Wireless Network Design: Optimization Models and Solution ...

13 Optimization of Wireless Broadband (WiMAX) Systems 313
N
∑ ck,nρ (pk,n,γk,n) ≥ ρ
n=1
min
k
∀k1 ∈ {1,K}
pk,n ≥ 0, ck,n = 0 or 1 ∀k ∀n
where p min
k is the minimum required data rate of user k or one service flow. With the
above constraints, BS first meets the minimum data rate requirement of each user
and also tends to maximize the system throughput through power and subcarrier
allocation.
13.4.4 WiMAX QoS Policy
The actual scheduler needs to consider the QoS, user channel condition and the overall
system throughput at the same time. In WiMAX, QoS is administrated through
the service flows. A service flow is a MAC transport service that provides unidirectional
transport of packets either to UL packets transmitted by the MS or to DL
packets transmitted by the BS. A service flow is characterized by a set of QoS parameters
such as latency, jitter, and throughput assurances. When a packet arrives, it
will be added into the proper service queues according to its QoS classifier. WiMAX
supports 5 types of services: Unsolicited grant service (UGS), Real -time polling services
(RtPS), Extended Real -time polling services (Ertps), Non Real-time polling
services (nrtPS), and Best Effort service (BE).
13.4.4.1 Unsolicited Grant Service (UGS)
The UGS is designed to support real-time service flows that transport fixed-size data
packets on a periodic basis, such as T1/E1 and Voice over IP without silence suppression.
BTS should offer fixed-size grants on a real-time periodic basis, which
eliminate the overhead and latency of MS requests and assure that grants are available
to meet the flow’s real-time needs.
13.4.4.2 Real-time Polling Services (rtPS)
The rtPS is designed to support real-time service flows that generate variable size
data packets on a periodic basis, such as streaming video. BS should offer real-time,
periodic, unicast request opportunities, which meet the flow’s real-time needs and
allow the MS to specify the size of the desired grant.