Microwave Journal

A WiMAX Design Library for System-level RF/DSP Co-design

November 14, 2005

The IEEE 802.16-2004 standard, generally referred to as WiMAX, specifies air interfaces for broadband wireless access (BWA) systems. The standard is expected to energize the BWA industry and create opportunities to deploy systems in applications that were previously cost-prohibitive. WiMAX enables multiple services in a wireless metropolitan area network (WMAN), such as wireless backhaul for telecommunications, high bandwidth/high reliability remote connectivity, E1/T1 replacements for small- and medium-sized businesses, and residential “wireless DSL” for broadband Internet at home.

WiMAX supports fixed broadband wireless access for both licensed and unlicensed spectra in the 2 to 11 GHz range. The mandatory PHY mode is 256-point FFT orthogonal frequency division multiplexing (OFDM). The WiMAX Forum certifies equipment supporting the OFDM PHY model.

WiFi 802.11a and 802.11g also use OFDM and have established an excellent performance record for robust wireless networking. WiFi uses 64-point OFDM. The much larger number of carriers for WiMAX helps it achieve greater range, because a receiver using 256 OFDM can tolerate delay spreads of up to 10 times that of systems using 64 OFDM. Also, 256 OFDM provides good non-line-of-sight capability.

The WiMAX Design Exploration Library

With competition heating up for WiMAX-related products, system designers are looking into EDA tools that can help them design products that achieve the best power performance at the least cost. This is challenging, especially as designers look for optimal system performance. Such efforts require a good understanding of the system design and the ability to optimize individual system block specifications. System blocks contain both analog/RF and DSP components. The WiMAX design exploration library provides preconfigured simulation setups, signal sources and fully coded BER analysis for simulation of the circuitry used in BWA designs. It speeds the development cycle by allowing system designers to analyze a system’s performance before all of its components are designed. It works within the ADS 2005A environment and with the Agilent Ptolemy simulator to streamline design and verification of OFDM-based, last-mile service designs. The WiMAX library can also be imported into Agilent’s RF design environment (RFDE), allowing RFIC designers to access WiMAX test benches within the Cadence Virtuo Custom IC platform through links developed as part of the ongoing alliance between Agilent Technologies and Cadence Design Systems. Transmitter measurements performed for both uplink and downlink subframes include EVM, constellation, CCDF, spectrum mask, waveform and spectral flatness.

Receiver measurements performed for both uplink and downlink subframes include receiver sensitivity, BER and PER in AWGN, BER and PER in fading channel, and adjacent channel rejection.

The fixed wireless access propagation channel model is included in the WiMAX Design Exploration Library. The multi-path fading is modelled as a tapped-delay line representing 6 SUI (Stanford University Interim) channel models.

Receiver Sensitivity Test Bench

One of the test benches in the WiMAX Design Exploration Library is the receiver sensitivity measurement. The standard dictates a BER/FER limit based on a receiver signal-to-noise ratio (SNR) and a maximum noise figure (NF) of 12 dB.

The test bench can introduce non-idealities due to the architecture of the circuits in the RF or DSP section. The measurement results can then easily identify circuit architectures that meet the standard requirements. For example, by changing the receiver SNR, one can realize the minimum received power needed to meet overall system frame error rate (FER). Table 1 shows the result of a sweep of a receiver SNR based on a double conversion receiver architecture. The RF and IF parameters shown in Figure 1 can easily be changed to adapt to the specific data rate of the WiMAX system. Furthermore, designers can select a different RF_RX_IF architecture to perform trade-off analysis. Timed components in Agilent Ptolemy are time-based signals that carry I, Q, Dt (time step resolution) and FCarrier information. This powerful signal representation is based on the timed synchronous data flow (Timed SDF) engine in Agilent Ptolemy. In this example, a double conversion receiver versus low IF architectures can be studied using Agilent Ptolemy built-in components from Timed RF subsystems selection available in Agilent Ptolemy, within the receiver test bench of the WiMAX design exploration library.

Fig. 1 RF architecture trade-off analysis in the WiMAX design exploration library.

UWB Interference on WMAN Signal

The actual operating environment of WiMAX transmitters and receivers includes transmitters based on other standards. Transmitters using the UWB standard are a potential source of interference with WiMAX, and one with which designers must be concerned. The interference of UWB signals in the 3.4 GHz band is of special concern when using the 802.16-2004 technology. Recent industry studies show that the UWB signal must be detected, and possibly moved to another RF frequency, to avoid destructive interference with fixed broadband wireless devices.1

The flexibility of Agilent Ptolemy Design Libraries allows designers to set up interference signals from various sources at different frequencies and signals. With the software that is available with Agilent’s vector signal analyzer (VSA) instrumentation, designers can quickly test the conditions that are destructive within the simulation environment.2 The power level setting for UWB interference, centered at 3432 MHz, can be set from the variables indicated on the UWB_Signal Source_RF component, as shown in Figure 2. After summing the UWB interference with the WiMAX signal, the signal centered at 3.4 GHz will be filtered and then analyzed with the VSA software running in Agilent Ptolemy.

Fig. 2 UWB interference on the WiMAX signal using VSA software running inside the Agilent Ptolemy simulator.

The results shown in Figure 3 indicate the increase in EVM due to the power increase in the UWB signal, from –9.9 to –2 dBm. The RF and base band filters from the Agilent Ptolemy filter design library can be selected to determine the filter characteristics that provide optimal interference rejection. Trade-offs between interference rejection and receiver sensitivity can be evaluated quickly with these simulations.

Fig. 3 VSA software analysis with the UWB interference signal set at –9.9 dBm (a) and –2 dBm (b).

Conclusion

The main challenge facing communications system designers is performing RF architecture selection, optimization and verification concurrently with digital base band design to make intelligent trade-off decisions and not over-design the system. Agilent Ptolemy provides a unique capability where timing synchronization (Timed SDF) enables digital base band models to be co-simulated with high fidelity RF behavioral models. Designers need to use this environment to catch problems early in the design cycle to prevent unnecessary hardware iterations later. The WiMAX Design Exploration Library offered with ADS 2005A enables designers to achieve easy design performance analysis of their RF and DSP components for WiMAX system designs.3

References

1. http://www.reed-electronics.com/electronicnews/article/CA6252881?nid=2019&rid=1752335370.

2. http://www.agilent.com/find/vsa.

3. http://eesof.tm.agilent.com/products/wimax_del_2005.html.

Agilent Technologies,
EEsof EDA Division,
Santa Rosa, CA
(800) 829-4444,
www.agilent.com/find/eesof.