An Introduction toTDMA , FDMA Wireless Systems Presented By: Mohsen Nadertehrani
Agenda • First generation (1 G) • Advanced mobile phone system (AMPS) • Other 1G standards • Second Generation • Global System for Mobile Communication (GSM) • Second Generation – IS-136 (DAMPS) • 2.5/3G TDMA : GPRS and EDGE
1G - Characteristics • Analogue transmission technology • Pioneered semiconductor and microprocessor technology • Focus on voice • Data services almost non-existent • Incompatible standards • Different frequencies and signaling • International roaming impossible • Inefficient use of the radio spectrum
AMPS • The first generation of wireless network (1983 , USA) • Anloge • FM modulation • Frequency devision duplex(FDD) • 25 MHz for uplink and 25 MHz for downlink • Uplink : 824-849 MHz Downlink : 869-894 MHz • Based on FDMA • Bandwidth of 30 KHz for each channel
1G - TACS • Total Access Coverage (TACS) • 1985 • UK • Adaptation of AMPS • TACS was adopted in other countries such as Hong Kong and Japan
1G - NMT-450 • Nordic Mobile Telephone 450 • 1982 • Sweden • First wireless communications standard deployed in Europe • Supported international roaming
1G - NTT • Nippon Telephone & Telegraph (NTT) • 1979 • Tokyo • World’s first operational cellular system
The 1G Landscape • A series of incompatible networks • Limited capacity for expansion • Limited support for roaming • Susceptible to interference • Poor security • No support for wireless data
Solution: 2G • Digital techniques rather than analogue • Increased flexibility • error control • compression • More efficient use of available bandwidth • Increased compatibility with the fixed component of the PSTN • Increased quality of service • Possibility of wireless data services
2G - GSM • Global System for Mobile Communication (GSM) • Conceived in 1982 • Deployed in 1992 in Europe • European Telecommunications Standards Institute (ETSI) • Most successful 2G system
2G - D-AMPS • Digital Advanced Mobile Phone Service (DAMPS) • Also called IS-54 (Interim Standard 54) • 1991 • IS-136 introduced in 1996
GSM: History • Developed by Group Speciale Mobile (founded 1982) which was an initiative of CEPT ( Conference of European Post and Telecommunication ) • Aim : to replace the incompatible analog system • Presently the responsibility of GSM standardization resides with special mobile group under ETSI ( European telecommunication Standards Institute ) Full set of specifications phase-I became available in 1990 • Under ETSI, GSM is named as “ Global System for Mobile communication “ • Today many providers all over the world use GSM (more than 135 countries in Asia, Africa, Europe, Australia, America) • More than 1300 million subscribers in world.
GSM Specifications • RF Spectrum • GSM 900 • Mobile to BTS (uplink): 890-915 Mhz • BTS to Mobile(downlink):935-960 Mhz • Bandwidth : 2* 25 Mhz • GSM 1800 • Mobile to BTS (uplink): 1710-1785 Mhz • BTS to Mobile(downlink) 1805-1880 Mhz • Bandwidth : 2* 75 Mhz
GSM – Radio Interface • TDMA + FDMA + optional Slow Freq. Hopping • 124 radio carriers, inter carrier spacing 200khz. • 890 to 915MHz mobile to base - UPLINK • 935 to 960MHz base to mobile - DOWNLINK • 8 channels/carrier 890MHz 915MHz 935MHz 960MHz 0 124 0 124
higher GSM frame structures 5 7 8 1 2 4 6 3 4.615 ms S user data tail tail user data S Training 1 3 1 57 bits 3 bits 57 bits 26 bits 546.5 µs 577 µs GSM – TDMA- Physical Channels 935-960 MHz 124 channels (200 kHz) downlink frequency 890-915 MHz 124 channels (200 kHz) uplink time GSM TDMA frame GSM time-slot (normal burst) guard space guard space
GSM – Speech Coding • Sampling Rate - 8K • Encoding - 13 bit Encoding (104 Kbps) • Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps BS Side 8 bit A-Law to 13 bit Uniform speech Encoder 8 K sps To Channel Coder 13Kbps MS Side speech Encoder 8 K sps, A/D LPF To Channel Coder 13Kbps
GSM – Channel coding • The 260 bits are divided into three classes: • Class Ia 50 bits - most sensitive to bit errors. • Class Ib 132 bits - moderately sensitive to bit errors. • Class II 78 bits - least sensitive to bit errors. • Class Ia bits have a 3 bit cyclic redundancy code added for error detection = 50+3 bits. • 132 class Ib bits with 4 bit tail sequence = 132 + 4 = 136. • Class Ia + class Ib = 53+136=189, input into a 1/2 rate convolution encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolution encoder thus outputs 378 bits, to which are added the 78 remaining class II bits. • Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.
3 3 3 3 3 3 3 3 57 bits 57 bits 57 bits 57 bits 57 bits 57 bits 57 bits 57 bits 1 1 1 1 1 1 1 1 26 26 26 26 26 26 26 26 1 1 1 1 1 1 1 1 57 bits 57 bits 57 bits 57 bits 57 bits 57 bits 57 bits 57 bits 3 3 3 3 3 3 3 3 GSM – Channel Interleaving • To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolution encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples. • 6 voice samples = 120ms • 26 TDMA Frames = 120 ms
IS-136 • As number of users using cellular phones increased in early 90s, companies looked for more bandwidth efficient cellular schemes. • Obvious choice was to use digital schemes with TDMA. • IS-136: complete digital.(48.6 kb/s digital control channel
Radio Transmission Base-station to mobile • Each time-slot carries 324 bits data rate per carrier 48.6 kb/s • IS-136 supports • Half-rate channel (8.1 kb/a) • Double full-rate channels (32.4 kb/s) • Triple full-rate channels (48.6 kb/s) 1.9 ms frequency Mobile to base-station 30 kHz slot Frame 40 ms
Radio Transmission • 30 KHz, 6 slots per frame, each user 2 slots, 40 msec frame • Some time offset between reverse and forward to not transmit and receive at same time, still do full duplex • 324 bits per slot, 6 slots/frame, in 40 msec=48.6 kbps • Full rate channel is 2 slots/frame = 16.2 kbps; also half rate, 2X, 3X • Uses DQPSK with possible 45 degree, 45+90, 45+180 and -45 degree shifts from each phase angle, so 4 possible next symbols, so 2 bits each, called pi/4 shifted DQPSK --- a 1.62 bps/Hz modulation spectral efficiency • Mobile transmits .25 mw up to 4 w, in 4 dB steps, but only 1/3 the time
2.5G • An enhancement to 2G networks that allows them to operate in a "packet switched" manner • 2.5G networks incorporate 2G technology with GPRS' higher speeds to support data transport. 2.5G is a bridge from the voice-centric 2G networks to the data-centric 3G networks. • GPRS (General Packet Radio Service) is a radio technology for GSM networks that adds packet-switching protocols. As a 2.5G technology, GPRS enables high-speed wireless Internet and other data communications. GPRS networks can deliver SMS, MMS, email, games, and WAP applications.
GPRS • The theoretical limit for packet switched data is approx. 170 kb/s. A realistic bit rate is 30-70 kb/s. It provides moderate speed data transfer, by using unused TDMA channels on a GSM network. • GSM circuit switch connections are still used for voice, but data is sent and received in "packets" in the same way as it would be in the fixed internet environment. • The advantage is that network resources are used more efficiently. Rather than maintaining a circuit for the duration of the connection, which ties up resources regardless of whether anything is actually being sent or received, GPRS only consumes resource when information packets are transmitted.
GPRS Airlink • General Packet Radio Service (GPRS) • Same GMSK modulation as GSM • 4 channel coding modes • Packet-mode supporting up to about 144 kbps • Flexible time slot allocation (1-8) • Radio resources shared dynamically between speech and data services • Independent uplink and downlink resource allocation
3G • 3G networks promise next-generation service with transmission rates of 144Kbps and higher that can support multimedia applications, such as video, video conferencing and Internet access. Both UMTS (WCDMA) and EDGE will support 3G services. 3G networks operate on a different frequency than 2G networks.
3G • UMTS (Universal Mobile Telecommunications System) or WCDM (Wideband Code Division Multiple Access) was selected as the successor to GSM. It is the European standard for 3G wideband digital radio communications, and it utilizes one 5 MHz channel for both voice and data, offering data speeds up to 2 Mbps. • EDGE is a mobile network radio technology that allows current GSM networks to offer 3G services within existing frequencies. As an evolution of GSM/GPRS, EDGE is an upgrade to GPRS' data and GSM's voice networks. EDGE provides data speed three times that of GPRS.
EDGE • Enhanced Data Rates for Global Evolution (EDGE) is a bolt-on enhancement to 2G and GPRS networks. This technology is compatible with TDMA and GSM networks. EDGE uses the same spectrum allocated for GSM850, GSM900, GSM1800 and GSM1900 operation. • Instead of employing GMSK (Gaussian minimum-shift keying) EDGE uses 8PSK (8 Phase Shift Keying) producing a 3bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE, like GPRS, uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) used to the quality of the radio channel, and thus the bit rate and robustness of data transmission. It introduces a new technology not found in GPRS, Incremental Redundancy, which, instead of retransmitting disturbed packets, sends more redundancy information to be combined in the receiver. This increases the probability of correct decoding.
EDGE Airlink • Extends GPRS packet data with adaptive modulation/coding • 2x spectral efficiency of GPRS for best effort data • 8-PSK/GMSK at 271 ksps in 200 KHz RF channels supports 8.2 to 59.2 kbps per time slot • Supports peak rates over 384 kbps • Requires linear amplifiers with < 3 dB peak to average power ratio using linearized GMSK pulses
EDGE vs GPRS • Same symbol rate – modulation bit rate differs • EDGE can transmit THREE times as GPRS during same period of time. Ref. Ericsson White paper on EDGE
References: • Mobile Wireless CommunicationsBy Mischa Schwartz • GSM, GPRS and EDGE Performance: Evolution Towards 3G/UMTS By Timo Halonen, Javier Romero Garcia, Juan Melero • GSM System EngineeringBy Asha Mehrotra