Code Division Multiple Access

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  • Topic: Mobile phone, Code division multiple access, Spread spectrum
  • Pages : 11 (3269 words )
  • Download(s) : 180
  • Published : April 27, 2013
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Code Division Multiple Access

INTRODUCTION:-

Mobile communications are rapidly becoming more and more necessary for everyday activities. With so many more users to accommodate, more efficient use of bandwidth is a priority among cellular phone system operators. Equally important is the security and reliability of these calls. One solution that has been offered is a CODE DIVISION MULTIPLE ACCESS SYSTEM. CDMA is one method for implementing a multiple access communication system. MULTIPLE ACCESS is a technique where many subscribers or local stations can share the use of the use of a communication channel at the same time or nearly so despite the fact originate from widely different locations. A channel can be thought of as merely a portion of the limited radio resource, which is temporarily allocated for a specific purpose, such as someone’s phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how the channels are allocated to the many users of the system. Since there are multiple users transmitting over the same channel, a method must be established so that individual users will not disrupt one another. There are essentially three ways to do this.

1. FREQUENCY DIVISION MULTIPLE ACCESS
In this technique, the available bandwidth is split up into non-overlapping frequency bands and these disjoint sub bands of frequency are allocated to the different users on a continuous time basis. In order to reduce interference between users allocated adjacent channel bands, channel bands are used to act as buffer zones, as illustrated in figure(1). These guard bands are necessary because of the impossibility of achieving ideal filtering for separating the different users. It could be compared to AM or FM broadcasting radio where each station has a frequency assigned.

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2. TIME DIVISION MULTIPLE ACCESS
In this technique, each user is allocated the full spectral occupancy of The channel, but only for a short duration of time called time slot. Buffers zones are in the form of guard times are inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arises due to system imperfections, especially in synchronization scheme.

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DRAWBACKS:
In both FDMA and TDMA, only one subscriber at a time is assigned to a channel. No other conversion can access this channel until the subscriber’s call is finished or until that original call to handed off to a different channel by the system. Voice data tends to be burst in nature. So much of the time, no data is being sent over the channel. This inefficiency tends to limit the capacity of the system.

3. CODE DIVISION MULTIPLE ACCESS
The above drawbacks are overcome in this third technique in which the users are spread across both frequency and time in the same Channel. This is a hybrid combination of FDMA and TDMA. For example, frequency hopping may be employed to ensure during each successive time slot, the frequency bands assigned to the users are recorded in random manner. During time slot 1, user 1 occupies frequency band 1, user 2 occupies frequency band 2, user 3 occupies band 3 and so on. During time slot 2, user 1 hops to frequency band 3, user 2 hops to band 1, user 3 hops to band 2, and so on. An important advantage of CDMA over FDMA and TDMA is that it can provide for secure communication.

MEANING OF CDMA:

Here, the users are spread across both frequency and time in the same channel. Here, unique digital codes, rather than separate RF frequencies or channels are used to differentiate subscribers. The codes are shared by both the mobile stations (cellular phone) and the base station, and are called “pseudo random code sequences” or “pseudo-noise code sequences”.

PN – SEQUENCE:

A PN – sequence...
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