Rapid growth of Wireless Interoperability Microwave Access (WiMAX) as a broadband wireless technology is expected due to increasing demand for convenient Internet access and highspeed data access. In order to construct wideband wireless systems such as WiMAX, however, effective modeling must be performed. In this report, wideband modeling was performed on key elements of a WiMAX system, notably the modulator, frequency upconverter, and transmit power amplifier. This modeling included the study of phase-noise effects. Extensive computations were carried out to study the effects of nonlinear effects as well as performance variations due to phase noise.

Wireless transmission of high data rates requires careful modeling of both the transmitter and receiver. A number of reports have addressed this previously. For example, in ref. 1, a multi-band multi-standard receiver architecture for wireless applications, such as IEEE 802.11a/b/g WLAN, and IEEE 802.16a wireless metropolitan-area network (WMAN) is presented. The targeted frequency bands include the licensed bands at 2.3 GHz, 2.5 to 2.7 GHz, and 3.5 to 3.7 GHz, and the unlicensed ISM 2.4- GHz and UNII 5-GHz bands. The main advantage of the proposed receiver include the use of a single frequency synthesizer for wideband frequency translation, with each frequency band sharing the same LNA and mixer for reduced chip area.

Reference 2 explains orthogonal frequency division multiplex (OFDM) and single-carrier (SC) physical layers as two separate and standalone compliant modes for the licensed bands between 2 to 11 GHz. The purpose of this article is to explore the benefits of mixed-mode operation with OFDM downlink and SC with Frequency Domain Equalization (SC-FDE) uplink; each is strictly compatible with currently available OFDM and SCFDE modes. It is not proposed as a new operating mode, but rather an integration of the existing OFDM and the SC modes.

Reference 3 describes equalizer adaptation in time-variant frequency-selective FRA-channels in the presence of oscillator phase noise. This article will explore the complexity of implementing that equalizer and compare it to a set of different equalizers. In addition, this article will examin the effects of a various parameters concerning the WiMAX baseband spectrum, such as the modulation scheme and the number of Fast Fourier Transform (FFT) points, and the effects of phase noise during modulation and frequency upconversion. By studying these parameters, a telecommunication systems designer can better understand how to increase the performance of a given model. Modeling of the telecommunications channels was performed with the aid of the Advanced Design System 2005A^TM simulation software from Agilent Technologies (www.agilent.com).

Figure 1 shows the transmitter model, consisting of a baseband modulator unit, and RF modulator and frequency upconverters, and a power-amplifier and phasenoise input. Figure 2 shows the baseband modulator unit, consisting of the input data modulation, the pilot data modulation, their mutual OFDM modulation, and an imported preamble to produce a complete IEEE 802.16 WiMAX baseband signal. Figure 3 shows the RF modulation and signal upconversion process. An amplifier, an in-phase (I)/quadrature (Q) modulator with an external oscillator and Chebyshev bandpass filter are used for this purpose. Figure 4 shows the power amplification and phasenoise input port. This involves the use of an RF gain amplifier, an external noise source (to provide controlled degradation of the external oscillator), and a power splitter for the timed measurements.

A computer-aided-engineering (CAE) software program based on Agilent ADS 2005A was used to develop a simulation program. Two cases were studied: a case free of phase noise and a case with a noisy modulator. For the first case, the focus was on the effects of various baseband parameters on the output baseband signal. These parameters include bandwidth, the number of Fast Fourier Transform (FFT) points, and the modulation scheme. The relationships established in the IEEE 802.16 WiMAX standard were used as the basis for the simulation, including:

Sampling frequency, Fs = floor[(n X BW)/8000] X 8000

Subcarrier spacing, ?f = Fs/NFFT

Useful OFDM symbol time, Tb = 1/?f

Cyclic prefix time, Tg = G X Tb

OFDM symbol time, Ts = Tb + Tg

Sampling time = Ts = Tb/NFFT

Samples per symbol = bandwidth X OFDM symbol time

Symbols per second = bandwidth/symbols per second or 1/OFDM symbol time

The subcarrier spacing is the sampling frequency divided by the number of the FFT points. Therefore, the useful symbol time is the inverse of the subcarrier spacing. By setting the step that the ADS algorithm will use for timed measurements, TStep, equal to the useful symbol time divided by the number of FFT points (the inverse of the sampling frequency), the algorithm will work properly.

One simulation was set up with a constant modulation scheme (16QAM with a 1/2 coding rate), a constant number of FFT points (256), a constant guard time (0.25 X the symbol time), and bandwidth as the variable. The starting bandwidth was 1.75 MHz and the stopping bandwidth was 28 MHz, using care to select channel bandwidths specified in the IEEE standard. By increasing the bandwidth, the ripple of the signal increased as well, while unwanted sidelobes were suppressed more than -35 dBc than the desired signal level (Fig. 5).