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System and method of time and frequency synchronization in a radiocommunication system
| Details |
Inventors: Dent, Paul W.;
Assignee: Ericsson Inc. (Research Triangle Park, NC)
Primary Examiner: Vu; Huy D.
Assistant Examiner:
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis, L.L.P.
A communication station such as a hand held telephone quickly identifies a desired carrier signal or time slot of a carrier signal and synchronizes itself in time and frequency to that carrier or slot. Synchronization is achieved even when frequency and time reference signals in the communication station are offset, e.g., due to drift, with respect to the carrier signal or slot by averaging successions of signal measurements that occur at times when corresponding signal values are expected. In one method, a plurality of measurements of the received signal taken at a group of predetermined time points where corresponding values are expected are averaged together, in order to enhance the signal to noise ratio. Averaging is carried out several times using different groups of time points, the groups corresponding to respective, different expected clock frequency errors. The results of the averaging steps are used in determining a best hypothesis for the received signal. Dynamic programming is used to test synchronization hypotheses based on different hypotheses of timing drift rate and to discriminate between different hypotheses of timing and timing drift rate in order to maximize sums of correlations. |
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DETAILED DESCRIPTION This invention seeks to determine a TDMA paging slot or any slot that includes a sequence of symbols having characteristics (e.
g.
, that the symbol sequence is one of a set of orthogonal codes) known a priori to the receiver.
In the case of paging, several paging slots are usually contained in each TDMA frame, and these may advantageously be transmitted, e.
g.
, by a satellite, in different directions with the aid of an electronically steered antenna, as described for example in allowed U.
S.
patent application Ser.
No.
08/179,953 filed on Jan.
11, 1994, entitled "Cellular/Satellite Communication System with Improved Frequency Re-Use" by Dent.
This allowed application is hereby incorporated by reference in this application.
For example, 128-slot TDMA signal frames such as those illustrated in FIG.
1 might allocate slot numbers 16, 32, 48, 64, 80, 96, 112, and 128 for paging.
These paging slots may be transmitted at a modestly higher power level than the other, traffic carrying, slots use on average.
In addition, one of these eight paging slots may be transmitted at a greatly higher power, for example 10 dB higher, than the other seven.
It is advantageous to stagger the higher power slots in time between different carriers and different directional beams radiated from the satellite transponder.
In particular, the one slot per frame transmitted at greatly higher power should not coincide with a similar slot on any other carrier or beam.
Using the exemplary format, 128 non-overlapping timing positions for the highest power slot are available, thus allowing one occurrence per frame in each of the 128 beams.
If more than 128 beams or carriers are required to occasionally radiate the greatly higher power burst, the power of one of the eight paging slots may be increased less often according to a systematic superframe structure known in advance to the terrestrial receivers.
Moreover, the paging slot that is greatly increased in power does not always have to be the same slot but can vary according to an agreed, possibly pseudorandom, pattern, such as 48, 128, 32, 16, 96
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