GSM Radio interface

GSM Radio interface

1. GSM Radio I nterface Asst.Prof. Rupali Lohar Dept. of Computer Science & Engineering B. R. Harne College Of Engineering & Technology,   Karav, Post Vangani (W Tal Ambernath , Mumbai, Maharashtra 421503

2. Radio Interface Another MSC HLR/AUC S M C PS TN ISDN O M C M S U m M S A - b i s B S C A B T S MS C / V LR

3. Radio Access Technology Fre quen cy T i m e P o w er F r e q u e n c y T im e Po w e r Fre quen cy T i m e Po w er F D M A T D M A C D M A U s e r U s e r U s e r U s e r U s e r U s U e s r er U s e r T i m e F D M A Frequency Time TDMA Frequency CD M A Frequency T i me C o de

4. 7 0 1 2 3 Physical Channel of Logical Channel different mes s a ge types Logical channel Logical channel Logical channel Logical channel Logical channel TDMA FRAME 1 2 3 4 5 6 200K 577  s Physical Channel Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved. Page21

5. Uplink the uplink (radio link from the mobile to the network-that is, mobile transmit, base receive)

6. Downlink downlink (from the network to the mobile-that is, base transmit, mobile receive) are transmitted on different frequency bands.

7. General Characteristics Currently, there are several types of networks in the world using the GSM standard, but at different frequencies. The GSM-900 is the most common in Europe and the rest of the world. Its extension is E-GSM. The DCS-1800 operates in the 1,800-MHz band and is used mainly in Europe, usually to cover urban areas. It was also introduced to avoid saturation problems with the GSM-900. The PCS-1900 is used primarily in North America. The GSM-850 is under development in America. The GSM-400 is intended for deployment in Scandinavian countries in the band previously used for the analog Nordic Mobile Telephony (NMT) system.

8. FDMA consists in dividing the frequency band of the system into several channels. In GSM, each RF channel has a bandwidth of 200 kHz, which is used to convey radio modulated signals, or carriers. Each pair of uplink/ downlink channels is called an absolute radio frequency channel (ARFC) and is assigned an ARFC number (ARFCN TDMA is the division of the time into intervals: within a frequency channel, the time is divided into time slots. This division allows several users (eight) to be multiplexed on the same carrier frequency, each user being assigned a single time slot. A packet of data information, called a burst, is transmitted during a time slot. The succession of eight time slots is called a TDMA frame, and each time slot belonging to a TDMA frame is identified by a time slot number (TN), from 0 to 7.

9. GSM 900 124 channels 200 KHz USED FOR FDMA Channels 1 and 124nare not used for transmission 32 channels are reserved for organizational data 90 are used for customer 248 channels separated in time via GSM TDMA frame Duration frame 4.615 ms Each frame again subdivided into 8 GSM time slot Data is transmitted small portion , called bursts

10. 8bit 41 synchronous bits 36 encrypted bits 3bit 68.25bit Guard interval Access burst (AB): Used in MS initial connection sretup Tail bit Data Tail bit 3bit 142bit 3bit 8.25bit Frequency correction burst (FB): Used in frequency synchronization between MS and BTS , avoid interference. Tail bit Data Tail bit Guard interval 3bit 39 encrypted bits 64 synchronous bits 39 encrypted bits 3bit 8.25bit Synchronous burst (SB): Used in timing synchronization between MS and BTS Tail bit Data Data Tail bit Guard interval Bu r s t Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved. Page22

11. 3bit 142 modulation bits 3bit 8.25bit Tail bit Tail bit Guard interval Bu r s t 3bit 57 encrypted bits 1 26bit 1 57 encrypted bits 3bit 8.25bit Tail bit Data Training sequence Data Tail bit Guard interval Frame stealing flag  Normal burst (NB)  Used to carry the information of the traffic channel and the control channel except for RACH  Dummy burst (DB)  Used if no data is available for a slot Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved. Page23

12. GSM normal burst This GSM burst is used for the standard communications between the base station and the mobile, and typically transfers the digitised voice data. Data is transmitted small portion , called bursts. 1 bit flag: This bit within the GSM burst indicates the type of data in the previous field.

13. Guard space- To avoid overlapping with other bursts due to different path delays. 3 tail bits: These tail bits at the start of the GSM burst give time for the transmitter ,are all set to 0 and can be used to enhance the receiver performance. 57 data bits: This block of data is used to carry information, and most often contains the digitized voice data

14. 26 bits training sequence: - This training sequence is used as a timing reference and for equalization. Used to adapt the parameter of receiver to the current path propagation . Select strongest signal in case of multipath propagation 1 bit flag S- indicates the type of data in the data field.

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