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4.2 WiMAX Specifications and Requirements
To support mobility, the air interface management must adequately be able to react quickly to
changing signal conditions and it also must hand over a connection between base stations when
the user is moving. For battery-driven devices, the air interface had to be optimized to be as
power-efficient as possible during times when no data is being transferred. To that extent, the
WiMAX air interface was enhanced to allow higher transmission speeds. Significantly, as
mentioned in the previous chapter, the signal transmission is based on OFDM technology [4-6]
making the network robust to high delay spreads of the wireless channel.
WiMAX can be implemented as fixed or mobile scenario. Regarding mobile WiMAX the
standard supports FDM and TDM multiplexing scheme, although initial deployments will only
make use of the TDM option [6]. The table below includes relevant physical parameters of
mobile WiMAX. Various channel bandwidths are supported depending on the regulatory issues
for certain deployment scenarios as detailed in chapter 2.
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Table 4-1: Mobile WiMAX parameters [7, 8]
Parameter
Value
Chanel Bandwidth
1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 20 MHz.
FFT size
Typical: 1024
Number of Antennas
Tx: 1, 2, 4
Rx: 1, 2, 4
Multiple Access Technology
DL: OFDMA
UL: OFDMA
Modulation Schemes
QPSK, 16QAM, 64QAM
Maximum BS output power
Not standardised. According to local regulations.
Assumptions:
5MHz: 43 dBm
10 MHz: 46 dBm
20 MHz: 49 dBm
Maximum EVM
64QAM: < -30 dB
In addition, Figure 4-2 shows the structure of a WiMAX frame (TDM) in more detail. In
general this WiMAX TDM frame has duration of 5 ms. The downlink sub-frame starts with the
preamble. It helps the mobile users to identify the cell and to roughly synchronize to the frame
timing [6]. Then the frame control elements are placed (FCH, downlink MAP and uplink
MAP). The FCH is the first element to be decoded by a mobile. It carries the length of and the
coding scheme used by the MAPs. They contain information elements regarding the placement
of the user data, the modulation and coding schemes and the zone switches. The FCH and the
MAPs are to be decoded by all users. The first zone within the downlink sub-frames must be in
partially used subcarrier (PUSC) mode [6]. The rest of the sub-frame is dedicated to the
transport of the user data.
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