Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1998-02-26
2002-03-26
Pham, Chi (Department: 2634)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C370S442000
Reexamination Certificate
active
06363130
ABSTRACT:
This invention relates to the detection of access bursts in TDMA (Time Division Multiple Access) cellular radio or wireless communications systems.
BACKGROUND OF THE INVENTION
In a cellular mobile radio communications system using TDMA, it is known to adjust the timing of time slot bursts transmitted from each mobile station to a base station so that bursts received by the base station from different mobile stations are aligned in time in their respective time slots. This time alignment typically comprises a fixed or standard offset time between reverse and forward time division multiplex frame timing (forward refers to the downlink transmission direction from the base station to the mobile station, and reverse refers to the opposite or uplink direction of transmission), and a time alignment advance (or retard) which varies according to the signal propagation time (i.e. the distance) between the mobile station and the base station.
In order to determine an initial time alignment for a mobile station newly communicating with a base station, the base station provides to the mobile station an initial traffic channel designation and instructs the mobile station to transmit access bursts, also referred to as shortened bursts, on this channel using the standard offset. The base station then attempts to monitor the access bursts and to determine from their timing an appropriate time alignment advance for the mobile station. This process is complicated by the fact that at this time the carrier offset, sample timing offset, and channel impairments are unknown at the base station receiver. The base station then communicates the time alignment advance to the mobile station as an initial time alignment for the mobile station to use for subsequent transmission of normal bursts to be received at the base station with the correct time relationship.
By way of example, in a TDMA cellular radio system that operates compatibly with EIA/TIA document IS-54-B: Cellular System Dual-Mode Mobile Station—Base Station Compatibility Standard (Rev. B), referred to herein simply as an IS-54 system, the access (shortened) burst has a predetermined form, described further below, comprising guard and ramp times, a sync word S, a CDVCC (coded digital verification colour code) word D, and fields V, W, X, and Y that comprise different numbers of zero bits. The sync word S is one of six defined sequences of 14 symbols identifying respective time slots and also used for synchronization and equalizer training. The CDVCC word D comprises 6 symbols (12 bits) having any of 255 distinct values used to identify a cell. Consequently, the entire content of the access burst, which comprises repeated occurrences of the words S and D, is known to the base station. The access burst has different numbers of symbols between any two occurrences of the sync word, so that the initial time alignment for the mobile station can be determined by the base station after detection of any two or more sync words of the access burst.
Having instructed a newly communicating mobile station to transmit access bursts with a particular sync word, the base station expects to detect this sync word in such an access burst. If, during the period that the base station is monitoring received signals to detect the expected sync word, this same sync word is produced as part of a burst from a different mobile station in a neighbouring cell of the system, then the base station may falsely lock onto this. In consequence, it may determine an incorrect initial time alignment for the newly communicating mobile station and transmit this with a message to the newly communicating mobile station to instruct it to transmit normal bursts, resulting in undesired interference and the risk of disrupted communications. This problem is exacerbated with increasing numbers of mobile stations (for example at busy times of the cellular communications system) and with reducing cell sizes.
This problem can be avoided or eliminated by having the base station check more thoroughly the form of the burst which it receives and interprets as the access burst, for example by checking for more occurrences of the sync word and for the correct CDVCC word D. However, this requires more computation in the base station, and this presents a particular problem at the system's most busy times when the computing resources of the base station may already be fully utilized.
A further problem can occur in that the accuracy with which the timing of the access burst, and hence the initial time alignment of the mobile station, is determined is adversely affected by multipath fading of the signal transmitted by the mobile station.
Accordingly, an improved method is desired for detecting access bursts, which avoids or eliminates the problems discussed above and does not increase, and advantageously decreases, the computing resources required of the base station.
An object of this invention is therefore to provide an improved method of and apparatus for detecting access bursts in a TDMA communications system.
SUMMARY OF THE INVENTION
According to one aspect, this invention provides a method of detecting an access burst in a TDMA communications system using differential modulation, the access burst including at least one predetermined sequence of N
T
consecutive identical symbols in a predetermined position in the burst, where N
T
is an integer greater than an integer L representing a number of symbol periods over which the system has channel dispersion, comprising sampling a received signal and detecting the predetermined sequence in the received signal by the steps of, for each of N
T
-L current symbol periods ending the predetermined sequence: predicting at least one sample of the received signal in the current symbol period from at least one sample of the received signal in the preceding symbol period assuming identity of symbols represented by the received signal in the current and preceding symbol periods; producing a squared distance between at least one sample of the received signal in the current symbol period and the predicted at least one sample; and comparing the squared distance with a threshold to determine said identity if the threshold is not exceeded.
In order to adjust the threshold for a fading channel such as in a wireless communications system, the method preferably includes the step of, for, each of the N
T
-L current symbol periods, producing the threshold in dependence upon a power of the received signal in the preceding symbol period.
The method preferably includes the step of accumulating the squared distance for a purality of the N
T
-L current symbol periods. The steps are desirably performed for each of a plurality of predetermined sequences in the access burst, and each of the predetermined sequences can for example represent a zero bit field in the access burst. Thus, in particular, at least one predetermined sequence can comprise at least one of zero bit fields W, X, and Y of an access burst of an IS-54 system, together with any zero bit symbols at the end of a CDVCC word preceding, and any zero bit symbols at the beginning of a sync word following, each of said fields W, X, and Y, and preferably said steps are carried out for each of a plurality of said fields W, X, and Y.
Another aspect of the invention provides a method of determining timing of an access burst received in a TDMA wireless communications system using differential modulation, the access burst including at least one predetermined sequence of N
T
consecutive identical symbols in a predetermined position in the burst, where N
T
is an integer greater than an integer L representing a number of symbol periods over which the system has channel dispersion, comprising the steps of: sampling a received signal; for each of a plurality of possible timings of the access burst, detecting whether the predetermined sequence is present in the respective position in the access burst by performing the following steps (a) to (d) either for N
T
-L current symbol periods which end the predetermined sequence if the a
Bontu Chandra Sekhar
Gu Yonghai
Al-Beshrawi Tony
Nortel Networks Limited
Pham Chi
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