IEEE 802.16

Air Interface for Fixed Broadband Wireless Access Systems

Kwangho Kook

IEEE 802 Standard

• 802.3 : CSMA/CD (Ehernet)• 802.4 : Token Bus• 802.5 : Token Ring• 802.6 : MAN• 802.11 : Wireless LAN• 802.12 : Gigabit LAN• 802.16 : Fixed Broadband Wireless Access

System

802.3

Medium

Access

802.3

Physical

802.2 Logical Link

802.1 Bridging

802.4

Medium

Access

802.4

Physical

802.5

Medium

Access

802.5

Physical

802.6

Medium

Access

802.6

Physical

802.11

Medium

Access

802.11

Physical

802.12

Medium

Access

802.12

Physical

802.16

Medium

Access

802.16

Physical

Data

Link

Layer

Physical

Layer

Fig. 1 The relationship between the standard and other members of the family

• 802.16 consists of the access point, BS(Base Station) and SSs(Subscriber Stations)

• All data traffic goes through the BS, and the BS can control the allocation of bandwidth on the radio channel.

• 802.16 is a Bandwidth on Demand system.

IEEE 802.16

SS

SS

SS

BS

Figure 1. Wireless Access Network

IEEE 802.16 [1]

• Scope :– Specifies the air interface, MAC (Medium Access

Control), PHY(Physical layer)

• Purpose : – to enable rapid worldwide deployment of cost-effective

broadband wireless access products– to facilitate competition in broadband access by

providing alternatives to wireline broadband access

• Main advantage : – fast deployment, dynamic sharing of radio resources

and low cost

• The spectrum to be used– 10 - 66 GHz licensed band

• Due to the short wavelength

– Line of sight is required

– Multipath is negligible

• Channels 25 or 28 MHz wide are typical

• Raw data rates in excess of 120 Mbps

– 2 -11 GHz

• IEEE Standards Association Project P802.16a

• Approved as an IEEE standard on Jan 29, 2003

IEEE 802.16

IEEE 802.16 MAC layer function[2]

• Transmission scheduling : – Controls up and downlink transmissions so that

different QoS can be provided to each user

• Admission control : – Ensures that resources to support QoS requirements of

a new flow are available

• Link initialization– Scans for a channel, synchronizes the SS with the BS,

performs registration, and various security issues.

• Support for integrated voice/data connections– Provide various levels of bandwidth allocation, error

rates, delay and jitter

IEEE 802.16 MAC layer function

• Fragmentation :

– Sequence number in the MAC header is used to reassemble at the receiver

• Retransmission : – Implement an ARQ(Automatic Repeat Request)

Basic Services

• UGS(Unsolicited Grant Service)– Supports real-time service flows that generate fixed

size data packets on a periodic basis, such as T1/E1 and Voice over IP

– The BS shall provide fixed size slot at periodic intervals.

• rtPS(Real-Time Polling Service)– Supports real-time service flows that generate variable

size data packets on a periodic basis, such as MPEG video

Basic Services

• nrtPS(Non-Real-Time Polling Service)– Supports non real-time service flows that generate

variable size data packets on a regular basis, such as high bandwidth FTP.

• BE(Best Effort service)– Provides efficient service to best effort traffic

Medium Application Degree of

Symmetry

Data rate Key performance parameters

And target values

One-way Delay

Delay variation

Information Loss

Audio Conversational

voice

Two-way 4 - 13 kbps <150 msec

Preferred <400 msec

< 1msec <3% FER

vidio Videophone Two-way 32 - 384 kbps <150 msec

Preferred <400 msec

Lip-sync

<100 msec

<1% FER

Data Telemetry

Two-way control

Two-way < 28.8 kbps <250 msec N.A. Zero

Data Interactive games

Two-way < 1 KB <250 msec N.A. Zero

Data Telnet Two-way < 1 KB <250 msec N.A. Zero

Table 1 End-user Performance Expectations – Conversational/Real-time Services

Medium Application Degree of

Symmetry

Data rate Key performance parameters

And target values

One-way Delay

Delay variation

Information Loss

Audio Voice messaging Two-way 4 - 13 kbps <1 sec

Playback

<2 sec

record

< 1 msec <3% FER

Data Web-browsing HTML

Two-way <4 sec/page N.A. Zero

Data Transactions Services

High priority

e-commerce, ATM

Two-way <250 msec N.A. Zero

Data Interactive games Two-way <250 msec N.A. Zero

Table 2 End-user Performance Expectations – Interactive Services

Medium Application Degree of

Symmetry

Data rate Key performance parameters

And target values

One-way Delay

Delay variation

Information Loss

Audio High quality

Streaming audio

PrimarilyOne-way

32 - 128 kbps <10 sec < 1msec <1% FER

vidio One-way One-way 32 - 384 kbps <10 sec <1% FER

Data Bulk data

Transfer/retrieval

PrimarilyOne-way

<10 sec N.A. Zero

Data Still image One-way <10 sec N.A. Zero

Data Telemetry

- monitoring

One-way < 28.8 kbps <10 sec N.A. Zero

Table 3 End-user Performance Expectations – Streaming Services

FDD based MAC protocol [3]

• Downlink – Broadcast phase : The information about uplink and

downlink structure is announced.

– DL-MAP(Downlink Map)• DL-MAP defines the access to the downlink information

– UL-MAP(Uplink Map)• UL-MAP message allocates access to the uplink channel

• Uplink– Random access area is primarily used for the initial

access but also for the signaling when the terminal has no resources allocated within the uplink phase.

MAC Frame MAC Frame MAC Frame

Broadcast Phase Downlink Phase

Movable boundary

DownlinkCarrier

Uplink Carrier Uplink Phase Random Access Phase

Broadcast Reserved

Movable boundary

Reserved Contention

Figure 4. FDD based 802.16 MAC Protocol

Downlink Subframe

Uplink Subframe

DL-MAP n-1

UL-MAP n-1

Frame n-1 Frame n

Round trip delay + T_proc

Bandwidth request slots

Figure 3. Time relevance of PHY and MAC control information

802.11• Wireless LAN Medium Access Control (MAC)

and Physical Layer(PHY) Specifications• 802.11a : up to 54 Mbps in 5GHz band• 802.11b : up to 11 Mbps in 2.4GHz band• 802.11 MAC protocol supports two kinds of

access method– PCF(Point Coordinated Function)

• Based on the polling controlled by AP(Access Point)• Intended for transmission of real-time traffic as well as that of

asynchronous data traffic

– DCF(Distributed Coordinated Function)• Designed for asynchronous data transmission • Based on CSMA/CA(Carrier Sense Multiple Access with Collision

Avoidance

Beacon D1+poll

U1+ack

D2+ack +poll

U2+ack

SIFS SIFSSIFSSIFSSIFS SIFS

D3+ack +poll

CF_End

PICF

Contention free period Contention period

Contention free period repetition interval (super frame)

Figure 5. Point Coordinator Function in IEEE 802.11 Standard

Downlink/Uplink Scheduling

• Radio resources have to be scheduled according to the QoS(Quality of Service) parameters

• Downlink scheduling: – the flows are simply multiplexed

– the standard scheduling algorithms can be used • WRR(Weighted Round Robin)• VT(Virtual Time)• WFQ(Weighted Fair Queueing)• WFFQ(Worst-case Fair weighted Fair Queueing)• DRR(Deficit Round Robin)• DDRR(Distributed Deficit Round Robin)

111VCC 1 (Source 1)

22VCC 2 (Source 2)

333VCC 3 (Source 3) 3 3

2

1

3

123111 2 33333

WRR scheduler

Counter Reset Cycle

• It is an extention of round robin schedulingbased on the static weight.

WRR

VT• VT : aims to emulate the TDM(Time Division Multiplexing) system [4]

– connection 1 : reserves 50% of the link bandwidth

– connection 2, 3 : reserves 20% of the link bandwidth

Connection 1 Average inter-arrival : 2 units

Connection 1 Average inter-arrival : 2 units

Connection 2 Average inter-arrival : 5 units

Connection 3 Average inter-arrival : 5 units

First-Come-First-Served service order

Virtual times

Virtual Clock service order

WFQ and WFFQ

• FFQ(Fluid Fair Queue) : head-of-the line processor sharing service discipline

– : guaranteed rate to connection i

– C : the link speed

– : the set of non-empty queue

– The service rate for a non-empty queue i

• WFQ : picks the first packet that would complete service in the corresponding FFQ system[4]

φi

CBj j

i

∑ ∈ )(τ φφ

)(τB

• WFFQ : picks the first packet that would complete service among the set of packets that have

started service in the corresponding FFQ system[4]

• Example– All packets have the same size 1 and link speed is 1

Guaranteed rate for connection 1 : 0.5

Guaranteed rate for connection 2-11 : 0.05

Connection 1 sends 11 back-to-back packets at time 0

Connection 2-11 sends 1 packet at time 0

– The completion time of connection 1 :

2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22

– The completion time of connection 2 – 11 : 20

Connection 1Connection 1

Connection 2

WFQ Service Order

Connection 11

… …

WFFQ Service Order

Figure 6. WFQ and WFFQ

VT and WFQ

• All packets are fixed size and require exactly one second to service

• Starting at time zero, 1000 packets from connection 1 arrive at a rate of 1 packet/second

• Starting at time 900, 450 packets from connection 2 arrive at a rate of 1 packet/second– The completion times of the 901, 902, 903, … packets

of connection 1 in FFQ system are 1802, 1904, 1806, …

– The completion times of the 1, 2, 3, … packets of connection 2 in FFQ system are 901, 902, 903, …

Connection 1Connection 1

Connection 2

Virtual Clock Service Order

WFQ Service Order

898 900 902 904

Figure 7. WFQ and Virtual Clock

898 900 902 904

Deficit Round Robin[5]

• Each connection is assigned a state variable called the DC(Deficit Counter).

• At the start of each round, DCi of queue i is incremented by a specific service share(quantum)

• If the length of the head of the line packet, Li, is less than or equal to DCi,, the scheduler allows the ith queue to send a packet.

• Once the transmission is completed DCi is decremented by Li.

• Deficit Round Robin Scheme

Qi

DCi

350035002800 7800 2000

1500

5000

700

1400

2800 7800 2000

2800 7800 2000

2800 7800 2000

2800 7800 2000

initializing(1st round)

serviced

Not serviced

serviced

(2nd round)

serviced

(3rd round)

(4th round)

Distrubuted Deficit Round Robin[6]

• Each connection is assigned a state variable called the DC(Deficit Counter)

• If the value of the DCi is positive then the scheduler allows the ith queue to send a packet.

• Once the transmission is completed DCi is decremented by Li, the length of the transmitted packet .

• At the start of the subsequent rounds, DCi is incremented by a specific service share(quantum)

• Distributed Deficit Round Robin Scheme

Qi

DCi

350035002800 7800 2000

1500

-6300

-2800

700

-2100

2800 7800 2000

2800 7800 2000

2800 7800 2000

2800 7800 2000

2800 7800 2000

initializing(1st round)

serviced

serviced

Not serviced

(2nd round)

Not serviced

(3rd round)

serviced

Uplink scheduling: – Responsible for the efficient and fair allocation of the

resources(time slots) in the uplink direction

– Uplink carrier : • Reserved slots

• contention slots(random access slots)

– The standard scheduling algorithms can be used

Downlink/Uplink Scheduling

Bandwidth allocation and request mechanisms• The method by which the SS(Subscriber Station)

can get the bandwidth request message to the BS(Base Station)– Unicast

• When an SS is polled individually, no explicit message is transmitted to poll the SS.

• The SS is allocated, in the UP-MAP(Uplink Map), bandwidth sufficient for a bandwidth request.

– Multicast• Certain CID(Connection Identifier) are reserved for multicast

groups and for broadcast messages.• An SS belonging to the polled group may request bandwidth

during any request interval allocated to that CID in the UP-MAP

– Broadcast

Bandwidth allocation and request mechanisms

• UGS : – The BS provides fixed size bandwidth at periodic intervals

to the UGS.

– The SS is prohibited from using any contention request opportunities.

– The BS shall not provide any unicast request opportunities.

• rtPS– The BS provides periodic unicast request opportunities.

– The SS is prohibited from using any contention request opportunities.

Bandwidth allocation and request mechanisms

• nrtPS– The BS provides timely unicast request opportunities.

– The SS is allowed to use contention request opportunities.

• BE– The SS is allowed to use contention request opportunities.

Contention Resolution• Collisions may occur during Request intervals.• Contention resolution is based on a truncated

binary exponential backoff, with the initial backoff window and the maximum backoff window controlled by the BS.

• A truncated binary exponential backoff– The SS shall randomly select a number within its

backoff window.– This value indicates the number of contention

transmission opportunities that the SS shall defer before transmitting

– If the contention transmission fails, the SS increases its backoff window by a factor of two.

The 4Gmobile system

• 4Gmobile system : Fourth-generation mobile wireless communications

• The vision of the 4Gmobile system– Providing broadband wireless access– Providing Internet-based communications– Ensuring seamless services provisioning across a

multitude of wireless systems and networks– Providing optimum delivery of the user’s wanted service

via the most appropriate network available• IEEE 802.16e

– Air interface for Fixed and Mobile Broadband Wireless Access Systems

– Started at December 11, 2002

Future Study

• Study on the scheduling method– Downlink scheduling method

– Uplink scheduling method

• Study on the relevant Fragment Size• Study on the criteria whether packing or non-

packing

References

[1] IEEE Std 802.16-2001.[2] B. Larish, “The MAC layer in Broadband Wireless Access Networks,”

http://www.eas.asu.edu/trace/eee459/Bryan%20Larish.doc[3] J. Bostie, G. Kandus, “MAC Scheduling for Fixed Broadband Wireless Access

Systems, COST263_v0_0.doc[4] Hui Zhang, “Service disciplines for guaranteed performance service in packet-

switching networks,” Proc. IEEE, vol. 83, Oct. 1995.[5] M. Shreedhar and G. Varghese, “Efficient Fair Queueing using deficit round

robin,” IEEE/ACM Transactions on Networking, Vol. 4, No. 3, June 1996, pp. 375-385.

[6] R.S. Ravindra, D. Everitt, and L.L.H. Andrew, “Fair Queueing Scheduler for IEEE 802.11 Based Wireless Multimedia Networks, http://www.ee.mu.oz.au/staff/lha/abstract/wlan_mmt99.html

[7] S. Lu, V. Bharghavan, and R. Srickant, “Fair Scheduling in Wireless Packet Networks,” IEEE Trans. on Networking, Vol. 7, No. 4 August 1999.

[8] Y. Cao and V.O.K. Li, “Scheduling Algorithms in Broad-band Wireless Networks,” Proc. IEEE, Vol. 89, No.1, January 2001, pp 76-87.