Multiple Access Techniques; types and their detailed study

Multiplexing Definition

In the field of telecommunication multiplexing or multiple access is the method of sharing a channel to many users. So the users get minimum or no interference.

Need of Multiplexing

Multiplexing is the basis of today’s communication networks. Multiplexing is done to avoid interference and for maximum utilisation of the channel. There is a need of multiplexing not only within a network but outside the network too. In other words communication networks can’t work at the same time within the same space without multiplexing. Let us understand this with an example.

Imagine there are three radio stations radio1, radio2 and radio3. Assume all three stations are broadcasting at 100Mhz. Radio1 broadcasting news, radio2 broadcasting an interview and radio3 broadcasting songs. If you want to listen to songs and you tune your radio device at 100MHz. My question is what will you listen? Definitely now your radio device can receive signals from all three radio stations. This will be messy and you will end up with no data or mixed data from all three stations.

To distinguish the data from all three radio stations it is necessary that all three stations should use different frequencies, codes, times or spaces.

Types of Multiple Access Techniques

Multiplexing can be done using different techniques. Some techniques are basics and some are improved and modified versions of the basic techniques. Basic techniques are given below

1. Space Division Multiple Access (SDMA)
2. Frequency Division Multiple Access (FDMA)
3. Time Division Multiple Access (TDMA)
4. Code Division Multiple Access (CDMA)

Today’s lecture is basically a comparison something like TDMA vs FDMA vs CDMA.

Space Division Multiple Access (SDMA)

In space division multiplexing different channels are separated by a large enough distance so they can’t interfere with each other. One advantage of SDMA is that we can use the same frequency multiple times. For example in the satellite communication highly directed multiple beams are sent to different countries. Each beam represents a channel and these channels are separated by spaces despite they have the same frequencies. (See fig-1)

Fig-1: Satellite Communication using SDMA

In the above figure space 1, space 2 and space 3 represent countries whereas circles represent the coverage area of the beams. The distance between the spaces is referred to as guard space. Guard space is necessary to avoid interference. This kind of guard space is also necessary in other types of multiplexing techniques.

Similarly in FM radio transmission different stations can use the same frequency around the world. Because FM broadcast region is limited. Even if one radio station provides specific services according to cities. It can use as SDMA technique. The problem arises when different radio stations work within the same city. So they have to use another multiple access technique.

The old telephone technology also uses SDMA where every subscriber is provided a separate copper wire line.

Advantages of Space Division Multiple Access

• same frequency channel can be used multiple times.
• bandwidth of the channel can be utilised fully.

Disadvantages of Space Division Multiple Access

• Multiple networks cannot work within the same space or geographical area using simultaneously.
• Two very close users can interfere with each other.

Frequency Division Multiple Access (FDMA)

Frequency division multiplexing definition describes the scheme to subdivide a bandwidth into several frequency bands called channels. Now each user can use his or her own channel for communication. This multiple access technique is very helpful if the users are communicating within the same geographical area or space.

These frequency bands are non-overlapping and a guard space is used between the adjacent frequency bands.

The various radio stations broadcasting within a city use FDMA. Each radio station has its own frequency. If someone wants to listen to a particular FM channel, he or she needs only to tune to that particular frequency of the channel.

We can use three dimensional model to differentiate FDMA, TDMA and CDMA. The fig-2 explains FDMA

Fig-2: FDMA Diagram

You can see in the above figure that there are four frequency bands f1, f2, f3 and f4. Each frequency band represents a channel. It doesn’t depend upon the time because all the channels are working at the same time but with different frequencies.

Advantages of FDMA

• It’s Simple to implement. The receiver only needs to tune to the particular frequency to receive the data.
• Many networks can work within the same space and at the same time.
• Each user has its own sub-channel , so he or she can use the full capacity of the sub-channel.
• No interference from the adjacent frequencies

Disadvantages of FDMA

• Capacity in terms of channel availability is low. After the channel being allocated to a user, the user can use the channel for a long time of his/her will. In case of high traffic the new users need to wait until the channel is free.
• Due to guard band between the adjacent frequencies, the overall bandwidth utilisation is poor because we are wasting some frequencies. I mean the guard band is not allocated to any user.

Time Division Multiple Access (TDMA)

In a typical mobile communication services TDMA is another multiplexing technique is being used. In TDMA whole channel is available to the user but for a certain amount of time. Its mean multiple users are using the same frequency but in different time slots. (See fig-3)

Fig-3: TDMA Diagram

Here the guard space in a time domain is also needed which is known as time gap. If two users overlap in time this is called co-channel interference. You can observe in the above figure that ‘user 1’ is using a frequency band in time t1, ‘user 2’ using the same band in time t2. ‘user 3’ using the same frequency band in time t3 and so on.

Both TDMA and FDMA can be combined. Its mean user can use a certain frequency band in one time slot and in second time slot the user is using another frequency band. In this scenario the time gap and the frequency guard band both are required. Another problem could come that two users have selected the same frequency at the same time. This issue can be resolved using the concept of frequency hopping.

Fig-4: Frequency Hopping

Advantages of TDMA

• One Advantage of TDMA over FDMA is that single frequency can be used by many users. So if more users are coming, you can increase the time slots and allocate them the same frequency. But remember there is always a limited number of time slots are available in every application.
• Due to time gap no interference from adjacent transmissions
• No frequency guard band is needed so you can use all the frequencies.

Disadvantages of TDMA

• Every user needs a precise synchronisation otherwise a user can create co-channel interference. On the receiver side a user must receive the data at exactly the same time.
• Because the channel is available for small amount of time, this reduces the overall data rate.

Code Division Multiple Access (CDMA)

In the code division multiple access (CDMA) each user can use the same channel but they are separated by a pseudo random codes. These codes are orthogonal to each other so the guard space is achieved in code domain between the users using the same channel simultaneously. The below fig-5 shows each user using different code and using the same frequency without time constraint.

Fig-5: Code Division Multiplexing

Receiver must know the code in order to extract the information. You can understand this by a simple example. Consider a room in which many people from different cultures and languages are talking at the same frequency (300Hz). I mean everyone is talking in his or her own language or code. Let’s say English has code1, Spanish has code2 and Persian has code3. If you want to listen Persian. Just tune your receiver at 300 Hz and use code3 to extract information available in Persian language. The English and the Spanish will be considered as background noise.

By the above example you can also observe how CDMA is a secure system. You are receiving the signal from every person but to extract information you have to know the code.

Advantages of CDMA

• More flexibility in terms of codes as compared to frequencies in FDMA system
• Only one frequency band can be used by multiple users all the time
• Codes are orthogonal to each other so Interference is minimum.

Disadvantages of CDMA

• Receiver needs to know the code and precise synchronisation is required.
• As all the users are communicating at the same frequency, a near user transmitting at higher power can damage information for other nearby users. So CDMA systems require precise power control between the users. its means a user should transmit at the minimum power it requires.

Which is better FDMA or TDMA?

This totally depends upon your needs. For example in FM transmission if you use TDMA, when one user is listening then other users are not listening. So the other users can miss data for that particular time. Because the data coming from the FM station is equally important to all the users that’s why TDMA is not suitable in FM transmission. Another aspect is that there are many radio stations working in a city. How will they synchronise with each to send data within their time slot?

But in the case of mobile communication system where higher data rate is not the priority and frequency resources are limited. You can use TDMA. For example GSM is an application of both FDMA and TDMA. FDMA divides 25 MHz bandwidth into 124 carriers.

These 124 carriers are further shared among the users by using the TDMA technique. One carrier is shared with eight users and each user can use this carrier for the duration of 0.577 millisecond. Eight such slots make one TDMA frame of duration 4.615 ms. This time is very very small that a person cannot realise the delay when talking to someone on the phone.

Which technologies use FDMA?

Technologies like GSM, LTE, Wi-Fi, cable TV, FM radio channels etc use FDMA

Are CDMA phones compatible with GSM?

GSM is a different protocol as compared to CDMA. If one phone is built for GSM it would not be compatible with CDMA unless you build a phone that can support both technologies. Samsung made some Galaxy S4 flagships that can support both GSM and CDMA . Today almost all the phones support 2G,3G and 4G technologies. Usually the companies are offering internet services over 4G LTE network and voice services either on 2G or 3G network.