Wireless Networks

1. Wireless Networks Instructor: Fatima Naseem Computer Engineering Department, University of Engineering and Technology, Taxila

2. Lecture # 13 Broadband Wireless

3. Broadband Wireless Background • Deregulation of telephone system in many countries • Competitors are now allowed to offer local voice & Internet service • Running Fiber, coax, or cat 5 UTP to millions of homes & businesses is Prohibitively expensive • Solution • Broadband Wireless

4. Broadband Wireless Access (BWA) • Technology for high-speed connection over the air • Uses radio waves to transmit and receive data • BWA is a point-to-multipoint system • Made up of base station and subscriber equipment. • Base station uses an outdoor antenna to send and receive high-speed data and voice to subscriber equipment

5. What is WiMAX?

6. Prior Attempts: LMDS & MMDS • Local Multipoint Distribution Service (1998) • 1.3 GHz around 28 GHz band (Ka Band) • 28 GHz ⇒ Rain effects • Multi-channel Multipoint Distribution Services (1999-2001) • 2.1, 2.5-2.7 GHz Band ⇒ Not affected by rain • Issues: Equipment too expensive, Roof top LoS antennas, short range (LMDS) or too small capacity (MMDS)

7. WiMax (Worldwide Interoperability for Microwave Access) – IEEE 802.16 • Based on Wireless MAN technology • WiMAX standard consist of • Fixed system (IEEE 802.16d-2004 Air Interface • standard) • Mobile system (IEEE 802.16e) • Define specifications for the PHY & MAC layer • PHY layer specs • Frame structure, OFDMA, modulation, and coding • MAC layer specs • Data and control plane, sleep mode for the terminals

8. Key Features of WiMAX • Works on many bands: 2.3 GHz, 2.5 GHz, 3.5 GHz, … • Scalable  Can use any available spectrum width: • 1.25 MHz to 28 MHz • Strong security • Open technology like WiFi • Reach and mobility like Cellular but much higher data rates • High data rate, up to 70Mbps • Long distance, up to 50kms • Mobility, up to 120 to 150 km/hour • Data rate vs Distance trade off using adaptive modulation. • 64QAM to BPSK • Offers non-line of site (NLOS) operation • Strong QoS  Guaranteed services for data, voice, and video

9. WiMAX • WiMAX ≠ IEEE 802.16 • Worldwide Interoperability for Microwave Access • 420+ members including Semiconductor companies, equipment vendors, integrators, service providers. Like Wi-Fi Alliance • Narrows down the list of options in IEEE 802.16 • WiMAX forum lists certified base stations and subscriber stations from many vendors

10. Effect of Frequency • Higher Frequencies have higher attenuation, • e.g., 18 GHz has 20 dB/m more than 1.8 GHz • Higher frequencies need smaller antenna • Antenna > Wavelength/2, 800 MHz ⇒ 6” • Higher frequencies are affected more by weather • Higher than 10 GHz affected by rainfall • 60 GHz affected by absorption of oxygen molecules • Higher frequencies have more bandwidth and higher data rate • Higher frequencies allow more frequency reuse • They attenuate close to cell boundaries. Low frequencies propagate far. • Mobility  Below 10 GHz

11. WiMax- Type of Service • Line-of-sight • A fixed dish antenna points straight at the WiMAX tower from a rooftop or pole • The line-of-sight connection is stronger and more stable, so it's able to send a lot of data with fewer errors • Line-of-sight transmissions use higher frequencies, with • ranges reaching a possible 66 GHz • Non-line-of-sight • A small antenna on your computer connects to the WiMAX tower • WiMAX uses a lower frequency range -- 2 GHz to 11 GHz

12. Comparison of 802.11 with802.16 • Why devise a new standard? Why not just use 802.11 • Reasons: • 802.16 provides service to buildings & buildings are not mobile. Do not migrate from cell to cell often • 802.16 runs over a part of city, distances involved can be several kilometers • In 802.16, each cell has more users than 802.11cell & these users are expected to use more BW than a typical 802.11. so more spectrum needed • 802.16 works in a 10-to-66 GHz range but these millimeter waves have different properties then the longer waves in ISM bands • Strongly absorbed by water • Line of sight communication

13. Data rate vs. Mobility

14. Why not WIFI • Scalability • Relative Performance • Quality of Service • Range • Coverage • Security

15. The 802.16 Protocol Stack • Differences with 802.11 • Transmission sublayer is used to hide the different technologies from data link layer • Security sublayer • MAC sublayer is completely connection oriented • Services-specific convergence sublayer takes place of LLC • Provides support for both connectionless & connectionoriented protocols

16. IEEE 802.16 PHYs

17. The 802.16 Physical Layer • Base station has multiple antennas • Each antenna pointing at different sector of surrounding terrain due to line of sight requirement for millimeter waves • Each sector has his own users & independent of surrounding sectors • De-Merits • Signal strength in millimeter band falls off sharply with distance from base station • SNR drops with distance from base station • Solution • Employs three different modulation schemes • Based on how far subscriber station is far from base station

18. The 802.16 Modulation • Close-in subscribers • QAM-64 is used with 6 bits/baud • Medium-distance subscribers • QAM-16 is used with 4 bits/baud • Distant subscribers • QPSK is used with 2 bits/baud • Example: for 25 MHz of spectrum • QAM-64 gives 150Mbps • QAM-16 gives 100Mbps • QPSK gives 50 Mbps • Farther the subscriber is from base station, lower the data rate • Transmission environment is shown pictorially on next slide

19. The 802.16 TransmissionEnvironment

20. The 802.16 Multiplexing • TDD: Time Division Duplexing • Base station periodically send out frames, each containing time slots; Refer next slide • 1st ones are for downstream traffic • Guard frames are used to switch direction • Number of time slots devoted to each direction can be changed dynamically to match the BW in each direction to the traffic • Downstream traffic is mapped into time slots by base station. • Upstream is more complex & depends upon on quality of service • Use of Hamming codes to do FEC in physical layer

21. Frames and time slots for TDD

22. 802.16 Frame Structure

23. Mobile WiMAX Frame

24. Frame Structure • DL Preamble: Time and frequency synchronization • Frame Control Header (FCH): MAPs lengths, modulation and coding, usable subcarriers • Downlink MAP: Burst profile (time, frequency, modulation, coding) to each user • Uplink MAP: Burst profile for transmission from each user. • MAPs can be compressed • Contention-based region: Ranging, bandwidth request, besteffort data • Ranging Channel: • Closed loop frequency, time, and power adjustments • Channel quality indicator channel (CQICH) • Ack Channel: subscriber stations • Initially, 5 ms frames only.

25. Subscriber Initialization

26. The 802.16 MAC SublayerProtocol … • Security sublayer • Encryption is used to keep secret all the data • Only payloads are encrypted; the headers are not • MAC sublayer common part • MAC frames occupy an integral number of physical layer time slots • Each MAC frame is composed of subframes, 1st two of which are downstream & upstream maps • These maps tells what is in which time slot & which time slots are free • Upstream channel allocation is closely tied to the quality of service issue

27. The 802.16 MAC SublayerProtocol: Service Classes • All services in 802.16 are connection-oriented • Each gets one of the above classes of service, determined when the connection is setup • Design is very different from that of 802.11 or Ethernet • Standard defines two forms of BW allocation • per station & per connection • Per station case • Subscriber station aggregates the needs of all users in the building & makes collective requests for them • Per connection case • Base station manages each connection directly

28. The 802.16 Frame Structure (a) A generic frame (b) A bandwidth request frame • EC bit: tells whether payload is encrypted • Type field: identifies frame type • CI field: presence or absence of final checksum • EK field: which of encryption keys is used (if any) • Length field: complete length of the frame including header • Connection ID: tells which connection this frame belongs to • Header CRC: checksum over header only • For more details, please consult the standard

29. WiMAX MAC: Key Features • Flexible and Extensible - Same MAC for all current and future PHYs • Modular: Several optional features. Negotiable SS/BS features • Multiple Topologies: PTP, PMP, mesh • Multiple Antenna Technologies: Adaptive Antennas, MIMO • Multiple Protocol Payloads: ATM, Packets (IP or Ethernet), W or w/o header suppression • Flexible Retransmission Policies: ARQ, HARQ • TDD and FDD Support • Variety of Subscribers: Several per subscriber or per connection parameters • Integrated QoS • Security

30. Base Station and Subscriber Stations • Base Station (BS): Controls the entire system, frame size, scheduling, admission control, QoS, Ranging, clock synchronization, power control and handoff. • All traffic goes through BS • Subscriber Station (SS): Find BS, Acquire PHY synchronization, Obtain MAC parameters, Generate bandwidth requests, make local scheduling decisions, follow transmission/reception schedule from BS, perform initial ranging, maintenance ranging, power control • Mobile Station (MS): Mobility management, Handoff, Power Conservation

31. Framing and Duplexing • Burst = n MPDUs with per burst CRC • Burst Profile: Modulation type, FEC, preamble type, guard time • Downlink Interval Usage Code (DIUC): Identifies burst profile • DL Channel Descriptor (DCD): Describes DL PHY. Broadcast periodically by BS. Frame duration, Defines DIUCs. • Uplink Interval Usage Code (UIUC): Identifies UL burst profiles • UL Channel Descriptor (UCD): Describes UL PHY.

32. Connections and Service Flows • Service Flows = Higher layer flows • Each Service flow has a connection • Extra connections for management and control • 16-bit CID ⇒ 65,535 connections • Each station has many connections with BS: • Initial Ranging CID • Basic CID • Primary Management CID • Secondary Management CID: Higher layer • Multicast Polling CID: Bandwidth requests

33. IEEE 802.16 – QoS Classes • Connection oriented: All traffic is assigned a connection • Five Service Classes: 1. Unsolicited Grant Service (UGS): CBR traffic, e.g., voice • Specified throughput, delay, and delay jitter 2. Enhanced Real-Time Polling Service (ertPS): • Silence suppressed voice. On/off UGS. 3. Real-Time Polling Services (rtPS): • rtVBR, e.g., streaming video. • Specified peak and average throughput, delay and delay jitter. 4. Non-Real-Time Polling Service (nrtPS): nrtVBR, • Specified peak and average throughput 5. Best Effort (BE); No throughput or delay guarantees

34. ARQ • Allows selective repeat (Stop and Wait, go back n ) • ARQ block size negotiated at connection setup • Depends upon the Type of Service (ToS), expected delay, etc • ARQ block cannot be fragmented • A fragment may contain blocks from multiple SDUs

35. IEEE 802.16 Protocol Structure • CS: All functions that are specific to a higher layer protocol • Classify SDUs based on MAC address, VLANs, priorities • Assigns Service Flow ID (SFID) and a connection identifier • Optional payload header suppression (PHS) • CPS: • Fragmentation and reassembly of large MAC SDUs • Packing and unpacking of several small MAC SDUs • QoS control, Scheduling • Bandwidth request • Automatic repeat request (ARQ

36. Benefits OF WiMax • Speed • Faster than broadband service • Wireless • Not having to lay cables reduces cost • Easier to extend to suburban and rural areas • Broad Coverage • Much wider coverage than WiFi hotspots

37. ENDOF LECTURE 13