Pulse or digital communications – Spread spectrum
Reexamination Certificate
2000-04-27
2004-01-27
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
C370S342000
Reexamination Certificate
active
06683903
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication systems and in particular, to a method and apparatus for synchronization within such communication systems.
BACKGROUND OF THE INVENTION
When a mobile unit is first powered on, there can exist a frequency offset between the mobile unit and the base station. This frequency offset is due to inaccuracies in the local frequency synthesizer (which are typically ±5 ppm). Because of this a mobile will have to perform a frequency acquisition stage by an automatic frequency control unit in order to synchronize to the base station. During frequency acquisition, a mobile unit typically estimates the frequency offset based on a phase offset between Walsh symbols. In particular, a frequency offset (&Dgr;ƒ) is usually determined as shown in
FIG. 1
by determining a phase difference (&Dgr;&thgr;) between two adjacent Walsh symbols
101
-
102
such that,
Δ
f
=
Δ
θ
2
π
T
,
-
π
<
Δ
θ
<
π
,
where T is the time interval between two adjacent Walsh symbols
101
-
102
. It is worth noting that condition −&pgr;<&Dgr;&thgr;<&pgr; is necessary to avoid phase ambiguity. Consequently, for a given &Dgr;ƒ, the maximum allowed T is then
T
<
π
2
π
×
&LeftBracketingBar;
Δ
f
&RightBracketingBar;
=
1
2
&LeftBracketingBar;
Δ
f
&RightBracketingBar;
.
If the Walsh symbol time duration (T) is larger than ½|&Dgr;ƒ|, frequency offset estimation will fail due to the phase ambiguity. Therefore, a need exists for method and apparatus for synchronization within a spread-spectrum communication system that reduces the chance of phase ambiguity.
REFERENCES:
patent: 5619524 (1997-04-01), Ling et al.
patent: 5640431 (1997-06-01), Bruckert et al.
patent: 5659573 (1997-08-01), Bruckert et al.
patent: 5767738 (1998-06-01), Brown et al.
patent: 5793759 (1998-08-01), Rakib et al.
patent: 5983113 (1999-11-01), Asanuma
patent: 6272168 (2001-08-01), Lomp et al.
patent: 6370129 (2002-04-01), Huang
Haas Kenneth A.
Motorola Inc.
Pace Lalita W.
Tran Khai
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