Frequency Allocation in Mobile WiMAX

In orthogonal frequency division multiple access (OFDMA), the frequency band is divided into subcarriers. These subcarriers are grouped into sets, called subchannels in WiMAX, which are allocated to users. We will consider in our numerical applications the case of an Fast Fourier Transform (FFT) size of 1024 subcarriers, with an intercarrier spacing of 7.8 kHz.

Two allocation modes are possible: distributed and adjacent.

Distributed Allocation

In the distributed subcarriers allocation, full channel diversity is obtained by distributing the allocated subcarriers to subchannels using a permutation mechanism. This mechanism is designed to introduce frequency diversity, thus minimizing the performance degradation due to fast fading which is characteristic of mobile environments. In addition to that, WiMAX standards specify two different distributed allocation modes: the fully used subchannelization (FUSC) mode where all subcarriers are used to form subchannels in each cell, and the partially used subchannelization (PUSC) mode where the frequency band is divided into three segments.

For illustration, with an FFT size of 1024 and after reserving the pilot and guard subcarriers, a FUSC allocation will correspond to 16 subchannels of 48 data subcarriers each, whereas a PUSC allocation will correspond to 30 subchannels, each containing 24 data subcarriers. Note that assigning subcarriers to subchannels in PUSC is a bit complicated, as it employs two permutations:

§  An outer permutation divides the subcarriers into six major groups of clusters using a specific renumbering sequence.

§  An inner permutation operates separately on each major group, distributing subcarriers to subchannels within the group and is based on the FUSC permutation with distinct parameters for the odd and even major groups.

This is illustrated in Figure 1, where two groups are assigned to one segment corresponding to a sector of the cell. Note that a segment can also be allocated to a cell in an omni-directional setting.

Figure 1: Construction of groups and segments in the partially used subchannelization allocation mode.

Adjacent Allocation

This method uses adjacent subcarriers to form subchannels. It corresponds to the WiMAX AAS (Advanced Array Systems) mode, designed to support Multiple-Input Multiple-Outpt (MIMO) techniques and adaptive modulation. Note that, to achieve a frequency diversity, mobiles using adjacent allocation may hop rapidly between different subchannels during their communication times.

Radio Coverage Area

Radio Coverage Area

Radio coverage occurs when a geographic area receives a radio signal above a specified minimum level. WiMAX can operate up to 50 km under line of sight (LOS) and up to 8 km under non-line of sight (NLOS) conditions. Practical cell sizes are limited to approximately 5 miles.

WiMAX radio coverage varies based on the options installed and used (such as diversity transmission) in the equipment and the modulation (such as QAM –vs- QPSK), frequency and the parameters that are set.

For the most part, there is a tradeoff between data transmission rate and distance. As the modulation type becomes more efficient (more bits per Hertz), the higher the channel quality has to be at the receiver which means the maximum distance that can be used from the transmitter is reduced.

Radio signal attenuation varies from approximately 20 dB per decade in free space to between 40 to 60 dB per decade when signals travel through objects (resulting in building penetration loss). As the distance increases by a factor of ten in freespace, the signal level drops by a factor of 1000, whereas when radio signals travel through objects (walls and floors), the signal may decrease by a factor of 100,000 or more.

Figure 1 illustrates the maximum distance and data transmission rates for fixed and mobile WiMAX communication in a geographic setting. A 20 MHz wide WiMAX radio channel can provide approximately 75 Mbps of data transfer (when it is close to the base station) while the data transmission rate decreases as the distance from the base station increases.

Figure 1: WiMax Carrier Serving Area

Frequency Bands

Frequency bands are the range of frequencies that are used or allocated for radio services. There are two primary frequency bands defined for WiMAX systems; 10 to 66 GHz (the original frequency band) and 2 to 11 GHz. The WiMAX system is designed to allow operation on licensed or unlicensed radio channels.

A licensed frequency band is a range of frequencies that requires authorization for use (a license) from a regulatory agency or owner of the frequency band in a geographic area for permission to transmit radio signals in that area. Unlicensed frequency bands are a range of frequencies that can be used by any product or person provided the transmission conforms to transmission characteristics defined by the appropriate regulatory agency.