Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-05-27
2003-04-29
Chin, Vivian (Department: 2682)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C370S320000, C370S342000
Reexamination Certificate
active
06556551
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to cellular wireless telephone systems and, in particular, to multi-frequency pilot beacons for hand-off of transceivers operating in Code Division Multiple Access (CDMA) systems.
BACKGROUND OF THE INVENTION
A cellular communication system is one in which coverage is provided in relatively small areas, commonly referred to as cells, that overlap. These overlapping cells form a grid of radio coverage that extends over a region of interest, e.g., an urban area.
In traditional cellular systems each call or radio connection between a mobile transceiver (telephone) and a cellular base station occupies a narrow segment of the frequency spectrum allocated to the provider of the cellular service. Since each call must have its own frequency segment the total number of simultaneous calls which can be handled is limited by the number of segments in the frequency spectrum.
When the coverage area is broken up into cells, frequency segments or frequencies can be reused in cells that are far enough apart so that the signals at the same frequency do not interfere with one another. In a typical cellular system the frequency reuse factor (how many cells have to be operating on different frequencies before frequency reuse can occur) is 7. At this reuse factor cells reusing the same frequency are two cells away from each other. This also means, that only a seventh of the allocated frequency spectrum can be used within any given cell.
While moving within the cellular grid a mobile transceiver is forced to switch its operating frequency between the channels allocated to the different cells. This process is called “hand-off”. In practice, the base station in one cell hands-off the transceiver call to a base station in another cell by forcing the transceiver to switch frequencies.
There are numerous problems with this traditional approach, often resulting in dropped calls and inefficient use of the frequency spectrum. Code Division Multiple Access (CDMA) technology is one of several alternative techniques for supporting cellular wireless communications in such a cellular system. CDMA systems have significant advantages over competing systems for multiple access communications such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and AM modulated systems such as Amplitude Companded Single Sideband (ACSSB) systems. Specifically, CDMA techniques result in a higher spectral efficiency than can be achieved using other multiple access techniques. In other words, more calls can be made in a given frequency band using CDMA than using other technologies.
In a prior art CDMA network
10
only one frequency band or carrier frequency, F
1
is used by all cells
12
, as shown in
FIG. 1. A
base station control
18
, which operates base stations
20
in cells
12
does not issue frequency hand-off commands. That is because a transceiver
14
of a mobile user
16
does not have to hand-off between different frequencies in network
10
.
Typically, a CDMA signal
22
in network
10
occupies a 1.25 MHz band (although other implementations can use more or less bandwidth). The band is centered at carrier frequency F
1
and several CDMA signals are superimposed upon each other within the band. As shown in
FIG. 2
, each CDMA signal
22
is created by multiplying a narrow band (about 10 kHz wide) baseband signal
24
containing the data (e.g., voice data) by a spreading code which increases the resulting bandwidth to 1.25 MHz. On the forward link, from base station
20
to transceiver
14
, CDMA signal
22
is prepared by spreading baseband signal
24
twice; once by a Walsh code and once by a pseudorandom noise sequence PN.
Baseband signal
24
is multiplied in a mixer
28
with a Walsh code W
i
provided from a Walsh code generator
26
to produce a coded signal
30
. Since individual Walsh codes are orthogonal their inner product satisfies the following condition:
W
i
*
W
j
=
{
0
i
≠
j
N
i
=
j
Thus, baseband signal
24
multiplied by Walsh code W
i
on the forward link can only be demodulated by a receiver by multiplying it with the same Walsh code, i.e., W
i
. Multiplication with any other Walsh code will not yield a signal. Hence, the receiver set to use Walsh code W
i
will reject all signals which are prepared with any Walsh code other than W
i
.
Walsh coded signal
30
is then multiplied with the aid of mixers
32
,
34
by a short pseudorandom noise sequence PN provided by a PN generator
36
. The PN sequence has a characteristic offset. In this case coded signal
30
is multiplied by an in-phase and a quadrature portion of the PN sequence in accordance with standard modulation techniques. The multiplied signals are converted from digital to analog and filtered by circuits
25
,
27
and then combined by a combining circuit
38
. The thus created CDMA signal
22
is up-converted by a mixer
23
to carrier frequency F
1
and sent to antenna
40
for transmission.
Since the same carrier frequency F
1
is used throughout CDMA network
10
base stations
20
are assigned unique offsets of the PN sequence to distinguish them. For example, base station
20
A uses sequence PN
A
which is the PN sequence with an offset A in generating its CDMA signals, base station
20
B uses sequence PN
B
, and so on. The various sequences PN
A
, PN
B
, . . . etc. are generated by shifting the standard PN sequence by varying offset amounts also referred to as PN offsets. The PN sequences are used to multiply each channel including a pilot channel. The pilot channel is defined as the unmodulated Walsh code zero. In other words, the pilot channel requires that generator
26
be set to zero and baseband signal
24
be zero. As a result, only the PN sequence with its given PN offset is transmitted in the pilot channel.
Just as in the case of frequency reuse, PN sequences with the same offsets can be reused in cells
12
which are sufficiently far apart to avoid interference, e.g., cells
12
A and
12
B use the same sequence PN
A
. Transceiver
14
will examine the different PN offsets to thus identify base stations
20
near it. As user
16
moves from one cell
12
to another, transceiver
14
can hand off to a neighboring base station in a soft hand-off process. The carrier frequency remains the same but the PN sequence of the new base station is used. The process is called soft because during the transition from one base station to another transceiver
14
is communicating simultaneously with both base stations.
As the number of mobile telephone users increases, more capacity than offered by CDMA network
10
will be required. One way to accomplish this goal is to use more frequency channels within the allocated frequency spectrum by adapting CDMA network
10
to operate at more than one carrier frequency. This means that CDMA network
10
will have to accommodate hard or frequency hand-off between different frequencies used in different cells
12
.
The prior art teaches the use of a pilot channel assigned Walsh code zero (0) to carry the PN offset information. The signal corresponding to the PN offset information is referred to as the pilot beacon. Knowledge of the PN offset allows the transceiver to identify with which base station they are communicating.
In U.S. Pat. No. 5,848,063 Weaver, Jr. et al. discusses the use of a pilot beacon for handing-off between dissimilar CDMA networks. The hand-off is not necessarily a frequency hand-off (hard hand-off) and the teaching is directed primarily at the hand-off algorithm and uses the measured time delay for the pilot beacon between the base station and the transceiver as a parameter for deciding when to execute a hand-off. U.S. Pat. No. 5,697,055 to Gilhousen et al. also discusses algorithms for determining hand-off between different cellular systems.
In U.S. Pat. No. 5,858,661 Weaver, Jr. et al. teach a method for creating areas where certain transceivers cannot communicate with certain base stations. These regions of silence are indicated by the presence of a pilot beacon with a specific
Chin Vivian
Lee John J
LGC Wireless Inc.
Lumen Intellectual Property Services Inc.
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