CDMA method with increased capacity

DETAILED DESCRIPTION The invention remedies the problem of this limit of messages if the encoding sequences are N bits long.
or N first messages the invention uses orthogonal encoding sequences of N bits.
This is easy if the number of messages is less than or equal to N.
For additional messages, random or pseudo-random (PN) encoding sequences are used that interfere with each other and with the orthogonal sequences.
The reception process preferably involves a number of steps.
The message signals encoded using orthogonal sequences and those using PN sequences are separated by separate decoding.
The proposed multiple access technique can thus accommodate N users without any mutual interference, or a greater number of users at the cost of a slightly degraded signal to noise ratio.
However, as shown in the remainder of the description, the effects of such noise can be neutralized by channel encoding methods already in use which provide transmission redundancy.
It is then as if M users could together use a channel theoretically limited to N users, when M is greater than N (in one example, M can be in the order of N plus 25% of N).
The invention therefore consists in a spread spectrum transmission method, in particular of the CDMA type, for transmitting a plurality of messages each containing a series of symbols, in which method: each symbol to be transmitted is encoded using a sequence of N bits to obtain for each symbol a sequence of N contributions known as chips, the chips of the various messages are combined to obtain a transmitted signal, and the transmitted signal is received, characterized in that: if N symbols at most are transmitted simultaneously, orthogonal sequences are chosen for the sequences of bits used, if M symbols are transmitted simultaneously, where M is greater than N, N orthogonal sequences of N bits are chosen for N symbols and random or pseudo-random sequences of N bits are chosen for the remaining M-N symbols, a sequence of N bits being random or pseudo-random if for subsequent symbols of the same message to be transmitted the sequences of N bits are reproduced only with a period greater than N