TDMA dan Kapasitasnya (pada GSM)

TDMA adalah teknik multiple access yang membagi frekuensi ke dalam time slot

TDMA

Overview

Frequency bands

GSM 900channels 0 – 124, 955 – 1023

876.2 to 914.8 MHz (uplink),921.2 to 959.8 MHz (downlink)

GSM 1800channels 512 – 885

1710.2 to 1784.8 MHz (uplink),1805.2 to 1879.8 MHz (downlink)

GSM 1900 (Americas)channels 512 – 810

1850.2 to 1909.8 MHz (uplink),1930.2 to 1989.8 MHz (downlink)

GSM 850 (Americas)channels 128 – 251

824.2 to 848.8 MHz (uplink),869.2 to 893.8 MHz (downlink)

Channel spacing 200 kHz

Modulation GMSK (Gaussian Minimum Shift Keying), phase shifts of π/2 (90°) on symbol transitions

Symbol rate 270.833 symbols/s

Bits per symbol 1

Access format TDMA (time-division multiple access)TDD/FDD (time and frequency division duplex)

Number of channels per carrier 8 time slots per TDMA frame (4.615 ms)148 bits per time slot, equivalent to 577 µs

Speech Digital transmission with one out of several possible vocoders:Full rate speech (13 kbit/s)RPE-LTP (regular pulse excited code with long term prediction)Enhanced full rate speech (12.2 kbit/s)ACELP (algebraic code excited linear prediction)Half rate speech (5.6 kbit/s)VSELP (vector sum excited linear predictor)Adaptive multirate (AMR), dynamic operation on full and half rate channel depending on capacity and reception quality

Power control Yes, in 2-dB steps over a range of 30 dB

Time Slots

Frekuensi dibagi menjadi 8 time slot, 0 sampai 7

GSM menggunakan Gaussian Minimum-Shift Keying (GMSK) sebagai modulasinya. GMSK memiliki modulation rate 270.833 kbps.

Dengan rate tersebut, maksimum 156.25 bits dapat ditransmisikan dalam tiap time slot (576.9 µs).

perhitungan:

     270.833 kb/s × 1000 = 270,833 bits/sec        270,833 b/sec ÷ 1,000,000 = .207833 b/µs         0.207833 b/µs × 576.9 µs = 156.25 bits   

     jadi , 156.25 bits dapat ditransmisikan dalam satu time slot

Data Rates

Bits per Time Slot

Data yang ditransmisikan dalam 1 time slot disebut burst. Tiap burst menggunakan 8.25 bit sebagai guard time,

untuk mencegah overlapping dan interferensi antar time slot.

Jadi sisa yang dapat digunakan ada 148 bit.

Ada empat tipe burst dalam TDMA:

Normal Burst (NB)Frequency Correction Burst (FB)Synchronization Burst (SB)Access Burst (AB)

Data Burst

Normal Burst

Tail Bits - Each burst leaves 3 bits on each end in which no data is transmitted. This is designed to compensate for the time it takes for the power to rise up to its peak during a transmission. The bits at the end compensate for the powering down at the end of the transmission.

Data Bits - There are two data payloads of 57 bits each.

Stealing Flags - Indicates whether the burst is being used for voice/data (set to "0") or if the burst is being "stolen" by the FACCH to be used for singalling (set to "1").

Training Sequence - The training sequence bits are used to overcome multi-path fading and propagation effects through a method called equalization.

This diagram illustrates a single burst inside a time slot. Remember that 8.25 bits are not used in order to allow for a guard time.

Digunakan untuk sinkronisasi frekuensi mobile station Memiliki guard time sama seperti normal burst (8.25

bits) Pada logical channel FCCH.

Frequency Correction Burst

Digunakan untuk sinkronisasi waktu mobile station. Bit data membawa TDMA Frame Number (FN) dan the

Base Station Identity Code (BSIC). Ini dipancarkan dengan frekiensi correction burst. Dipancarkan pada Synchronization Channel (SCH).

Synchronization Burst

Digunakan mobile station untuk random access. Memliki guard time yang lebih lama (68.25 bits) Digunakan ketika MS akan mengakses BTS yang

baru, dimana tidak diketahui timing advancenya.

Access Burst

Karena tiap burst memiliki 57 bit segmen data, maka satu burst memiliki muatan data 114 bit

Tiap burst 576.9 µs , jadi bit rate dalam satu time slot :

     114 bits ÷ 576.9 µs = 0.1976 bits/µs   

    0.1976 bits/µs × 1,000,000 = 197,607 bits/sec

Since there are 8 time slots per carrier frequency, each time slot would only get 1/8 of this bit rate, so...

     197,607 bits ÷ 8 = 24,700 bits   

     24,700 bits ÷ 1000 = 24.7 kbits/sec   

Perhitungan Data Throughput

TDMA Frame Each sequence of 8 time slots is known as a TDMA

frame. The duration of a TDMA frame is 4.615 milliseconds (ms) (576.9 µs × 8).

TDMA Frame Structure & Hierarchy

Multiframe merupakan kumpulan beberapa frame TDMA

ada dua tipe multiframe:

     Control Channel Multiframes     Traffic Channel Multiframes

Multiframe

Control Channel Multiframe     composed of 51 TDMA frames     duration = 235.4 ms

Traffic Channel Multiframe     composed of 26 TDMA frames     duration = 120 ms

Single Time Slot Allocated

Superframe merupakan kumpulan beberapa multiframe

Ada dua tipe superframe :

Control Channel Superframe     composed of 26 Control Channel (CCH) multiframes (each CCH multiframe has 51 TDMA frames)     duration = 6.12 seconds

Traffic Channel Superframe     composed of 51 Traffic Channel (TCH) multiframes (each TCH) multiframe has 26 TDMA frames)     duration = 6.12 seconds

Each superframe, whether it is a CCH or TCH frame, consists of 1326 TDMA frames (51 * 26)

Superframe

A hyperframe is composed of 2048 superframes.     duration = 3h 28m 53s 76ms (12,533.76 seconds)     consists of 2,715,548 TDMA frames

Each TDMA frame is numbered according to its sequence within the hyperframe, starting from 0 and ending at 2,715,547.

The TDMA frame number within a hyperframe is abbreviated FN. The FN is one of the variables used in GSM encryption algorithms.

Hyperframe

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