Simultaneous plural code series generator and CDMA radio...

Details Simultaneous plural code series generator and CDMA radio... Simultaneous plural code series generator and CDMA radio...

2002-06-21

2004-04-27

Chin, Stephen (Department: 2634)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S347000, C375S349000

Reexamination Certificate

active

06728305

ABSTRACT:

TECHNICAL FIELD
The invention relates to a code generator which simultaneously generates a plurality of code series which are in mutually orthogonal relationship to each other, and also a CDMA radio receiver using same.
PRIOR ART
A code series generator of the kind described is used for various purposes in the field of communication. For example, in order to preserve secrecy of information being transmitted in a radio communication, digital data from the transmitting side is multiplied by a code series, referred to as a secret code, in a scrambler circuit while received data is multiplied by a secret code which is of the same type and of the same phase as the secret code used on the transmitting side in a descrambler circuit of the receiving side. In the CDMA radio communication, a code series, called a spread code, is used to provide a multiple access. On the transmitting side, data is multiplied by a spread code before transmission while on the receiving side, a received signal is multiplied by the spread code to derive data. Different codes are orthogonal to each other, and a communication is enabled by the use of the spread code of the same type and of the same phase on both the transmitting and the receiving side.
If a code series used on the transmitting side is unknown in the secret code communication or CDMA radio communication, it is necessary on the receiving side to retrieve the type of the code series used on the transmitting side. Because the number of the types of the code series used is fixed, a plurality of code series may be used in parallel during the retrieval to reduce a length of time required for the retrieval. Also, in the CDMA radio communication, if a single user simultaneously uses a plurality of code series (multicode), a high rate of transmission is enabled, thus allowing a variety of services to be offered.
FIG. 1A
shows an example of an arrangement for a receiver of a mobile unit in a conventional CDMA radio communication. Describing a flow of a received signal in this arrangement, a signal received at an antenna
11
is demodulated in a radio unit
12
to be converted into a baseband signal, which is then converted into a digital signal in an A/D converter
13
. Supplied to code generators
14
1
-
14
4
are spread code information which is detected by a multi-path searcher
15
as well as a frame timing. This allows the code generators
14
1
-
14
4
to generate a code each which is synchronized with a spread code used on the transmitting side. A multiplication between the baseband signal from the A/D converter
13
and the spread codes from the code generators
14
1
-
14
4
then takes place in correlators
16
1
-
16
4
to effect a despreading, thus recovering an original signal. Subsequently, the reversely spread signals are detected in detectors
17
1
-
17
4
and are then synthesized by a RAKE receiver
18
to be delivered to a signal processing unit
19
. The despreading process which takes place in the correlators
16
1
-
16
4
permits a separation of signals from different transmission paths (or multi-path) which are contained in the received signal, and thus this reception processing represents a RAKE reception with path diversity effect.
Gold code series, for example, which exhibit an excellent orthogonality between codes is used for the code series mentioned. Gold code series generator comprises a first M-series generator
14
A, a second M-series generator
14
B, and an exclusive OR
14
X which calculates an exclusive OR of both outputs from the M-series generators
14
A,
14
B, as shown in FIG.
2
A. It is to be understood that throughout subsequent Figures, the symbol including “+” mark in a circle indicates an exclusive OR. The first and the second M-series generator
14
A,
14
B generates mutually different M-series, that is, codes which are distinct from each other if they are compared against each other at any phase relationship.
M-series generator
14
A comprises a shift register, SR and an exclusive OR XR, and a value in each shift stage is shifted by one stage toward a last stage S
5
in response to each shift clock, whereby M-series is delivered as an output from the last stage S
5
of the shift register SR. Simultaneously, the exclusive OR XR forms an exclusive OR of the output from the last stage and an output from an intermediate shift stage, with its output being fed back to an initial stage S
1
of the shift register SR. In the second M-series generator
14
B, each of a plurality of shift stages of its shift register is connected with an exclusive OR XR to form an exclusive OR with an output to be fed back to the initial stage. Thus, mutually different M-series are generated by changing the locations and/or the number of shift stages which are connected with the exclusive OR's. A register value in a single shift register which defines M-series changes for every clock, but retains a periodicity such that it returns to its original initial value after the register value has gone a round through all possible combinations. Denoting the number of shift stages of the shift register SR by n, the length of the period is equal to 2
n
−1 chips. By adding together (or forming an exclusive OR of) two such different M-series in the exclusive OR
14
X, there is obtained Gold code series having the same period as the M-series.
Different Gold code series can be formed by changing an initial value loaded into a shift register which defines M-series. A user can use a plurality of different Gold code series simultaneously to achieve a reduction in the length of time required to identify Gold code series used on the transmitting side or a high rate data transmission.
It has been customary in the art of radio communication to use a separate code generator for each code series in order to generate a plurality of different code series simultaneously. Accordingly, as the number of code series generated increases, there results an increase in both circuit scale and power dissipation.
Japanese Laid-Open Patent Application No. 264,098/1995 discloses a spread spectrum communication system including a code generator for simultaneously generating a plurality of spread codes and which is constructed as shown in FIG.
2
B. Specifically, an output from an M-series generator
14
is supplied to an end bit adding unit
14
C
0
and to a shift register
14
SR, an output from each shift stage of which is supplied to an end bit adding unit
14
C
1
-
14
Cn, respectively, the end bit adding units delivering a series of codes which are substantially orthogonal to each other. The M-series generator
14
, the shift register
14
SR and the end bit adding units
14
C
0
-
14
Cn are operated by a same clock. In this manner, (n+1) spread codes are generated.
With the code generator shown in
FIG. 2B
, the circuit scale and the power dissipation are both reduced than when a code generator is used for each code series.
In the code generator shown in
FIG. 2B
, the mutual orthogonality is given as a result of adding one bit to the end of each input code series or to the end of a code series having a sequential phase offset by one bit (or to the end of each code period) in each of the end bit adding units
14
C
0
-
14
Cn. However, a phase offset by one bit produces a significant cross-correlation between the codes. Assume that output codes delivered from the end bit adding units
14
C
0
,
14
C
1
are code
1
and code
2
(“1” being added to the end of code
1
and code
2
) as shown in
FIG. 2C
, for example. If one bit shift to the right is caused as by a transmission path delay to the code
1
, there would be only two bit differences for the leading bit and the trailing bit between the code
1
which is shifted by one bit to the right and the code
2
, producing a remarkable magnitude of cross-correlation between the codes, which can no longer be treated as distinct codes.
Thus, a plurality of codes generated by a code generator as shown in
FIG. 2B
involves a likelihood of causing an interchannel interference when they are used in a communication s

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