Time Division Multiple Access TDMA

With TDMA we work in the time domain rather than the frequency domain of FDMA. Each user is assigned a time slot rather than a frequency segment and, during the user's turn, the full frequency bandwidth is available for the duration of the user's assigned time slot.

Let's say that there are n users and so there are n time slots. In the case of FDMA, we had n frequency segments and n radio carriers, one for each segment. For the TDMA

Transmit

Guard band

Receive

Transmit

60 kHz

Receive

60 kHz

Figure 18.7 A conceptual drawing of FDMA.

Time slot

1

2

3

4

5

N -1

N

I Tall

Payload

Training

Payload

Tail

Guard 1

| bits

sequence

bits

period 1

Figure 18.8 A typical TDMA frame.

Figure 18.8 A typical TDMA frame.

case, only one carrier is required. Each user gains access to the carrier for 1/n of the time and there is generally an ordered sequence of time slot turns. A TDMA frame can be defined as cycling through n users' turns just once.

A typical TDMA frame is illustrated in Figure 18.8. One must realize that TDMA is only practical with a digital system such as PCM or any of those discussed in Section 18.5.2. As we said in Section 9.3.5.2, TDMA is a store and burst system.

Incoming user traffic is stored in memory, and when that user's turn comes up, that accumulated traffic is transmitted in a digital burst.

Suppose there are 10 users. Let each user's bit rate be R, then a user's burst must be at least 10R. Of course, the burst will be greater than 10R to accommodate a certain amount of overhead bits as shown in Figure 18.8.

We define downlink as outbound, base station to mobile station(s), and define uplink as mobile station to base station. Typical frame periods are:

North American IS-54 40 msec for six time slots European GSM 4.615 msec for eight time slots

One problem with TDMA, often not appreciated by many, is delay. In particular, this is delay on the uplink. Consider Figure 18.9, where we set up a scenario. A base station receives mobile time slots in a circular pattern and the radius of the circle of responsibility of that base station is 10 km. Let the velocity of a radio wave be 3 x 108 m/sec. The time for the wave to traverse 1 km is 1000 m/(3 x 108) or 3.333 ^sec. In the uplink frame we have a mobile station right on top of the base station with essentially no delay and another mobile right at 10 km with 10 x 3.33 ^sec or 33.3 ^sec delay. A GSM time slot is about 576 ^sec in duration. The terminal at the 10-km range will have its time slot arriving 33.3 ^sec late compared to the terminal with no delay. A GSM bit period is about 3.69 ^sec so that the late arrival mutilates about 10 bits, and unless something is done, the last bit of the burst will overlap the next burst (Refs. 2 and 11).

Figure 18.9 A TDMA delay scenario.
1TDMA frame = 8 time slots

0 1

2 3 4 5 6

1 time slot = 156.25 bit durations

Normal Burst

Tail bits 3

Encrypted bits Training sequence 58 26

Encrypted bits 58

Tail bits 3

Guard period 8.25

Frequency Correction Burst

Tail bits 3

Fixed bits 142

Tail bits 3

Guard period 8.25

Synchronization Burst

Tail bits 3

Encrypted sync bits Extended training sequence 39 64

Encrypted sync bits 39

Tail bits 3

Guard period 8.25

Access Burst

Tail bits

Synchro sequence

Encrypted bits

Tail bits

Guard period

8

41

36

3

68.25

Figure 18.10 GSM frame and burst structures. (From Figure 8.7, Ref. 2. Reprinted with permission.)

Figure 18.10 GSM frame and burst structures. (From Figure 8.7, Ref. 2. Reprinted with permission.)

Refer now to Figure 18.10, which illustrates GSM burst structures. Note that the access burst has a guard period of 68.25 bit durations or a slop of 3.69 x 68.25 ^sec, which will well accommodate the later arrival of the 10-km mobile terminal of only 33.3 ^sec.

To provide the same long guard period in the other bursts is a waste of valuable "spectrum."6 The GSM system overcomes this problem by using adaptive frame alignment. When the base station detects a 41-bit random access synchronization sequence with a long guard period, it measures the received signal delay relative to the expected signal from a mobile station with zero range. This delay, called the timing advance, is transmitted to the mobile station using a 6-bit number. As a result, the mobile station advances its time base over the range of 0-63 bits (i.e., in units of 3.69 ^sec). By this process the TDMA bursts arrive at the base station in their correct time slots and do not overlap with adjacent ones. As a result, the guard period in all other bursts can be reduced to 8.25 x 3.69 ^sec or approximately 30.46 ^sec, the equivalent of 8.25 bits only. Under normal operations, the base station continuously monitors the signal delay from the mobile station and thus instructs the mobile station to update its time advance parameter. In very large traffic cells there is an option to actively utilize every second time slot only to cope with the larger propagation delays. This is spectrally inefficient but, in large, low-traffic rural cells, admissible (from Ref. 2).

18.6.3.1 Comments on TDMA Efficiency. Multichannel FDMA can operate with a base station power amplifier for every channel, or with a common wideband amplifier for all channels. With the latter, we are setting up a typical generator of intermodulation (IM) products as these carriers mix in a comparatively nonlinear common power amplifier. To reduce the level of IM products, just like in satellite communications discussed in Chapter 9, backoff of the power amplifier is required. This backoff can be in the order of 3-6 dB.

6We are equating bit rate or bit durations to bandwidth. One could assume 1 bit/Hz as a first-order estimate.

With TDMA (downlink), only one carrier is present on the power amplifier, thus removing most of the causes of IM noise generation. Thus with TDMA, the power amplifier can be operated to full saturation, a distinct advantage. FDMA required some guardband between frequency segments; there are no guardbands with TDMA. However, as we saw previously, a guard time between uplink time slots is required to accommodate the following situations:

• Timing inaccuracies due to clock instabilities

• Delay spread due to propagation7

• Transmission delay due to propagation distance (Section 18.6.3)

• Tails of pulsed signals due to transient response

The longer the guard times are extended, the more inefficient a TDMA system becomes.

18.6.3.2 Advantages of TDMA. The introduction of TDMA results in a much improved transmission system and reduced cost compared to an FDMA counterpart. Assuming a 25-MHz bandwidth, up to 23.6 times capacity can be achieved with North American TDMA compared to FDMA, typically AMPS (see Ref. 2, Table II.)

A mobile station can exchange system control signals with the base station without interruption of speech (or data) transmission. This facilitates the introduction of new network and user services. The mobile station can also check the signal level from nearby cells by momentarily switching to a new time slot and radio channel. This enables the mobile station to assist with handover operations and thereby improve the continuity of service in response to motion or signal fading conditions. The availability of signal strength information at both the base and mobile stations, together with suitable algorithms in the station controllers, allows further spectrum efficiency through the use of dynamic channel assignment and power control.

The cost of base stations using TDMA can be reduced if radio equipment is shared by several traffic channels. A reduced number of transceivers leads to a reduction of multiplexer complexity. Outside the major metropolitan areas, the required traffic capacity for a base station may, in many cases, be served by one or two transceivers. The saving in the number of transceivers results in a significantly reduced overall cost.

A further advantage of TDMA is increased system flexibility. Different voice and nonvoice services may be assigned a number of time slots appropriate to the service. For example, as more efficient speech CODECs are perfected, increased capacity may be achieved by the assignment of a reduced number of time slots for voice traffic. TDMA also facilitates the introduction of digital data and signaling services as well as the possible later introduction of such further capacity improvements as digital speech interpolation (DSI).

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Responses

  • abraham
    Why frame period in tdma is integral multiple of 125usev?
    1 year ago

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