Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-12-09
2002-12-24
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C375S236000
Reexamination Certificate
active
06498789
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a code division multiple access (hereinafter referred to simply as a “CDMA”) system employing a spread spectrum (SS) communication scheme in the field of mobile communication.
Since spread spectrum communication employed in the field of mobile communication permits code division multiple access and has superior noise resistance, the spread spectrum communication is used for a CDMA communications system or radio LAN communications. A CMDA communications system is currently in actual use in North America and Korea (hereinafter referred to as a “North American Scheme”) and is standardized as (TIA/EIA/IS95 and TIA/EIA/IS98) scheduled to be put into service by Japanese communication service companies. Further, another CDMA scheme is scheduled to be adopted in Japan as a third-generation mobile communications scheme (hereinafter referred to simply as a “wideband scheme”).
A spread spectrum scheme comprises a spread spectrum direct sequence (direct spread spectrum) scheme and a spread spectrum frequency hopping scheme. All currently employed as CDMA schemes are direct sequence schemes. In a spread spectrum communications scheme, multipath components are combined at the maximum rate through use of a receiver called a RAKE system, thereby producing a diversity effect. The RAKE receiver is described in, e.g., U.S. Pat. No. 5,109,390.
Use of the RAKE receiver complying with the CDMA scheme enables a mobile communications device to communicate with base station other than the base station with which the device is currently communicating, thus allowing a hand-off without interruption of communication (i.e., a soft hand-off).
In order to realize a soft hand-off, the North American scheme causes all the base stations to be in synchronization with one another through use of the Global Positioning System (GPS). In the wideband scheme, the base stations are not in synchronization with one another. Accordingly, the North American scheme enables easier detection of a base station for soft hand-off purpose. In the North American scheme, all base stations share a common code (e.g., a long code), and the base stations are brought into synchronization with one another by activating a generator for producing the common code through use of a GPS clock signal.
FIG. 6
shows the configuration of a mobile communications device complying with the North American scheme. The mobile communications device shown in
FIG. 6
is roughly divided into a transmission section A and a receiving section B. The transmission section A comprises a transmission data preparation section
1
; an error detection/correction encoding section
2
; a long code generator
3
; a long code modulation section
4
which performs a first spreading operation through use of the long code; an in-phase component direct sequence section
5
which directly spreads a long code modulation signal through use of a spread code of an in-phase component; a quadrature component direct sequence section
6
which directly spreads a long code modulation signal through use of a spread code of a quadrature component; and a radio transmission section
7
which converts the frequency of a directly-spread base band signal into a radio frequency band, amplifies the signal, and transmits the signal from an antenna.
The receiving section B comprises a radio receiving section
8
which receives a signal of radio frequency band by way of an antenna, amplifies the signal, and converts the frequency of the received signal into a base band range; a RAKE receiving section
9
which combines multipath components at the maximum rate and effects a soft hand-off; a symbol combining section
10
which combines a received symbol at the maximum rate; the long code generator
3
that is identical in configuration with that provided in the transmission section A; a decimating section
11
which decimates the long code; a long code demodulation section
13
which descrambles the long code through use of the decimated long code; an error detection/correction decoding section
14
; and a demodulation data processing section
15
which segments the decoded received data into an audio signal and a control signal. The RAKE receiving section
9
comprises in-phase component despread sections
9
a
to
9
c
; quadrature component despread sections
9
d
to
9
f
; and combining sections
9
g
to
9
i
. In-phase components and quadrature components are despread and combined for every path, whereby a despread signal (i.e., a received symbol) is output for every path.
In the existing mobile communications device complying with the North American scheme, the mobile communications device produces at different timings, a long code for transmission purpose and a long code for receiving purpose. Therefore, as shown in
FIG. 6
, the communications device requires separate long code generators for transmission and receiving purposes. The reason for this will now be described.
FIG. 7
shows the configuration of a long code generator used in the existing North American scheme mobile communications device. As shown in
FIG. 7
, the long code generator comprises
42
flip-flops, XOR circuits provided for all the outputs of the flip-flops, and seven XOR circuits necessary for feed-back operations (dividing operations) of a shift register (
42
AND circuits are also required if a long code masking operation is performed). A clock frequency of 1.228 MHz is used as a clock signal for the long code generator.
In a North American scheme mobile communications device, an output from the long code generator
3
of the transmission section is used as a long code for the receiving section while being decimated. In the mobile communications device, a transmission timing is matched with the fastest path by means of the antenna's edge according to the North American standards. The transmission timing is usually faster than the receiving timing because of a delay in the radio section and a delay for multipath combining (or a difference in timing between the fastest path and the slowest path), thereby posing no problem. However, the transmission timing is sometimes delayed (see EIA/TIA/IS-95A), and hence the receiving timing may become faster than the transmission timing, thereby posing a problem. The mobile communication device receives from the base station a string of long codes having a certain absolute time interval. From that point on, the mobile communications device produces a long code through use of a clock signal in synchronization with the base station. Accordingly, the mobile communication device cannot hold a string of long codes prior to receipt of a string of long codes from the base station. Accordingly, if the receiving timing becomes faster than the transmission timing, the receiving section cannot receive the long code from the transmission section. The despread timing of the RAKE receiving section of the receiver changes every time a receiving path changes, thereby making it difficult to adjust the transmission and receiving timings.
For this reason, the receiver is required to have a long code generator which operates at a timing independent of the transmission section and is identical with the long code generator of the transmission section (see the configuration of the long code generator shown in FIG.
6
). For this reason, the mobile communications device becomes larger in scale, and an electric current dissipated in the communications device is increased.
Under the North American standards, the symbol combining section
10
is required to combine a delay path spreading over a cycle of several symbols without involvement of phase shift. Paths used for combining symbols are frequently switched in order to improve a receiving performance. If the receiving timing of each path frequently changes over a cycle of several symbols, the delay path cannot be combined without involvement of phase shift. For this reason, the configuration of the symbol combining section
10
becomes complicated.
SUMMARY OF THE INVENTION
The present i
Elallam Ahmed
Kizou Hassan
Pearne & Gordon LLP
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