CDMA CONCEPTS-2

Spread Spectrum Technique

All of the modulation and demodulation techniques strive to achieve greater power and/or bandwidth efficiency in a stationary additive white Gaussaian noise channel. Since bandwidth is a limited resource, one of the primary design objectives of all modulation schemes is to minimize the required transmission bandwidth. Spread spectrum techniques on the other hand employ a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. While this system is very bandwidth inefficient for a single user, the advantage of spread spectrum technique is that many users can simultaneously use the same bandwidth without significantly interfering with one another. In a multiple-user multiple access interference environment, spread spectrum systems becomes very bandwidth efficient.

There are two types of spread spectrum techniques are used

* Direct Sequence
* Frequency Hopping.

Direct Sequence is adopted by CDMA
Direct Sequence Spread Spectrum

1 chip period

The base band data since is multiplied by a PN code, which is multiplication of each bit of base band data with the PN code, will result into a spread bandwidth but the spectral power density remains the same, because before spreading the energy was concentrated in a small bandwidth, now it is spread over a large bandwidth. Now when this spread signal is transmitted it is received at the receiver, with signal from other user of the same spectrum, which became noise for this noise, plus interference from other sources. The receiver in these systems will have a correlate, which use the replica of the PN code to extract the same information.

Advantages of Spread Spectrum

1. As the signal is spread over a large frequency band, the power spectral density is getting very small, so other communications systems do not suffer from this kind of communications. However the Gaussian Noise level is increasing.
2. Multiple accesses can be deal with, as large number of codes can be generated, which will permit a large number of users.
3. The maximal number of users is not spectrum or resource limited, like other access systems like FDMA, here they are only interference limited.
4. Security: without knowing the spreading code, it is nearly impossible to recover the transmitted data.
5. Fading rejection: as large bandwidth is used the system is less susceptible to distortions.