Multiple access refers to multiple users accessing the same medium for their transmission needs. In any cellular system or cellular technology, it is necessary to have a scheme that enables several multiple users to gain access to it and use it simultaneously. As cellular technology has progressed different multiple access schemes have been used. They form the very core of the way in which the radio technology of the cellular system works.
There are four main multiple access schemes that are used in cellular systems ranging from the very first analogue cellular technologies to those cellular technologies that are being developed for use in the future.
Broadly, all Multiple Access schemes can be classified into:
- SDMA (Space Division Multiple Access)
- FDMA (Frequency Division Multiple Access)
- TDMA (Time Division Multiple Access)
- CDMA (Code Division Multiple Access)
- A combination of these, like TDMA/FDMA is also possible.
Requirements for a multiple access scheme
In any cellular system it is necessary for it to be able have a scheme whereby it can handle multiple users at any given time. There are many ways of doing this, and as cellular technology has advanced, different techniques have been used.
There are a number of requirements that any multiple access scheme must be able to meet:
- Ability to handle several users without mutual interference.
- Ability to be able to maximise the spectrum efficiency
- Must be robust, enabling ease of handover between cells.
Space-division multiple access (SDMA)
Multiple access is made possible by segmenting the physical space, and using each region for a different communication. In traditional mobile cellular network systems, the base station has no information on the position of the mobile units within the cell and radiates the signal in all directions within the cell in order to provide radio coverage. This results in wasting power on transmissions when there are no mobile units to reach, in addition to causing interference for adjacent cells using the same frequency, so called co-channel cells. Likewise, in reception, the antenna receives signals coming from all directions including noise and interference signals. By using smart antenna technology and differing spatial locations of mobile units within the cell, space-division multiple access techniques offer attractive performance enhancements. The radiation pattern of the base station, both in transmission and reception, is adapted to each user to obtain highest gain in the direction of that user. This is often done using phased array techniques.
Space division can be in the form of:
- Cells separated from each other, and using the same frequency (frequency reuse principle of cellular networks)
- Sectors around a common point, and the use of directed antennas.
- Generally used in combination with FDMA/TDMA/CDMA.
Frequency Division Multiple Access (FDMA)
FDMA is the most straightforward of the multiple access schemes that have been used. As a subscriber comes onto the system, or swaps from one cell to the next, the network allocates a channel or frequency to each one. In this way the different subscribers are allocated a different slot and access to the network. As different frequencies are used, the system is naturally termed Frequency Division Multiple Access. This scheme was used by all analogue systems.
Multiple access is made possible by dividing the available frequency spectrum, and allotting each band for a different communication channel.
The frequency allocation can be:
- Permanent (Ex: Radio broadcast stations)
- Temporary (Ex: Cellular networks)
In the olden days, PSTN used FDM to combine messages on its trunk lines.
Time division multiple access (TDMA)
The second system came about with the transition to digital schemes for cellular technology. Here digital data could be split up in time and sent as bursts when required. As speech was digitised it could be sent in short data bursts, any small delay caused by sending the data in bursts would be short and not noticed. In this way it became possible to organise the system so that a given number of slots were available on a give transmission. Each subscriber would then be allocated a different time slot in which they could transmit or receive data. As different time slots are used for each subscriber to gain access to the system, it is known as time division multiple access. Obviously this only allows a certain number of users access to the system. Beyond this another channel may be used, so systems that use TDMA may also have elements of FDMA operation as well.
- Each user is allotted a time-slot during which an assigned frequency can be used to transmit the messages between a sender and a receiver.
- Different users use the same frequency in their respective time-slots.
- Time-slot allocation can be a fixed pattern or demand driven.
- Time synchronization is important; Guard time used.
- Modern day PSTN uses TDM to combine messages on its trunk lines.
Code Division Multiple Access (CDMA)
CDMA uses one of the aspects associated with the use of direct sequence spread spectrum (DSSS). It is therefore possible to allocate different users different codes, and use this as the means by which different users are given access to the system.
The scheme has been likened to being in a room filled with people all speaking different languages. Even though the noise level is very high, it is still possible to understand someone speaking in your own language. With CDMA different spreading or chip codes are used. When generating a direct sequence spread spectrum, the data to be transmitted is multiplied with spreading or chip code. This widens the spectrum of the signal, but it can only be decided in the receiver if it is again multiplied with the same spreading code. All signals that use different spreading codes are not seen, and are discarded in the process. Thus in the presence of a variety of signals it is possible to receive only the required one.
In this way the base station allocates different codes to different users and when it receives the signal it will use one code to receive the signal from one mobile, and another spreading code to receive the signal from a second mobile. In this way the same frequency channel can be used to serve a number of different mobiles.
- Each user uses the entire frequency band all the time. There is no demarcation in time or in frequency.
- However, each user is allotted an unique code, which is used to encode the data before transmission.
- The binary codes belong to a class of codes known as Orthogonal codes.
- In order to decode the message, the receiver has to know the code used by the transmitter.
CDMA – The technique
- Encoding the data before transmission, using an unique code, is called Spread Spectrum technique.
- Spread Spectrum systems spread the spectrum of the baseband (pulse) data over a much wider range.
- Even though a signal mixes with other signals in time and frequency, it can still be extracted by using the code.
Orthogonal Frequency Division Multiplexing (OFDM)
Is a special case of multicarrier transmission, where a single datastream is transmitted over a number of lower rate subcarriers. In July 1998, the IEEE standardization group decided to select OFDM as the basis for their new 5-GHz standard, targeting a range of data stream from 6 up to 54 Mbps. This new standard is the first one to use OFDM in packet-based communications, while the use of OFDM until now was limited to continuous transmission systems. In this project, transmitter and receiver were simulated according to the parameters established by the standard, to evaluate the performance and different possibilities in the implementation. Also, some considerations about forward error correction coding, synchronization and channel estimation are given oriented to improve the system performance.