The explanation of time slot, channel, burst sequence and frame in GSM

Just from the forum to see someone asked GSM time slot, channel, burst sequence, frame knowledge. Today we digital communication right up to this piece, I based on what I know and online search answer!

1, Time Division multiplexing technology

FDMA: Frequency division Multiple Access TDMA: Time division Multiple Access CDMA: Code Division Multiple access in order to improve the utilization of the communication channel, so that a number of independent signals along the same channel, without interfering with each other communication mode

Multi-Site Technology Introduction

There are three main applications of multiple access technologies:

(1) Frequency Division Multiple Access (FDMA) technology: In the traditional radio broadcast, are used frequency division multiple access (FDMA mode, each broadcast channel has a frequency point, if you want to listen to a broadcast channel, you must tune your radio to this frequency point. This technique is also used in analog cellular systems, where a customer call in a cell takes up a frequency, i.e. one channel, and no other user can occupy it. (2) Time Division Multiple Access (TDMA) Technology: Time Division Multiple access (TDMA) at each frequency to produce a plurality of gaps, in different slots for communication, Europe's GSM, Japan's PDC have adopted the TDMA technology. (3) CDMA Technology: Code Division Multiple Access (CDMA) technology is the use of a set of orthogonal pseudo-random code sequence to the useful signal spread spectrum processing technology, compared with the first two technologies, this technology is the highest frequency utilization, is the future of digital mobile communication system mainstream technology.

2. TDMA Technology in GSM

In GSM system, the wireless interface uses time division multiple address (TDMA) and frequency division multiple access (FDMA) combination method. Users communicate on different channels, and each channel (TRX) can be divided into 8 slots, each with one channel, so a TRX can be used by up to 8 full-speed (or 16 half-rate) mobile customers simultaneously.

The TDMA system has the following features:

 (1  ) multiple channels per carrier frequency. As mentioned above, the TDMA system forms a frequency time matrix, which produces multiple timeslot at each frequency, and each point in the matrix is a channel that can provide telephone or non-speech services to any mobile customer under the control allocation of the base station ( 2   3  ) high transfer rate, adaptive equalization. Each carrier has more time slots, the frequency interval is wide, the transmission rate is high, but the digital transmission brings time dispersion, so the delay expands, so we must adopt the adaptive equalization technology. ( 4  ) high transport overhead. Because the TDMA is divided into the time slot transmission, so that the receiving machine in each burst pulse sequence will have to regain synchronization. In order to separate one time slot from another, the protection time is also necessary. As a result, TDMA systems typically require more overhead than FDMA systems. ( 5  ) is open to new technologies. For example, when the speech coding algorithm is improved and the bit rate is reduced, the channel of the TDMA system is easily reconfigured to accommodate the new technology. ( 6  ) the cost of sharing a device is low. Because each carrier is serving multiple customers, the average cost per customer for a TDMA system shared device is significantly reduced compared to the FDMA system. ( 

3. Time division Multiple access TDMA frame structure

1. TDMA frame Definition

See Figure 1 in the TDMA of GSM, the frame is defined as 8 consecutive time slots (ts0-7) contained in each carrier frequency, equivalent to one channel in the FDMA system. In each timeslot, the signal is sent in the form of a burst pulse series.

2. Frame structure

Figure 2 shows the complete structure of the TDMA frame, which is essentially a duplicate physical frame on the wireless link.

（1) TDMA frames-each TDMA frame has 8 slots, the entire frame length is about 4.615ms, each time slot contains 156.25 code elements, the time slot length is 0.577ms. (2TDMA Complex Frame (Multi frame)-Multiple TDMA frames constitute a complex frame, with two structures: successive 26 TDMA frames, called 26-complex frames, with a period of 120ms, for the business channel and the On-road control channel (TCH and Sacch/facch). Successive frames of 51 TDMA frames, called 51-complex frames, are used for the control channel (CCH), with a period of 3060/ -≈235. 385ms. (3) TDMA (Superframe)-Multiple successive TDMA to form a frame, which is a continuous 51x -TDMA frame, the duration of a frame is 6.12s. (4TDMA super high frame (Hyper frame) It includes 2048 frames, each of which includes 2,715,648 TDMA frames with a time period of 3 hours, 28 minutes, 53 seconds, 760 milliseconds. The TDMA frame number is a cycle number with a TDMA frame (2,715,648 TDMA frames). Then why use the TDMA frame number? This is because the GSM system's confidentiality to the customer is achieved by encrypting the information before sending it. The algorithm for calculating an encryption sequence is to have a TDMA frame number as an input parameter, so each frame must have a frame number. With the TDMA frame number, the mobile station can judge which kind of logical channel is transmitted on the control channel TS0.

The explanation of time slot, channel, burst sequence and frame in GSM