Wireless Networks a.y. 2010-2011 Channel partitioning protocols Giacinto Gelli DIBET gelli@unina.it 1
Outline Introduction Duplexing techniques FDD TDD Channel partitioning techniques FDMA TDMA CDMA Hybrid techniques 2
Introduction Medium access control (MAC) schemes allow many users/stations to share simultaneously the same communication resource: Cable, fiber in wired systems Radio spectrum in wireless systems Multiple channels can be obtained by partitioning the available communication resource along one or many directions (e.g., time, frequency, code, space) => multiplexing Another important issue is how to separate uplink/downlink transmissions => duplexing 3
Two-way communications Simplex systems communication is unidirectional e.g. broadcast systems, paging systems Half-duplex systems communication is bidirectional, but not simultaneous e.g. push-to-talk systems, CB radio Full-duplex systems communication is bidirectional and simultaneous e.g. cellular systems, WLAN Many wireless systems are full duplex => duplexing techniques are needed to support simultaneous bidirectional communications on the same medium 4
Duplexing techniques Uplink channel Downlink channel Uplink channel Downlink channel f u frequency f d f t time t u t d separation separation Frequency Division Duplexing (FDD) DL/UL use a pair of different frequencies Analog and digital systems e.g. TACS, GSM Time Division Duplexing (TDD) DL/UL use a pair of different time slots Only digital systems e.g. DECT 5
Basic classification of MAC protocols Channel partitioning protocols: The communication resource is partitioned in N channels that are assigned to stations is a quasi-static way TDMA, FDMA, CDMA and combinations Random access protocols: Each station tries to access the full communication resource in a random, uncoordinated manner => collisions occur ALOHA and variants Taking-turns protocols: Usage of the communication resource is disciplined by some turning mechanisms Polling, token-passing, etc. 6
Ideal characteristics of MAC protocols Broadcast channel of rate R b bps 1) When one station wants to transmit, it can send at rate R b 2) When N stations want to transmit, each can send at average rate R b /N 3) Fully decentralized: - no special station to coordinate transmissions - no synchronization of clocks, slots 4) Simple and scalable 7
Channel partitioning protocols Channels are obtained by partitioning the communication resource along one or more dimensions: FDMA=Frequency Division Multiple Access TDMA=Time Division Multiple Access CDMA=Code Division Multiple Access Hybrid schemes (TDMA+FDMA, TDMA+CDMA, etc.) Channel partitioning techniques are mainly used in voiceoriented networks 8
FAMA vs. DAMA Fixed Assignment Multiple Access (FAMA): each user is permanently allocated a channel: Inefficient use of resources when the number of users >> number of channels Demand Assignment Multiple Access (DAMA): the available channels are allocated to users only on demand, after the use the channels are returned to a central pool Efficient use of resources when the number of users >> number of channels and the percentage of occupancy of the channel is low 9
FDMA code The available bandwidth is divided in N frequency bands In any band, only one user is allowed to transmit/receive 1 2 3 4 5 frequency Utilized in analog cellular systems (AMPS, TACS) time 10
FDMA (cont d) Pros: No temporal synchronization needed Can be employed also with analog systems Cons: Not flexible => one user is limited to transmit at rate R b /N even when he/she is the only user of the system Small efficiency when traffic is bursty Guard bandwidths needed to reduce the interference between channels (ACI=Adjacent Channel Interference) Vulnerable to non-linearities (channel intermodulation) 11
Example: TACS Total Access Communication System 1G analog cellular system f 960 MHz Duplexing: FDD, multiplexing: FDMA UL bandwidth: 890-915 MHz, DL bandwidth:935-960 MHz. DL Every FDMA channel is 25 khz wide Fixed separation of 45 MHz between each DL and UL channel Maximum number of duplex channel is (25MHz /25 khz) = 1000 Evolved in E-TACS (Extended TACS) 45 MHz UL 935 MHz 915 MHz 25 khz channel uplink 872-888 MHz, downlink 917-933 MHz, further 640 channels 890 MHz 12
TDMA code The available time is divided in N time slots In any slot, only one user can transmit/receive 1 2 3 4 5 frequency The slots constitute a structure that repeats periodically => frame Rarely used alone, often in combination with other techniques (TDMA+FDMA, TDMA+CDMA) time 13
TDMA (cont d) Pros: Discontinuous (burst) transmission => power saving More flexible to accommodate users with different rates => assign more slots to high-speed users Cons: One user is limited to transmit at rate R b /N even when he/she is the only user of the system => same limitation of FDMA Accurate synchronization required (difficult in uplink) Only for digital systems Guard times required (similar to guard bandwidths in FDMA) 14
FDMA/TDMA code 24 19 14 9 4 23 18 13 8 3 22 17 12 7 2 21 16 11 6 1 25 20 15 10 5 frequency Combination of FDMA and TDMA In any slot and in any band, only one user can transmit/receive Utilized in 2G digital cellular systems (IS-54, GSM) Utilized in DECT time 15
FDMA/TDMA (cont d) Pros: Capacity increase over TDMA and FDMA Increased protection against eavesdropping Increased protection against interference and frequency-selective fading Cons: One user is limited to transmit at rate R b /N even when he/she is the only user of the system => same limitation of FDMA and TDMA Accurate synchronization and coordination Only for digital systems 16
Example: GSM 45 MHz DL UL f 960 MHz 200 khz channel 935 MHz 915 MHz 890 MHz frame=4.615 ms 0 1 2 3 4 5 6 7 time slot = 577μs 0 1 2 3 4 5 6 7 Global System for Mobile Communications t 2G digital cellular system Duplexing FDD, multiplexing FDMA/TDMA UL bandwidth: 890-915 MHz, DL bandwidth: 935-960 MHz (like TACS), extensions to 1.8 GHz and 1.9 GHz (USA) Every FDMA channel is 200 khz wide and multiplexes 8 TDMA channels Maximum number of duplex channel is (25MHz /200 khz) 8 = 1000 (like TACS) 17
Example: DECT Digital Enhanced Cordless Telecommunications 2G digital cordless system Duplexing TDD, multiplexing FDMA/TDMA UL/DL bandwidth: 1880-1900 MHz f 1897.344 MHz DL 1.728 MHz channel 1881.792 MHz frame=10 ms UL 0 1 2 3 11 12 13 14 15 23 time slot = 416.7 μs 10 FDMA channels with channel spacing 1.728 MHz multiplexing 12 TDMA channels => total number of channels = 120 Each frame of 10 ms contains 24 slots: the first 12 are for UL, the last 12 for DL t 18
CDMA time 5 4 3 2 1 code frequency Every user is characterized by a different code Users transmit in the same time/band possibly without coordination Transmission bandwidth is much larger than information bandwidth => spread spectrum techniques (DSSS, FHSS) Utilized in 2G and 3G cellular systems (IS-95, W-CDMA) 19
CDMA (cont d) Pros: User coordination not required Increased protection against eavesdropping Capacity degrades gradually with increasing number of users Flexibility to accommodate multirate users Robustness to fading multipath, NBI and ISI => overlay with narrowband systems Cons: accurate power control required (near-far effect) high transmission rates (chip-rate) 20
References A. S. Tanenbaum. Computer Networks (4th edition). Prentice-Hall, 2003 (chap. 4) A. Goldsmith. Wireless Communications. Cambridge University Press, 2005 (chap. 14) J. H. Schiller. Mobile Communications. Addison-Wesley, 2003 (chap. 3 & 7) 21