Doppler learning phase lock loop for burst demodulator

Details Doppler learning phase lock loop for burst demodulator Doppler learning phase lock loop for burst demodulator

2001-07-20

2003-08-05

Lee, Benny (Department: 2817)

Demodulators

Phase shift keying or quadrature amplitude demodulator

C375S340000, C375S327000

Reexamination Certificate

active

06603349

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to demodulators which demodulate burst communications and more specifically, to phase lock loops (PLL) which track the phase signal which is being demodulated.
2. Description of the Prior Art
In communication systems, particularly digital communication systems comprising a transmitter for digital data transmissions and a receiver for reception thereof, it is customary to modulate a carrier with any one of many different modulation techniques, including phase shift keying (PSK). Specific examples include binary phase shift keying (BPSK) modulation or quaternary phase shift keying (QPSK) modulation. When information is modulated onto a carrier, the receiver generally differs in timing from the transmitter due to a frequency difference between the local oscillators at the transmitter and receiver and the effect of varying delays and frequency shifts in the propagation path therebetween.
To track and coherently demodulate PSK modulated signals received from a transmitter, it is necessary for the receiver to form an estimate of the transmitter's phase so that the tumbling received signals may be transformed back into the fixed phase space of the transmitter. This process is known as “phase tracking.” Conventionally, there are a number of phase tracking loops employing phase locking principles, such as squaring loops, Costas tracking loops, and digital decision-directed feedback loops for performing phase tracking of either a BPSK or QPSK modulated signal. A commonly used method for performing this type of phase tracking is the digital decision directed phase locked loop (DD-PLL). The basic principle of DD-PLLs is well known as described in the classic “
Telecommunication Systems Engineering
” text by William C. Lindsey and Marvin K. Simon, originally published by Prentice-Hall in 1973, and the “
Digital Communications
” text by Kamilo Fehere, originally published by Prentice-Hall in 1983 and republished by Noble Publishing Corp. in 1997. The input to a DD-PLL is typically a sequence of complex data sample pairs obtained by down converting the incoming BPSK or QPSK modulated signal to a baseband quadrature (orthogonal) pair in a form of an I,Q digit combination which is passed through matched filters with sampling of the results occurring at the symbol rate. This sampled pair is considered a complex variable in rectangular form. The complex variable is converted to polar form to produce the equivalent polar variable pair. The apparent incoming phase is referenced to the currently estimated phase (i.e. the tracked phase) to form a phase difference. The phase difference between the incoming phase and the estimated phase is influenced by the true difference between the phase systems of the transmitter and the receiver, by phase and thermal noise present at the receiver, and by a symbol's data content which changes in angle by a multiple of &pgr;/2 for QPSK or of &pgr; for BPSK. The polar form is then transformed back into the rectangular form, for subsequent processing, including soft decision decoding when error control is being utilized.
In contemporary phase tracking circuits, the effect of the data content on the phase difference between the incoming phase and the estimated phase is compensated by making a “hard” decision on the data content of each individual BPSK or QPSK symbol on the rectangular coordinates. A standard phase detector generates phase error measurements for each BPSK or QPSK symbol, based on the hard decision of each symbol. In the absence of noise in the baseband quadrature pair, the estimated phase decision, which is based on each individual BPSK or QPSK symbol, is usually correct so that the resultant phase error measurement equals the true difference between the phase systems of the transmitter and the receiver. The value of the resultant phase error measurement is then filtered to yield an updated estimate for use at the next symbol epoch in a classical servo loop.
In practice, noise is always present so that the resultant phase error measurement may be grossly distorted, especially when an incorrect decision is made in converting the phase difference between the incoming phase and the estimated phase to the resultant phase error measurement. As long as the error rate is small, symbol-by-symbol DD-PLLs perform well. However, at low signal-to-noise ratios, the Bit Error Rate (BER) can be relatively high which means that the phase detector can also be unreliable. The effect of incorrect decisions, together with the large amount of noise entering the loop, causes the tracking loop performance to degrade. The deviation of the tracked phase variable increases faster than the signal to noise ratio degrades.
A common architectural structure in a data communication systems to which the present invention is applicable is for a central point to communicate with many individual user terminals (UTs). Examples include:
1. Current VSAT (very small aperture terminals) networks where the hub (i.e., the single point) communicates with many individual terminals, via a conventional (i.e., bent pipe) satellite.
2. Digital cellular radio systems where a base station (the single point) communicates with many users—who may be mobile. GSM and IS-54 are specific cases in point.
3. Processing satellite (PS) systems which are similar in principle to VSATs but where the central point is located in space, typically in a geosynchronous satellite system.
As the list above reveals, there are many types of prior art systems to which the subject invention has application. For specificity, the case of a processing satellite is hereinafter used, but it should be understood that the description of the prior art in the context of a satellite is not to be construed as a limitation of the invention to satellite applications. In a satellite application, the forward and reverse links are called the downlink and uplink, respectively, and these terms may be used interchangeably.
Systems with this basic architecture frequently use a Time Division Multiplex: Frequency Division Multiple Access/Time Division Multiple Access (TDM: FDMA/TDMA) architecture. That is, the downlink (PS to UT) uses a high speed single carrier with time division multiplexing to share its downlink capacity among the many user terminals: the uplinks (user UT to PS) typically include many frequency divided low/medium speed carriers (“FDMA”) which in turn are further shared in time using well known TDMA techniques. The TDM downlink is a continuous flow: the uplinks are bursty.
In systems with the foregoing structure, it is common practice for the UTs to reference the frequency they use in the uplink to that observed in the downlink, typically by extracting the down link symbol rate for use as a local frequency standard. By so doing, the frequencies received by the PS in the uplinks are derived from the frequency transmitted by the PS on the downlink. Among other advantages, this technique may reduce the cost of the UTs by eliminating the need for each UT to have a stable, autonomous, frequency source.
As is well known in the field of digital communications, recovery of TDMA bursts is easier and more reliable when the frequency of the arriving burst is close to that of the demodulator. Thus, the technique of locking the reverse frequency to the forward frequency has these performance advantages as well as the UT cost benefit previously mentioned.
When there is relative motion present between the PS and the UT, frequencies seen by the UT are shifted by the well-known Doppler effect. That is, if the satellite uses frequency f
PS,d
in its downlink, then the UT sees the frequency f
UT,d
=f
PS,d
*(1+v/c) where “v” is the relative (radial component) velocity and is positive when the satellite is moving away from the terminal, and “c” is the speed of light. For typical modern practice in a PS system, the relative motion is quite small, being of the order of one meter per second, so that the apparent frequency shift is of the order of a

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