Multiple Access Techniques In Cellular Systems

Hi this is an article about the Multiple Access Techniques used in the cellular systems.

                                                                                                                                                                The goal in the design of a cellular system is to be able to handle as many calls as possible in a given bandwidth with the specifi ed blocking probability (reliability).

Multiplexing deals with the division of the resources to create multiple channels. Multiplexing can create channels in frequency, time, etc., and the corresponding terms are then frequency division multiplexing (FDM), time division multiplexing (TDM), etc. Since the amount of spectrum available is limited, we need to fi nd ways to allow multiple users to share the available spectrum simultaneously. Shared access is used to implement a multiple access scheme when access by many users to a channel is required.

For example, one cancreate multiple channels using TDM, but each of these channels can be accessedby a group of users using the ALOHA multiple access scheme. The multiple access schemes can be either reservation-based or random.

Multiple access schemes allow many users to share the radio spectrum. Sharing the bandwidth effi ciently among users is one of the main objectives of multiple access schemes.

The variability of wireless channels presentsboth challenges and opportunities in designing multiple access communicationssystems. Multiple access strategy has an impact on robustness and interference levels generated in other cells. Therefore, multiple access schemes are designed to maintain orthogonality and reduce interference effects.

Multiple access schemes can be classifi ed as reservation-based multiple access (e.g., FDMA, TDMA, CDMA) and random multiple access (e.g., ALOHA, CSMA) . If data traffi c is continuous and a small transmission delay is required (for example in voice communication) reservationbased multiple access is used. The family of reservation-based multiple access includes frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In many wireless systems for voice communication, the control channel is based on random multiple access and the communication channel is based on FDMA, TDMA, or CDMA.

The reservation-based multiple access technique has a disadvantage in that once the channel is assigned, it remains idle if the user has nothing to transmitwhile other users may have data waiting to be transmitted. This problem is critical when data generation is random and has a high peak-rate to average-rate ratio. In this situation, random multiple access is more effi cient, because a communication channel is shared by many users and users transmit their data in a random or partially coordinated fashion.

ALOHA and carrier sense multiple access (CSMA) are examples of random multiple access. If the data arrives in a random manner, and the data length is large, then random multiple access combined with a reservation protocol will perform better than both random- and reservationbased schemes.