Mobile Communication Systems. Part 7- Multiplexing

Transcription

1 Mobile Communication Systems Part 7- Multiplexing Professor Z Ghassemlooy Scholl of Computing, Engineering and Information Sciences University of Northumbria U.K.

2 Contents Multiple Access Multiplexing SDM FDM TDM CDM Wideband Schemes Duplex Method

3 Multiple Access In today s data communications systems there is a need for several users to share a common channel resource at the same time. The resource could be: high speed optical fibre links between continents frequency spectrum in a cellular telephone system twisted pair ethernet cable in the office

4 Multiple Access For multiple users to be able to share a common resource in a managed and effective way, it requires: Some form of access protocol Defines how or when the sharing is to take place and the means for identifying individual messages. Process is known as multiplexing in wired networks and multiple access in wireless digital communications.

5 Multiplexing/Multiple Access There four possible ways to divide the frequency spectrum among many channels: Space-division multiplexing (SDM) Frequency-division multiplexing (FDM) / Frequency Division Multiple Access (FDMA) Time-division multiplexing (TDM) / Time Division Multiple Access (TDMA) Code-division multiplexing (CDM) / Code Division Multiple Access (CDMA)

6 SDM channels k i k 1 k 2 k 3 k 4 k 5 k 6 c t c t s 1 c f s 2 t f s 3 f

7 Frequency Multiplex I Dividing the entire frequency spectrum into smaller bands A frequency band is allocated per channel for the entire transmission time FDM, used in 1st generation systems, wastes spectrum Advantages: lower channel bit rate (than TDM) means less susceptible to multi path ISI Requires coordination works also for analog signals code k 1 k 2 k 3 k 4 k 5 k 6 f t

8 Frequency Multiplex II Disadvantages: In-efficient use of bandwidth if the traffic is distributed unevenly Requires guard band between channels Cannot readily support variable user data rates, fixed channel width means fixed bit rate

9 Time multiplex I Entire spectrum is allocated for a channel some of the time For 2nd generation Advantages: Only one carrier in the medium at any given time High throughput even for many users Common TX component design, only one power amplifier k 1 code k 2 k 3 k 4 k 5 k 6 f Disadvantages: t precise synchronization necessary requires terminal to support a much higher data rate than the user information rate Time slots

10 Example TDMA System GSM is a good example of a TDMA system GSM handsets transmit data at a rate of 270 kbit/s in a 200 khz channel using GMSK modulation. each frequency channel is assigned 8 users, each having a basic data rate of around 13 kbit/s

11 TDMA Frame TDMA used for the 3G air interface A frame length: ms and it consist of 64 1/64 time slots of length /16 time slots of length 288 Downlink Uplink 72μs 288μs Switching point between uplink and downlink

12 Time and Frequency Multiplex I Combination of both methods A certain frequency band for a given amount of time is allocated per channel Example: GSM Advantages: Improved protection against tapping and frequency selective interference k 1 Higher data rates compared to code code multiplex Disadvantages: Requires precise coordination k 2 k 3 k 4 k 5 k 6 f t

13 Code Multiplex I Each channel has a unique code. All channels use the same spectrum at the same time. Advantages: bandwidth efficient and good power control no need for coordination and synchronization good protection against interference and tapping k 1 k 2 k 3 k 4 k 5 k 6 coding Disadvantages: lower user data rates more complex signal regeneration f t Implemented using spread spectrum technology

14 CDMA Classification CDMA : direct sequence (DS) CDMA : frequency hopping (FH) Carrier frequency changes periodically, after T secs Hopping pattern determined by spread code CDMA : time hopping (TH) Data transmitted in rapid bursts Time intervals determined by code Frequency Direct sequence Frequency hopping Time hopping Time

15 Direct Sequence CDMA Directly modulated, discrete time, discrete valued code signal Analogue or Digital Code bits are chips (±1) Rate of Code >> Rate of Data PSK, BPSK, D-BPSK, QPSK or MPSK Data DS-SS Transmitter Data modulator Spreading modulation Carrier generator Code generator

16 DS-SS Transmitter & Receiver Binary Data X Wideband modulator Despreading Data demodulator Binary Data Code generator Carrier generator Code Synchronisation/trac king Code generator Carrier generator

17 CDMA Evolution Early Stages Narrowband Wideband 1949 John Pierce : time hopping spread spectrum 1949 Claude Shannon and Robert Pierce : basic ideas of CDMA 1950 De Rosa-Rogoff : direct sequence spread spectrum 1956 Price and Green antimultipath : RAKE patent 1961 Magnuski : near-far problem 1970s Several developments for military field and navigation systems 1978 Cooper and Nettleton : cellular application of spread spectrum 1980s Investigation of narrowband CDMA techniques for cellular applications 1986 Formulation of optimum multiuser detection by Verdu 1993 IS-95 standard Europe : FRAMES FMA2 Japan : Core-A WCDMA USA : cdma2000 Korea : TTA I, TTA II 2000s Commercialization of wideband CDMA systems

18 Wideband-CDMA Framing structure Radio Frame (10ms) frame #i frame #i+1 Time Slot (2560*T c ) timeslot #0 timeslot #1 timeslot #2 timeslot #13 timeslot #14 T c = chip time = 1 / 3.84 μs

19 High Speed Wireless Access Mobile communication system Up to 30 Mbps Using the SHF and other band (3-60 GHz) Used for mobile video telephone conversations

20 Ultra High Speed Wireless LAN Wireless LAN Up to 156 Mbps Using the millimeter wave radio band ( GHz) Used for high quality TV conferences.

21 5GHz Band Mobile Access Two types ATM type Wireless Access Ethernet type Wireless LAN Using 5GHz band Each system can transmit at up to 20-25Mbps Used for multimedia information

22 High Data Rate Wireless LAN Evolution Gigabit Ethernet (1G bit/s) Fast Ethernet (100M bit/s) Ethernet (10M bit/s) ATM Ethernet (10M bit/s) Conventional 2.4GHz Ethernet Wireless LAN 2M bit/s Future 5GHz ATM Wireless LAN 25M bit/s 5GHz Ethernet Wireless LAN (IEEE802.11) 36Mbit/s IMT kbit/s 2Mbit/s ARIB, Japan, 1999

23 Wireless Home-Link Wireless Home-Link Up to 100Mbps Using the SHF and other band(3-60ghz) Between PCs and Audio Visual equipments Multimedia information.

24 Home Link Concept Display Personal Computer 5GHz Display CATV DVD VTR Telephone line Personal Computer 5GHz Display Personal Computer Satellite Tuner

25 Duplex Methods Separating the send and receive signals (remember full duplex). Two approaches: Frequency Division Duplex (FDD) Uses a pair of frequency bands one for uplink and another for downlink used in all second generation cellular systems requires good frequency separation filters - diplexer Time Division Duplex (TDD) Uses a single frequency band for both uplink and downlink sharing the transmission time propagation delay limits cell size very efficient for asymmetric traffic, e.g. internet download used in cordless systems (DECT) and wireless LANs

26 What is Universal Mobile Telecommunication System? European name for third generation (3G) radio system (1G = analog, 2G = digital voice and low speed data (GSM)) Key features with respect to 2G: Integration of fixed and mobile networks Expanded range of services (Packet, Internet, Multimedia) Bit rates: Rural outdoor: 144 kb/s, 500 km/h Suburban outdoor: 384 kb/s, 120 km/h Indoor, low range outdoor: 2Mb/s, 10 km/h Flexibility: Variable bit rates Circuit switched and packet oriented bearers Negotiation of bearer service attributes (bearer type, bit rate, delay BER, up/down symmetry, protection) Adaptability to quality, traffic, network load & radio conditions

27 Summary Multiple Access - sharing resources Frequency Division Multiple Access - FDMA Time Division Multiple Access - TDMA [Code Division Multiple Access CDMA] Duplex Methods Frequency Division Duplex - FDD Time Division Duplex - TDD

28 Questions and Answers Tell me what you think about this lecture