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CS 117 Winter 2004 Lecture #9 March 9, 2004

CS 117 Winter 2004 Lecture #9 March 9, 2004. Cellular Wireless Networks AMPS (Analog) D-AMPS (TDMA) GSM Reference: Tanenbaum Chpt 2 (pg 153-169). Cellular Wireless Network Evolution. First Generation: Analog Voice (1946) AMPS : Advance Mobile Phone Systems (1982)

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CS 117 Winter 2004 Lecture #9 March 9, 2004

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  1. CS 117 Winter 2004Lecture #9March 9, 2004 • Cellular Wireless Networks • AMPS (Analog) • D-AMPS (TDMA) • GSM Reference: Tanenbaum Chpt 2 (pg 153-169)

  2. Cellular Wireless Network Evolution First Generation:Analog Voice (1946) AMPS: Advance Mobile Phone Systems (1982) Residential cordless phones Second Generation:Digital Voice (1990) IS-54; IS-136: North American Standard - TDMA (1996) IS-95: CDMA (Qualcomm) (1993) GSM: Pan-European Digital Cellular (1991) Iridium , 2002:Mobile Communication Satellite(1998) Third Generation: Digital Voice and Data (2000) IMT-2000 (cdma2000, W-CDMA/UMTS-Combines the functions of: cellular, cordless, wireless LANs, paging, Bluetooth, etc. Supports multimedia services: data, voice, video, image)

  3. BS BS BS Backbone Network BS BS BS Cellular Concept • Geographical separation • Capacity (frequency) reuse • Backbone connectivity

  4. Invented by Bell Labs; installed In US in 1982; in Europe as TACS

  5. 1G; Advanced Mobile Phone System(AMPS) (1982) • Frequencies are not reused in in a group of 7 adjacent cells • To add more users, smaller cells can be used. • In each cell, 57 channels each for A-side and B -side carrier • about 800 channels total (across the entire AMPS system) • FDMA: one frequency per user channel 100 cells 10-20 km

  6. BS BS BS Publ. Swit. Tel. Net. BS BS BS Cellular Concepts Hand-held telephone 0.6 watts; Car transmitters are 3 watts, Maximum. by the FCC. • Geographical separation • Capacity (frequency) reuse • Backbone connectivity The base station consists of a computers and transmitter/receiver connected to an antenna. MTSO (Mobile Telephone Switching office)- MSC (Mobile Switching Center).

  7. ;Concepts If MS leave a cell, its BS asks the surrounding BS how much power they are getting from it. The BS then transfers ownership to the cell getting the strongest BS Nerve center of the system MS PacketSwitching Telephone Network (PSTN)

  8. AMPS 1G; Channel Categories 832 channels are divided into four categories: 1. Control(base to mobile) to manage the system (PSK) (=21channels are reserved for control, and are wired into a Programmable Read-Only Memory (PROM)). 2. Paging(base to mobile) to alert users to calls for them. 3. Access(bidirectional) for call setup and channel assignment. 4. Data(bidirectional) for voice, fax, or data. (FM) Since the same frequencies cannot be reused in nearby cells, the actual number of voice channels available per cell is much smaller than 832, about 45.

  9. To register and make a phonecall • When phone is switched on , it scans a preprogrammed list of 21 control channels, to find the most powerful signal. • It transmits its ID number on it to the MSC – which informs the home MSC (registration is done every 15 min) • To make a call, user transmits dest Ph # on random access channel; MSC will assign a data channel • At the same time MSC pages the destination cell for the other party (idle phone listens on all page channels)

  10. Handoff (Cellular Concepts) Handoff:Transfer of a MS from one cell to another(300 msec.) Each BS constantly monitors the received power from each MS. When power drops below given threshold, BS asks neighbor station (with stronger receivedpower) to pick up the MS, on a new channel (the old is not reused in adjacent cells) - handoff. Hard handoff: User must switch from one frequency to another. The old BS drops the MS before the new one acquires it. If the new one is unable to acquire it, the cell is disconnected abruptly. Soft Handoff: The MS is acquired by the new BS before the previous one drops, the MS needs to be able to tune to two frequencies at the same time (available only with CDMA). Neither first nor second generation devices can do this.

  11. Soft - Handoff (Cellular Concepts) • Soft Handoff: simultaneous radio link between MS and different BSs • Hard handoff: The old BS drops the MS before the new one acquires it.

  12. AMPS 1G;Channels AMPS uses FDM to separate the channels. Each simplex channels is 30 KHz wide (A) 832 channels (B) 832 channels (Freq Division Duplex)

  13. FDD & TDD duplexing

  14. AMPS 1G;

  15. AMPS 1G;

  16. AMPS

  17. 2G-USDigital Cellular: (Int.Stand:IS-54; IS-136)=USDC • Initially, Same frequency as AMPS (simplex Transmit: 824 to 849 MHz; simplex receive: 869 to 894 MHz). • Each 30 kHz band RF channel is used at a rate of 48.6 kbps(to co-exist with AMPS,one channel can be analog and the adjacent ones can be digital-MTSO determines) • 3 TDM slots/RF band • 8 kbps voice coding • 16.2 kbps TDM digital channel • 4 cell frequency reuse (instead of 7 as in AMPS) • 3 x 416 / 4 = 312 channels (57 in AMPS) D-AMPS

  18. D-AMPS Conceptually, it works like AMPS its successor

  19. D-AMPS 2G; Phones: Digital Voice The second generation was digital and four systems are in use now:D-AMPS, GSM, CDMA, and PDC (Japan) FDD:(1850-1910 MHz Upstream; 930-1990 MHz Downstream) Waves are 16 cm long, so ¼-wave antenna is only 4 cm long. D-AMPS can use both the 850 MHz and 1900 MHz bands to get a wider range of available channels • D-AMPS mobile phone, the voice signal picked up by the microphone is digitized and compressed. Compression is done from the standard 56-kbps PCM encoding to 8 kbps, or less. The compression is done in the telephone. • In D-AMPS, three users can share a single frequency pair using TDM. Each frequency pair supports 25 frames/sec of 40 msec each. Each frame is divided into six time slots of 6.66 msec each,(6.66msec x 6=3.996 msec); 25 frames/sec x 6 slots = 150 slots Data Modulation:Pulse Code Modulation

  20.  FDMA TDM s1(t) S1(f) S2(f) SN(f) f1 + f1 f2 fN f Buser sN(t) fN • Narrow single-user bandwidth

  21. TDM frame 40 msec TDM frame 40 msec 1850.01 Mhz MS-BS 1930.05 Mhz BS-MS Upstrm Downst 1 2 3 1 2 3 1 2 3 4 5 6 1850.01 Mhz MS-BS 1930.05 Mhz BS-MS 3 1 2 3 1 2 6 1 2 3 4 5 324 bit slot= 64 bits of control+ 101 bits of error correction+ 159 bits of speech data. Speed=50 slots/sec D-AMPS (a)D-AMPS Datachannel with three users. (b) D-AMPS channel with six users users 1/3 of time a MS is idle = line quality measurement 1/6 of time a MS is idle = line quality measurement

  22. D-AMPS G-Guard Time; R-Ramp Time RSVD-Reserved for Future Use

  23. Frame 1944 bits in 40 ms( 48600 b/s) SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5 SLOT 6 DATA122 DATA122 DVCC12 SYNC28 SACCH12 R6 G6 DATA16 MOBILE TO BASE G:GUARD TIME R:RAMP TIME DVCC: DIGITAL VERIFFICATION COLOR CODE RSVD: RESERVE FOR FUTURE USE RSVD 12 DATA130 DATA130 DVCC 12 SYNC28 SACCH 12 D-AMPS IS-54 slot and frame structure BASE TO MOBILE

  24. D-AMPS

  25. D-AMPS Control structure, conceptually, like AMPS D-AMPS Groups of 16 frames form a superframe, with control information used: • system configuration, • real-time control, • non real-time control, • paging, • access response, • short messages. When a mobile is switched on, it makes contact with the base station to announce itself and then listens on a control channel for incoming calls. Having picked up a new mobile, the MTSO informs the user’s home base where he is, so calls can be routed correctly. One difference between AMPS and D-AMPS is how handoff is handled. A. In AMPS, the MTSO manages it completely without help from MS; B. D-AMPS, 1/3 of the time a mobile is neither sending nor receiving. It uses these idle slots to measure the line quality. As in AMPS, it still takes about 300 msec to do the handoff. This technique is called: MAHO (Mobile Assisted Handoff).

  26. D-AMPS; IS-54, IS-136 D-AMPS

  27. GSM 2G; GSM (Group Special Mobile)(Global System for Mobile) • Pan-European Cellular Standard: 2G;Digital • FDD: (890-915 MHz Upstr;935-960 MHz Downstr.) • 124 frequency carriers; 8 channels per carrier • Carrier spacing: 200 KHz (Narrowband TDM) • Speech coder: linear coding (rate = 13 Kbps) • Modulation: PSK • Slow FHSS modulation (217.6 hops/s) to overcome multipath fading. • First approximation, GSM is similar to D- • AMPS.

  28. First approximation, 2G; GSM is similar to 2G; D-AMPS. D-AMPS is used in the U.S. and in Japan (modified). Everywhere else in the world used GSM

  29. TDM frame 959.8 MHz 935.4 MHz 935.2 MHz 914.8 MHz 890.4 MHz 890.2 MHz Simplex. Chann. 124 BS to 2MS 1 124 MS To 2BS 1 Frequency Time GSM uses 124 frequency channels, each of which uses an 8-SlotTDM system. (In the Figure one TDM frame is absent). 992 Ch/cell. GSM GSM-The Global System for Mobile Communications

  30. TDMA s1(t) T frame T frame t T frame t 1 2 N sN(t) t T frame N *T frame • Broade single-user bandwidth (1/Tb) • GSM = FDMA 200 KHz + TDMA 8 slot/frame

  31. 32,500 bit multiframe sent in 120 msec. 1 2 3 4 11 Ctl 13 14 15 23 24 148-Bit data frame sent in 4.615 msec 0 1 2 3 4 5 6 7 8.25-bit 30 mk. sec guard time 148-Bit frame sent in 547 mk.sec 000 Information Sync Information 000 Bits 3 57 26 57 3 Voice/data bit A portion of the GSM framing structure GSM

  32. GSM

  33. GSM

  34. GSM GSM system • A GSM system has 124 pairs of simplex channels; • Each 200 kHz wide. • Supports eight separate connections on it, using TDM. • Each active station is assigned one time slot on one channel pair. • 992 channels can be supported in each cell, but many of them are not available, to avoid frequency conflict with neighboring cells. • The eight shaded time slot all belong to the same connection. • Four of them in each direction. • Transmitting and receiving does not happen in the same time slot because the GSM radios cannot transmit and receive at the same time and it takes time to switch from one to the other.

  35. GSM GSM system (Cont) • A data frame is transmitted in 547 mksec, but a transmitter is only allowed to send one data frame every 4.615 msec, since it is sharing the channel with seven other stations. • The gross rate of each channel is 270,833 bps, divided among eight users. This gives 33.854 kbps gross, more than double D-AMPS, 324 bits, 50 times per second for 16.2 kbps. • With AMPS, the overhead cats up a large fraction of the bandwidth, ultimately leaving 24.7 kbps worth of payload per user before error correction. • After error correction, 13 kbps is left for speech, giving better voice quality than D-AMPS (using correspondingly more bandwidth). See pp.30, eight data frames make up a TDM frame and 26 TDM frames make up a 120-msec multiframe. Of the 26 TDM frames in a multiframe, slot 12 is used for control and slot 25 is reserved for future use, so only 24 are available for user traffic.

  36. GSM Control Channels (CC) • CC used to manage the system. • The broadcast control channel (BCC) is a continuous stream of output from the BS containing the BS’s identity and the channel status. All MS monitor their signal strength to see when they moved into a new cell. • The dedicated control channel (DCC) is used for location updating, registration, and call setup. In particular, each BS maintains a database of MS. Information needed to maintain this database and is sent on the dedicated control channel. • The common control channel (CCC), which is split up into three logical sub-channels: • Is the paging channel CC3a), which the BS uses to announce incoming calls. Each MS monitors it continuously to watch for call it should answer. • Is the random access channel (CC3b). This allows users to request a slot on the dedicated control channel. If two requests collide, they are garbled and have to be retried later. • Is the access grant channel (CC3c). The announced assigned slot.

  37. Some of these slots are used to hold several control channels used to manage the system Broadcast control channel (BCC) Continuous stream of output from the BS, containing the BS’s identity and the channel status. All MS monitor their signal strength to see when they moved into a new cell Dedicated control channel (DCC) For location updating, registration, and call setup Common control channel (CCC) 3 logical subchannels: 1. paging channelTo announce incoming calls 2. random access channelTo request a slot on the dedicated control channel GSM 3. access grant channelannounced assigned slot

  38. GSM GSM Signalling channels • BCCH: Broadcast Control Channel • point-to-multipoint unidirectional control channel broadcasting system information to MS • CCCH: Common Control Channel • up-link: RACH (Random Access Channel) • down-link: PCH (Paging Channel) AGCH (Access Grant Channel) • DCCH: Dedicated Control Channel • point-to-point bidirectional control channel • SACCH (Slow Associated Control Channel) • FACCH (Fast Associated Control Channel) • SDCCH (Stand Alone Dedicated Control Channel)

  39. GSM

  40. GSM

  41. GSM

  42. GSM;

  43. CDMA (Code Division Multiple Access): IS-95QUALCOMM, San Diego • Based on DS spread spectrum • Two frequency bands (1.23 Mhz), one for forward channel (cell-site to subscriber) and one for reverse channel (sub to cell-site) • CDMA allows reuse of same spectrum over all cells. Net capacity improvement: • 4 to 6 over digital TDMA (eg. GSM) • 20 over analog FM/FDMA (AMPS)

  44. Access techniques for mobile communications FDMA (TACS) P F TDMA (GSM, DECT) ATDMA (UMTS) T P F CDMA (UMTS) T P F P - Power T - Time F - Frequency T

  45. CDMA (Code Division Multiple Access) • unique “code” assigned to each user; i.e., code set partitioning • all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data • Note: chipping rate >> data rate (eg, 64 chips per data bit) • encoded signal = (original data bit) X (chipping sequence) • decoding: inner-product of encoded signal and chipping sequence • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

  46. CDMA Encode/Decode

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