TDMA: Time Division Multiple Access

TDMA stands for Time division multiple access. It is mainly a channel access method that is used for shared medium networks as well as a digital transmission technology that allows its users to access a single radio-frequency channels without any interference . and it allocates to its users unique time slots within each channel.TDMA is the widespread type of service that is used in North America.Mainly .TDMA is used in 2G cellular system(GSM,PDC,IDEN,IS-136,DECT) . In this case a single channel is divided into six time slots and each signal is using two time slots . To make the transmission possible each caller is assigned a specific time slot. TDMA offers high performance with respect to capacity, courage and mobility.TDMA mainly transmit data in a buffer-and-burst method and the transmission of any user is non- continuous.

TDMA Frame Structure

In TDMA frame structure a data stream divided into frames and those frames divided into time slots.

and we explain the concept of TDMA frame structure with the help of an example:-

In this case there is the TDMA frame shown which use seven set of frequencies that are sufficient to cover the large area that provides the maximum distance d that is larger than twice the maximum radius r , the radius that is being covered by each transmitter.and the length of each time slots that are generated from the frequency channels is 0.577ms and these 8 slots make a TDMA frame of length 4.615ms.

There are main three burst present in the TDMA:-

1. Normal Burst(NB)
2. Frequency Correction Burst(FB)
3. Synchronization Burst(SB)
4. Access Burst(AB)

Normal Burst:- The transmission of the data during the single time slot is known as a burst.and the guard time that is used to prevent the burst suffering from the overlapping is 8.25 bits.

In the Normal Burst there are four types of bits as given below:-

1. Tail Bits :- In the diagram of TDMA there is 3 bits at both of the two ends , in which there is no transmission of the data.At the beginning the the bit is used to rise up the power upto its peak during the transmission and the end it is used to down the power used at the end of the transmission.
2. Data Bit:- Around the training bits there are two data bits each of 57 bits.
3. Stealing Flags :- These are the flags around the training bits that are of length 1 bit and it is used to indicate that whether the burst is being used for voice or the data (set to 0)or whenever it is used for signalling (set to 1 ).
4. Training Sequence :- In the training bits a method is being used to overcome the problem of multi-path fading and the other propagation effects .and in this 26 bits are used.

Frequency Correction Burst:- FCB is mainly an unmodulated carrier which then after shift in frequency . The guard time used in the FCB is same as that of the normal bit . It is used for frequency synchronization of the mobile stations.

Synchronization Burst:- The time synchronization of the mobile is possible with the help of synchronization burst .The payload used in this carries the FDMA frame number (FN) and Base Station Identity Code (BSIC). and the broadcasting in this case is with the frequency correction burst.and also broadcasting occurs on the synchronization channel.

Access Burst:- Firstly the guard burst used in this very longer as compared to the normal burst , so it is 68.25bits . The mobile stations use this type of the burst for random access. It also helps to measure the unknown distance of any mobile station from the tower , when the MS wants to access to the new BTS.

Features of TDMA

1. In case of TDMA a single carrier frequency is shared among several users. and each user use the time slots. The time slots used by the users depends on several factors such as modulation techniques and the available bandwidth.
2. Data Transmission in the case of the users of TDMA is not continuous but it occurs in bursts which results in low use of the battery consumption because the subscriber is turned off , when it is not in the use.
3. The Handoff process is much simpler in the case of TDMA because of its discontinuity and it is able to listen to the other base stations during its idle slot time.
4. The Transmission rates are very high in the case of TDMA as compared to the FDMA because of the Adaptive Equalization which is necessary in the case of TDMA.
5. The guard time present in the case of TDMA should be minimum.
6. The main advantage of TDMA is that it is very cost-effective technology which is used for upgrading a current analog system to digital.
7. In this the battery used is of extended life and the talk time , since the mobile is only transmitting the portion of the time of the time during conversations.
8. TDMA technology is used to separate users in time and ensure that there will not be any interference which is presented from any other simultaneous transmissions.

Disadvantages of TDMA

The main disadvantages of TDMA are given as below:-

1. The main disadvantage of the TDMA is that each user that is used in the TDMA has a predefined time slot that is defined by the system and if the users roaming from one cell to another cell or from one place to any other place and are not alloted a time slot or if all the time slot available for the user are already occupied , then the user will not receive any call.
2. The another main disadvantage of TDMA systems is interference that is created by it at a frequency which is directly connected to the time slot length.In this case or problem with the TDMA is the multiple distortion , so whenever the signal coming from one place or tower to another tower, then there are many buildings that create problem in the signal for arriving at the destination because of the interference.
3. Efficient time schedule that is possible in the case of TDMA but it is not always easy to find it.
4. Clock synchronization needed.
5. Whenever there is the case of dynamic status changes of nodes on the scheduling is also handled by it.
6. The channel utilization in the case of TDMA is very low as compared to CSMA during low contention.

TDMA in Mobile Phone Systems

2G Systems:- Many 2G Systems are based on the TDMA.So we can say the TDMA as TDMA cellular systems which include the main examples of the TDMA cellular systems are GSM, IS-95, D-AMPS, PDC,iDEN and PHS. As in the case of GSM it combines the TDMA with frequency hopping and the wideband transmission , so as to minimize the interference .So in the case of GSM system,base station send the timing commands which is then helpful in the synchronization of the mobile phones which are helpful in easier transmission and decide the time .There is the guard interval in the case of GSM.There is the RACH in GSM. Whenever the mobile is in the next base ststion , then the call arrived sucessfully without any delay.and if the mobile is located beyond the 35 km range in GSM, then the RACH used in this will arrive in the time slot that is in the neighboring.So there is no special technique to limit the range of the GSM cell to 35 km .So to overcome this problem , the synchronization is changed between the uplink and downlink in the base station.

3G Systems:- 3G system use the combination of both CDMA and TDMA . So it take the advantage of both the techniques.The most popular 3G systems are UMTS which use CDMA rather then TDMA, and in TD-CDMA and TD-SCDMA , the TDMA is combined with CDMA and TDD.

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Comparison Between GSM,TDMA and CDMA

TDMA

Firstly we all know that the TDMA become a technology of the past.In the case of TDMA the data is compressed in to packets within its same bandwidth.Some important things about the TDMA are given as below:-

1. It act as a bridge between the analog and the digital technologies which led to the development of the GSM and iDEN.

2. iDEN:- It is a Integrated Digital Enhanced Network and it is similar to as that of the GSM technology and it is operating at 800 MHz,1900MHz and 1.5 GHz in the US. It is used to handle the CDMA and GSM technology.It can handle three users per channel and for the purpose of more capacity and demand , it is require the use of more towers which is costly.

3. The Data capacity in the case of TDMA is is around the 56 kbps.

GSM

1. GSM stands for Global System for Mobile Communication uses a narrow – band modification of TDMA.
2. GSM can handle up to eight users per channel.
3. GSM mainly operates on 1900MHz.
4. GSM is incapable of the reliable data speeds which requires the alternate technologies.
5. GSM is more widespread around the world

CDMA

CDMA stand s for Code Division Multiple Access which is a technology that sent as a constant flow across larger bandwidth instead of sending a signal into packets and sent on cycles, CDMA transmissions.

1. CDMA can handle ten users per channel, but it require more frequency to provide service.
2. In the US, CDMA uses the standard 800MHz and 1900MHz.
3. The data capacity in the CDMA,is fastest, which is equal to the 144 kbps.