Lecture 13 – Evolution of Cellular Systems Introduction 1st Generation cellular systems 2nd Generation cellular systems 3rd Generation cellular systems Ben Slimane

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)

Cellular System Evolution

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

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

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

1G: AMPS

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

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

2G cellular Systems

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 (V m ): 250 km/hr Maximum coding delay: approx. 20 ms Maximum delay spread (  m ): 10  s

Steps in Design of TDMA Timeslot

GSM Frame Format Transmission bit rate = /0.577 = kbps

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

GSM Signal Processing

GSM Network Architecture

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

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

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

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

2.5G GPRS: General Packet Radio Service –Bitrates from 9.05 to 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

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

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)

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

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

The RAKE Receiver

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

Spreading in Cellular CDMA

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

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 Mbps for office use Symmetrical / asymmetrical data transmission rates Support for both packet switched and circuit switched data services

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

Alternative Interfaces

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

UMTS Wideband CDMA Uplink MHz Downlink 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

W-CDMA

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