Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-12-04
2003-12-16
Ton, Dang (Department: 2666)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C375S147000, C375S148000
Reexamination Certificate
active
06665282
ABSTRACT:
BACKGROUND
The invention relates to receiving spread spectrum radio signals, such as digitally modulated signals in a Code Division Multiple Access (CDMA) mobile radio telephone system, and more particularly, to configuring a RAKE receiver.
FIG. 1
illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system
10
. Base station
30
transmits a signal
40
that has a maximum signal strength that is limited so as to reduce interference with other base stations. The maximum signal strength of the base station's transmission creates a foot print or a region within which mobile stations
50
and
60
can communicate with base station
30
. If base station
30
uses a single omni-directional antenna, the foot print extends in an unlimited direction (360 degrees). While each footprint is an irregular shape that overlaps with adjacent foot prints, a foot print is often depicted as a hexagon
20
and is usually referred to as a cell.
In most systems, the base station
30
transmits a broadcast signal that is transmitted to all the mobile stations in cell
20
. The mobile stations use different traffic signals, but the same broadcast channel. The broadcast signal contains, for example, paging messages that are needed by all the mobile stations in the cell. The base station can control the power of each traffic signal, but the broadcast signal has to be able to reach as far as the cell's border. Therefore, the broadcast channel usually contains more signal power than the individual traffic channels.
FIG. 2
is a schematic diagram of an example of a CDMA system. A transmitter
30
can transmit input user data to multiple users. In a traditional CDMA system, each symbol of input user data
31
is multiplied by a short code or chip sequence
33
. There is a unique short code for each input user. Input user data is then spread by a long code or chip sequence
35
. While the short codes eliminate multiple access interference among users in the same cell, the long code is used to eliminate multiple access interference among the transmitters. An accumulator
36
adds the spread signals to form a composite signal
37
. Composite signal
37
is used to modulate a radio frequency carrier
38
which is transmitted by a transmitting antenna
39
.
A receiver
50
has a receiving antenna
59
for receiving signal
40
. Receiver
50
uses a carrier signal
58
to demodulate signal
40
and to obtain composite signal
58
. Composite signal
57
is multiplied by a synchronized long code or chip sequence
55
. Long code
55
is a locally generated complex conjugated replica of long code
35
.
The despread signal
54
is then multiplied by a synchronized short code or chip sequence. Short code
53
is a locally generated complex conjugated replica of short code
33
(or one of the other N short codes used by transmitter
30
). The multiplication by short code
53
suppresses the interference due to transmission to the other users. A digital logic circuit
52
(e.g., a summation and dump unit) can be used to provide an estimate of input user data
31
.
It will be evident to those skilled in the art that receiver
50
can not reconstruct input user data
31
unless it can (1) determine long code
35
and synchronize a locally generated complex conjugated replica of long code
35
with the received signal
57
, and (2) determine short code
33
and synchronize a locally generated complex conjugated replica of short code
33
with the despread signal
54
. It is for this reason that many CDMA signals contain a pilot signal or a periodic code (synchronization code). The synchronization codes can be found by using a matched filter or a correlation scheme and by identifying the correlation peaks.
FIG. 3
is a schematic diagram of an exemplary frame structure. Channel
40
has multiple frames
42
. Each frame
42
has a constant number of slots
44
. Each slot
44
contains one or more pilot symbol(s)
46
. The long code
35
is repeated each frame so that, for example, the first pilot symbol in each frame is multiplied by the same portion of long code
35
, and successive pilot symbols are multiplied by the same successive portions of long code
35
. While the receiver can use the pilot signal to synchronize the received signal and search for multipath delays, in some systems, the pilot signal is a relatively small portion of each frame and does not contain much energy. A broadcast channel may use the same, or a different, frame structure. The broadcast channel may contain a pilot signal that is considerably longer. In either case, the broadcast channel usually contains more energy than a traffic channel.
FIG. 4
a
illustrates the use of three directional antennas to divide a cell into three 120° sectors. Cell
20
has three sectors
21
,
22
, and
23
.
FIG. 4
b
illustrates the use of six directional antennas to divide a cell into six 60° sectors. Cell
20
has six sectors
21
,
22
, . . . , and
26
. As discussed above, the long code
55
suppresses the interference due to other transmitters, and the short code
53
suppresses the interference due to other users. However, as the number of users increases so does the interference. In some systems, it is necessary to use directional antennas to subdivide each cell.
If base station
10
uses directional antennas, base station
10
can transmit multiple signals to smaller groups. When a base station uses directional antennas, each directional antenna transmits to a smaller number of mobile stations than a single antenna would. As a result, the amount of interference decreases and the base station can support a larger number of mobile stations without exceeding an acceptable level of interference noise. If each of the mobile stations uses the same broadcast channel, the base station can use an omnidirectional antenna to transmit the broadcast signal, and directional antennas to transmit the traffic signals.
In mobile communication systems, signals transmitted between base and mobile stations typically suffer from echo distortion or time dispersion (multipath delay). Multipath delay is caused by, for example, signal reflections from large buildings or nearby mountain ranges. The obstructions cause the signal to proceed to the receiver along not one, but many paths. The receiver receives a composite signal of multiple versions of the transmitted signal that have propagated along different paths (referred to as “rays”). The rays have different and randomly varying delays and amplitudes.
Each distinguishable “ray” has a certain relative time of arrival, T
n
seconds. A receiver can determine the relative time of arrival of each ray by using a matched filter, a search finger that is shifted, or any other correlation scheme. The output of the matched filter or the correlation scheme is usually referred to as the multipath profile (or the delay profile). Because the received signal contains multiple versions of the same signal, the delay profile contains more than one spike.
FIG. 5
is an example of a multipath profile. The ray that propagates along the shortest path arrives at time T
o
with amplitude A
0
, and rays propagating along longer paths arrive at times T
1
, T
2
, . . . , T
N
with amplitudes A
1
, A
2
, . . . , A
N
, respectively. In order to optimally detect the transmitted signal, the spikes must be combined in an appropriate way. This is usually done by a RAKE receiver, which is so named because it “rakes” different paths together. A RAKE receiver uses a form of diversity combining to collect the signal energy from the various received signal paths (or rays). The term “diversity” refers to the fact that a RAKE receiver uses redundant communication channels so that when some channels fade, communication is still possible over non-fading channels. A CDMA RAKE receiver combats fading by detecting the echo signals individually, and then adding them together coherently.
FIG. 6
is a schematic diagram of a RAKE receiver with four fingers. A radio frequency (RF) receiver
110
demodulates a receiv
Eriksson Håkan B.
Jonson Martin J.
Urabe Kenzo
Burns Doane Swecker & Mathis L.L.P.
Telefonaktiebolaget L M Ericsson (publ)
Ton Dang
Tran Phuc
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