In terms of operating carrier frequency and transmission bandwidth, IEEE 802.16-2004 provides a wide variety of options. However, the proposed baseline PHY receiver algorithm design is not affected by variation in carrier frequency and bandwidth. However, advanced receiver algorithms might take advantage of these variations and employ better baseband algorithms depending on the operating frequency and bandwidth. In this work, a generic baseline receiver algorithm design is proposed that works fairly well over different operation frequencies. Improvements on the receiver algorithms, which can take a priori information into account, are left for a future study.

The WiMAX Forum (www.wimaxforum.org) has the mission of ensuring the interoperability, promoting the IEEE 802.16 standard, and helping grow the overall market acceptance for IEEE 802.16 as a BWA standard. However, one of the Forum's first deliverables is to establish specific conformance procedures and testing labs to administer the testing. This process will result in "WiMAX certified" products that have guaranteed interoperability with other WiMAX certified solutions.

In developing a WiMAX system model, the transmitter's signal format must be adequately described, including frame formats (downlink and uplink subframes), OFDM symbol formats (for preamble and data symbols), standards based baseband transmitter blocks, and basic model of the transmitted signal itself. The frame format is like a traditional packet-based structure that uses a preamble and header followed by data bursts. In a standard downlink frame [DL] (Fig. 1), the base-station transmits a preamble (long preamble that has a two OFDM symbol length), header, and multiple downlink bursts that are assigned to different users. Optionally, midambles (not shown in Fig. 1) can also be inserted before some bursts. In the uplink (UL), a preamble (short preamble which has only one OFDM symbol length) is used for each uplink burst transmitted by a user. Each uplink transmitter is assigned a time slot to transmit its burst in a time-division-multiplexing (TDM) manner. Similarly, in the downlink, a TDM type approach is employed (Fig 1). As mentioned previously, the uplink and downlink subframes can be configured as TDD, FDD, or H-FDD. In TDD mode, a short gap is placed between the DL and UL subframes (transmit-receive-transition-gap or TTG). After completion of the UL subframes, another short gap is added between this subframe and the next DL subframe (receiver-transmit-transition-gap or RTG).

Each preamble, header, and burst is made up of one or more OFDM symbols. Modulation on the OFDM carriers is binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16-state quadrature amplitude modulation (16QAM), or 64QAM. Depending on the link quality between the transmitter and receiver, an appropriate modulation is selected for individual data bursts. Particularly interesting is the ability of using different modulation formats on each data burst. The information bits are mapped into data symbols depending on the used modulation type. The serial data symbols are then demultiplexed into parallel blocks and inverse-fast-Fourier-transform (IFFT) is applied to these parallel blocks to obtain the time-domain OFDM symbols. In OFDM-256, the number of subcarriers is 256. There are three types of subcarrier assignments: data, pilot, and null. In a regular OFDM symbol, 200 subcarriers are used for data and pilot, and the remaining 56 carriers are nulled for providing guard bands and dealing with carrier leakage which renders the DC carrier unusable. Of the 200 subcarriers, eight of these are used as pilot subcarriers, and these pilots are inserted at regular interval among the other data subcarriers which make up the remaining 192 active carriers. Figure 2 shows the carriers in the frequency domain.

Figure 3 shows the block diagram of a basic digital baseband transmitter. The information bits from the upper layers of the protocol stacks are first passed through a channel coding which consists of three blocks: randomizer, forward-error-correction (FEC) block, and interleaver. Data randomization is performed on each burst of data on the downlink and uplink. A FEC block, consisting of the concatenation of a Reed-Solomon (RS) outer code and a rate-compatible convolutional inner code, must be supported on both uplink and downlink. On the other hand, support of block-turbo coding (BTC) and convolutional-turbo-coding (CTC) are optional.