Overview of Wireless Medium Access Techniques

1. Wireless Medium Access Techniques“ETN785” Dr. Safdar H. Bouk Room#327, Dept. of EE COMSATS Institute of Information Technology, Islamabad

2. Course Outline Wireless Issues MAC Layer Categories IEEE 802.11 Standard QoS Management (IEEE 802.11e) Vehicular MAC (IEEE 802.11p) WiMAX Zigbee

3. Background • Physical layer functions • Get stream of bits and transport them to the other end. • Modulation/Demodulation • It is not an easy task • Large-scale path loss and Small-scale fading and multipath effects causes the received power at the receiver to • Fluctuate (hard to decode the symbols (bits)) • To decrease (Affects of interfering sources increases) • Received signal power level affect the quality of the signals (information) that is transported. • Received signal power level defines the Signal-to-Noise (SNR) ratio

4. Background • We know: • That SNR and bandwidth of a channel affects • Datarate – bps) of the wireless channel by Shannon limit • The bit error rate (BER) on the channel. • That multipath fading results in a wireless channel error model that changes states between good (low-error rate) and bad (high error-rate) • Large-scale path loss defines the range of stations for different environments (LOS, urban,…) • The above factors are important channel characteristics that affect the design of wireless systems architectures and design of the protocols and applications for wireless links/networks • These are some of the Fundamental Concepts of Wireless Communication.

5. Questions!!! • In broadcast networks, How the channel is divided between competing users? • What is Medium Access Control (MAC)? • What protocols are used for allocating a multiple access channel ?

6. Motivation – Hidden Terminal Problem A B C A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C

7. Motivation – Exposed Terminal Problem D A B C B sends to A, C wants to send to D C has to wait, CS signals a medium in use since A is outside the radio range of C waiting is not necessary C is “exposed” to B

8. Motivation - Near and Far Terminals A B C • Terminals A and B send, C receives • the signal of terminal B hides A’s signal • C cannot receive A • precise power control required

9. Who gets the Channel Access? • When there are two ormore users trying to use a shared single channel there should be an algorithm to control this access. • This problem occurs in broadcast networks which are known as multi-access channels.

10. What is MAC? • Wireless spectrum (frequency band) is a very precious and limited resource. • We need to use this resource very efficiently • We also want our wireless system to have high user capacity • A lot of (multiple) users should be able to use the system at the same time. • For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system. • The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…)

11. What is MAC? • Medium Access Control (MAC) is a sublayer of the Data-link layer. • The protocols used to determine who goes next on a multiaccess channel belongs to a MAC sublayer. • MAC is important in LAN which use a multiaccess channel as the basis for communication.

12. Data link layer divided into two functionality-oriented sublayers

13. Taxonomy of multiple-access protocols discussed in this chapter

14. Wireless Media Access Control • Random Schemes (Less-Coordinated) • Examples: MACA, MACAW, Aloha, 802.11 MAC,… • More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs • Coordinated Schemes • Examples: TDMA, FDMA, CDMA • More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,… • Polling based Schemes • Examples: Bluetooth, BlueSky,… • Access is coordinated by a central node • Suitable for Systems that wants low-power, aims to carry voice and data at the same time.

15. Duplexing • It is sharing the media between two parties. • If the communication between two parties is one way, the it is called simplex communication. • If the communication between two parties is two- way, then it is called duplex communication. • Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver. • Duplex communication achieved by: • Time Division (TDD) • Frequency Division (FDD) • Some other method like a random access method

16. Duplexing • Usually the two parties that want to communication in a duplex manner (both send and receive) are: • A mobile station • A base station • Two famous methods for duplexing in cellular systems are: • TDD: Time Division Duplex • FDD: Frequency Division Duplex

17. Duplexing - FDD F • A duplex channel consists of two simplex channel with different carrier frequencies • Forward band: carries traffic from base to mobile • Reverse band: carries traffic from mobile to base M B R Base Station Mobile Station Reverse Channel Forward Channel frequency fc,,F fc,R Frequency separation Frequency separation should be carefully decided Frequency separation is constant

18. Duplexing - TDD • A single radio channel (carrier frequency) is shared in time in a deterministic manner. • The time is slotted with fixed slot length (sec) • Some slots are used for forward channel (traffic from base to mobile) • Some slots are used for reverse channel (traffic from mobile to base) M B Mobile Station Base Station Slot number 0 1 2 3 4 5 6 7 … F R F R F R F R …. channel Reverse Channel Forward Channel time Ti+1 Ti Time separation

19. Duplexing – TDD versus FDD • FDD • FDD is used in radio systems that can allocate individual radio frequencies for each user. • For example analog systems: AMPS • In FDD channels are allocated by a base station. • A channel for a mobile is allocated dynamically • All channels that a base station will use are allocated usually statically. • More suitable for wide-area cellular networks: GSM, AMPS all use FDD • TDD • Can only be used in digital wireless systems (digital modulation). • Requires rigid timing and synchronization • Mostly used in short-range and fixed wireless systems so that propagation delay between base and mobile do not change much with respect to location of the mobile. • Such as cordless phones…

20. Multiple Access - Coordinated • We will look now sharing the media by more than two users. • Three major multiple access schemes • Time Division Multiple Access (TDMA) • Could be used in narrowband or wideband systems • Frequency Division Multiple Access (FDMA) • Usually used narrowband systems • Code Division Multiple Access • Used in wideband systems.

21. Narrow- and Wideband Systems • Narrowband System • The channel bandwidth (frequency band allocated for the channel is small) • More precisely, the channel bandwidth is small compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel) • AMPS is a narrowband system (channel bandwidth is 30kHz in one-way) • Wideband Systems • The channel bandwidth is large • More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel. • A large number of users can access the same channel (frequency band) at the same time.

22. Frequency Division Multiple Access B • Individual radio channels are assigned to individual users • Each user is allocated a frequency band (channel) • During this time, no other user can share the channel • Base station allocates channels to the users fN,F f1,F f2,F f2,R f1,R fN,R … M M M

23. Features of FDMA • An FDMA channel carries one phone circuit at a time • If channel allocated to a user is idle, then it is not used by someone else: waste of resource. • Mobile and base can transmit and receive simultaneously • Bandwidth of FDMA channels are relatively low. • Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low) • Lower complexity systems that TDMA systems.

24. Capacity of FDMA Systems Frequency spectrum allocated for FDMA system … Guard Band channel Guard Band Bt : Total spectrum allocation Bguard: Guard band allocated at the edge of the spectrum band Bc : Bandwidth of a channel AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz channel bandwidth (simplex): Number of channels is 416.

25. Time Division Multiple Access • The allocated radio spectrum for the system is divided into time slots • In each slot a user can transmit or receive • A user occupiess a cyclically repeating slots. • A channel is logically defined as a particular time slot that repeats with some period. • TDMA systems buffer the data, until its turn (time slot) comes to transmit. • This is called buffer-and-burst method. • Requires digital modulation

26. TDMA Concept Downstream Traffic: Forward Channels: (from base to mobiles) 1 2 3 … N 1 2 3 …. N … Logical forward channel to a mobile Base station broadcasts to mobiles on each slot Upstream Traffic: Reverse Channels: (from mobile to base) 1 2 3 … N 1 2 3 …. N … Logical reverse channel from a mobile A mobile transmits to the base station in its allocated slot Upstream and downstream traffic uses of the two different carrier frequencies.

27. TDMA Frames • Multiple, fixed number of slots are put together into a frame. • A frame repeats. • In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels. • In TDMA/FDD: a different carrier frequency is used for a reverse or forward • Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa). • Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame.

28. General Frame and Time Slot Structure in TDMA Systems One TDMA Frame Preamble Information Trail Bits Slot 1 Slot 2 Slot 3 … Slot N Guard Bits Sync Bits Control Bits Information CRC One TDMA Slot A Frame repeats in time

29. A TDMA Frame • Preamble contains address and synchronization info to identify base station and mobiles to each other • Guard times are used to allow synchronization of the receivers between different slots and frames • Different mobiles may have different propagation delays to a base station because of different distances.

30. Efficiency of a Frame/TDMA-System • Each frame contains overhead bits and data bits. • Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits. bT: total number of bits in a frame Tf: frame duration (seconds) bOH: number of overhead bits Number of channels in a TDMA cell: m: maximum number of TDMA users supported in a radio channel

31. TDMA • TDMA Efficiency • GSM: 30% overhead • DECT: 30% overhead • IS-54: 20% overhead. • TDMA is usually combined with FDMA • Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other. • There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels) • For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA.

32. Contemporary TDMA Systems