Other Advanced PHY Layer Features

Adaptive modulation and coding (AMC), Hybrid Automatic Repeat Request (HARQ)

and Fast Channel Feedback (CQICH) were introduced with Mobile WiMAX to enhance

coverage and capacity for WiMAX in mobile applications.

Support for QPSK, 16QAM and 64QAM are mandatory in the DL with Mobile WiMAX.

In the UL, 64QAM is optional. Both Convolutional Code (CC) and Convolutional Turbo

Code (CTC) with variable code rate and repetition coding are supported. Block Turbo

Code and Low Density Parity Check Code (LDPC) are supported as optional features.

Table 2 summarizes the coding and modulation schemes supported in the Mobile

WiMAX profile the optional UL codes and modulation are shown in italics.
The combinations of various modulations and code rates provide a fine resolution of data
rates as shown in Table 3 which shows the data rates for 5 and 10 MHz channels with
PUSC sub-channels. The frame duration is 5 milliseconds. Each frame has 48 OFDM
symbols, with 44 OFDM symbols available for data transmission. The highlighted values
indicate data rates for optional 64QAM in the UL.

The base station scheduler determines the appropriate data rate (or burst profile) for each
burst allocation based on the buffer size, channel propagation conditions at the receiver,
etc. A Channel Quality Indicator (CQI) channel is utilized to provide channel-state
information from the user terminals to the base station scheduler. Relevant channel-state
information can be fed back by the CQICH including: Physical CINR, effective CINR,
MIMO mode selection and frequency selective sub-channel selection. With TDD
implementations, link adaptation can also take advantage of channel reciprocity to
provide a more accurate measure of the channel condition (such as sounding).
Hybrid Auto Repeat Request (HARQ) is supported by Mobile WiMAX. HARQ is
enabled using N channel “Stop and Wait” protocol which provides fast response to packet
errors and improves cell edge coverage. Chase Combining and optionally, Incremental
Redundancy are supported to further improve the reliability of the retransmission. A
dedicated ACK channel is also provided in the uplink for HARQ ACK/NACK signaling.
Multi-channel HARQ operation is supported. Multi-channel stop-and-wait ARQ with a
small number of channels is an efficient, simple protocol that minimizes the memory
required for HARQ and stalling [8]. WiMAX provides signaling to allow fully
asynchronous operation. The asynchronous operation allows variable delay between
retransmissions which gives more flexibility to the scheduler at the cost of additional
overhead for each retransmission allocation. HARQ combined together with CQICH and
AMC provides robust link adaptation in mobile environments at vehicular speeds in
excess of 120 km/hr.
3. MAC Layer Description
The 802.16 standard was developed from the outset for the delivery of broadband
services including voice, data, and video. The MAC layer is based on the time-proven
DOCSIS standard and can support bursty data traffic with high peak rate demand [9]
while simultaneously supporting streaming video and latency-sensitive voice traffic over
the same channel. The resource allocated to one terminal by the MAC scheduler can vary
from a single time slot to the entire frame, thus providing a very large dynamic range of
throughput to a specific user terminal at any given time. Furthermore, since the resource
allocation information is conveyed in the MAP messages at the beginning of each frame,
the scheduler can effectively change the resource allocation on a frame-by-frame basis to
adapt to the bursty nature of the traffic.