Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-03-13
2001-06-19
Ton, Dang (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
Reexamination Certificate
active
06249517
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for spreading the information signals in a CDMA (Code Division Multiple Access) system supporting various information rates, and more particularly, to an apparatus for selecting a channel-separating and a PN (Pseudo-random Noise) spreading method according to the rate of the symbol to spread.
2. Description of Related Art
In DS-CDMA (Direct sequence CDMA) systems, there are two different channel spreading methods, one of which is to spread the signal with different codes from one another, the other of which is to use one long PN code with different time offsets. The conventional CDMA systems have used one of these methods and a combination thereof.
The conventional CDMA system, as developed by QUALCOMM Incorporated, has used the first method to separate the forward channels in the same cell or sector. In other words, the Sync Channel, the Paging Channels and the Traffic Channels of the forward link are spread and separated by the orthogonal Walsh functions with distinct indexes. In the case of a single-path ideal channel and one cell environment, interference between the channels does not exist because of the orthogonal property of the Walsh functions.
In the real mobile radio channel, there is multiple access interference (MAI) between the channels even though the orthogonal sequences are used because the multipath fading characteristics of the channel lead to distortion of the signal. However, the MAI, which is caused by the fading effect of the channel, is still lower than that in the case of channel-separation with non-orthogonal sequences.
In the conventional CDMA systems, the second method above has been used to identify a forward CDMA channel from the adjacent cells or sectors. PN binary codes are used to distinguish signals received at a mobile station from different base stations. All CDMA signals in the system share a quadrature pair of PN codes. Signals from different cells or sectors are distinguished by time offset from the basic code. This relies on the property of PN codes that the autocorrelation averages to zero for all the time offsets greater than a single code chip time.
The conventional CDMA systems that provide voice-based services have a small number of data rates so it is natural to make the channels to be orthogonal using orthogonal sequences. The channel-separation and PN spreading of the traffic channel in the conventional CDMA systems is shown in
FIG. 1. A
traffic information bit is input into a traffic channel that passes through a channel encoder, such as a convolutional encoder
11
, a symbol repeator
12
and a block interleaver
13
, in this order. Thereafter, the output symbols from the block interleaver
13
are spread in an orthogonal spreader
14
by a Walsh function with an index allocated to the channel during the call setup procedure and the output from the orthogonal spreader
14
is spread in the pilot PN spreader
15
and
16
(I channel and Q channel), respectively, by the pilot codes with a time offset which is allocated to the corresponding cell or sector. All the forward channels within one cell or sector share a pair of pilot PN codes with the same time offset.
One of the typical multiple accesses applied to the mobile communication systems is the CDMA communication system. In IS-95, a CDMA common air-interface (CAI) adopted in the United States as a cellular and personal communications service (PCS) standard, the forward channels in one cell or sector are spread by the Walsh functions as channel-separating orthogonal codes and a pair of PN sequences of period 2
15
with a given time offset as pilot PN codes. Therefore, there is a little intra-cell interference at a mobile station because all the forward channels can be spread with orthogonal sequences and transmitted synchronously.
However, in the systems in which the Walsh functions are used, it is difficult to transmit information data with various rates because of the relationship between the symbol rate at the input to the orthogonal spreader and the chip rate at the output of the PN spreader. That is, one symbol at the input to the orthogonal spreader should be spread with one or more periods of the Walsh sequence to guarantee orthogonality at the demodulator of the mobile receiver.
A spreading method, which uses the long codes only for channel-separation without orthogonal spreading, is considered to support a variety of rates of user information. This method gives a lot of flexibility in system design, but there is no orthogonality between the channels even among the channels satisfying the above Walsh condition, which lowers the bit error performance.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus for spreading the information signals that is capable of supporting various information rates.
Another object of the present invention is to provide an apparatus for reducing the interference in a specific rate in a CDMA system.
In accordance with an aspect of the present invention, there is provided an apparatus for spreading information signals in a CDMA system supporting various information rates, comprising: a first data path for transmitting traffic data from users; at least one orthogonal spreading means formed on the first data path for spreading the traffic data using an orthogonal code; a second data path for by-passing the first data path; control means for controlling a data transmission path by selecting one of the first and the second data paths in response to a digital bit rate, a length of an orthogonal code and a chip rate, wherein the control means selects the first data path to spreading the traffic data through the orthogonal spreading means if a product of the digital bit rate and the length of the orthogonal code is equal to the chip rate, and, if not, the control means selects the second data path to spread the traffic data; and a long PN code generating means for spreading an output from the orthogonal spreading means using a long PN code having a fixed code offset when the first data path is selected or for directly spreading the traffic data using a long PN code having different code offsets when the second data path is selected.
In accordance with another aspect of the present invention, there is provided an apparatus for spreading information signals in a CDMA system supporting various information rates, comprising: switching control means for receiving a digital bit rate, a length of an orthogonal code and a chip rate, and for generating first and second control signals, wherein the switch control means generates the first control signal if a product of the digital bit rate and the length of the orthogonal code is equal to the chip rate, and, if not, the switch control means generates the second control signal; at least one orthogonal spreading means for spreading the received traffic data using an orthogonal code; first switching means for transferring the traffic data to the orthogonal spreading means in response to the first control signal; second switching means for selectively transferring the traffic data or an output from the orthogonal spreading means in response to the second control signal; a first long PN code generating means for generating a long PN code having a fixed code offset or different code offsets; and a first logic gate for EX-ORing an output from the orthogonal spreading means and the long PN code having the fixed code offset or EX-ORing the traffic data directed input and the long PN code having the different code offsets in response to the second control signal.
In accordance with further another aspect of the present invention, there is provided an apparatus for spreading information signals in a CDMA system supporting various information rates, comprising: first means for determining whether a digital bit rate input into a user channel satisfies a given condition; second means for spreading traffic data using an orthogonal code and along PN code having a fixed code offset, if the di
Choi Yang Ho
Pyo Hyun Myung
Roh June Chul
Korea & Telecom
Staas & Halsey , LLP
Ton Dang
LandOfFree
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