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WiMAX and IEEE 802.16. (parts of) Chapter 11. Wireless Local Loop (WLL). Wired technologies are responding to demand for reliable, high-speed access from residential, business, and government subscribers ISDN, xDSL, cable modems…

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Wimax and ieee 802 16

WiMAX and IEEE 802.16

(parts of) Chapter 11

Wireless local loop wll
Wireless Local Loop (WLL)

  • Wired technologies are responding to demand for reliable, high-speed access from residential, business, and government subscribers

    • ISDN, xDSL, cable modems…

  • Increasing interest shown in competing wireless technologies for subscriber access

  • Wireless local loop (WLL)

    • Narrowband – offers a replacement for existing telephony services

    • Broadband – provides high-speed two-way voice and data service (>2 Mbps, up to 100s of Mbps)

Advantages of wll over wired approach
Advantages of WLL over Wired Approach

  • Cost – wireless systems are less expensive due to avoided cost of cable installation

  • Installation time – WLL systems can be installed in a small fraction of the time required for a new wired system

  • Selective installation – radio units installed for subscribers who want service at a given time

    • With a wired system, cable is typically laid out in anticipation of serving every subscriber in a given area

Alternatives to wll
Alternatives to WLL?

  • Wired approach, using existing installed cable

    • Many users do not have cable TV

    • Many cable providers do not offer data services

    • WLL is now cost-competitive

  • Mobile cellular technology

    • Too expensive

    • 3G less functional than broadband WLL

    • WLL subscriber units are fixed, so directional antennas can be pointed at the base station

Propagation considerations for wll
Propagation Considerations for WLL

  • Most high-speed WLL schemes use millimeter wave frequencies (10 GHz to about 300 GHz, i.e. wavelengths of 30mm to about 1mm)

    • There are wide unused frequency bands available above 25 GHz

    • At these high frequencies, wide channel bandwidths can be used, giving high data rates

    • Small size transceivers and adaptive antenna arrays can be used

Propagation considerations for wll1
Propagation Considerations for WLL

  • Millimeter wave systems have some undesirable propagation characteristics

    • Free space loss increases with the square of the frequency; losses are much higher in millimeter wave range than in microwave systems

    • Above 10 GHz, attenuation effects due to rainfall & atmospheric or gaseous absorption are significant

    • Multipath losses can be quite high

  • Therefore WLL cells have limited radius (few km), and there should be an unobstructed LOS between transmitter and receiver

Fresnel zone
Fresnel Zone

  • How much space around the direct path between transmitter and receiver should be clear of obstacles?

    • Objects within a series of concentric circles around the line of sight between transceivers have constructive/destructive effects on communication

  • For a point along the direct path,

    radius of first Fresnel zone:

    • S = distance from transmitter

    • D = distance from receiver

If no obstruction within 0.6R along path, then attenuation due to obstructions is negligible

Atmospheric absorption
Atmospheric Absorption

  • Radio waves at frequencies above 10 GHz are subject to molecular absorption

    • Peak of water vapor absorption at 22 GHz

    • Peak of oxygen absorption near 60 GHz

  • Favourable windows for communication:

    • From 28 GHz to 42 GHz

    • From 75 GHz to 95 GHz

Effect of rain
Effect of Rain

  • Attenuation due to rain

    • Presence of raindrops can severely degrade the reliability and performance of communication links

    • The effect of rain depends on drop shape, drop size, rain rate, and frequency

  • Estimated attenuation due to rain:

    • A = attenuation (dB/km)

    • R = rain rate (mm/hr)

      • Table 11.7: statistics on R for various climate zones

    • a and b depend on drop sizes and frequency

      • Table 11.6: typical values for a and b as a function of frequency

Effects of vegetation
Effects of Vegetation

  • Trees near subscriber sites can lead to multipath fading

  • Multipath effects from the tree canopy are diffraction and scattering

  • Measurements in orchards found considerable attenuation values when the foliage is within 60% of the first Fresnel zone

  • Multipath effects highly variable due to wind

Ieee 802 16 standards
IEEE 802.16 Standards

  • Use wireless links with microwave or millimeter wave radios

  • Use licensed spectrum

    • Other modes also envisaged (license-exempt, lightly licensed)

  • Are metropolitan in scale

  • Provide public network service to fee-paying customers

  • Use point-to-multipoint architecture with stationary rooftop or tower-mounted antennas

  • Provide efficient transport of heterogeneous traffic supporting Quality of Service (QoS)

  • Are capable of broadband transmissions (>2 Mbps)

Ieee 802 16 standards1
IEEE 802.16 Standards

  • IEEE 802.16 group formed in 1998

    • 802.16 standard

      • Air-interface for wireless broadband.

      • LOS

      • Operated in 10-66GHz range

    • 802.16a amendment

      • Included NLOS application in 2-11GHz

      • PHY layer used orthogonal frequency division multiplexing (OFDM)

      • MAC layer supported Orthogonal Frequency Division Multiple Access (OFDMA)

    • 802.16-2004 (also called 802.16d)

      • Further amended standard

      • Formed basis for first WiMAX solutions

    • Above standards supported fixed wireless applications

      • No mobility support

Ieee 802 16 standards2
IEEE 802.16 Standards

  • 802.16e-2005 (also called 802.16e)

    • Added to 802.16-2004 to support mobility and improve performance

      • soft and hard handover between Base Stations

      • Introduces Scalable OFDMA

        • higher spectrum efficiency in wide channels

        • cost reduction in narrow channels

      • Improves coverage using:

        • Antenna diversity schemes

        • Hybrid ARQ (hARQ)

      • Improving capacity and coverage using:

        • Adaptive Antenna Systems (AAS)

        • Multiple Input Multiple Output (MIMO) technology

Ieee 802 16 standards3
IEEE 802.16 Standards

  • 802.16e continued

    • Introduces high-performance coding techniques to enhance security and NLOS performance

      • Turbo coding

      • Low density parity check

    • Introduces downlink sub-channelization, allowing administrators to trade-off coverage with capacity

    • Increases resistance to multipath interference using Enhanced Fast Fourier Transform algorithm, which can tolerate larger delay spreads

    • Adds an extra QoS class (enhanced real-time Polling Service) more appropriate for Voice over IP (VoIP) applications.

Ieee 802 16 standards4
IEEE 802.16 Standards

  • The standards offer a variety of different design options

    • Therefore interoperability is not guaranteed

    • A number of choices are offered at:

      • PHY layer

        • Single carrier WirelessMAN-Sca

        • OFDM-based WirelessMAN_OFDM

        • OFDMA-based WirelessMAN-OFDMA

      • Also:

        • MAC layer

        • Duplexing

        • Frequency band

        • etc.

Wimax standards
WiMAX Standards

  • WiMAX Forum: industry-based consortium with task of taking basic standard and

    • Ensuring interoperability

    • Certifying compliance

      • First certified product based on IEEE 802.16-2004 in January 2006

  • Interoperability is ensured by selecting a limited number of system and certification profiles from within the standard.

    • A system profile defines a subset of mandatory & optional PHY & MAC layer features selected from 802.16-2004 or 802.16e-2005 standard.

    • A certification profile further defines operating frequency, channel bandwidth, and duplexing mode

      • Equipment certified against specific certification profile

    • 5 fixed and 14 mobile certification profiles have been defined

    • Equipment tested against 2 fixed profiles

Wimax standards1





Core network

Mobile Client


Upper layer



Upper layer

IP Stack


Mobility agent


CS = convergence sublayer










IEEE 802.16e

Fixed client





WiMAX Standards

Wimax standards2
WiMAX Standards

  • Currently define PHY and MAC layers

    • MAC subdivided into three functions

      • Security sublayer: providing encryption and authentication

      • MAC layer: providing access, ARQ, and QoS

      • MAC convergence sublayer: providing interface to various networks

        • ATM, Ethernet, IP…


MAC convergence



MAC common

part sublayer

MAC security




Wimax technical challenges
WiMAX: Technical Challenges

  • Developing reliable transmission and reception schemes

  • Achieving high spectral efficiency and coverage to deliver broadband services to a large number of users using limited available spectrum

  • Supporting and efficiently multiplexing services with a variety of QoS requirements

  • Achieving low power consumption to hand-held devices

  • Providing robust security

  • Adapting IP-based protocols and architecture for the wireless environment to achieve lower cost & convergence with wired network

  • Supporting mobility through seamless handover and roaming

Phy layer ofdm 1
PHY layer: OFDM (1)

Orthogonal Frequency Division Multiplexing (OFDM)

Multicarrier modulation technique

Idea: divide a high bit rate stream into parallel low bit rate streams and modulate each stream on a separate carrier.

Reduces intersymbol interference by reducing the data rate by a factor of N i.e. symbol time is increased by a factor of N

Therefore equalisers may not be necessary

Also: frequency selective fading only affects some channels, not whole signal

Phy layer ofdm 2
PHY layer: OFDM (2)

OFDM eliminates the need for non-overlapping channels, by selecting subcarriers that are orthogonal to each other over the duration of a bit.

This orthogonality allows us to recover our original signal without distortion from other signals.

The OFDM subcarriers can be packed tightly together because there is little interference between adjacent subcarriers

Phy layer ofdma 1
PHY layer: OFDMA (1)

So far, we’ve presented OFDM as transmitting a single data stream

Split into N reduced bit rate streams

Transmit each of these N streams using a different tone

Clearly, in terms of signal recovery, it does not matter if the N data streams come from the same or different sources

Thus OFDM can be used to provide subchannels…called OFDMA

OFDMA allows users to share both subcarriers and time slots.

This allows resources to be allocated according to both user and system requirements

Phy layer ofdma 2
PHY Layer: OFDMA (2)





User 3

User 6

User 9

User 2

User 5

User 8

User 1

User 2

User 3

User 1

User 4

User 7

Block of






Phy layer ofdma 3
PHY layer: OFDMA (3)

In reality, the time slot & frequency allocation is more complex

Slots are defined as one subchannel and a number of OFDM symbols

Users are assigned data regions either as

A contiguous series of slots…

Called Adaptive Modulation and Coding

Good for fixed or low mobility applications

…or using distributed subcarriers to enhance frequency diversity

Better for high mobility situations

Allocations according to Demand, QoS requirements, Channel conditions

In addition, Time Division Duplexing (TDD) means time slots are divided into Uplink and Downlink subframes.

Downlink to uplink ratio varies from 3:1 to 1:1 depending on traffic profiles.

Frequency Division Duplexing (FDD) is also supported.

Adaptive modulation and coding 1
Adaptive modulation and coding 1

Various modulation and coding allowed: BPSK, QPSK, 16 QAM, 64 QAM

Modulation and coding can change on burst-by-burst basis

Channel quality indicator used to inform BS about channel quality in downlink direction

BS estimates channel quality in the uplink direction

BS chooses modulation and coding to maximize throughput for available signal-to-noise ratio

Total of 52 combinations of modulation and coding schemes are defined in WiMAX as burst profiles

Wimax features 1
WiMAX Features 1

  • OFDM-based PHY layer

    • Allows operation in NLOS conditions

    • Offers good resistance to multipath effects.

  • Very high peak data rates

    • Modulation, error-correction coding, MIMO, multiplexing

    • Typically peak PHY rates 25Mbps

  • Scalable bandwidth and data rate support

    • Data rates can be scaled to bandwidth availability

  • Adaptive modulation and coding

    • Modulation and forward error coding schemes can change on a per user and per frame basis based on channel conditions.

      • Allows highest possible data rates based on SNR and SIR

Wimax features 2
WiMAX Features 2

  • Link layer retransmissions

    • When enhanced reliability required, ARQ provided

    • Hybrid ARQ (hARQ) combines FEC and ARQ techniques

  • Support for TDD and FDD

    • uplink and downlink channels supported either by dividing time slots, or using frequency subchannels

  • Orthogonal Frequency Division Multiple Access (OFDMA)

    • Users are allocated different OFDM channels

  • Support for advanced antenna techniques

    • Beam-forming, space time coding, spatial multiplexing…

Wimax features 3
WiMAX Features 3

  • Quality of Service Support

    • Connection-oriented MAC architecture allows support of QoS

    • Constant bit rate, variable bit rate, real-time, non-real-time, best effort

    • Terminal can have multiple connections with different QoS requirements

  • Robust Security

    • Strong encryption and authentication capabilities.

  • Support for Mobility

    • Secure seamless handover for delay-tolerant applications

    • Support for power-saving mechanisms

    • PHY layer enhancements

      • Channel estimation

      • Uplink subchannelisation

      • Power control

  • IP-based architecture

    • Convergence with other networks

    • Use of available protocols

Mac layer channel access mechanisms
MAC layer Channel Access Mechanisms

  • WiMAX is a connection-oriented solution: the Base Station is responsible for allocation of bandwidth to the user in both the uplink and downlink direction.

    • Downlink allocation based on needs of incoming traffic

    • Uplink allocation requested by Mobile Station

      • If an MS has multiple connections with the BS, BS allocates bandwidth as a block which the MS can use as it sees fit.

      • BS uses polling to solicit requests

        • Unicast: for each individual MS

        • Multicast: a group of MSs must compete for slot.

          • Contention resolution scheme used

        • If MS already has allocation, must use this allocation for additional requests

    • Allocation decisions strongly tied to QoS considerations.

Mac layer qos 1
MAC layer QoS 1

  • The connection-oriented MAC architecture provides strong support for QoS

    • Allows base station to control bandwidth allocated to individual sessions

    • Connections are unidirectional (so MS will have multiple connections with BS)

      • Separate connections established to send data and control information.

    • Also define Service Flow – unidirectional flow of packets with specific QoS parameters

      • SFID—service flow id

      • CID—connection id

      • ProvisionedQosParamset—provisioned


      • AdmittedQosParaSet—QoS parameters for

        which MS and BS have reserved resources.

      • ActiveQoSParaSet—QoS parameters being

        provided at any given time

      • Authorisation Module—check limits of QoS


QoS Parameters include:

traffic priority

maximum sustained traffic rate

maximum burst rate

minimum tolerable rate

scheduling type, ARQ type

maximum delay

tolerated jitter

service data unit type and size

bandwidth request mechanism

transmission PDU formation rules

Mac layer qos 2
MAC layer QoS 2

  • (1) Unsolicited Grant Service

    • Support fixed-size data packets at a constant bit rate

      • e.g. voice services such as VoIP without silence suppression

    • Mandatory service flow parameters:

      • Maximum sustained traffic rate

      • Maximum latency

      • Tolerated jitter

      • Request/retransmission policy

    • Fixed size grants offered on a periodic basis with no need to explicitly request bandwidth each time.

      • Eliminates request overhead and latency

Mac layer qos 3
MAC layer QoS 3

  • (2) Real-Time Polling Service

    • Support real-time service flows generating variable size packets periodically e.g. MPEG video

    • Mandatory service flow parameters:

      • Minimum reserved traffic rate

      • Maximum sustained traffic rate

      • Maximum latency

      • Request/transmission policy

    • BS provides unicast polling opportunity to MS to request bandwidth.

      • Polling frequent enough to ensure latency requirements met.

Mac layer qos 4
MAC layer QoS 4

  • (3) Non-real-Time Polling Service

    • Support delay tolerant data streams which require minimum guaranteed rate e.g. FTP

    • Mandatory service flow parameters:

      • Minimum reserved traffic rate

      • Maximum sustained traffic rate

      • Traffic priority

      • Request/transmission policy

    • Same polling opportunities as for RTPS, but less frequent

      • Average duration between opportunities a few seconds.

      • MSs can also request resources during contention-based polling

Mac layer qos 5
MAC layer QoS 5

  • (4) Best Effort service

    • No guarantee of minimum level of service required

      • e.g. Web Browsing

    • Mandatory service flow parameters:

      • Maximum sustained traffic rate

      • Traffic priority

      • Request/transmission policy

    • MS must use contention-based polling to request resources

Mac layer qos 6
MAC layer QoS 6

  • (5) Extended real-time variable rate services

    • Support variable data rate real-time applications that require guarantees on data rate and delay.

      • e.g. VoIP with silence suppression

    • Mandatory service flow parameters:

      • Minimum reserved traffic rate

      • Maximum sustained traffic rate

      • Maximum latency

      • Jitter tolerance

      • Request/transmission policy

    • Periodic UL allocations provided, but MS can also request additional resources during UL allocation

Mac layer power saving
MAC layer: Power Saving

  • Power saving is achieved by allowing the MS to go into either sleep or idle mode.

    • In sleep mode, MS becomes unavailable for predetermined periods negotiated with the BS

      • Power Save Class 1: sleep window increases exponentially from minimum to maximum value (e.g. best-effort non-real-time traffic)

      • Power Save Class 2: sleep window is fixed-length (UGS service)

      • Power Save Class 3: sleep window is one time only (e.g. multicast or management traffic – if MS knows when next traffic expected)

      • MS will still scan base stations for handoff-related information

    • In idle mode, the MS turns off and does not need to register with a BS, but still receives downlink traffic.

      • MS assigned a paging group of BSs which will page it if it has data

      • MS hands over between paging groups, but not BSs

      • Optional to support idle mode in WiMAX

Mac layer mobility support 1
MAC layer: Mobility Support 1

  • WiMAX envisages 4 mobility scenarios:

    • Nomadic: user moves from a fixed point to another fixed point

      • Requires reconnection

    • Portable: nomadic access for portable device with best-effort handover

    • Simple mobility: user can move at speeds of up to 60kmph with <1sec interruption for handover

    • Full mobility: user can move at speeds of up to 120 kmph

      • Handover with <50msec latency and <1% packet loss.

Mac layer mobility support 2
MAC layer: Mobility Support 2

  • IEEE 802.16e-2005 defines signalling mechanism for tracking MSs

    • From coverage area to coverage area, when MS is active

    • From paging group to paging group, when MS is idle

  • Three handoff mechanisms supported in 802.16e:

    • Hard Handover (mandatory for mobile WiMAX)

      • Abrupt transfer from one BS to another

      • Decision made according to signal quality data gathered by MS via RF scanning

      • Decision can be made by BS, MS, or other entity

      • Can have undelivered packets from previous BS

        • Retained until a timer expires

Mac layer mobility support 3
MAC layer: Mobility Support 3

  • Two other handoff mechanisms are supported in 802.16e, but are optional for mobile WiMAX

    • In both, MS maintains valid simultaneous connection to multiple BSs

    • Fast Base Station Switching (FBSS)

      • MS maintains a list of BS involved called the active set

      • Connection is maintained with each BS

      • MS communicates with the anchor BS only

      • Can change anchor BS without explicit handover signalling

    • Macro Diversity Handover (MDHO)

      • Similar to FBSS, except data exchanged simultaneously with a number of BSs known as a diversity set.

  • Note that BSs in the active or diversity set must synchronise and communicate on the same channels.

Protocol architecture 2
Protocol Architecture 2

  • Physical and transmission layer functions:

    • Encoding/decoding of signals

    • Preamble generation/removal (for synchronisation)

    • Bit transmission/reception

    • Specification of transmission medium and frequency band

  • Medium Access Control layer functions:

    • On transmission, assemble data into a frame with address and error detection fields

    • On reception, disassemble frame, and perform address recognition and error detection

    • Govern access to the wireless transmission medium




Protocol architecture 3
Protocol Architecture 3

  • Convergence Sublayer functions:

    • Encapsulate PDU framing of upper layers into native 802.16 MAC/PHY frames

    • Map upper layer’s addresses into 802.16 addresses

    • Translate upper layer QoS parameters into native 802.16 MAC format

    • Adapt time dependencies of upper layer traffic into equivalent MAC service

  • Some upper-layer services (e.g. digital audio and video) don’t need to use the CS – the stream of digital data is presented directly to the Transmission Layer

Physical layer upstream transmission
Physical Layer – Upstream Transmission

  • Multipoint-to-point channel

  • Uses a DAMA-TDMA technique

    • DAMA = demand assignment multiple-access

    • Adapts to demand changes among the users

  • Error correction uses Reed-Solomon code

  • Modulation scheme based on QPSK

Physical layer downstream transmission
Physical Layer – Downstream Transmission

  • Continuous downstream mode

    • For continuous transmission stream (e.g. audio, video)

    • Simple TDM scheme is used for channel access

    • Duplexing technique is frequency division duplex (FDD)

      • A different frequency band is used for transmission in each direction

  • Burst downstream mode

    • Targets burst transmission stream (e.g. IP-based traffic)

    • DAMA-TDMA scheme is used for channel access

    • 3 available duplexing techniques:

      • FDD with adaptive modulation (can adapt modn. & FEC schemes)

      • frequency shift division duplexing (FSDD)

      • time division duplexing (TDD)

Wimax security
WiMAX security

  • WiMAX supports a variety of credentials: username/password, digital certificates, smart cards

  • Each SS has a X.509 certificate, which is used to authenticate the SS to the BS (includes digital certificate and MAC address)

  • If OK: BS will send Authorisation Key (AK) to SS, which allows SS to encrypt transmissions

  • SS can then register with the network

  • Privacy is based on Privacy Key Management (PKM)

  • Accommodates Advanced Encryption Standard (AES)

  • 128 or 256-bit key used for deriving the cipher is generated during the authentication phase - the key is refreshed periodically for additional protection