WiMAX, or Worldwide Interoperability for Microwave Access, is defined by the WiMAX Forum™ with the objective to promote conformance and interoperability of products based on IEEE 802.16, known as WirelessMAN®.
WiMAX is to 802.16 what WiFi is to 802.11. It defines a selection of the requirements from the IEEE standard, groups them in well-defined profiles, and establishes a set of conformance tests for these profiles. WiMAX also brings compliance and interoperability to the industry with the WiMAX Forum Certified™ Testing and Certification Program. This stamp of approval guarantees conformance and a minimum set of performance criteria and interoperability with other certified products.
WiMAX enables the delivery of last-mile wireless broadband access as an alternative to cable and DSL. With the addition of the mobility variant, WiMAX offers solutions that go beyond replacing existing wired access media. It provides fixed, nomadic, portable, and true mobile wireless broadband point-to-point and point-to-multipoint connectivity without the need for direct line-of-sight with a base station.
WiMAX comes in two flavors: the fixed version which specifies profiles based on the WirelessMAN-OFDM PHY from the 802.16-2004 standard and the mobile version based on the WirelessMAN-OFDMA PHY from the 802.16e-2005 amendment.
In addition to the profiles, WiMAX defines Radio Conformance Test (RCT) specifications for each system version, which contain more details about the physical-layer requirements and the conformance tests. Successfully passing these tests in a WiMAX Forum accredited lab is a requirement for WiMAX Forum Certified products.
Korea's telecom industry has developed its own standard for mobile broadband wireless access, called WiBro. WiBro was developed by the Koreans as a regional and, potentially, broader alternative to 3.5G to 4G systems. While WiBro-compliant products and services are available on the market today, the lack of momentum for this standard forced WiBro to join WiMAX and harmonize with the mobile version of the standard included as one or more of the mobile profiles.
For WiMAX to become successful, the availability of low-cost devices will be critical. WiMAX subscriber station (SS) functionality is expected to be available at roughly the same price as WiFi solutions.
This not only places pressure on chipset vendors and system builders to reduce hardware costs, but it also greatly impacts how these devices and systems will be tested, especially in manufacturing where test time directly affects product cost. Fast, accurate, and low-cost manufacturing test methodologies are essential for WiMAX technology to be productized, gain widespread acceptance, and meet customer expectations.
The WiMAX Signal Structure
Both fixed and mobile WiMAX use orthogonal frequency division multiplexing (OFDM) as their physical-layer modulation scheme. With OFDM, the occupied bandwidth is divided into a large number of closely spaced sub carriers (Figure 1).
The subcarriers are individually modulated using binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-QAM, or 64-QAM modulation. Compared with traditional single-carrier modulation, OFDM has longer symbols and, as a result, a lower symbol rate. However, data is transmitted simultaneously over all OFDM subcarriers resulting in similar spectral efficiency.
In the time domain, the OFDM data payload is extended with a cyclic prefix. In mobile WiMAX, this prefix is 1/8th of the payload. The cyclic prefix protects the data payload from interference caused by multipath channels and is a critical OFDM advantage when compared to traditional modulation.
In addition, OFDM lends itself to much simpler equalizer implementations. This is because the multipath channel can be assumed flat over the narrow subcarrier bandwidth, and the equalizer design is reduced to linear algebraic operations.
The mapping between the time and frequency domains is conveniently performed using FFTs. The emergence of low-cost and low-power signal processing functionality in silicon to perform FFT functions is one of the enablers of OFDM technology.
In both fixed and mobile WiMAX cases, a fixed frame duration is divided into downlink and uplink portions, and those portions are further assigned among the SSs registered with the base station. The allocations are dynamically adjusted on a per-frame basis and encoded at the start of each downlink portion.
Mobile WiMAX further adds orthogonal frequency division multiple access (OFDMA). With OFDMA, the subcarriers within a symbol are further assigned to multiple SSs. The subdivision is performed using logical subchannels, which are mapped to physical subcarriers using a variety of mapping schemes. Available mapping schemes include PUSC, FUSC, and AMC.
Fixed WiMAX is built on a 256-bin FFT where 200 bins are active and map to data and pilot subcarriers. With fixed WiMAX, the channel bandwidth is adjusted by changing the subcarrier frequency spacing. In contrast, mobile WiMAX has fixed subcarrier spacing, and the channel bandwidth is adjusted by changing the FFT size and the number of active subcarriers. Mobile WiMAX is said to have scalable OFDMA. The constant subcarrier spacing and symbol duration simplify SS design.
The WiMAX Forum promotes interoperability among WiMAX devices through system profiles, certification profiles, and the WiMAX Forum certified equipment certification program. System profiles specify the features that the equipment must support. These capabilities are drawn from the larger set of mandatory and optional features described in the IEEE specifications.
Certification profiles further define equipment classes, which typically include the RF frequency band of operation, the duplex type (either time division duplex or frequency division duplex), and the channel bandwidth.
Fixed WiMAX currently has one approved system profile and five approved certification profiles. Further releases will address features such as additional quality of service (QoS) and encryption functionality.
In addition to the features already described, WiMAX includes support for various smart antenna technologies. The Release 1 system profile requires that the SS supports two streams and spatial multiplexing in the downlink and two-user collaborative spatial multiplexing in the uplink, implying a minimum configuration of one transmit and two receiver antennas. Two-user collaborative spatial multiplexing is a 2 x 2 system where two SSs are used, each transmitting one stream.
Device Design and Test Challenges
The advanced features and technologies involved in the standard present many challenges in designing, developing, and manufacturing WiMAX products. These are especially prevalent in the RF section of a design.
One of the biggest is the close spacing of the OFDM subcarriers. Wireless systems with closely spaced OFDM subcarriers are more susceptible to carrier frequency error and phase noise produced by the local oscillator of the radio. Phase noise can cause inter-carrier interference (ICI) and degrade system performance. The issue is complicated further since the subcarrier frequency spacing is different in mobile and fixed WiMAX.
In mobile WiMAX system design, it is important to consider the carrier frequency shift or spread due to the Doppler effects caused by the relative movement of the mobile station in the surrounding environment, which has similar impact on system performance as phase noise.
Other WiMAX design challenges include high transmit power requirements in conjunction with tight spectrum mask requirements and a stricter error vector magnitude (EVM) requirement for Tx. Also, WiMAX requires a wide power control range of at least 30 dB.
To achieve a balance between lower equipment cost and high-efficiency digital modulation, a zero IF (ZIF) or low IF (LIF) radio architecture with I-Q modulation often is used. This radio architecture is sensitive to analog implementation such as PCB trace layout and component variation. Imbalances between I and Q signal paths will directly affect modulation accuracy and, consequently, transmit signal quality.
To optimize overall system performance, it is necessary for the baseband IC to compensate for any I-Q imbalances. Sophisticated WiMAX systems design-in IQ imbalance calibration and compensation capability. Other impairments such as carrier frequency error, phase noise, LO leakage, spurious interference, and power-amplifier compression also will affect WiMAX system performance.
EVM provides an indication of the overall transmit signal quality and is a direct measure of the average symbol constellation error. Through analysis of the unique characteristics of the symbol constellation diagram, a designer can uncover relationships between constellation distortion and deterministic impairments such as IQ imbalance. Random phase noise has a unique impact on the symbol constellation. Nonlinearity effects due to the compression of the amplifier also can be seen from the symbol constellation distortion.
Through measurements of transmit power spectrum density, complementary cumulative distribution function (CCDF), EVM, and carrier phase noise, design engineers can identify the source of impairments that degrade WiMAX transmitter performance. For example, with phase imbalance, an 8 x 8 square constellation diagram showing 64-QAM modulation states becomes slightly trapezoidal. In contrast, amplitude imbalance broadens each of the states, but the overall constellation remains square.
Test solutions such as IQmax from LitePoint that combine vector signal analyzer and vector signal generator capabilities are powerful tools to identify and troubleshoot WiMAX designs. A vector signal analyzer (VSA) can perform measurements in the time domain, frequency domain, and modulation domain. VSAs enable designers to determine the impacts of amplitude, phase, and group delay imbalances among I and Q channels, phase noise, spurious signals, transient effects, and signal compression on transmitter performance.
WiMAX signals occupy a wider signal bandwidth than typical cell phone signals and are more sensitive to group delay. The subcarriers farther away from the RF carrier frequency on both the lower and higher side will experience more delay than those close to the center frequency, causing more EVM degradation at these subcarriers (Figure 2).
System engineers also need to take phase noise into consideration. It usually is introduced during frequency conversion when a baseband signal is mixed with a local oscillator (LO) to translate to RF frequency.
LO phase noise consists of contributions from the frequency stability of the reference crystal oscillator, the frequency stability of the free-running voltage-controlled oscillator (VCO) used by the phase-locked loop (PLL), and the loop bandwidth and noise from the PLL used in the frequency synthesizer. The impact of phase noise can be seen as a circular distortion of the signal points around the center of the symbol constellation diagram (Figure 3).
Phase noise affects both modulation accuracy and EVM. Due to the random nature of phase noise, it usually is much more difficult to compensate for.
In portable wireless device design, it is necessary to consider DC power consumption. This often requires RF power amplifiers to operate near their compression point where the amplifier can achieve the highest efficiency. As a result, the peak signal will experience some distortion due to nonlinear properties of the amplifier.
Signal distortion due to amplifier nonlinearity will cause degradation of the EVM, and that distortion point will determine the limit for the maximum transmitted power of a device. A WiMAX signal typically has peak to average ratio of about 10 dB.
A WiMAX device often is rated based on its rms power. It is important to have sufficient power below the compression point to achieve a balance between the transmit signal quality and DC power consumption.
Signal compression can be measured from the CCDF curve. Figure 4a and b show CCDF in conjunction with a constellation diagram, giving the designer a visual relationship between compression and impact on constellation error.
In a typical WiMAX device where there are impairments of the I-Q imbalance and phase noise and amplifier compression are present, the symbol constellation is even more distorted. Yet, an experienced designer can isolate the main source of distortion and find out the necessary trade-off to optimize overall system performance.
Other key parameter measurements required for WiMAX device design include the following:
- Transmitter Power (peak and rms)
- Transmitter Spectral Mask
- Spectral Flatness and Delta
- Symbol Constellation Diagram
- Transmit EVM vs. Carrier
- Transmit EVM vs. Time
- Compression (CCDF)
- Phase Error Over Time
- Phase Noise Spectrum and Integrated Phase Noise
- Transmitter Carrier Frequency Error
- Transmitter Carrier Leakage
- Receiver Sensitivity
- Received Packet and Bit Error Rates
WiMAX is a promising new technology well positioned to be the next-generation wireless broadband access network for both fixed and mobile applications. Advanced WiMAX test systems that combine vector signal analysis and generation capabilities in one instrument offer design engineers a powerful tool to perform detailed analysis and troubleshooting of all critical parameters to meet the challenges of WiMAX device design.
About the Authors
Fan Liang is the senior member of the technical staff and director of application engineering at LitePoint and has more than 17 years experience in wireless networks and system design. Prior to joining LitePoint, he served in various management and engineering roles while working at Intel, Texas Instruments, Agilent Technologies, and Hewlett-Packard. Mr. Liang earned an M.S.E.E. from Xi'an Jiaotong University. e-mail: [email protected]
Onno Harms is product manager for WiMAX test solutions at LitePoint. Mr. Harms has more than 15 years experience in wireless systems, IC, and product development and volume manufacturing. He holds an M.S.E.E. from Twente University in The Netherlands. e-mail: [email protected] LitePoint, 575 Maude Ct., Sunnyvale, CA 94085, 408-456-5000
FOR MORE INFORMATION
on the IEEE 802.16