Ambiguity resolution system in direct sequence spread spectrum m

Pulse or digital communications – Repeaters – Testing

Patent

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Details

375208, 375261, 375279, 375298, 375325, 375329, 375343, H03B 1707

Patent

active

058090622

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates specifically to direct sequence spread spectrum modulation systems and more particularly to an ambiguity resolution system therefor.
The usual form of modulation which applies in such systems, is phase shift keying. This requires a phase reference for coherent demodulation. This can be derived from the data modulated signal in a decision directed fashion but always has an associated 180 degree phase uncertainty (ambiguity).
Conventional techniques for the resolution of phase ambiguity for phase shift keying (eg binary PSK - BPSK) require the additional transmission of known pilot information. In the context of direct sequence, several methods of providing the pilot information are known.
The transmission of a separate independent pilot code is one example. This is suitable for a base station transmitter in which different signals are being transmitted to many different mobile units and a single common pilot can be shared. In this case the proportion of the total transmit power applied to the pilot can be small while the signal to interference ratio of the pilot can be high. Moreover, since the transmitted signal constitutes the sum of many signals it is already a non-constant envelope, so that the addition of a further pilot signal does not affect this. However, for a mobile transmitter, the pilot energy applies only to one signal and subtracts directly from the energy available for transmission of data. Moreover, the addition of a separate pilot would add undesirable envelope fluctuations to the transmitted signal.
The transmission of short pilot bursts-lasting several-bits at regular intervals is another example. This technique is well established for coherent modulation but does not allow for efficient averaging of the pilot with time for the time varying channel. It is also inflexible in that, as the speed of the mobile unit changes, it is not straightforward to alter the proportion of time applied to the pilot.
The transmission of individual pilot symbols at regular (or pseudo random intervals) is a further example. This method is better than the transmission of short pilot bursts in allowing for averaging of the channel conditions and is appropriate for non spread spectrum systems. However it is still rather inflexible in terms of adaptive variation of the proportion of pilot signals. Moreover, in the existing approach, the pilot symbols are used to provide the phase reference (and amplitude reference for optimum weighting of components according to maximal ratio combining in a Rake receiver) directly. This is inefficient and leads to the need for relatively high pilot energy.


SUMMARY OF THE INVENTION

The disadvantages of the above techniques are overcome by the present invention by replacing part of the modulated spread spectrum sequence with an unmodulated spread spectrum sequence. Thus the pilot component may be as little as one chip per bit, or even one chip for every several bits. This allows the pilot level to be reduced to the absolute minimum while still providing regular updates.
The signal to noise/interference ratio of the individual pilot chips is, of course, too low to provide a phase/amplitude reference. An advantage of the present invention is that decision directed channel estimation is used to provide the basic carrier reference, and the pilot chips are used merely to provide phase ambiguity resolution. Indeed, the decision directed carrier reference extraction is used to combine the pilot chips across the multipath components in order to obtain the ambiguity resolution signal.
It will also be seen that this approach leads to low complexity and allows for simple implementation of adaptive pilot level variation.
According to the present invention a direct sequence spectrum modulation system is provided. The system has means for inserting a predetermined number of chips in a bit period to form a pilot signal to resolve phase ambiguity, and channel estimation is provided by means for removing the modulation.
Various embodiments of t

REFERENCES:
patent: 4435822 (1984-03-01), Spencer et al.
patent: 5218620 (1993-06-01), Mori et al.
patent: 5519730 (1996-05-01), Jasper et al.
C.D.'Amours et al., RAKE Receiver Structures for Differential and Pilot Symbol-Assisted Detection of DS-CDMA Signals in Frequency-Selective Rayleigh Fading Channels, IEEE Global Telecommunications Conference, vol. 3, Nov. 29, 1993, New York, pp. 1798-1802.

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