Multiplex communications – Fault recovery
Reexamination Certificate
1998-02-27
2001-03-27
Ngo, Ricky (Department: 2731)
Multiplex communications
Fault recovery
C375S326000, C375S341000, C375S348000
Reexamination Certificate
active
06208617
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to signal detection and, more particularly, to channel tracking techniques in mobile receivers adapted to receive phase-modulated signals.
One well-known technique for transmitting information to mobile receivers is to convert the signal to digital symbols, to map those symbols onto a two-dimensional space, to modulate a carrier with the mapped symbols, and to transmit the modulated carrier to the receiver. The modulation of a symbol mapped onto a two-dimensional space (having x and y coordinates) takes place by amplitude-modulating the x component of the symbol by a carrier signal, amplitude-modulating the y component of the symbol by the carrier signal shifted by 90 degrees, and adding the two modulation products. In some applications the mapping is restricted to a circle, and that, effectively, results in phase modulation of the carrier.
A mobile unit receives signals that are corrupted by inter-symbol interference (ISI) as well as by thermal noise, and the challenge is to detect the so-distorted symbols. The ISI is a non-stationary process when the mobile unit is moving. That is, the characteristics of the channel are based on the location of the mobile unit relative to the transmitter, and when that location changes, the channel characteristics change. Prior art systems allow for adapting a receiver's response to the channel characteristics, but this adapting requires processing, and the processing requires time. As long as the channel characteristics change slowly, there is no problem. When the channel characteristics change rapidly, such as when the mobile unit changes its location rapidly (e.g. the mobile unit is in a car, or a plane), the currently-used adapting processes are able to keep up with the changes under ideal conditions.
The challenge to track the changing channel characteristics is compounded by the fact that the mobile unit has no information about the precise time when symbols are applied to the transmitter's modulator, and therefore does not know precisely when to sample the received signal. Furthermore, although the receiver nominally knows what the transmitter's carrier frequency is, the actual carrier frequency may be off and, in any event, the receiver's local frequency may be off from its specified value because of normal manufacturing tolerance issues, temperature variations, etc.
When the receiver's local oscillator is not equal to the transmitter's oscillator, an offset in frequency is said to exist. When there is no offset in frequency, the received signal is sampled, converted to digital form, and applied to a detection algorithm. The detection algorithm must remove the ISI introduced by the channel and must also compensate for the changing characteristics of the channel due to the movements of the mobile unit (e.g., in a car moving at 60 miles per hour, the channel characteristics change fairly rapidly). One technique that accomplishes channel tracking is the Least-Mean-Squared (LMS) algorithm. The LMS algorithm, however, is not thought of as being able to handle changing channel characteristics when there is a significant frequency offset.
When the receiver's frequency does have a significant offset, conventional differential detectors can be used to estimate the frequency offset and to compensate therefor. Differential detectors are described, for example, by Proakis in “Digital Communication,” McGraw Hill, 1989, Chapter 4.2.6. However, differential detectors fail when the channel characteristics change rapidly.
To overcome the problem of both a frequency offset and a rapidly changing channel, practitioners have included a training word in the symbol sequence, and once the training word is detected and its position is ascertained, the frequency offset can be extracted. An algorithm for accomplishing this, which is quite complex, is presented, for example, by Bahai and Sarraf in “A Frequency Offset Estimation for Nonstationary Channels,”
Proc. of ICASSP
97, pp. 3897-3900, April, 1997.
A simpler solution would obviously be advantageous.
SUMMARY
A simpler solution is, indeed, available where, in accord with the principles disclosed herein, a given algorithm, such as the LMS algorithm, computes an estimate of the frequency offset, and that very same algorithm is also used to estimate the channel characteristics. When the LMS algorithm is used, a frequency offset estimate can be derived from signals derived in the course of executing algorithm. A frequency compensation factor is then developed and applied to the incoming signal to create a signal that does not have an appreciable frequency offset. That signal is then applied to a process that also employs the LMS algorithm, in combination with a detection algorithm, such as, for example, the Viterbi algorithm, to recover from the incoming signal the information signals that had been encoded into the incoming signals. In addition to carrying out the disclosed process during training intervals, the process can be carried out during normal transmission of data.
Thus, in at least one embodiment of this invention, a fairly simple algorithm is employed to estimate the frequency offset. Moreover the algorithm employed for estimating channel characteristics is the very same as the algorithm employed to compensate for frequency offsets. This simplifies the receiver's construction and reduces its cost.
REFERENCES:
patent: 5204878 (1993-04-01), Larsson
patent: 5303263 (1994-04-01), Shoji et al.
patent: 5602881 (1997-02-01), Shiino et al.
patent: 5684836 (1997-11-01), Nagasu et al.
patent: 5751776 (1998-05-01), Shiino et al.
A. Bahai et al, “A Frequency offset estimation technique for nonstationary channels”, Proc. of ICASSP97, pp. 3897-3900, Apr. 1997.
Brendzel Henry T.
Lucent Technologies - Inc.
Ngo Ricky
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