MOB sujet 4 Transmission techniques - DS CDMA

Master of Computer Science 1 - MOB Mobile Internet and Surrounding

1/10 Subject 4

MOB sujet 4

Transmission techniques - DS CDMA

1. CDMA Exercises

The principle of the spread spectrum modulation consists in coding the information by the way

of a bandwidth signal larger than the band strictly necessary, with a goal of fighting the interfering

signals and the distortions tied with the propagation.

The signal is coded at the beginning: a code is assigned with each user in order to allow

decoding in the arrival. The spread is ensured by a pseudo-random signal called spread code. At

the reception, the signal is seen as the noise if the receiver does not have the code.

The spread spectrum modulation is optimized to fight against the noise, by which it limit better

the effects.

1.0.1 Mathematical reminds

- Correlation measure the manner whose two signals vary simultaneously. A strong

correlation implies a great resemblance between the two compared signals.

- Auto-correction quantifies the correlation of a signal with a version of itself shifted in

time, according to this temporal shift.

- Cross-correlation quantifies the correlation of two shifted signals the one in comparing

with the other in time, according to this temporal shift.

- Orthogonality: two periodic signals from period T are known as orthogonal if their

cross-correlation is null for a null temporal shift.

CDMA modulation (Code Division Multiple Access) is a spread spectrum modulation. It

allows transmitting two signals simultaneously in the same band, their separation being realized at

the reception in getting profit of their orthogonality. Several mobile telephony systems (IS95,

UMTS…) have chooses this technique. The CDMA is then also used in order to manage the

multi-users aspect. Among the spread spectrum methods are:

- Frequency Hopping - CDMA (FH-CDMA), seemed to be the most interesting technique

at the beginning, but its difficult implementation took the interest back on spread spectrum

by directed sequence;

- Direct Sequence - CDMA (DS-CDMA), is used in particular in the 3G European UMTS

system (Mobile Universal Telephone System).

The principle of the DSCDMA is as follows. On each transmitting station, is allocated a

single code of m chips called "chip sequence" constituted from "-1" and from "+1". To

transmit a bit to "1", the station sends its sequence chip. To transmit a bit to “0”, the station

sends the opposite of its sequence chip. The duration Tc of transmission of a chip is m time

lower than the duration Tb of a bit.

Example: let us suppose that the chip sequence of A is "-1-11-1111-1 " (m = 8). To transmit