CDMA spread spectrum basics

- overview, information or tutorial about CDMA spread spectrum and the way that the system operates.

CDMA is based around the use of direct sequence spread spectrum techniques. Essentially CDMA is a form of spread spectrum transmission which uses spreading codes to spread the signal out over a wider bandwidth then would normally be required. By using CDMA spread spectrum technology, many users are able to use the same channel and gain access to the system without causing undue interference to each other.

Although as the number of users increases care has to be taken to ensure that interference levels do not rise to the extent that performance falls, it is still possible to provide access to a large number of different users and allow them access.

CDMA spread spectrum basics

The key element of code division multiple access CDMA is its use of a form of transmission known as direct sequence spread spectrum, DSSS.

Direct sequence spread spectrum is a form of transmission that looks very similar to white noise over the bandwidth of the transmission. However once received and processed with the correct descrambling codes, it is possible to extract the required data.

When transmitting a CDMA spread spectrum signal, the required data signal is multiplied with what is known as a spreading or chip code data stream. The resulting data stream has a higher data rate than the data itself. Often the data is multiplied using the XOR (exclusive OR) function.

CDMA spreading technique

CDMA spreading

Each bit in the spreading sequence is called a chip, and this is much shorter than each information bit. The spreading sequence or chip sequence has the same data rate as the final output from the spreading multiplier. The rate is called the chip rate, and this is often measured in terms of a number of M chips / sec.

The baseband data stream is then modulated onto a carrier and in this way the overall the overall signal is spread over a much wider bandwidth than if the data had been simply modulated onto the carrier. This is because, signals with high data rates occupy wider signal bandwidths than those with low data rates.

CDMA spread spectrum generation

CDMA spread spectrum generation

To decode the signal and receive the original data, the CDMA signal is first demodulated from the carrier to reconstitute the high speed data stream. This is multiplied with the spreading code to regenerate the original data. When this is done, then only the data with that was generated with the same spreading code is regenerated, all the other data that is generated from different spreading code streams is ignored.

CDMA spread spectrum decoding

CDMA spread spectrum decoding

The use of CDMA spread spectrum is a powerful principle and using this CDMA technique, it is possible to transmit several sets of data independently on the same carrier and then reconstitute them at the receiver without mutual interference. In this way a base station can communicate with several mobiles on a single channel. Similarly several mobiles can communicate with a single base station, provided that in each case an independent spreading code is used.

CDMA spread spectrum encode / decode process

In order to visualise how the CDMA spread spectrum process operates, the easiest method is to show an example of how the system actually operates in terms of data bits, and how the data is recovered from the CDMA spread spectrum signal.

The first part of the process is to generate the CDMA spread spectrum signal. Take as an example that the data to be transmitted is 1001, and the chip or spreading code is 0010. For each data bit, the complete spreading code is used to multiple the data, and in this way, for each data bits, the spread or expanded signal consists of four bits.

1 0 0 1 Data to be transmitted
0010 0010 0010 0010 Chip or spreading code
1101 0010 0010 1101 Resultant spread data output

With the signal obtained and transmitted, it needs to be decoded within the remote receiver:

1101 0010 0010 1101 Incoming CDMA signal
0010 0010 0010 0010 Chip or spreading code
1111 0000 0000 1111 Result of de-spreading
1 0 0 1 Integrated output

NB: 1 x 1 = 0     1 x 0 = 1

In this way it can be seen that the original data is recovered exactly by using the same spreading or chip code. Had another code been used to regenerate the CDMA spread spectrum signal, then it would have resulted in a random sequence after de-spreading. This would have appeared as noise in the system.

The spreading code used in this example was only four bits long. This enabled the process to be visualised more easily. Commonly spreading codes may be 64 bits, or even 128 bits long to provide the required performance.

CDMA spreading gain

The bandwidth of the CDMA spread spectrum signal will be much wider than the original data stream. To quantify the increase in bandwidth, a term known as the spreading gain is used. If the bandwidth of the CDMS spread spectrum signal is W and the input data bit length or period 1/R then the CDMA spreading gain can be defined:

Spreading gain   =   W   /   R

It is found that the larger the spreading gain of the CDMA spread spectrum signal, the more effective the performance of the system is. This is because the wanted signal becomes larger. In the example shown above, the spreading gain is four, as seen by the fact that four "1"s are generated for each required data bit. Data produced by other dispreading codes would appear as noise and can be discarded as it would be lower in value.

The principle behind CDMA spread spectrum communications is relatively straightforward. The same code must be sued within generation and decoding of the CDMA spread spectrum signal to enable the data to pass unchanged through the system. The use of a different code in transmission and reception results in a signal similar in character to noise being generated and this can be discarded.

By Ian Poole

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