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AIR INTERFACE FOR FIXED BROADBAND WIRELESS ACCESS SYSTEMS

IEEE Std 802.16-2001

Poll-Me

Bit Usage

Piggybacking

No

 

Attempt

& BW Stealing

 

 

piggybacking &

exhausted?

 

 

BW stealing first

 

 

 

 

 

Yes

No

UGS Packet

 

 

available?

 

Yes

 

Set PM=1 in

 

Grant

 

Management

 

sub-header

 

Done

Figure 36—Poll-Me bit usage

6.2.7 MAC support of PHY

Several duplexing techniques are supported by the MAC protocol. The choice of duplexing technique may affect certain PHY parameters as well as impact the features that can be supported.

The MAC is able to support both a framed and a nonframed PHY specification. For a framed PHY, the MAC aligns its scheduling intervals with the underlying PHY framing. For an unframed PHY, the scheduling intervals are chosen by the MAC to optimize system performance.

6.2.7.1 Unframed Frequency Division Duplexing (FDD)

In an Unframed FDD PHY, the uplink and downlink channels are located on separate frequencies and each subscriber station can transmit and receive simultaneously. In addition, uplink and downlink transmissions use no fixed duration frame. In this type of system, the downlink channel is “always on” and all subscriber stations are always listening to it. Therefore, traffic is sent in a broadcast manner using time division multiplexing (TDM) in the downlink channel. The uplink channel is shared using time division multiple access (TDMA), where a centralized scheduler controls the allocation of uplink bandwidth.

The current version of this standard does not define an Unframed FDD PHY.

6.2.7.2 Framed (burst) FDD

In a framed (burst) FDD system, the uplink and downlink channels are located on separate frequencies and the downlink data can be transmitted in bursts. A fixed duration frame is used for both uplink and downlink transmissions. This facilitates the use of different modulation types. It also allows simultaneous use of both full-duplex subscriber stations (which can transmit and receive simultaneously) and optionally half-duplex subscriber stations (which cannot). If half-duplex subscriber stations are used, the bandwidth controller shall not allocate uplink bandwidth for a half-duplex subscriber station at the same time that it is expected to receive data on the downlink channel.

Copyright © 2002 IEEE. All rights reserved.

93

IEEE Std 802.16-2001

LOCAL AND METROPOLITAN AREA NETWORKS—PART 16:

Figure 37 describes the basics of the Framed FDD mode of operation. The fact that the uplink and downlink channels utilize a fixed duration frame simplifies the bandwidth allocation algorithms. A full-duplex subscriber station is capable of continuously listening to the downlink channel, while a half-duplex subscriber station can listen to the downlink channel only when it is not transmitting in the uplink channel.

Downlink

Uplink

frame

time

 

Broadcast

 

Half Duplex SS #1

 

Full Duplex Capable SS

 

Half Duplex SS #2

 

 

 

 

 

 

 

 

Figure 37—Example of Burst FDD bandwidth allocation

6.2.7.3 Time Division Duplexing (TDD)

In the case of TDD, the uplink and downlink transmissions occur at different times and usually share the same frequency. A TDD frame (see Figure 38) also has a fixed duration and contains one downlink and one uplink subframe. The frame is divided into an integer number of physical slots (PSs), which help to partition the bandwidth easily. The TDD framing is adaptive in that the bandwidth allocated to the downlink versus the uplink can vary. The split between uplink and downlink is a system parameter and is controlled at higher layers within the system.

94

Copyright © 2002 IEEE. All rights reserved.

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