Signal transmitting method, transmitter, receiver, and spread-sp

Details Signal transmitting method, transmitter, receiver, and spread-sp Signal transmitting method, transmitter, receiver, and spread-sp

1997-11-03

2000-12-26

Kizou, Hassan

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

370342, 370441, 370503, H04B 7216

Patent

active

061670375

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a signal transmission method, transmitter, receiver and spreading code synchronization method in mobile communication systems to which is applied a direct sequence code division multiple access (DS-CDMA) scheme carrying out multiple access using direct sequence.


BACKGROUND ART

The DS-CDMA communication method is a scheme that transmits information data after spreading their bandwidth using a code with a rate much higher than the information data rate, and its research and development have been intensively conducted to be applied to cellular systems. This is because the DS-CDMA systems have such characteristics as facilitating flexible cell design which will enable the capacity in terms of the number of users to be increased as compared with the conventional frequency division multiple access (FDMA) or time division multiple access (TDMA).
The DS-CDMA systems include two spreading methods: One carries out spreading using a spreading code called a "short code" with a period equal to that of the information symbols; and the other carries out spreading using a spreading code called a "long code" with a period much longer than that of the information symbols. As the spreading codes, Gold codes or others are used. The Gold codes consists of two M (maximum) sequences, and the Gold codes belonging to the same group can be generated by the number corresponding to its period.
Accordingly, the number of the Gold codes that can be generated is no more than that corresponding to the processing gain (PG) or spreading ratio. As a result, in the cellular systems, the same spreading code cannot be used within several cells because of interference from other cells, and this presents a reuse problem in spreading code assignment.
On the other hand, using a long code enables to generate a great number of codes by lengthening its period. Thus, each cell can assign spreading codes to users independently of the other cells in the multicellular configuration. This is because the probability is very small that the same code is used at the same time in another cell thanks to the great number of the codes.
In the cellular systems, besides the incoming radio wave traveling through the shortest path from the transmitting point, there are delayed waves resulting from reflection and refraction due to obstacles or configuration of ground such as surrounding buildings, mountains, towers, etc. Since the delayed waves usually become interference signals against desired waves, they will degrade received characteristics. In the DS-CDMA system, the information signals are transmitted as very fast signals. Thus, when they are spread to 1 MHz band, the desired waves can be separated from delayed waves with a delay of one microsecond by carrying out correlation detection at a resolution of one microsecond. Combining these waves after demodulating independently, which is called RAKE combining, has an advantage of making full use of the power of the delayed waves.
In this case, since each one of consecutive information symbols is spread by a spreading code of the same pattern in the short code system, the delayed waves with a delay beyond one information symbol cannot be combined. On the contrary, since the consecutive information symbols are spread with different portions of a long code in the long code system, the delayed waves with a delay beyond one information symbol can be RAKE combined.
Although the long code has various merits as described above, it has a demerit that it takes a long time to establish the synchronization of the spreading code. Specifically, a DS-CDMA receiver must establish synchronization of the phase of a spreading code replica at the receiver side with that of the spreading code in a received signal at the beginning of communications. Since the long code has a much longer spreading code phase to be searched for compared with the short code, much longer time is required for establishing the synchronization.
The receiver conducts the correlation detection using a matched

REFERENCES:
patent: 5111478 (1992-05-01), McDonald
patent: 5329547 (1994-07-01), Ling
patent: 5345469 (1994-09-01), Fulghum
patent: 5535239 (1996-07-01), Padovani et al.
patent: 5790588 (1998-08-01), Fukawa et al.

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