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Gsm (3)
The document discusses GSM radio coverage and the air interface in 3 main points: 1. It describes the geometry of GSM cells and frequency reuse techniques used to divide a service area into smaller cells to improve coverage and capacity. Different cluster sizes such as 3/9, 4/12 and 7/21 are discussed. 2. It explains the GSM frequency bands and channel structure, including physical channels divided into timeslots, and logical channels for traffic and control data. 3. It outlines the structure of bursts transmitted on the air interface, including normal bursts containing encrypted speech blocks, training sequences, and guard periods between bursts.
Gsm (3)
- 1. GSM Global System forMobile Communication By: Mohammed Hassan E-mail:
- 2. GSM Radio Coverage Chapter Objectives Bythe End of this Chapter you will : • Know The Geometrical Theoretical shape of the cells • Know the frequency band allocated for GSM • Know what is meant by frequency Reuse • Know when to use different cluster sizes
- 3. Dead Spots Problem ofomni directional antennas GSM Radio Coverage
- 4. R R To solvethe dead spot problem • The number of cells required to cover a given area. • The cell transceiver power. Tradeoffs R Cell Geometrical Shape GSM Radio Coverage
- 5. Omni-Directional AntennaSectorial Antenna TransceiverAntenna GSM Radio Coverage
- 6. The cells willtake the form of overlapping circles. Due to the obstacles in the coverage area the actual shape of the cells would be Random. Sectorial Antenna Sectorial Antenna GSM Radio Coverage
- 7. To provide coveragefor a large service area of a mobile network we have two Options: (A) Install one transceiver with high radio power at the center of the service area Drawbacks • The mobile equipments used in this network should have high output power in order to be able to transmit signals across the coverage area. • The usage of the radio resources would be limited. (B) Divide the service area into smaller areas (cells) Advantages • Each cell as well as the mobile handsets will have relatively small power transceivers. • The frequency spectrum might be “reused” in two far separated cells. This yields: Unlimited capacity of the system. Good interference characteristics GSM Coverage Plan GSM Radio Coverage
- 8. Spectrum Allocation (GSM900) GSM 900 Frequency Allocation F (MHz)915890 Uplink 1 2 3 4 121 122 123 124 F (MHz) Downlink 960935 1 2 3 4 121 122 123 124 890.2 890.4 890.6 935.2 935.4 935.6 200 KHz 1 1 121 121 Downlink 935 – 960 MHz Uplink 890 – 915 MHz ARFCNAbsolute Radio Frequency Channel Number GSM Radio Coverage
- 9. GSM 1800 FrequencyAllocation F (MHz)17851710 Uplink 1 2 3 4 371 372 373 374 F (MHz) Downlink 18801805 1 2 3 4 371 372 373 374 1710.2 1710.4 1710.6 1805.2 1805.4 1805.6 200 KHz Downlink 1805 – 1880 MHz Uplink 1710 – 1785 MHz Spectrum Allocation (GSM 1800) GSM Radio Coverage
- 10. 890 915 935 960 GSM900 With 124 ARFCN 25 MHz 45MHz Uplink Downlink 1710 1785 1805 1880 GSM 1800 With 374 ARFCN 75 MHz 95MHz Uplink Downlink Comparison GSM Radio Coverage
- 11.
- 12. • Cell shape GSMRadio Coverage
- 13.
- 14. cell sectorization GSMRadio Coverage
- 15. cell sectorization Sector1 Sector 2 Sector 3 GSM Radio Coverage
- 16. F1 F1 F1 F1 F1 F1 F1 cellsectorization GSM Radio Coverage
- 17. cell sectorization Sector1 Sector 2 Sector 3 Sector 4 Sector 5 Sector 6 GSM Radio Coverage
- 18. GSM Radio Coverage cellsectorization
- 19. Total no ofchannels (frequencies) = 124 Every channel can be shared between a maximum of 8 subscribers. Maximum no of simultaneous calls = 8 X 124 = 992 !! Why do we need frequency reuse? The frequency reuse is performed by dividing the whole available frequencies between a group of neighboring cells which is called frequency reuse pattern or a “Cluster”, and then repeat this cluster over the whole network on 2 conditions: The group of frequencies allocated to a given cell must not be used in the adjacent cells. Enough distance between the cells where the same group of frequencies are reused. Frequency Reuse GSM Radio Coverage
- 20. A3 A2 A1 B3 B2 B1 C3 C2 C1 A3 A2 A1 B3 B2 B1 C3 C2 C1 A3 A2 A1 B3 B2 B1 C3 C2 C1 A3 A2 A1 B3 B2 B1 C3 C2 C1 A3 A2 A1 B3 B2 B1 C3 C2 C1 A3 A2 A1 B3 B2 B1 C3 C2 C1 A3 A2 A1 B3 B2 B1 C3 C2 C1 A3 A2 A1 B3 B2 B1 C3 C2 C1 3/9 cluster inwhich the available frequencies are divided into 9 groups and distributed between 3 sites A3 A2 A1 B3 B2 B1 C3 C2 C1 3/9 Cluster GSM Radio Coverage
- 21. 4 / 12Cluster A3 A2 A1 B3 B2 B1 C3 C2 C1 D3 D2 D1 4/12 cluster in which the available frequencies are divided into 12 groups and distributed between 4 sites A3 A2 A1 B3 B2 B1 C3 C2 C1 D3 D2 D1 A3 A2 A1 B3 B2 B1 C3 C2 C1 D3 D2 D1 A3 A2 A1 B3 B2 B1 C3 C2 C1 D3 D2 D1 A3 A2 A1 B3 B2 B1 C3 C2 C1 D3 D2 D1 GSM Radio Coverage
- 22. A3 A2 A1 C3 C2 C1 D3 D2 D1 B3 B2 B1 E3 E2 E1 F3 F2 F1 G3 G2 G1 A3 A2 A1 C3 C2 C1 D3 D2 D1 B3 B2 B1 E3 E2 E1 F3 F2 F1 G3 G2 G1 A3 A2 A1 C3 C2 C1 D3 D2 D1 B3 B2 B1 E3 E2 E1 F3 F2 F1 G3 G2 G1 7/21 cluster inwhich the available frequencies are divided into 21 groups and distributed between 7 sites 7/21 Cluster GSM Radio Coverage
- 23. Carrier to interferenceratio It’s the difference in power level between the carrier in a given cell and the same carrier received from the nearest cell that reuses the same frequency. Number of frequencies per site Traffic Channels C/I Ratio 3/9 High High Low 4/12 Medium Medium Medium 7/21 Low Low High Which Cluster Size to use? GSM Radio Coverage
- 24.
- 25. Chapter Objectives By theEnd of this Chapter you will: Know How the Physical and Logical Channels are classified Know the different types of Traffic Channels Know the different types of Control Channels Know the Structure of each Control Channel Know how control and traffic data is Mapped in the air interface Know the terminology of different TDMA Frames Air Interface
- 26. • GSM FrequencyBands System P-GSM 900 E-GSM 900 GSM(DCS) 1800 GSM(PCS) 1900 Uplink (MS BS) Downlink(BS MS) 890 – 915 MHz 935 – 960 MHz 880 – 915 MHz 925 - 960 MHz 1710 – 1785 MHz 1805 - 1880 MHz 1850 – 1910 MHz 1930 - 1990 MHz Wavelength 33 cm 33 cm 17 cm 16 cm Bandwidth 25 MHz 35 MHz 75 MHz 60 MHz Duplex distance 45 MHz 45 MHz 95 MHz 80 MHz Carrier separation 200 kHz 200 kHz 200 kHz 200 kHz No. of carriers 124 174 374 299 Channel rate 270.8 kbps 270.8 kbps 270.8 kbps 270.8 kbps Air Interface
- 27. COW BTS AirInterface
- 28. Types ofchannels 1. Physical channels 2. Logic channels Air Interface
- 29. Physical Channels Time GSM bandis divided into 124 RF channels, and each channel is divided into 8 time slots using TDMA. These time slots are called “physical channels”. CH 1 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 CH 2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 CH 3 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 CH 124 0 1 2 3 4 5 6 7 0 1 2 3 4 5 Air Interface
- 30. Logical Channels Logical Channels Aphysical channel may be occupied by a traffic channel or a control channel, both of them are classified as “logical channels”. Traffic Channels Half Rate Full Rate Control Channels SCH BCCH FCCHPCH RACH AGCH SDCCH SACCH FACCH BroadcastCommon Dedicated Used to carry signaling or synchronization Data Air Interface
- 31. Logical Channels-Traffic Channels TCH Traffic Channels Speech TCH/FS Data TCH/HS TCH/9.6TCH/2.4 TCH/4.8 Normal Burst TCH Traffic Channel TCH/FS Full rate Speech Channel TCH/HS Half rate Speech Channel TCH/9.6 Data Channel 9.6kb/s TCH/4.8 Data Channel 4.8kb/s TCH/2.4 Data Channel 2.4Kb/s Air Interface
- 32. Traffic Channels Carrieseither encoded speech or user data up and down link between a single mobile and a single BTS. Types of traffic channel: Full rate (TCH) Transmits full rate speech (13 Kbits/s). A full rate TCH occupies one physical channel. Half rate (TCH/2) Transmits half rate speech (6.5 Kbits/s). Two half rate TCHs can share one physical channel, thus doubling the capacity of a cell. Air Interface
- 33. Logical Channels-Control Channels FCCHSCH CCH ControlChannels DCCH SDCCH BCH BCCH Synch. CH. ACCH SACCHFACCH CCCH RACH CBCH PCH/AGCH Air Interface
- 34. Traffic Channels Carries eitherencoded speech or user data up and down link between a single mobile and a single BTS. Full Rate = 13 Kbit/S Half Rate = 6.5 Kbit/S Enhanced Full Rate = 15.1 Kbit/S Air Interface
- 35. Random Access CHannel(RACH) Access Grant CHannel (AGCH) It’s used to page (search) for a specific mobile To or from a certain BTS to a single mobile Transfer control information between all MSs and BTS. Necessary for call origination and paging. Request allocation of SDCCH Allocate SDCCH to the mobile station. Common Control Channels Paging CHannel (PCH) Air Interface
- 36. Frequency Correction ControlCHannel (FCCH) From Single BTS to all the mobiles in the area Monitored by MSs periodically (every 30 sec). Carries information for frequency correction of the mobile Synchronization CHannel (SCH) Carries 2 important pieces of information • TDMA frame number (max = 2715684 ) • Base station identity Code (BSIC) Broadcast Control CHannel (BCCH) Carries Cell specific data Broadcast Channels Air Interface
- 37. Dedicated ControlChannels Standalone Dedicated Control CHannel (SDCCH) Carries system signaling during: A call setup before allocating a TCH. Registration & Authentication. Transmission of SMS in idle mode. MS paging Response. Cell Broadcast Control CHannel (CBCCH) MS must be setup to receive this channel. It displays general information. It uses one of the SDCCH channels Carry messages between MS and network. Air Interface
- 38. TDMA inGSM Air Interface
- 39. TDMA framestructure -Bit rate of the radio carrier is 270.833 Kbps -Bit duration =1/270833=3.69 µsec -One time slot =148 bits+8.25 guard bits=156.25 bits -Time slot duration =156.25x3.69 µsec= 0.577 msec -Frame duration=0.577x8= 4.615 msec Air Interface
- 40. Burst Structure Types Normal Burst Structure Tail Bits Encrypted Bits Training Sequenc e Encrypted Bits Tail Bit s Guar d Perio d 3 57 1 26 1 57 3 8.25 0 1 2 3 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec The tail bits help the equalizer to determine the start and stop points of the transmitted bits. They are three bits at the beginning and at the end of the burst and they are always zeros Air Interface
- 41. Tail Bits Encrypted Bits Training Sequenc e Encrypted Bits Tail Bit s Guar d Perio d 3 57 126 1 57 3 8.25 0 1 2 3 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec The subscriber speech or data is encrypted into 57 bit blocks. Each burst will contain two 57 bits blocks from two different speech segments(20m Sec). Burst Structure Types Normal Burst Structure Air Interface
- 42. Tail Bits Encrypted Bits Training Sequenc e Encrypted Bits Tail Bit s Guar d Perio d 3 57 126 1 57 3 8.25 0 1 2 3 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec One bit “stealing flag” will be added to each block to indicate whether the burst is stolen for the FACCH signaling or used as a normal traffic channel Burst Structure Types Normal Burst Structure Air Interface
- 43. Tail Bits Encrypted Bits Training Sequenc e Encrypted Bits Tail Bit s Guar d Perio d 3 57 126 1 57 3 8.25 0 1 2 3 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec The guard period of 8.25 bits length, which is equivalent to about 30 s, is left at the end of each burst, to prevent overlapping between consecutive bursts and to facilitate burst sync. Burst Structure Types Normal Burst Structure Air Interface
- 44. It carries informationof all logical channels except RACH, SCH and FCCH Tail Bits Encrypted Bits Training Sequenc e Encrypted Bits Tail Bit s Guar d Perio d 3 57 1 26 1 57 3 8.25 0 1 2 3 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec Burst Structure Types Normal Burst Structure Air Interface
- 45. Tail Bits Fixed Bits Tail Bit s Guar d Perio d 3142 3 8.25 This is the one used by the channel (FCH) for frequency correction of the mobile. It consists of a long sequence of bits called the fixed bits which are all equal to zeros, leading to a constant frequency output from the GMSK modulator 0 1 2 3 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec Burst Structure Types Frequency Correction Burst Structure Air Interface
- 46. 0 1 23 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec Tail Bits Encrypted Bits Synchronization Sequence Encrypted Bits Tail Bit s Guar d Perio d 3 39 64 39 3 8.25 The SCH burst consists of a long synchronization sequence along with the important information being encrypted and divided into two blocks. The TDMA frame number is sent on the SCH channel, which carries also the Base station Identity code (BSIC). The TDMA frame number is used by the mobile to determine which control channels will be transmitted on that frame. Burst Structure Types Synchronization Burst Structure Air Interface
- 47. Tail Bits Synchronization Sequence Encrypte d Bits Tail Bits GuardPeriod 8 41 36 3 68.25 The Access Burst is used by the RACH channel. The mobile sends this burst when it does not know the distance to its serving BTS, which is the case when the mobile is switched on or after it makes a handover to a new cell. So this burst must be shorter in order to prevent it from overlapping with the burst on the next time slot. 0 1 2 3 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec Burst Structure Types Access Burst Structure Air Interface
- 48. 0 1 23 4 5 6 7 TDMA Frame 156.25 bits in 577 u Sec Tail Bits Encrypted Bits Training Sequenc e Encrypted Bits Tail Bit s Guar d Perio d 3 58 26 58 3 8.25 The dummy burst is sent from the BTS when there is nothing else to be sent. It carries no information and it has the same structure of a normal burst with the encrypted bits replaced by a known bit pattern to the mobile. Burst Structure Types Dummy Burst Structure Air Interface
- 49. 3 57 126 1 57 3 8.25 0 1 2 3 4 5 6 7 156.25 bits in 577 m Sec Normal burst 1 2 e.g. TCH 26 1 2 e.g. BCCH 51 1 e.g. BCCH 26 1 2 e.g. TCH 51 1 2 e.g. BCCH 2048 1 Hyper frame = 2048 Super frames = 2,715,648 TDMA Frames = 3hrs 28 min and Hyper Frame Structure of TDMA Frames 12 1 2 2 1 Super frame = 51 TCH Multi frames 1 Super frame = 26 BCCH Multi frames 1 BCCH Multi frame = 51 TDMA Frames1 TCH Multi frame = 26 TDMA Frames Air Interface
- 50.
- 51. Adaptive FrameAlignment The BS-MS delay depends on distance. If BS–MS separation= 10km, Propagation delay=33 sec. Long guard interval is inefficient. GSM uses adaptive time alignment system whereby BS transmits timing advance at the MS. This instructs the mis-aligned MS to transmit earlier TDMA frame Propagation delay TS0 TS2TS1 B: Far away from BS B A A: Close to BS Air Interface
- 52. Rayleigh fading isfrequency-selective. BS&MS hop from one frequency to other. Frequency hopping is optional. Rate of hopping =1/4.615msec=217 hop/sec. Slow Frequency Hopping Air Interface