114 9. WIMAX & LTE
by Qualcomm. It is interesting to note that some important service providers (e.g., Verizon), who
have been on the 3GPP2 evolution path, have announced plans to evolve to LTE.
WiMAX networks are based on IEEE 802.16 standards. The MAC Layer of these standards is
based on the MAC Layer of the networks based on Data Over Cable Service Interface Specification
(DOCSIS). There have been a number of different revisions of IEEE 802.16. The original IEEE
802.16 standard, published in December 2001, was intended for the fixed wireless usage in the 10-66
GHz spectrum. Subsequently, two most notable revisions have been IEEE 802.16d (59) intended
for the fixed wireless usage and IEEE 802.16e (60) intended for the mobile wireless usage, both
focusing on the 2-11 GHz spectrum.WiMAX networks are designed to work with 1.25 to 20 MHz
of bandwidth. IEEE 802.16m, which has ambitious performance requirements and is currently
under development, will be the next evolution of this series of standards.
In this section, we discuss some of the key ideas on WiMAX. Although the standards allow
for the Mesh mode of operation (i.e., direct communication between two WiMAX devices), this
mode is not expected to be prevalent in the near future. We only focus on the Point-to-Multipoint
(PMP) mode of operation where all communication in a cell takes place via a centralized node
referred to as the Base Station (BS). The IEEE 802.16 standards cover a number of different tech-
nologies for the Physical Layer. However, because of its expected prevalence in WiMAX networks,
we limit our Physical Layer related discussion to the Orthogonal Frequency Division Multiplexing
Access (OFDMA) technology. Furthermore, as suggested in Figure 9.1, although Frequency Di-
vision Duplex (FDD) is supported, WiMAX networks are primarily designed for Time Division
Duplex (TDD) for two-way communication.Therefore,the WiMAX discussion below assumes the
TDD mode of operation.
9.2.1 OFDMA
Orthogonal Frequency Division Multiplexing (OFDM) is a Frequency Division Multiplexing (FDM)
scheme that uses a large number of closely placed subcarriers. Orthogonality among subcarriers is
ensured by fixing the subcarrier spacing to be the reciprocal of the symbol time defined as the time
interval over which a given modulated state of a subcarrier is maintained. The benefits of OFDM
include higher spectral efficiency and better protection against multi-path distortion. In WiMAX,
subcarriers are grouped in subchannels. OFDMA is a multiple access method that builds on the
OFDM based Physical Layer. In this method, WiMAX devices share access to the frequency and
time domain resources of the system by using different subchannels and symbol times. Figure 9.2
shows an example of a WiMAX OFDMA frame. Although other values are allowed, a WiMAX
frame, which includes downlink and uplink sub-frames as well as guard times, is typically 5 ms long.
The ratio of downlink to uplink sub-frame durations is fixed in a given network operation (i.e., it
does not change dynamically).
DL Burst #6
DL Burst #5
DL Burst #4
DL Burst #3
DL Burst #2
UL Burst #1
UL Burst #2
UL Burst #3
UL Burst #4
UL Burst #5
UL Burst #6
DL Burst #1 (UL-MAP)
OFDMA Symbol Number
k k+1 k+3
k+27 k+30
Subchannel Number
5 ms
Figure 9.2: Example of WiMAX OFDMA Frame.
Figure 9.2, adapted from (61), shows the time and frequency resources of a sample WiMAX
frame in time (horizontal) and frequency (vertical) dimensions, respectively. The time dimension is
divided in units of symbol times, while the frequency dimension is divided in units of subchannels.
A WiMAX frame is divided in downlink (DL) and uplink (UL) sub-frames, which are separated by
Transmit-Receive Transition Gap (TTG) and Receive-Transmit Transition Gap (RTG) as shown.
Each WiMAX frame starts off with a preamble symbol followed by Frame Control Header (FCH),
DL-MAP (Downlink Map) and UL-MAP (Uplink MAP). FCH provides frame configuration
information, and the MAP messages (DL-MAP and UL-MAP) provide information regarding
location of each instance of downlink or uplink data or control information in the frame as well
as the corresponding modulation and coding. ACK, CQI and Ranging in the uplink sub-frame
denote respectively the allocations for the control information related to acknowledgments for H-
ARQ (Hybrid-ARQ), Channel Quality Indicator for channel quality feedback, and ranging (e.g.,
messaging required for joining the network or requesting uplink allocation).

Get Communication Networks now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.