For years, there exists a continuing challenge for service providers to satisfy the growing demand for broadband wireless access (BWA) in underserved business and residential markets [8]. They are seeking a solution to build systems that support infrastructure build outs comparable to cable, digital subscriber lines (DSL), and fiber. Recently, the IEEE 802.11x or WiFi wireless LAN technology has been used in BWA applications; however, it was evident that they are not suitable for outdoor BWA use for their limited capacity in terms of bandwidth and subscribers, range and other issues [8].
The IEEE conducted a multi-year effort to develop this standard, culminating in final approval of the 802.16a air-interface specification in January 2003. The 802.16a standard delivers carrier-class performance in terms of robustness and QoS and has been designed
Fig. 2.4: Carrier allocation of an OFDMA symbol (modified from [9]).
from the ground up to deliver a suite of services over a scalable, long range, high ca-pacity “last mile” wireless communications for carriers and service providers around the world [8]. The 802.16a standard specifies a protocol that among other things supports low latency applications such as voice and video, provides broadband connectivity with-out requiring a direct line of sight between subscriber terminals and the base station and will support hundreds if not thousands of subscribers from a single BS [8].
The IEEE 802.16a is an amendment of the 802.16 standard to cover frequency bands in the range between 2 and 11 GHz, and it specifies a metropolitan area networking pro-tocol that enables a wireless alternative for cable, DSL and T1 level services for last mile broadband access [8]. The major reason for using 2–11 GHz bands is that they have the ability to deal with non-line-of-sight (NLOS) operation. The longer wavelengths allow
for non-directional NLOS operation with the ability to serve much broader geographic re-gions, allowing underserved customers to take advantage of this technology. Compared to the higher frequencies, such spectra offer the opportunity to reach many more customers less expensively, although at generally lower data rates [10].
The 2–11 GHz spectrum does not require line-of-sight and directionality, and there-fore requires multiplexing techniques supporting multi-path propagation. Because resi-dential applications are expected, rooftops may be too low for a clear sight line to a BS antenna. Therefore, significant multipath propagation is expected [10]. As a result, the 802.16a did some major changes to the PHY layer specification, which includes a sin-gle carrier PHY, a 256-point FFT OFDM PHY, and a 2048-point FFT OFDMA PHY, to address the needs of 2–11 GHz bands. In this thesis, we consider the 2048-point FFT OFDMA.
The glossary we will often use in the following sections is introduced here. 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 usually known as the mobile station or the user. The BS is a generalized equipment set providing connectivity, management, and control of the SS.
2.3.1 UL Carrier Allocation
The number of subcarriers in one OFDMA symbol is 2048. These carriers are divided into as three types: data carriers for data transmission, pilot carriers for various estimation purposes, and null carriers (guard bands and DC carrier) which transmit nothing at all.
The data and pilot carriers together are termed the used carriers for they transmit useful information. The allocation is as shown in Fig. 2.5 for UL. Among the 2048 subcarriers, there are 1696 used carriers, composed of 1536 data carriers and 160 pilot carriers. The rest 352 subcarriers are unused subcarriers as the guard band distributed on the edges of the symbol, and one DC carrier right in the middle of the band of the OFDMA symbol.
Group 1 Group 2
Guard band Guard band
Group53
The 1696 used carriers = 1536 data carriers + 160 pilot carriers DC carrier
pilot pilot
32 used carriers (including pilot carriers)
subchannel 2 subchannel 1
Fig. 2.5: Illustration of carrier usage in OFDMA UL.
In 802.16a, the used subcarriers are divided into 32 subchannels, where each subchannel contains 48 data carriers, 1 fixed pilot carrier, and 4 variable location pilot carriers. The carrier allocation for UL is listed in Table 2.2.
The carrier index of the fixed-location pilots never change in different symbols. The variable-location pilots, however, shift their locations every symbol periodically every 13 symbols, according toLk = 0, 2, 4, 6, 8, 10, 12, 1, 3, 5, 7, 9, 11, where k = 0 to 12. Lk
is the amount of carrier spacing which will be added to L0 to shift to the right of the subcarrier position. Fork = 0, the variable-location pilots are positioned at indexes 0, 13, 27, and 40. For other values ofk, these locations change by adding Lkto each index.
2.3.2 OFDMA Data Mapping
A PHY burst in OFDMA is allocated a group of contiguous subchannels, in a group of contiguous OFDMA symbols using an FEC block as a unit. Note that one FEC block spans one OFDMA subchannel in the subchannel axis and three OFDM symbols in the time axis. Fig. 2.6 illustrates the order in which FEC blocks are mapped to OFDMA subchannels and OFDM symbols [1].
Table 2.2: OFDMA UL Carrier Allocation
Parameter UL Value
Number of DC carriers 1
Number of guard carriers, left 176 Number of guard carriers, right 175 Nused, number of used carriers 1696
Total number of carriers 2048 Number of data carriers per subchannel 48
2.3.3 OFDMA Frame Structure for TDD
The 802.16a is designed to operate in the frequency band between 2 to 11 GHz. The duplexing method of OFDMA system in this band shall be either frequency division du-plexing (FDD) or time division dudu-plexing (TDD) in licensed bands and TDD in license-exempt bands. We consider the TDD mode in this thesis, since TDD is better suited to data communications, which is often highly asymmetric. TDD flexibility permits efficient allocation of the available traffic transport capacity, and thus the uplink and downlink traf-fic transport ratio can vary with time.
Fig. 2.7 shows the frame structure of TDD OFDMA. A frame consists of one DL sub-frame and one UL subsub-frame, and they are transmitted by the BS and the SS, respectively.
The allowed duration of a frame is from 2 to 20 ms and is specified by the frame duration code. A subframe contains several transmission bursts, which are composed of multiples FEC blocks. In each frame, the Tx/Rx transition gap (TTG) and Rx/Tx transition gap (RTG) shall be inserted between the downlink and uplink and at the end of each frame respectively to allow the BS and the SS to turn around. TTG and RTG shall be at least
Fig. 2.6: Mapping of FEC blocks to OFDMA subchannels and symbols (from [1]).
5 µsec and an integer multiple of four samples in duration [1].
From the UL-MAPs, the SSs know their usable subchannels and transmission time.
The first symbol is an all-pilot preamble where the SS should send on all its allocated subchannels. The number of symbols of the UL subframe is 3N + 1, where N is a positive integer, one for the preamble, and the others for data bursts.