Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
1999-04-07
2003-05-06
Marcelo, Melvin (Department: 2663)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C375S130000
Reexamination Certificate
active
06560194
ABSTRACT:
The present invention is generally concerned with spread spectrum communication systems, i.e. systems in which spectrum spreading is applied to a sequence of data to be transmitted, on transmission, using a spreading code, and, conversely, despreading is effected on reception, in order to find the original sequence.
BACKGROUND OF THE INVENTION
Spreading an incident data sequence of N symbols, denoted (d
1
, d
2
, . . . , dN), using a code of length (or spreading factor) Q, denoted c
Q
=(c
1
, c
2
, . . , cQ), produces a sequence of length Q.N, which can be represented as follows:
(d
1
.c
1
, d
1
.c
2
, . . . , d
1
.cQ, d
2
.c
1
, d
2
.c
2
, . . . , dN.cQ)
where d
1
.c
1
represents the multiplication of d
1
by c
1
.
An alternative representation of the spread sequence of length Q.N is (d
1
.cQ, d
2
.c
Q
, . . . cN.c
Q
) where d
1
.c
Q
represents the product of the symbol d
1
by the spreading code c
Q
.
A more general approach to spreading consists in having a different spreading code for each symbol of the input sequence, in which case the resulting spread sequence can be expressed in the form: (d
1
.c
Q
(1)
,d
2
.c
Q
(2)
, . . . ,dN.c
Q
(N)
), c
Q
(1)
being the spreading code associated with the symbol d
1
. All the spreading codes preferably have the same length Q so that the resulting spread sequence has the length Q.N.
FIG. 1
outlines the principle of spreading, Ts designating the basic period (or symbol period) of a non-spread sequence and Tc designating the basic period (or chip, period) of a spread sequence, Ts and Tc being related by the equation Ts=Q.Tc. In the figure dn and dn+1 correspond to two successive symbols of a non-spread incoming sequence and d′l and d′l+1 correspond to two successive basic symbols (or “chips”) of the same spread symbol of the incoming sequence.
One benefit of the above systems is enabling a plurality of users to share the same frequency band by allocating different users different codes.
One important application is Code Division Multiple Access (CDMA) cellular mobile radio systems.
In the above systems, the codes are advantageously orthogonal two by two within a cell to reduce interference between users in that cell (interference between cells can also be reduced by other techniques known per se).
Mathematical tools have been developed for generating orthogonal codes. Walsh functions, which are known per se, generate orthogonal codes of length Q recursively from orthogonal codes of length Q/2, Q/4, etc. To be more precise, two orthogonal codes of length q are obtained from a code of length q/2 by juxtaposing with that code of length q/2, respectively that code, and the complementary code.
In systems using orthogonal codes of fixed length Q, the allocation of orthogonal codes can then be a relatively simple operation, consisting in selecting a code that is not being used at the time in question from Q possible orthogonal codes of length Q.
There is a need in these cellular mobile radio systems to be able to transmit data at a varying bit rate, in particular when the bit rate of the user itself varies (for example in the case of transmitting multimedia type data), or because, depending on the conditions for propagation of radio signals, a higher or lower degree of redundancy has to be introduced into the data to be transmitted in order to obtain a higher or lower degree of protection against transmission errors.
Increasing the bit rate of data to be transmitted by a user, for the same allocated frequency band (i.e. for the same duration Tc) by reducing the length of the code allocated to that user so that they continue to send on only one code is known per se.
FIG. 2
shows the principle of a technique of this kind, and uses the same type of representation as
FIG. 1
, but for two different bit rates of the incoming sequence, respectively identified by the suffices 1 and 2, in this instance for three successive symbols dn, dn+1 and dn+2 of the incoming sequence, the symbol period corresponding to Ts
1
for symbol dn and Ts
2
for symbols dn+1 and dn+2 and the code length corresponding to Q
1
for symbol dn and Q
2
for symbols dn+1 and dn+2.
Mobile radio systems using codes of variable length, typically Walsh codes, of length Q, Q/2, Q/4, etc. are therefore known per se.
These codes can be represented in the form of a tree, each code giving rise within the tree to two codes of double the length, in the manner outlined above.
FIG. 3
shows a tree of the above kind in which the codes are denoted c
Q
h
, where Q represents the length of the code and h represents the order of that code in the set of possible codes of length Q.
Accordingly, in
FIG. 3
, the various codes can be denoted as follows:
c
Q−1
h−0
=(1)
c
Q=2
h=0
(1,1)
c
Q=2
h=1
(1,−1)
c
Q=4
h=0
(1,1,1,1)
c
Q=4
h=1
(1,1−1,−1)
c
Q=4
h=2
(1,−1,1,−1)
c
Q=4
h=3
(1,−1,−1,1)
c
Q=8
h=0
(1,1,1,1,1,1,1,1)
c
Q=8
h=1
(1,1,1,1,−1,−1−1,−1)
c
Q=8
h=2
(1,1,−1,−1,1,1,−1−1)
c
Q=8
h=3
(1,1,−1,−1,−1,−1,1,1)
c
Q=8
h=4
(1,−1,1,−1,1,−1,1,−1)
c
Q=8
h=5
(1,−1,1,−1,1,−1,1,−1)
c
Q=8
h=6
(1,−1,−1,1,1,−1,−1,1)
c
Q=8
h=7
(1,−1,−1,1,−1,1,1,−1)
In a tree of the above kind the orthogonal relationship between codes of different length is preserved only on condition that certain allocation rules are complied with. Accordingly, a given code cannot be allocated at a given time if it is recursively tied to a code that has already been allocated.
Allocating orthogonal codes therefore constitutes a relatively more complex operation in systems using codes of variable length than in systems using codes of fixed length.
Thus document U.S. Pat. No. 5,751,761 teaches, on each new request for allocation of a code of given length, drawing up a list including codes already allocated at that time and codes tied to them in a recursive manner, the code to be allocated then being a chosen one of the codes not appearing in the list.
A solution of the above kind has the particular drawback that establishing a list of the above kind is a lengthy operation and entails multiple write operations, the number of which increases with the depth of the tree, i.e. with the maximum permitted code length.
OBJECTS AND SUMMARY OF THE INVENTION
A particular aim of the present invention is to avoid this drawback.
Thus in one aspect the present invention consists in a method of allocating orthogonal codes in a code division multiple access mobile radio system using codes of variable length which are generated in such a way that they can be represented in the form of a tree, said method essentially including, in response to a code allocation request, a first step of determining, in accordance with the tree capacity used, corresponding to codes already allocated, and the tree capacity required, corresponding to the code allocation requested, if such allocation is possible.
In accordance with another feature, if such allocation is found to be possible at the end of said first step said allocation method includes a second step of searching for a code to allocate.
In accordance with another feature, if such allocation is found to be possible at the end of said first step and if no code to be allocated has been found at the end of said second step said allocation method includes a third step of rearranging allocated codes within said tree.
Accordingly, in contrast to the document cited previously, it is not necessary, on each new allocation request, to draw up a complete list of all the codes that are not available, and in particular to look for all the codes tied in a recursive manner to codes already allocated. All that is required is to see if a code having the required
Gourgue Frederic
Roosen Fabienne
Alcatel
Ferris Derrick W
Marcelo Melvin
LandOfFree
Method of allocating orthogonal codes in a code division... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of allocating orthogonal codes in a code division..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of allocating orthogonal codes in a code division... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3035605