Mobile Communication Systems Part 7- Multiplexing Professor Z Ghassemlooy Faculty of Engineering and Environment University of Northumbria U.K. http://soe.ac.uk/ocr Z. Ghassemlooy

Contents Multiple Access Multiplexing Wideband Schemes Duplex Method SDM FDM TDM CDM Wideband Schemes Duplex Method Z. Ghassemlooy

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 Z. Ghassemlooy

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. Z. Ghassemlooy

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) Z. Ghassemlooy

Space-division Multiplexing (SDM) f t c k2 k3 k4 k5 k6 k1 channels ki The spatial dimension is used for multiplexing Data stream are transmitted over, non-overlapping transmission channels Uses spot beam antennas Base station tracks user when moving Cover areas with same frequency as TDMA, CDMA, and FDMA Can be achieved using: Beam forming Sectorization But, needs perfect adaptive antenna system: infinitely large antenna needed Compromise needed Z. Ghassemlooy

Frequency Multiplexing I Dividing the entire frequency spectrum into smaller bands A frequency band (carrier) is allocated per channel for the entire transmission time FDM, used in 1st generation systems (wastes spectrum), GSM and UMTS FDD Mode Two bands of frequencies for every user Advantages: lower channel bit rate (than TDM) means less susceptible to multi path ISI requires coordination works also for analog signals in contrast to TDM, each stream can continuously transmit within its sub- band k2 k3 k4 k5 k6 k1 f t code Z. Ghassemlooy

Frequency Multiplexing II OFDM Is an efficient FDMs, which offers minimum spacing of the sub-bands without ISI Frequency Division Multiple Access In multiple access schemes, where different data streams belong to different users 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 Number of channels in a FDMA system N … number of channels Bt … total spectrum allocation Bguard … guard band Bc … channel bandwidth Z. Ghassemlooy

Time multiplexing I Disadvantages: code f t Entire spectrum is allocated for a channel some of the time. multiple users share a single radio channel For 2nd generation Uses time for forward and reverse link 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 Disadvantages: precise synchronization necessary requires terminal to support a much higher data rate than the user information rate f t code k2 k3 k4 k5 k6 k1 Time slots Z. Ghassemlooy

Example TDMA System GSM is a good example of a TDMA system GSM (850-900 MHz) 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 Signal is divided by time using the fundamental unit of a burst period. This burst period = 15/26 ms and is grouped together by 8 bursts into a frame. A single traffic channel is defined by grouping 26 frames together; giving a total timeframe of 120 ms. These traffic channels are used to transfer speech and data. Number of channels Z. Ghassemlooy

Switching point between uplink and downlink TDMA Frame TDMA used for the 3G air interface A frame length: 4.615 ms and it consist of 64 1/64 time slots of length 72 usec 16 1/16 time slots of length 288 usec Downlink Uplink 72ms 288ms Switching point between uplink and downlink Efficiency: It is a measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme f: Frame efficiency bOH: Number of overhead bits per frame bT: Total number of bits per frame f = (1-bOH/bT)*100% Z. Ghassemlooy

Time and Frequency Multiplexing 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 Higher data rates compared to code multiplex Disadvantages: Requires precise coordination f t code k2 k3 k4 k5 k6 k1 Z. Ghassemlooy

Code Division Multiplexing Each channel has a unique spreading code. All channels use the same spectrum at the same time. Spreading codes should as far as possible be orthogonal to each other to reduce interference The codes, one/zero sequences, used to differentiate signals, are designed and generated at a much higher rate than the baseband information.   This rate is referred to as a chip rate rather than a bit rate. k2 k3 k4 k5 k6 k1 f t coding Spreading factor = Chip rate/Data rate

Code Division Multiplexing Advantages: bandwidth efficient and good power control no need for coordination and synchronization good protection against interference and tapping Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology

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 Direct sequence Frequency hopping Time hopping Time Frequency First I will introduce you to 3G communication systems then I will mention my research objectives. Z. Ghassemlooy

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 Spreading modulation Data modulator DS-SS Transmitter Code generator Carrier Z. Ghassemlooy

DS-SS Transmitter & Receiver X Wideband modulator Binary Data Code generator Carrier Despreading Data demodulator Binary Code generator Carrier Synchronisation/tracking Z. Ghassemlooy

CDMA Evolution Early Stages Narrowband Wideband 1949 John Pierce : time hopping spread spectrum 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 Z. Ghassemlooy

Wideband-CDMA Tc = chip time = 1 / 3.84 ms Framing structure Z. Ghassemlooy

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 Z. Ghassemlooy

Ultra High Speed Wireless LAN Up to 156 Mbps Using the millimeter wave radio band (30-300 GHz) Used for high quality TV conferences. Z. Ghassemlooy

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

High Data Rate Wireless LAN Evolution ATM Gigabit Ethernet （1G bit/s) Fast Ethernet （100M bit/s) Ethernet （10M bit/s) Ethernet (10M bit/s) Conventional 2.4GHz Ethernet Wireless LAN 5GHz (IEEE802.11) Future 5GHz ATM Wireless LAN 25M bit/s IMT 2000 384kbit/s〜2Mbit/s 36Mbit/s 2M bit/s ARIB, Japan, 1999 Z. Ghassemlooy

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. Z. Ghassemlooy

Home Link Concept Satellite Tuner CATV DVD VTR Telephone line 5 GHz Personal Computer Display Z. Ghassemlooy

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 Z. Ghassemlooy

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 Z. Ghassemlooy

Summary Multiple Access - sharing resources Duplex Methods 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 Z. Ghassemlooy

Questions and Answers Tell me what you think about this lecture fary@ieee.org Z. Ghassemlooy