Why does wireless communication system need multiple access techniques?

The radio channel is a communication medium shared by many subscribers in one cell. Mobile stations compete with one another for the frequency resource to transmit their information streams. Without any other measures to control simultaneous access of several users, collisions can occur (multiple access problem). Since collisions are very undesirable for a connection-oriented communication like mobile telephony, the individual subscribers/mobile stations must be assigned dedicated channels on demand. In order to divide the available physical resources of a mobile system, i.e. the frequency bands, into voice channels, special multiple access procedures are used which are presented as below.

Multiple Access

For cellular communications, a change in generation has generally meant a change in the multiple access scheme that is implemented. The first generation of cellular systems used frequency division multiple access (FDMA); the majority of second generation systems use time division multiple access (TDMA) and most of the third generation schemes use code division multiple access (CDMA). In addition, a shift has been made from the original analogue system to a digital communications system.

Also see What are frequency division duplex and Time Division Duplex in telecommunication

Frequency Division Multiple Access (FDMA)

A wireless system has the resource of frequency to share among many users. The first approach to solving this problem is to split the available frequency into a number of channels, each with a narrow slice of the frequency.

Frequency Division Multiple Access scheme (FDMA)

Each user in the system that wishes to communicate is allocated a frequency channel, and each channel has a certain gap, known as a guard band, between it and the next channel so that the two do not interfere with each other. Once all the channels are in use, a new user to the system must wait for a channel to become free before communication can commence.
Therefore, the system is limited in capacity as it can only support as many simultaneous users as there are channels.This is known as a hard capacity system.
Another problem is that if there is any external interference at a particular frequency, then a whole channel may be blocked.

The concept of FDMA can be considered in the context of radio broadcasting. There is a certain allocation of frequency resources, for example 88 MHz to 108 MHz for FM, and each radio station in a particular region is given one channel within this on which it transmits.

Time Division Multiple Access (TDMA)

As wireless communications systems are expected to support more and more simultaneous users, there are clearly severe limitations with the FDMA scheme. A more efficient channel usage is required. With TDMA, a frequency channel is divided up into a number of slices of time.

Time Division Multiple Access scheme (TDMA)

A user is allocated a particular time slot, which repeats periodically. In the diagram, the frequency is split into six time slots; a user is allocated one slot in every six. Providing that the time slices are small enough and occur frequently enough, a user is oblivious to the fact that they are only being allocated a discrete, periodic amount of time. In this manner, the capacity can be dramatically increased and hence the efficiency of our system. Again, this is referred to as a hard capacity type network.

Code Division Multiple Access (CDMA)

The above two multiple access schemes are considered in terms of efficiency, each of them involves only one user transmitting on a particular channel at a particular time, which is clearly inefficient. For example, with GSM, in a given cell, only one user is transmitting at any time; all other active users are waiting for their time slot to come around. If a mechanism could allow more than one user to transmit at a time; then the resource usage could be dramatically improved. CDMA is such a scheme, where all users are transmitting at the same frequency at the same time. The effect of interference that users cause to each other is discussed under the heading of noise. Having a system that is limited by a noise target rather than specifically allocating resources for the sole use of a particular mobile device is known as a soft capacity system.
CDMA is part of a general field of communications known as spread spectrum. Spread spectrum describes any system in which a signal is modulated so that its energy is spread across a frequency range that is greater than that of the original signal and it is the codes that perform this spreading function, and also allow multiple users to be separated at the receiver. Most common form used in CDMA ( Frequency Hopping and Direct sequence).

Code Division Multiple Access scheme (CDMA)