GSM lectures_1, giving details about architecture.pptx

GSM lectures_1, giving details about architecture

1. Govt. of India Department of Telecommunications (DOT) Ministry of Communications, Global System for Mobile Communications B y- Atul Joshi Director (WA)

2. 4 Interfaces in GSM

3. Channels in GSM

4. 1. Q: What are the two main types of channels in GSM? A: Traffic Channels (TCH) and Control Channels (CCH) 2. Q: What is the primary function of a Traffic Channel (TCH)? A: To carry user data such as voice or SMS. 3. Q: Name any two types of Control Channels in GSM. A: Broadcast Control Channel (BCCH), Common Control Channel (CCCH), Dedicated Control Channel (DCCH) 4. Q: Which channel in GSM carries system information from the BTS to all mobile stations? A: Broadcast Control Channel (BCCH) 5. Q: What does the Random Access Channel (RACH) do ? A: It allows a mobile station to request access to the network (used during call initiation).

5. Q1: What does DCCH stand for in GSM? A. Direct Control Channel C. Duplex Control Channel D. Digital Control Channel Explanation: DCCH stands for Dedicated Control Channel, which is used for dedicated signaling between the mobile and the network . B . Dedicated control Channel

6. Q2: Name the three types of Dedicated Control Channels in GSM. A . SDCCH, SACCH, FACCH C. RACH, PCH, AGCH D. TCH, SCH, FCCH Explanation: The three types of DCCH in GSM are SDCCH (Standalone), SACCH (Slow Associated), and FACCH (Fast Associated). B. BCCH, CCCH, DCCH

7. Q3: Which DCCH is used for signaling during call setup before a traffic channel is allocated? A. FACCH B. SACCH C. BCCH Explanation: SDCCH is used before a traffic channel is allocated, such as during call setup or location update. D . S D CCH

8. Q4: What is the function of the SACCH? A. Authentication B. Urgent handover D. SMS delivery Explanation: SACCH carries slow, periodic signaling like timing advance and power control during a call. C . Power control & timing advance

9. Q5: How is the FACCH different from the SACCH? B. FACCH is for SMS C. SACCH is urgent D. FACCH works in idle mode Explanation: FACCH is used for urgent signaling and temporarily takes over a traffic channel, unlike SACCH which transmits in reserved slots. A. FACCH steals TCH capacity; SACCH does not

10. Q6: Which DCCH delivers call setup info like authentication and location update? A. BCCH C. SACCH D. FACCH Explanation: SDCCH handles signaling related to authentication, call setup, and location update. B . S D CCH

11. Q7: During an ongoing call, which channel handles link maintenance information? A. BCCH B. SDCCH D. FACCH Explanation: SACCH is responsible for maintaining the radio link by providing timing and power control during an active call. C. SACCH

12. Q8: What happens when the network sends an urgent handover command during a call? A. Uses SACCH C. Uses BCCH D. Uses SDCCH Explanation: FACCH is used to send urgent commands like handover by replacing voice data temporarily on a traffic channel. B . Uses FA CCH

13. GSM Uplink & Downlink Frequency Bands GSM 900 Mhz (Global System for Mobile Communications) DCS 1800 MHz (a.k.a. Digital Cellular System) B T S UPLINK DOWNLINK 15

14. GSM Specifications GSM 900 Mobile to BS (UP-LINK) - 890 to 915 MHz BS to Mobile (DOWN -LINK) - 935 to 960 MHz Bandwidth - 25 MHz GSM 1800 ( DCS ) : Mobile to Cell (UP-LINK) -1710 to 1785 MHz Cell to Mobile (DOWN -LINK) - 1805 to 1880 MHz Bandwidth - 75 MHz RF Spectrum : 16

15. GSM Specifications Carrier Separation - 200 kHz No. of RF Carriers - 124 Access Method - TDMA/FDMA Modulation Method - GMSK Transmission Rate - 270.833 Kbps Speech Coding - Full rate 13 Kbps Half rate 6.5 Kbps Duplex Distance - 45 MHz 17

16. GSM - MULTIPLE ACCESS GSM uses both FDMA & TDMA Freq Mhz. 890.2 1 890.4 2 890.6 3 890.8 4 891.0 5 6 91 4.8 124 FDMA Access along Frequency axis Each RF carrier 200khz apart Total 124 RF Channels available. One or more carrier assigned to each base station ……... 18 So, therotically , how many subscribers can be latched to the 900 MHZ band at a time for one scenario? 124 *8 = 992?? Subscribers at a time 10% career for control = 112* 8 == 896 1800 MHz?

17. Absolute Radio Freq Carrier Number (ARFCN) 1 and 124 not used until it is co-ordinate d with Non -GSM operators in adjacent freq. bands. Thus for practical purposes only 12 2 RF C arriers are available . F up-link (n) = 890.2 +0.2* ( n-1 ) MHz F down-link (n) = 935.2 +0.2* ( n-1 ) MHz Frequency for any ARFCN ( n) can be calculated from : Here 124 . GSM - MULTIPLE ACCESS 19

18. GSM - MULTIPLE ACCESS 20 ✅ GSM 1800 Band Details: Uplink: 1710 – 1785 MHz Downlink: 1805 – 1880 MHz Total bandwidth: 75 MHz (uplink) + 75 MHz (downlink) Carrier spacing: 200 kHz ✅ Subscribers per Carrier: Each carrier → 8 time slots (TDMA) → 8 subscribers max ✅ Total Subscribers on GSM 1800 Band: ( therortically ) 375 carriers×8 slots=3000 subscribers at a time Practical Estimate: Some carriers used for BCCH/SACCH/SDCCH (control channels) Assume ~10–12% reserved for control purposes Usable carriers ≈ 330–340 So, practical max subscribers ≈ 2640–2720

19. GSM FDMA 25 MHz 25 MHz Mobile to Base 1 2 890.2 890.4 890.6 ( MHz) Base to Mobile [phone] 200 kHz 45MHz Channel layout and frequency bands of operation 890 935 960 915 200 kHz 21

20. GSM TDMA 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 45 MHz Frequency F2’ F1’ (Cell transmit) F2 F1 (Cell Rx) Amplitude Typical TDMA/ FDMA frame structure 22

21. Digital Voice Transmission - In GSM speech coding a block of 20 ms is encoded in one set of 260 bits. - This calculates as 50X 260 = 13 kbps. Thus GSM speech coder produces a bit rate of 13 kbps per subscriber. - This provides speech quality which is acceptable for mobile telephony and comparable with wire-line PSTN phones. Speech Coding 23

22. Speech Coding Parameters like tone, length of tone, pitch are transmitted Sampling=50 times/sec of 260 bits each overall bit rate= 50x260x8 subs=104kbps 24

23. GSM Digital Voice Transmission - It uses 260 bits from speech coding as input and outputs 456 encoded bits. - In one burst one block of 57 bits from one sample and another block from another sample are sent together. - These 456 bits for every 20 ms of speech are interleaved forming eight blocks of 57 bits each. Channel Coding Interleaving 25

24. GSM Digital Voice Transmission To counteract the problems encountered in radio path: Burst Formatting - Additional bits as training sequence added to basic speech/data. - Total of 136 bits added, bringing overall total to 592 bits. - Each TS of TDMA frame is 0.577 ms long and during this time 156.25 bits are transmitted. - One burst contains only 148 bits. Rest of the space, 8.25 bits time, is empty and is called Guard Period ( GP ). - GP enables MS/BTS to “ramp up” and “ ramp down”. 26

25. 8.25 3 57 1 26 1 57 3 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 D D D D D D D D D D D D D D D D 456 bits Sample 1 456 bits Sample 2 Normal Burst Stream of Time Slots Interleaving & Burst Formatting 1 st Sample of 20 ms speech 2 nd Sample of 20 ms speech 27

26. GSM: Gaussian Minimum Shift Keying (GMSK) The modulation method in GSM is GMSK which facilitates the use of narrow bandwidth and coherent detection capability. Rectangular pulses are passed through a Gaussian filter prior to their passing through a modulator. The modulation scheme almost satisfies the adjacent channel power spectrum density requirements of -60dB specified by CCIR. 28

27. GSM: Speech to Radio waves Speech Coding Channel Coding Interleaving Burst formatting Ciphering Burst formatting Deciphering Analog Modulation 200kHz BW Speech Decoding Analog 200kHz BW Demodulation De-interleaving Channel Decoding 29

28. Equalization, Diversity and Channel Coding EQUALIZATION: Compensate intersymbol interference (ISI) created by multipath within the time dispersive channels. DIVERSITY : It is used to compensate for fading channel impairment and is usually implemented by using two or more receiving antennas, CHANNEL CODING : it improves the small scale link performance by adding redundant data bits in the transmitted message so that if an instantaneous fade occur in the channel the data may still be recovered at the receiver.(ex: Block codes, Convolution code and turbo codes) Block Codes : Block codes are forward error correction (FEC) codes that enable a limited numbers of error to be detected without retransmission.( Channel Coding) INTERLEAVER : Function of interleaver to spread these bit out in time so that if there is a deep fade or noise bursts the important bits from a block of source data do not corrupted at the same time.( Time Diversity) 30