Lecture 13 – Evolution of Cellular Systems

1. Lecture 13 – Evolution of Cellular Systems • Introduction • 1st Generation cellular systems • 2nd Generation cellular systems • 3rd Generation cellular systems Ben Slimane slimane@kth.se

2. Multiple Access/Multiplexing Methods • Frequency Division Multiple Access (FDMA) • Time Division Multiple Access (TDMA) • Frequency-Hop Code Division Multiple Access (FH-CDMA) • Direct Sequence-Code Division Multiple Access (DS-CDMA)

3. Cellular System Evolution

4. 1G Cellular Systems • Appeared in late 1970s and deployed in early 1980s • All based on analog techniques • All used FDMA and FM modulation • Date rate: 8-10 kbps • Low system capacity (reuse N=7) • Large cells with omni-directional base station antennas

5. 1G: NMT • 1981 Nordic Mobile Telephone • First generation analog technology • NMT450 and NMT900 • Free standard ready 1973, 1977 • Network open 1981 in Sweden and Norway • Based on FDMA • Channel bandwidth: 25/12.5 kHz • Total number of channels: 1999 • Analog traffic channel, digital control channel

6. 1G: AMPS • Advanced Mobile Phone System (AMPS) • Appeared late 1970s, • First deployed in 1983, US, South America, China, and Australia • Based on FDMA • Channel bandwidth: 30 kHz • Total number of channels: 832 channels

7. 1G: AMPS

8. 1G versus 2G • Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital • Encryption – all second generation systems provide encryption to prevent eavesdropping • Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception • Channel access – second-generation systems allow channels to be dynamically shared by a number of users

9. 2G Cellular Systems • Deployed in mid 1990s • 2G cellular systems all use digital voice coding and digital modulation • Can provide advanced call capabilities and a better system capacity • More users per unit bandwidth • Designed before the widespread of the Internet • Voice services and limited data services (SMS, FAX) • Data rate: on the order 10 kbps

10. 2G cellular Systems

11. 2G: GSM • Global Systems for Mobile Communications (GSM) • Based on TDMA • Channel bandwidth: 200 kHz • Traffic channels (slots) per RF channel: 8 • Maximum cell radius (R): 35 km • Frequency: 900/1800 MHz • Maximum vehicle speed (Vm): 250 km/hr • Maximum coding delay: approx. 20 ms • Maximum delay spread (m): 10 s

12. Steps in Design of TDMA Timeslot

13. GSM Frame Format • Transmission bit rate = 156.25/0.577 = 270.833 kbps

14. Time Slot Fields • Trail bits: allow synchronization of transmissions from mobile units • Encrypted bits: encrypted data (ciphertext bits) • Training sequence • A known bit pattern • used to estimate the multi-path radio channel • Stealing bit: block contains data or ”stolen” for control • Guard bits • used to avoid overlapping with other bursts • Speech information • The actual information data

15. GSM Signal Processing

16. GSM Network Architecture

17. Mobile Station • Mobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit • Mobile equipment (ME) – physical terminal, such as a telephone or PCS • ME includes radio transceiver, digital signal processors and subscriber identity module (SIM) • GSM subscriber units are generic until SIM is inserted • SIMs roam, not necessarily the subscriber devices

18. Base Station Subsystem (BSS) • BSS consists of a base station controller and one or more base transceiver stations (BTS) • Each BTS defines a single cell • Includes radio antenna, radio transceiver and a link to a base station controller (BSC) • BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging

19. Network Subsystem (NS) • NS provides link between cellular network and public switched telecommunications networks • Controls handoffs between cells in different BSSs • Authenticates users and validates accounts • Enables worldwide roaming of mobile users • The Mobile Switching Center (MSC) is the central element of the NS • The MSC controls four databases

20. The MSC Databases • Home location register (HLR) database – stores information about each subscriber that belongs to it • Visitor location register (VLR) database – maintains information about subscribers currently physically in the region • Authentication center database (AuC) – used for authentication activities, holds encryption keys • Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station

21. 2.5G • GPRS: General Packet Radio Service • Bitrates from 9.05 to 171.2 kbit/s • Multiple Time slots allocated to user • Link adaptations. • EDGE: Enhanced data rates for GSM evolution • Data rates up to 384 kbit/s • Two modulation schemes (GMSK, 8PSK) • Link adaptations

22. Cellular CDMA • Frequency diversity – resolve multi-paths by means of the RAKE receiver • Multipath resistance – chipping codes used for CDMA exhibit low cross-correlation and low autocorrelation • Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals • Graceful degradation – system only gradually degrades as more users access the system

23. Cellular CDMA • Self-jamming – non-orthogonal codes create interference between users • Near-far problem – weak users jammed by strong users • Soft handoff – smooth handoff from one cell to the next • more complex than hard handoff • Frequency reuse of 1 • No frequency planning needed (N=1)

24. Cellular CDMA • The RAKE receiver • Resolves multi-path components and combine them coherently • A diversity gain with order equals to the number of resolved paths is obtained • Soft Handoff • Mobile station temporarily connected to more than one base station simultaneously • Require more radio resources

25. The RAKE Receiver • Spreading codes with low correlation properties allow the separation of the different radio paths • The RAKE receiver uses this property and locks on the different paths

26. The RAKE Receiver

27. Soft handoff in CDMA • When a mobile unit is in soft handover • Two codes are needed on the downlink • Only one code is needed on the uplink

28. Spreading in Cellular CDMA

29. 2G: IS-95

30. 2G: IS-95 • Downlink • Pilot (0) – cell detection, channel estimation • Synchronization (32) – identification information • Paging (1-7) – messages to mobiles • Traffic (8-31, 33-63) – 55 traffic channels with data rate of 9600 bps • A unique channel for each user • Uplink • Access channels • Traffic channels

31. ITU’s View of 3G • Voice quality comparable to the public switched telephone network • 144 kbps data rate available to users in high-speed motor vehicles over large areas • 384 kbps available to pedestrians standing or moving slowly over small areas • Support for 2.048 Mbps for office use • Symmetrical / asymmetrical data transmission rates • Support for both packet switched and circuit switched data services

32. ITU’s View of 3G • An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound traffic • More efficient use of the available spectrum in general • Support for a wide variety of mobile equipment • Flexibility to allow the introduction of new services and technologies

33. Alternative Interfaces

34. CDMA Design Considerations • Bandwidth – limit channel usage to 5 MHz • Chip rate – depends on desired data rate, need for error control, and bandwidth limitations; 3 Mcps or more is reasonable • Multi-rate – advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service

35. UMTS • Wideband CDMA • Uplink 1920-1980 MHz • Downlink 2110-2170 MHz • Bandwidth 4,4-5 MHz • HSDPA: High Speed Downlink Packet Access • Data rates: 1,8, 3,6, 7,2 and 14,4 Mbit/s

36. W-CDMA

37. LTE • Long Term Evolution (LTE) • Advanced OFDM for downlink • Single carrier FDMA for uplink • Data rates exceeding 100 Mbps in the downlink with full mobility • Scalable bandwidth (1.25 to 20 MHz) • Frequency-reuse 1 • Multiple transmit and receive antennas