Elements of 802.16 - 3 [Under WiMAX]
» Request/Allocation/Polling/Contention resolution schemes (procedures)
Preprovisioned service flows are activated during MS initialisation. The service flows can also be dynamically established (created, admitted) and activated. Based on QoS parameters, BS employ 5 types of scheduling mechanisms for a service flow (as mentioned in earlier article). In effect, as per QoS parameter values, BS need to provide allocation (called grants) or opportunities to request allocation at appropriate times. Current radio load on BS also plays a part here. BS designer has to implement appropriate algorithms to cater to different combinations of traffic that may be possible in field.
For UGS service flow, BS provide fixed "uplink grants" on real time periodic basis. This way MS do not have to send any "bandwidth request". This is not the case with all services, including when MS has management message exchange to do. 802.16 specify a "polling mechanism" to make possible for MS to send a bandwidth request. Polling could be unicast (to particular MS only), multicast (to group of MSs) or broadcast (to all MSs). As per the polling, MS sends a bandwidth request at alloted burst. If polling is broadcast/multicast, MS has to invoke "contention" procedures. Of course, MS should not make a bandwidth request if it does not need bandwidth. For unicast polling, basic CID is used whereas multicast/broadcast polling use multicast/broadcast CIDs respectively. It seems broadcast/multicast polling is not applicable to OFDMA PHY (i.e. mobile WiMAX). As per the request, BS may provide uplink grants. Bandwidth requests are made for particular CID. The uplink grants are always for basic CID.
802.16 also use concept of piggyback request wherein bandwidth request can be made along with user data payload. If MS already has UGS allocation, it can tell BS to poll it by setting header PM (Poll Me) bit.
» Retransmission mechanisms
For error correction, 802.16 support optional retransmission mechanisms. Both ARQ and HARQ (Chase Combining and Incremental Redundancy) are specified. The parameters are negotiated during connection establishment.
In conclusion 802.16 specify a flexible radio organisation and protocol to support various kinds of services and control of the same efficiently.
» Mobility
To support mobility, 802.16 specify basic Handover mechanism along with optional mechanisms like Macro Diversity HO (MS received same data from two or more BSs), Fast BS Switching (BSs change frame by frame, so MS receives from different BSs, but one frame at a time).
» Power saving mechanisms
One of the major additions in 802.16e was "power saving mechanisms". Simply speaking, MS when not needed can go for intermittent sleeps thus saving battery power. During sleep, BS will not try communicating with MS. Before MS start procedure for intermittent sleeps, it negotiates sleep parameters with BS. Note that sleep periods are negotiated per connection (e.g. UGS require different sleep periods than BE service). It is interesting to see effective sleep periods when more than one transport connections are present: Availability period of one connection may overlap with sleep period of another connection, so MS can go to sleep mode only when sleep periods of all ongoing connections overlap.
» Mesh networking
In addition to PMP (Point-to-MultiPoint, one BS talking directly to SSs) communication, 802.16 also specify optional feature of Mesh networking. In Mesh networking, though there is Mesh BS, communication between BS and SS can go through multiple SSs. In effect, intermediate act like routers. SSs can even talk to each other and exchange data. Mesh networking is possibly meant for LOS frequencies (fixed WiMAX).
Here are bullet points for 802.16 based on OFDMA PHY (Mobile WiMAX):
o OFDMA PHY
o Burst/slot based use of radio resources
o MAC
layer for media control
o Frame structure with DL/UL maps
o
Step wise network entry (Ranging, Security procedures etc.)
o 5
types of transport services
o
Request/Allocation/Polling/Contention procedures for efficient use
of resources
o Different handover procedures.
o Power saving
o
Optional Mesh networking
In next article, we will look at 802.16 layer diagram and how the functions are split in layers and sublayers.
References: WiMAX Handbook (edited) by Ahson and Ilyas, Fundamentals of WiMAX by Andrews, Ghosh, and Muhamed, WiMAX forum web site, Latest 802.16-2009 (covering amendments 802.16e-2005, 802.16-2004/Cor1-2005, 802.16f-2005 & Std 802.16g-2007 made on 802.16-2004).
| Copyright © Samir Amberkar 2010 | § § |
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