Initial frequency synchronization mechanism

Details Initial frequency synchronization mechanism Initial frequency synchronization mechanism

1998-01-23

2001-01-16

Eisenzopf, Reinhard J. (Department: 2745)

Telecommunications

Transmitter and receiver at separate stations

Optimum frequency selection

C455S164200

Reexamination Certificate

active

06175722

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to frequency acquisition in general and to frequency acquisition in the presence of high power adjacent channels, in particular.
BACKGROUND OF THE INVENTION
Reference is now made to
FIGS. 1A and 1B
.
FIG. 1A
is a schematic illustration of frequency versus power, describing the initial stage of a initial frequency synchronization procedure, known in the art. The present example describes a closed loop automatic frequency control (AFC) mechanism.
FIG. 1B
is a schematic illustration of frequency versus power, describing the final stage of the initial frequency synchronization procedure of FIG.
1
A.
Arrow
14
represents the frequency of a mobile unit which detects and attempts to lock and synchronize with the carrier frequency
10
of a base unit transmitter having a value of F
BASE
, which is located near by. In the present example the mobile unit further detects a carrier frequency
12
provided by a neighbor transmitter, having a value of F
NEIGHBOR
. The value of the mobile unit F
MOBILE
is located between the values of the base unit frequency F
BASE
and the neighbor mobile transmitter frequency F
NEIGHBOR
.
In the present example the mobile unit
14
detects the signals provided by base
10
and the neighbor
12
wherein the received power of the neighbor
12
is higher than the received power of the base unit
10
.
According to conventional initial synchronization procedures, the mobile unit frequency is synchronized with the frequency having the highest received power, which in the present example is the neighbor frequency
12
.
It will be noted that often the received frequencies are filtered so as to exclude undesired signals. Such a filter is represented by arc
16
. These techniques often fail when the power of the undesired signal is significantly high.
Accordingly the synchronization mechanism of the mobile unit sets synchronization path towards the neighbor frequency F
NEIGHBOR
and starts progressing its frequency
14
towards F
NEIGHBOR
. Finally the synchronization mechanism allows the frequency of the mobile unit
14
to acquire and synchronize with the frequency of the neighbor unit
12
. This is shown in
FIG. 1B
by aligning line
12
and arrow
14
. As can be seen, at this stage the frequency
10
of the base transmitter is filtered out by the filter
16
.
A conventional synchronization mechanism provides frequency shifts within a limited range, determined by its structure, such as VCO voltage and the like. It will be appreciated by those skilled in the art that the F
NEIGHBOR
can be located outside this range. in such a case, F
MOBILE
, might get stuck at the boundary frequency value which is closest to F
NEIGHBOR
.
It will be appreciated by those skilled in the art that such situations, where the frequency of the mobile unit
14
is synchronized with the frequency of neighbor unit
12
instead of the frequency of the base unit
10
, is not acceptable.
SUMMARY OF THE PRESENT INVENTION
It is an object of the present invention to provide a novel method for performing accurate initial frequency acquisition in the presence of high power adjacent channels.
It is a further object of the present invention to provide a novel device for performing accurate initial frequency acquisition in the presence of high power adjacent channels.
In accordance with the present invention there is thus provided a method for acquiring frequency of a desired channel having a carrier frequency F
MAIN
, for a dynamic receiver frequency F
MOBILE
, from a starting frequency F
START
, in the presence of high power adjacent interfering channels.
The starting frequency F
START
is shifted from F
MAIN
by not more than a predetermined frequency gap &Dgr;F. The method includes the steps of:
determining a first frequency boundary and a second frequency boundary;
detecting channels within a filtering bandwidth;
selecting a dominant channel from the detected channels;
progressing the dynamic receiver frequency F
MOBILE
towards the carrier frequency of the dominant channel;
detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary;
restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary; and
repeating from the step of detecting channels.
According to another aspect of the present invention, one of the first frequency boundary and the second frequency boundary is F
START
−&Dgr;F, while the other is F
START
+&Dgr;F.
The method of the invention can also include the step of determining a frequency advance direction. The frequency advance direction can be fixed at the beginning of each frequency acquisition cycle, wherein the frequency acquisition cycle is determined from the point where F
MOBILE
shifts from F
START
until the point where F
MOBILE
returns to F
START
.
The step of progressing can be performed in a frequency step F
STEP
. The value of the frequency step F
STEP
can be infinitesimal with comparison to the predetermined frequency gap &Dgr;F, or adjustable. Accordingly, the method can further include the step of adjusting the frequency step F
STEP
after each step of detecting channels.
In accordance with another aspect of the present invention, there is provided a device for acquiring frequency of a desired channel having a carrier frequency F
MAIN
, for a dynamic receiver frequency F
MOBILE
, from a starting frequency F
START
, in the presence of high power adjacent interfering channels.
The device is connected to an antenna via a receiver and to a reference frequency F
REFERENCE
source. The device includes controllable frequency generating means for generating an internal frequency F
INTERNAL
, frequency shift means connected to the controllable frequency generating means, and to the receiver, for shifting received frequency F
RECEIVED
, of a received channel, according to the internal frequency F
INTERNAL
.
The device also includes a frequency shift detector, connected to the frequency shift means, for detecting a frequency difference between the internal frequency F
INTERNAL
and the received frequency F
RECEIVED
, with respect to the reference frequency F
REFERENCE
, thereby producing a frequency shift value F
SHIFT
.
The device further includes loop filtering means, connected to the frequency shift detector, for filtering the frequency shift value F
SHIFT
, thereby producing a filtered frequency shift value F
SHIFT-FILTERED
, and controlling means, connected to the controllable frequency generating means and to the loop filtering means, for determining a frequency step F
STEP
from the filtered frequency shift value F
SHIFT-FILTERED
.
The controlling means provide the frequency shift value F
SHIFT
to the controllable frequency generating means. The controllable frequency generating means adjust the internal frequency F
INTERNAL
according to the frequency shift value F
SHIFT
, and the controlling means control the controllable frequency generating means to generate frequency in a range from a first frequency boundary F
FIRST
and a second frequency boundary F
SECOND
.
According to one aspect of the invention, the controlling means set the frequency shift value F
SHIFT
to be F
SECOND
−F
INTERNAL
, when |F
INTERNAL
−F
START
|>|F
INTERNAL
−F
FIRST
| while the controlling means set the frequency shift value F
SHIFT
to be F
FIRST
−F
INTERNAL
, when |F
INTERNAL
−F
START
|>|F
INTERNAL
−F
SECOND
|.
According to another aspect of the invention, the device further includes frequency filtering means, connected between the frequency shift detector and frequency shift means.
The controlling means reset the loop filtering means when setting the frequency shift value F
SHIFT
to be F
SECOND
−F
INTERNAL
or F
FIRST
−F
INTERNAL
.


REFERENCES:
patent: 4342007 (1982-07-01), Elliot
patent: 4715001 (1987-12-01), Deem et al.
patent: 5509034 (1996-04-01), Beukema
patent: 59330

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