WH ITE PAPER
Index
• Using Channel Emulation to Accurately Measure Over-the-Air Conditions • Emulation with Multiple Antenna Connection Support is Critical to the Evaluation of MIMO-based WiMAX and Wi-Fi Technologies • WiMAX and Wi-Fi Channel Emulation Highs and Lows: High Dynamic Range, Low Noise Floor • Channel Control Aids Troubleshooting of Bad Behavior • Accurate and Repeatable Emulation with a Controlled RF Environment • Test Complexity Demands Automated Channel Emulation Test Systems • Cost-Effective and Easy-to-Use Channel Emulation Test Systems Require System Integration • Summary
Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation
As WiMAX and Wi-Fi become increasingly popular, the stakes increase for vendors servicing the market and the engineers developing new products. Both standards are in the midst of a MIMO technology transition, providing further incentive to find new design and verification tools that can accelerate development of higher performance products. Multiple-Input Multiple-Output (MIMO) technology is the foundation of the next generation of mobile WiMAX and Wi-Fi products. By leveraging multiple transmit and receive antennas to employ techniques such as spatial multiplexing, adaptive antenna processing, and beam forming, MIMO-enabled products deliver greater wireless throughput and range enabling ubiquitous high-speed voice, video and data services. In-lab controlled channel emulation, using a channel emulator is required to accurately characterize the effect of multi-channel RF interactions on the conformance, performance and interoperability of MIMO and Single-Input Single-Output (SISO) WiMAX and Wi-Fi systems. This white paper explains the critical test requirements for comprehensive design of MIMO-enabled WiMAX and Wi-Fi devices. The understanding of key technical requirements and features that increase test effectiveness will assist engineers and engineering managers in the selection of the channel emulator that best meets their needs.
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Using Channel Emulation to Accurately Measure Over the Air Conditions
To accurately test real-world, over-the-air conditions in a controlled lab environment, a channel emulator that reproduces these conditions is required. A channel emulator therefore must have:
• dynamic emulation to mimic the constantly changing over-the-air channel conditions; • a real-time path to precisely represent the inherent bi-directional nature of the
device and base station path. Over-the-air conditions are constantly changing due to many variables, including device movement, the environment, people, cars etc. Channel emulation technology utilizes sophisticated channel models to recreate conditions that occur in real-world wireless transmission. Standards bodies and industry forums define channel models to represent certain classes of channel conditions, which serve as statistical characterizations of specific environments. The conditions provided by the channel model are based on random processes that create a specific instance of a channel condition due to fading, multipath and correlations. The models are dynamic in the sense that the conditions are constantly changing. To accurately represent all the conditions, the emulator must also be dynamic in order to change in time, and provide long intervals of non-repeating channel conditions, as the real world would. This provides the devices under test with a very large number of unique channel instantiations similar to real-world conditions, resulting in more test coverage. There are a number of channels that are defined by standards organizations to create a baseline. For example, ITU M.1225 Pedestrian B and Vehicular A channel models provide a baseline for testing WiMAX devices today while IEEE 802.11n channel models A through F form the baseline of Wi-Fi testing. Table 1 below provides key parameters of the ITU M.1225 Pedestrian B and Vehicular A channel models with proposed changes by the Wimax forum for spatial correlations. Table 2 provides key parameters of IEEE 802.11n channel models A through F.
Models
Parameters Max Doppler speed (km/h) RMS Delay Spread (ns) Maximum Delay (ns) Number of Taps Total Angular Spread (BS) Total Angular Spread (MS) Antenna Configurations ITU, Pedestrian B 3 375 3700 6 5° 68° ITU, Vehicular A 60 595 2510 6 5° 70° ITU, Vehicular A (long tap) 120 530 10000 6 5° 70°
Table 1 Parameters of
ITU M.1225 Pedestrian B and Vehicular A channel models
BS 4l, MS l/2 (High correlation) BS, MS cross polarized (Med. Correlation) BS 4l, MS l/2 Cross Pol (Low correlation)
Models
Parameters Avg 1st Wall Distance (m) RMS Delay Spread (ns) Maximum Delay (ns) Number of Taps Number of Clusters Rx and Tx Antenna Spacing A 5 0 0 1 N/A B 5 15 80 9 2 C 5 30 200 14 2 1/2,1, 4 λ D 10 50 390 18 3 E 20 100 730 18 4 F 30 150 1050 18 6
Table 2 Parameters of
IEEE 802.11n channel models
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In addition, test organizations, product development labs and others may have their own models that they feel better represent the conditions in which devices are expected to operate. A channel emulator must have the ability to use standard models as well as custom, user-defined models. In the case of standard models, channel emulators should offer them as a built-in feature. To allow for extensible emulation of channels, a channel emulator must offer the ability to program both spatial and temporal parameters as part of defining custom model parameters. In real-world device operation, a bi-directional channel exists between the mobile station/ client and the base station/Access Point. This bi-directional channel is used as part of the normal communication that takes place between these devices. Sometimes these channels are half duplex as in the case of WiMAX Time Division Duplexing (TDD), but sometimes they are full duplex as is the case with WiMAX Frequency Division Duplexing (FDD). These channels are often described as downlink and uplink relative to the base station/Access Point. Both the bi-directional downlink and uplink channels undergo fading and multipath conditions. For a system test that closely represents real-world channel conditions, a channel emulator must provide bi-directional channels with full fading and multipath in both the downlink and uplink paths. The real channel from the base station to mobile station is reciprocal with the real channel from the mobile station to the base station. For a channel emulator to accurately reproduce real-world conditions, the emulated downlink and uplink channels must be reciprocal. Adaptive Antenna System (AAS) technologies such as beam forming rely on this principal to work properly. This further implies an inherent “balance” between these channels. Without the emulator providing such balance, accurately testing these beam forming techniques is not possible. To accurately represent the real-world over-the-air conditions in which WiMAX and Wi-Fi systems will operate, an effective channel emulator must be dynamic and provide very long intervals of non-repeating channel conditions. In addition, the channel emulator must have the ability to use both built-in standard channel models (ITU M.1225 Pedestrian B and Vehicular A for WiMAX or IEEE 802.11n channel models A through F for Wi-Fi) as well as custom, user-defined models. Finally, accurate system testing of current and future MIMO techniques requires bi-directional (with full fading and multipath in both downlink and uplink paths) and reciprocal channel emulation technology.
Why is beam forming a critical feature to WiMAX and Wi-Fi devices?
Beam forming is believed to be a critical enabler of cost effective WiMAX network installations. Without taking advantage of the range extension capabilities of beam forming, typical WIMAX installations will require a large number of WiMAX basestations to provide adequate network coverage for a given area thus increasing the capital and operating costs of a WiMAX network. Similarly, beam forming is an optional feature that also increases the range of Wi-Fi devices may prove to be a critical feature in many enterprise and home infrastructure and client devices.
Multiple Antenna Connection Support is Critical for Most Radio Technologies
Most next-generation wireless data systems make use of multiple antenna technologies to improve range, performance and capacity. There are different techniques used to achieve these enhancements. Some examples include MIMO spatial multiplexing, AAS, Space Time Coding (STC) and Maximal Ratio Combining (MRC). Such techniques are often described by their multiple antenna configurations, such as SISO, MISO (Multiple-Input Single-Output),
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SIMO (Single-Input Multiple-Output), and MIMO. Figure 1 shows SISO, MISO, SIMO and MIMO antenna configurations and indicates the performance enhancing techniques that each configuration may enable. Spatial multiplexing (MIMO) typically provides performance improvements by increasing the capacity of the system, defined effectively as bits per second per hertz (bps/hz). AAS improves the range of the network by steering the signal power to the user. STC, a form of transmitter diversity, and MRC, a form of receiver diversity, are techniques that respectively transmit and receive multiple copies of the same user data in an effort to combat impairments, such as fading. Employing any of these techniques requires multiple antenna connections for proper testing of the system. Further, as the antennas are often correlated on the equipment under test, a system that accurately provides for considerations such as cross correlation, angle of arrival and departure, and angular spread is necessary. For WiMAX systems, AAS techniques that use many antennas at the base station are common to extend the range of the system, reducing the number of base stations required. A WiMAX MISO system with four antennas and MIMO-enabled Wi-Fi systems with a minimum of three antennas is not uncommon. At mobile stations, where battery life is a major concern, techniques like MRC help improve overall performance without costly multiple transmitters (SIMO). Both of these techniques result in the need for channel emulation with a large number of antenna connections. Effective channel emulation requires at least 4x4 capabilities to handle the many modes that are being deployed today, as well as to be ready for the technologies defined by the IEEE 802.16e and draft IEEE 802.11n standards upon which mobile WiMAX technology and MIMO-enabled Wi-Fi are respectively based.
WiMAX and Wi-Fi Channel Emulation Highs and Lows: High Dynamic Range, Low Noise Floor
Data communications technologies, as employed in WiMAX and Wi-Fi, present very demanding requirements on system dynamic range and fidelity. Most modern radio systems employ advanced digital modulation technologies to increase capacity, as defined by bits per symbol. An example is 64QAM (Quadrature Amplitude Modulation) that offers a capacity of six bits per symbol. But such high order modulations also demand high dynamic range and linearity. An OFDM 64QAM signal is capable of a peak to average power ratio (PAPR) of 13 dB and requires a high signal-to-noise ratio (SNR) (> 26 dB). As is the case with products that employ advanced modulation, most will have some rate adaptation that allows the device to change to less aggressive modulation when the conditions do not support a more aggressive modulation. The implementation of rate adaptation combined with transmit power control results in a signal power that can change over a significant range (> 10 dB) during normal radio operation. The summation of all
Figure 1 From the top: SISO,
MISO, SIMO and MIMO antenna configurations
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these factors requires a test device that has a very wide dynamic range of operation. For OFDM 64QAM, 13 dB (PAPR) + 26 dB (to maintain adequate SNR) + 20 dB (rate adaptation and power control) = a dynamic range of at least 59 dB for the expected input signal. As the device signal is then “processed” by the channel emulator, the high dynamic range of the signal requires even greater fidelity on the channel emulator to avoid introducing any unwanted distortions. IEEE 802.16, the technical standards body which defines the technical requirements for WiMAX, specifies that a WiMAX transmitter should have output fidelity, as described by error vector magnitude (EVM) of -31 dB. It can be shown mathematically that if a channel emulator offers the same fidelity as the device under test, the fidelity of the signal out of the channel emulator will be 3 dB less, -28 dB. Ideally, test equipment will introduce as little distortion as possible when passing the signal. Test equipment should minimize the distortion of the signals that it passes. A channel emulator that has an EVM of -41dB, 10dB better than a device at the specification mentioned above, will pass a signal at -30.6dB, very close to the original signal. With the burst nature of WiMAX and Wi-Fi transmissions, devices go from offering full power output to no power output on a transmission by transmission basis. This requires a relatively high dynamic range and low noise floor for proper receiver evaluations. The channel emulator should faithfully reproduce this range. The inherent thermal noise in a 20 MHz wide channel (20 MHz is the maximum defined WiMAX channel bandwidth and the mandatory Wi-Fi channel bandwidth) is given by -174 dBm/Hz * 20 MHz = 101 dBm in 20 MHz (at room temperature). A receiver with a real noise figure of 10 dB would then have a noise threshold or noise floor of -91 dBm at 20 MHz. The test equipment should not be the limitation in receiving low power signals.
Channel Control Aids Troubleshooting of Bad Behavior
As previously discussed, effective WiMAX and Wi-Fi testing requires running channel models for long periods of time, often several hours or even days. As the channel evolves over time, events may occur long into a run of the channel model that cause a significant loss in signal throughput. Investigating the conditions that created this loss in throughput requires advanced control of the emulator. If an engineer’s control of the emulator is limited to the simple act of starting it, the engineer would have to wait possibly several hours or days until the specific channel condition is recreated. A channel emulator that provides the user with channel control commands such as fast forward, rewind, pause and play, enable the engineer to start the emulator and immediately fast forward to the time period of interest (which may have occurred hours or even days earlier in the test run). Similarly, if a test is in progress and an anomaly is observed as having occurred in the past, pause and rewind controls allow the engineer to pause the emulator and rewind to the time period of interest. Once the channel conditions under investigation are being run, the engineer may need to debug the radio. Advanced channel controls such as looping on a point in time, single stepping the channel and
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observing the actual channel parameters during those steps, significantly aid the engineer in debugging the issue and provides data that can be used in system simulations. Figure 2 shows the results of a throughput vs. time performance test conducted on a SISO device. The test plot identifies significant throughput reductions throughout the test. Using the advanced channel emulator controls the user “plays” the specific time period of interest to help understand the cause(s) of the throughput reductions.
Accurate and Repeatable Emulation with a Controlled RF Environment
The value of wireless device design and quality assurance testing increases when test results are both accurate and repeatable. Accurate and repeatable testing of wireless equipment is achieved in a controlled RF environment that is not subjected to external RF interference. To obtain accurate and repeatable results when using a channel emulator that has high dynamic range, the channel emulator must be integrated with a controlled RF environment. Cost effective channel emulation in a controlled RF testing environment can be done with an engineering bench top set-up that uses RF isolated enclosures for the devices under test coupled with a well designed channel emulator. Such set-ups can replace the need for costly screen rooms or standard test house environments which lack repeatability across multiple locations.
Throughput versus Time for a SISO device
Figure 2
Plot shows throughput reduction during changing channel conditions. Control commands can facilitate efficient and effective troubleshooting.
Test Complexity Demands Automated Channel Emulation Test Systems
The need to run time-consuming, statistically significant tests with a channel emulator has been discussed. That, coupled with the need to run several models as well as the need to adjust parameters like range (of the client device from the base station) and motion, results
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in many hours or days of testing effort. Without a completely automated test environment, engineers either need to be physically present or at least log in frequently to the test set-up to make all the necessary changes. By using a channel emulator that is completely coupled to a test automation environment, which not only controls the emulator, but can also command and control the base station and clients, many tests can be batched up and run without human intervention. Test automation systems that automatically store the results in a database, with appropriate timestamps and product information, provide a complete record of the testing performed. They also provide a basis for comparing test results as system hardware/software is changed, or alternate devices are tested. Such automated testing improves time to market, test coverage, repeatability and the collection as well as archival of the results.
Cost-Effective and Easy-to-Use Channel Emulation Test Systems Require System Integration
The cost of ownership for a channel emulator includes the cost of the procurement, installation, usage and maintenance of the entire channel emulation test system. Channel emulation test systems that require complicated off-site integration and calibration of the channel emulator and controlled RF environment can be costly to purchase and maintain, suffer significant time delays in the lead up to operational status as well as periodic down-time for required recalibration. A turnkey solution that integrates the channel emulator, controlled RF environment, test automation software and test results database can significantly reduce installation and ongoing costs. In addition, a single console that controls device configuration, test equipment, test execution and output make operation simple and efficient for the user.
Summary
The complexity of WiMAX and Wi-Fi technologies, particularly those employing MIMO, as well as the performance and interoperability demands of voice and video applications is driving the need for comprehensive testing of WiMAX and Wi-Fi devices. Channel emulation enables vendors and service providers to test devices using re-created, real-world channel conditions, thus minimizing the time and expense of testing in the “actual” real-world. Technical specifications are a critical consideration in choosing a channel emulator. To accurately re-create the over-the-air conditions of the real-world, a channel emulator must be dynamic and bi-directional with a phase balanced reciprocal channel calibration. The noise performance of an emulator must be better than the device under test, and the noise floor must be low. In addition, a 4x4 MIMO capable channel emulator is required. Channel emulators also vary in the efficiency with which they can be used. Features such as sophisticated channel model control as well as seamless integration with a controlled RF test environment, system level test automation and a results management database, all help to optimize the effectiveness of a channel emulator as an engineering tool. In selecting a channel emulator, engineers should look for the combination of technical specifications and end-user features that meet the demanding requirements of their new WiMAX and Wi-Fi product designs.
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The table below summarizes the critical technical and use requirements for channel emulation of WiMAX and Wi-Fi devices along with the corresponding channel emulator or test system feature that addresses each requirement.
Critical Requirements for Effective Channel Emulation
Comprehensive WiMAX and Wi-Fi system testing requires accurate representation of over- the-air conditions
Key Channel Emulator System Feature(s)
Dynamic with very long intervals of statistically non-repeating data Use standard channel models (ITU M.1225 Pedestrian B and Vehicular A for WiMAX, IEEE 802.11n™ channel models A through F for Wi-Fi) as well as custom, user defined models Bi-directional and reciprocal channel emulation
Testing of WiMAX and Wi-Fi devices with many antennas True assessment of multi-antenna WiMAX and Wi-Fi device performance Simplify troubleshooting WiMAX and Wi-Fi device behavior Obtaining accurate and repeatable channel emulation test results Regression and interoperability certification testing completed without human intervention Easy and cost-effective set-up and use
4x4 MIMO channel emulation EVM better than the device under test, high dynamic range and low noise floor Sophisticated channel model play/forward/rewind/pause control Integration with a controlled RF test environment Integration with system level test automation and results database
Seamless integration of calibrated channel emulator and controlled environment managed through a single console
About Azimuth Systems
Azimuth Systems is a leading provider of wireless data communications test solutions. Azimuth’s products are used by the world’s foremost wireless semiconductor, system vendors and service providers to speed time-to-market and improve wireless product quality. The company offers the ACE™ 400NB channel emulator for Wi-Fi product testing and the ACE 400WB channel emulator for WiMAX product testing. Both platforms are purpose-built MIMO channel emulators, designed to meet the critical technical and ease-of-use requirements for engineers choosing a channel emulator.
Corporate Headquarters
Azimuth Systems, Inc. 31 Nagog Park Acton, MA 01720 Phone: 978. 263. 6610 info@azimuthsystems.com
West Coast
Azimuth Systems, Inc. 2890 Zanker Road, Suite 205 San Jose, CA 95134 Phone: 408. 943. 8300 www.azimuthsystems.com
©2007 Azimuth Systems, Inc. All rights reserved. All specifications subject to change without notice. Azimuth, the Azimuth Logo, and ACE are trademarks of Azimuth Systems, Inc. All other trademarks are the property of their respective holders.
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