Since the publication of the IEEE 802.16 standard for fixed broadband wireless access in 2001, a number of revision and amendments have taken place. Like other IEEE 802 standards, the 802.16 standards are primarily concerned with physical (PHY) layer and medium access control (MAC) layer functionalities. The idea originally was to provide broadband wireless access to buildings through external antennas communicating with radio base stations (BSs).
To overcome the disadvantage of the line-of-sight (LOS) requirement between trans-mitters and receivers in the 802.16 standard, the 802.16a standard was approved in 2003 to support nonline-of-sight (NLOS) links, operational in both licensed and unlicensed fre-quency bands from 2 to 11 GHz, and subsequently revised to create the 802.16d standard (now code-named 802.16-2004). With such enhancements, the 802.16-2004 standard has been viewed as a promising alternative for providing the last-mile connectivity by radio link. However, the 802.16-2004 specifications were devised primarily for fixed wireless users. The 802.16e task group was subsequently formed with the goal of extending the 802.16-2004 standard to support mobile terminals.
The IEEE 802.16e has been published in Febuary 2006. It specifies four air inter-faces: WirelessMAN-SC PHY, WirelessMAN-SCa PHY, WirelessMAN-OFDM PHY, and WirelessMAN-OFDMA PHY. This study is concerned with WirelessMAN-OFDMA PHY in a mobile communication environment.
Some glossary we will often use in the following is as follows. The direction of transmission from the base station (BS) to the subscriber station (SS) is called downlink (DL), and the opposite direction is uplink (UL). The SS is considered synonymous as the mobile station (MS). It is sometimes termed the user. The BS is a generalized equipment set providing connectivity, management, and control of the SS.
2.3.1 OFDMA Basic Terms
In the OFDMA mode, the active subcarriers are divided into subsets of subcarriers, where each subset is termed a subchannel. The subcarriers forming one subchannel may, but need not be, adjacent. The concept is shown in Fig. 2.5.
Three basic types subchannel organization are defined: partial usage of subchannels (PUSC), full usage of subchannels (FUSC), and adaptive modulation and coding (AMC);
among which the PUSC is mandatory and the other two are optional. In PUSC DL, the entire channel bandwidth is divided into three segments to be used separately. The FUSC is employed only in the DL and it uses the full set of available subcarriers so as to maximize the throughput.
Slot and Data Region
The definition of an OFDMA slot depends on the OFDMA symbol structure, which varies for uplink and downlink, for FUSC and PUSC, and for the distributed subcarrier permu-tations and the adjacent subcarrier permutation.
• For downlink PUSC using the distributed subcarrier permutation, one slot is one subchannel by two OFDMA symbols.
• For uplink PUSC using either of the distributed subcarrier permutations, one slot is one subchannel by three OFDMA symbols.
• For downlink FUSC and downlink optional FUSC using the distributed subcarrier permutation, one slot is one subchannel by one OFDMA symbol.
Figure 2.5: OFDMA frequency description (3-channel schematic example, from [1]).
In OFDMA, a data region is a two-dimensional allocation of a group of contiguous sub-channels, in a group of contiguous OFDMA symbols. All the allocations refer to logical subchannels. This two-dimensional allocation may be visualized as a rectangle, such as the 4×3 rectangle shown in Fig. 2.6.
Segment
A segment is a subdivision of the set of available OFDMA subchannels (that may include all available subchannels). One segment is used for deploying a single instance of the MAC.
Permutation Zone
A permutation zone is a number of contiguous OFDMA symbols, in the DL or the UL, that use the same permutation formula. The DL subframe or the UL subframe may con-tain more than one permutation zone. The concept of permutation zone will be further elaborate later.
2.3.2 OFDMA Symbol Parameters
Some OFDMA symbol parameters are listed below.
• BW : Nominal channel bandwidth.
• Nused: Number of used subcarriers.
Figure 2.6: Example of the data region which defines the OFDMA allocation (from [1]).
• n: Sampling factor. This parameter, in conjunction with BW and Nused, determines the subcarrier spacing and the useful symbol time.
• G: Ratio of cyclic prefix (CP) time to useful time.
• NF F T: Smallest power of two greater than Nused.
• Sampling frequency: Fs = bn · BW/8000c × 8000.
• Subcarrier spacing: ∆f = Fs/NF F T.
• Useful symbol time: Tb = 1/∆f .
• Cyclic prefix (CP) time: Tg = G · Tb.
• OFDM symbol time: Ts= Tb+ Tg.
• Sampling time: Tb/NF F T.
2.3.3 Scalable OFDMA [7]
One feature of the IEEE 802.16e OFDMA is the selectable FFT size, from 128 to 2048 in multiples of 2, excluding 256 to be used with OFDM. This has been termed scalable OFDMA (S-OFDMA). One use of S-OFDMA is that if the channel bandwidths are allo-cated based on integer power of 2 times a base bandwidth, then one may consider making the FFT size proportional to the allocated bandwidth so that all systems are based on the same subcarrier spacing and the same OFDMA symbol duration, which may simplify system design. For example, Table 2.2 lists some S-OFDMA parameters proposed by the WiMAX Forum [8]. S-OFDMA supports a wide range of bandwidth to flexibly address the need for various spectrum allocation and usage model requirements.
When designing OFDMA wireless systems the optimal choice of the number of sub-carriers per channel bandwidth is a tradeoff between protection against multipath, Doppler shift, and design cost/coplexity. Increasing the number of subcarriers leads to better im-munity to the ISI caused by multipath; on the other hand it increases the cost and com-plexity of the system (it leads to higher requirements for signal processing power and
Table 2.2: S–OFDMA Parameters Proposed by WiMAX Forum
Parameters Values
System Channel Bandwidth (MHz) 1.25 5 10 20 Sampling Frequency (MHz) 1.4 5.6 11.2 22.4
FFT Size 128 512 1024 2048
Subcarrier Spacing (∆f ) 10.94 kHz
Useful Symbol Time (Tb=1/∆f ) 91.4 µsec
Guard Time (Tg=Tb/8) 11.4 µsec
OFDMA Symbol Duration (Ts=Tb+Tg) 102.9 µsec
power amplifiers with the capability of handling higher peak-to-average power ratios).
Having more subcarriers also results in narrower subcarrier spacing and therefore the sys-tem becomes more sensitive to Doppler shift and phase noise. Calculations show that the optimum tradeoff for mobile systems is achieved when subcarrier spacing is about 11 kHz [9] .