback loop.
FIG. 4
illustrates that, in a feedforward arrangement, sam

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