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Time Division Multiple Access (TDMA)

In TDMA, the entire radio spectrum is divided into time slots and in each slot only one user is allowed to transmit or receive. Each user gets a cyclically repeating slot. When one user is transmitting it’s information, the other users have to buffer their data i.e. TDMA users have to transmit data in a buffer and burst method, thus the transmission for any user is non-continuous.

Figure 3 : Illustration of TDMA

As shown in Figure 3, A particular user gets time slots which are non continuous in nature. The next time slot is assigned to him/her only when the other users sharing the same spectrum have each done one burst of transmission atleast. Collection of time slots which are assigned to unique users is called as a Frame. In other words, a user gets only one time slot per frame to transmit. In TDMA/TDD, half of the time slots in the frame would be used for forward link and the other half would be used for reverse link. In TDMA/FDD systems, since two separate physical channels exist, transmit and receive frames are different and on different carrier frequencies.

The technical definition of the TDMA system is given as,

“TDMA is an assigned frequency band shared among a few users. However, each user is allowed to transmit in predetermined time slots. Hence, channelization of users in the same band is achieved through separation in time. (Qualcomm, 1997)”

Features:

1. TDMA systems divides the radio spectrum into time slots, and in each time slot only one user is allowed to either transmit or receive. TDMA effectively shares a single carrier frequency with several users.
2. Transmission for any user is non-continuous, bursty in nature. This reduces the battery consumption if the subscribers transmitter can be switched off when not in use.
3. In TDMA, the handoff process is much simpler as transmission mode is discontinuous. The mobile can work in slotted mode and is able to monitor the RSSI of other Base Stations when it is idle and not transmitting/receiving anything. Thus MAHO (Mobile Assisted Hand Off) can be implemented easily.
4. TDMA uses different time slots for transmission and reception, thus duplexers are not required. Even if FDD is used, a switch rather than a duplexer inside the subscriber unit is all that is required to switch between transmitter and receiver using TDMA.
5. A TDMA frame consists of data for many users. Hence, guard times are necessary to separate users. TDMA system requires high synchronization overhead bits due to bursty transmission. In each TDMA frame, the preamble contains the address and synchronization information. Thus, TDMA systems have larger overheads as compared to FDMA.
6. TDMA could allocate varied number of time slots per frame to different users.
7. In TDMA, the allotted spectrum is not divided into narrow channels. Hence , during propagation, the TDMA channel bandwidth is comparatively more than the coherence bandwidth of the medium. This results in frequency selective fading. Hence, Equalization is necessary on the receiver side.

Efficiency of a TDMA FRAME

The frame efficiency is the percentage of bits per frame which contains useful transmitted data. It is a measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme. It should be noted here that the transmitted data may include source and channel coding bits, so the raw end-user efficiency of a system is generally less than the calculated efficiency of a frame.

Frame efficiency can be expressed as,

$$\eta_f = (1 -\frac{b_{OH}}{b_T}) \times 100 \%$$.

Where, $b_{OH}$: Number of overhead bits, and

$b_{T}$ : Total number of bits.

Number of channels In TDMA system:

lt can be found by multiplying the number of TDMA slots per channel by the number of channels available. It is given as,

$$N = \frac{m(B_{tot} -2B_{guard}}{B_{c}})$$

Where, $m :$ The maximum number of TDMA users supported on each radio channel.

$B_{tot}$ : Total bandwidth

$B_{guard}$ : Guard band

$B_{c}$ : One channel's bandwidth

Example:

If a normal GSM time slot has 40.25 overhead bits and 116 data bits. Calculate the frame or time slot efficiency.

Solution:

$\eta_f = (1 -\frac{b_{OH}}{b_T}) \times 100 \%$

$\eta_f = (1 -\frac{40.25}{116 + 40.25}) \times 100 \%$

$= 74.24 \%$

Figure 4: (a) Ilustration of FDMA/TD/A/FDD system (b) Illustration of TDMA/TDD system

Figure 4(a) shows an FDMA/TDMA/FDD system used widely in 2G systems. The spectrum is divided into frequency channels (FDMA). Each channel is divided into time slots ( TDMA) . Each user gets one time slot from the uplink channels to transmit and one timeslot from downlink channels to receive (FDD). More details will be discussed in GSM technology.

Figure 4(b) shows an implementation of FDMA/TDMA/TDD systems. It was used in the PAN –European digital PCS standard DEC. As, the distances are short, a TDD format allows using the same frequency for forward and reverse operation.

Depending on how the available bandwidth is allocated to the users, FDMA and TDMA techniques can be classified as narrowband and wideband systems.

Narrowband Systems

Systems operating with channels substantially narrower than the coherence bandwidth are called as Narrowband systems. Duplexing techniques normally used in these systems are FDD. These types of systems are called as FDMA/FDD. To minimize the interference between the forward and reverse channels, the frequency separation between the two is made as large as possible. Narrowband TDMA allows users to use the same channel but allocates a unique time slot to each user on the channel, thus separating a small number of users in time on a single channel. These systems can use either TDD or FDD and each channel is shared using TDMA. Such systems are called TDMA/TDD or TDMA/FDD.

Wideband Systems

In wideband systems, the transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel. Thus, multipath fading doesn’t greatly affect the received signal within a wideband channel, and frequency selective fades occur only in a small fraction of the signal bandwidth. Most of the TDMA and CDMA systems which allow many users in the same bandwidth, fall under this category. Either FDD or TDD duplexing techniques can be used.