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

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)

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7 Cellular Concept Geographical separation Capacity (frequency) reuse Backbone connectivity BS Backbone Network

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

9 1G; Advanced Mobile Phone System (AMPS) 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 km 100 cells (1982)

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

11 Packet Switching Telephone Network (PSTN) ;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 MS Nerve center of the system

12 1G; Channel Categories 832 channels are divided into four categories: 1. Control (base to mobile) to manage the system (PSK) (=21 channels 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. AMPS

13 To register and make a phone call 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)

14 1G; Call Management: To register and make a phone call A. Each MS in AMPS has a 32-bit serial number and a 10- digit phone number in its PROM. 1. When phone is switched on, it scans a PROM list of 21 control channels to find the most powerful signal. 2. PROM transmits its ID number on it to the Mobile Switching Center – which informs the home MSC (registration is done every 15 min). B. To make a call, user transmits destination Ph # on random access channel; MSC will assign an idle data channel 1. At the same time MSC pages the destination cell for the other party (idle phone listens on all page channels). 2. All the control information in AMPS, is sent in digital form, multiple times, and with an error-correcting code, even though the voice channels themselves are analog. AMPS

15 1G; Incoming calls C. To accept Incoming calls, all idle phones continuously listen to the paging channel to detect messages directed at them. When a call is sent to a mobile phone (either from a fixed phone or another mobile phone), a packet is sent to the caller’s home MTSO to find out where it is. A packet is then sent to the BS in its current cell, which then sends a broadcast on the paging channel of the form “Unit 14, are you there?” The called phone then responds with “Yes” on the access channel. The BS then says something like “Unit 14, call for you on Data channel 3”. At this point, the called phone switches to channel 3 and starts making ringing sound. AMPS

16 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 received power) 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.

17 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.

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

19 FDD & TDD duplexing

20 AMPS 1G;

21 AMPS 1G;

22 Channel multiplexingFDMA+TDM Uplink MHz-832 channels Downlink MHz-832 channels Channel Bandwidth30 kHz FDD separation45 MHz ModulationFM (traffic, voice); FSK (control) ChannelsControl, Paging, Access, Data AMPS

23 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

24 Conceptually, it works like AMPS D- AMPS its successor

25 2G; Phones: Digital Voice The second generation was digital and four systems are in use now: D-AMPS, GSM, CDMA, and PDC (Japan) FDD: ( MHz Upstream; 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 D- AMPS

26  Narrow single-user bandwidth f  f1f1 s1(t)s1(t) sN(t)sN(t) S 1 (f)S 2 (f) S N (f) B user f2f2  + f1f1 fNfN fNfN FDMA TDM

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

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

29 IS-54 slot and frame structure BASE TO MOBILE SLOT 1 SLOT 2 SLOT 3SLOT 4SLOT 5 SLOT 6 Frame 1944 bits in 40 ms( b/s) G6G6 R6R6 DATA 16 SYNC 28 DATA 122 SACCH 12 DVCC 12 DATA 122 MOBILE TO BASE DATA 130 DVCC 12 SACCH 12 SYNC 28 RSVD 12 G:GUARD TIME R:RAMP TIME DVCC: DIGITAL VERIFFICATION COLOR CODE RSVD: RESERVE FOR FUTURE USE D- AMPS

30 D- AMPS

31 D-AMPS Control structure, conceptually, like AMPS Groups of 16 frames form a superframe, with control information used: 1.system configuration, 2.real-time control, 3.non real-time control, 4.paging, 5.access response, 6.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). D- AMPS

32 Channel multiplexingFDM+6 TDM slots Uplink (initially) (later) MHz; 832 channels (x6=4992) MHz Downlink (initially) (later) MHz; 832 channels MHz Channel Bandwidth30 kHz FDD separation45 MHz ModulationFM (traffic, voice); FSK (control) PCM ChannelsControl, Paging, Access, Data Channel Rate48.6 kbps Voice compression56 kbps to 8 kbps; PCM TDM frames25 frames, 40 msec each Time slots6 slots, 6.67 msec each; (25x6=150) D-AMPS; IS-54, IS-136 D- AMPS

33 2G; GSM (Group Special Mobile) (Global System for Mobile) Pan-European Cellular Standard: 2G; Digital FDD: ( MHz Upstr; 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. GSM

34 First approximation, 2G; GSM is similar to 2G; D-AMPS. GSMD-AMPS FDM Dual simplex channels Single frequency pair is split by TDM Channel 200 kHz wideChannel 30 kHz wide Higher data rate D-AMPS is used in the U.S. and in Japan (modified). Everywhere else in the world used GSM

35 GSM-The Global System for Mobile Communications GSM

36  Broade single-user bandwidth (1/T b )  GSM = FDMA 200 KHz + TDMA 8 slot/frame t t s1(t)s1(t) sN(t)sN(t) 12N t N * T frame T frame TDMA T frame

37 A portion of the GSM framing structure 1114 Ctl ,500 bit multiframe sent in 120 msec Bit data frame sent in msec 8.25-bit 30 mk. sec guard time 000 Information Sync Information Bit frame sent in 547 mk.sec Bits Voice/data bit GSM

38 GSM

39 GSM

40 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. GSM

41 GSM system (Cont) A data frame is transmitted in 547 mksec, but a transmitter is only allowed to send one data frame every 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 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. GSM

42 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: 1.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. 2.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. 3.Is the access grant channel (CC3c). The announced assigned slot. GSM

43 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) 1. paging channel To announce incoming calls 2. random access channel To request a slot on the dedicated control channel 3. access grant channel announced assigned slot 3 logical subchannels: GSM

44 BCCH: Broadcast Control Channel ä 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 ä point-to-point bidirectional control channel ä SACCH (Slow Associated Control Channel) ä FACCH (Fast Associated Control Channel) ä SDCCH (Stand Alone Dedicated Control Channel) GSM Signalling channels GSM

45 GSM

46 GSM

47 GSM

48 Channel multiplexingFDM+8 TDM slots Uplink (GSM900) (DCS1800) MHz; 125 channels (x8=1000) MHz Downlink (900) (DCS1800) MHz; 125 channels (x8=1000) MHz Channel Bandwidth200 kHz FDD separation45 (900) / 95 (1800) MHz ModulationFSK ChannelsBrdcst. Cont; Ded. Cont; Comn. Cont.= =Paging.+Rndm. Access+Acc. Grnt. Channel Rate13 kbps TDM frames24 frames, 120 msec each Time slots8 slots, msec each; (24x8=192) GSM;

49 CDMA (Code Division Multiple Access): IS-95 QUALCOMM, 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)

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

51 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”)

52 CDMA Encode/Decode

53 CDMA: two-sender interference

54 CDMA (cont’d) One of 64 PS (Pseudo Random) codes assigned to subscriber at call set up time RAKE receiver (to overcome mpath- fading) Pilot tone inserted in forward link for: –power control –coherent reference Speech activity detection Voice compression to 8 kbps (16 kbps with FEC)

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