Link to Article in Nikkei Electronics Asia
Next-generation mobile wireless technology represents an important step in the drive to broaden access to high-speed wireless services and multiple input multiple output (MIMO) technology is the foundation of this next generation of mobile wireless broadband products. By leveraging multiple transmit and receive antennas to employ spatial multiplexing and adaptive antenna systems (AAS) such as beamforming, MIMO-enabled WiMAX and Wi-Fi products deliver greater throughput and range that drives cost-effective, high-speed wireless products and services.
As a result, great incentive exists to find design and test tools that can accelerate the delivery of high performance products. In-lab controlled channel emulation is central to accurately characterizing the effect of multi-channel RF interactions on the conformance, performance and interoperability of WiMAX and Wi-Fi systems for both MIMO and SISO (single input single output) implementations.
Channel Emulation
Using statistical models, channel emulators recreate the complex, constantly changing, over-the-air wireless conditions - often called impairments - so the transmitting and receiving sides of the communication can be subjected to the real life signal quality of different environments. To achieve this effectively, the channel emulator must have: a dynamic emulation to replicate the constantly changing over-the-air channel conditions; and a bi-directional real-time path to precisely represent the inherent bi-directional nature of the client device and base station path.
Based on random processes that create specific instances of a channel condition caused by signal fading, signal multipath and antenna correlations, channel models serve as statistical characterizations of environments where wireless devices may be used.
To accurately represent dynamic channel conditions, a channel emulator must be able to run endless dynamic models and provide long intervals of non-repeating channel conditions. This provides devices under test (DUT) with a large number of unique channel instantiations similar to real-world conditions, resulting in adequate test coverage.
To represent certain classes of channel conditions typical to usage scenarios, standard bodies and industry forums have defined several channels for baseline testing. Current WiMAX baseline testing uses the ITU M.1225 Pedestrian B and Vehicular A channel models, while IEEE 802.11n channel models A through F provide the baseline for Wi-Fi.
Since many test organizations and labs have their own permutations of these standard models, a channel emulator must have the ability to use standard models and custom, user-defined models.
Multiple Antenna Connection Support
Next-generation broadband wireless systems make use of multiple antenna technologies to improve range, performance and network capacity. Examples of these technologies include spatial multiplexing, AAS schemes, space time coding (STC) and maximal ratio combining (MRC), all loosely described as MIMO or by their multiple antenna configurations such as SISO, multiple input single output (MISO) and single input multiple output (SIMO).
As a result, multiple antenna connections over a real or emulated channel is the only way to properly test. Furthermore, since the antennas have some correlation on the DUT, a test system must accurately consider correlation factors like line of site components, angle of arrival and departure, angular speed and cross correlation. Therefore, effective channel emulation also needs to offer scalable system support of up to four antennas (4 x 4) to handle the many modes being deployed today and in the future.
Recreating Real World Conditions
In the real world, a bi-directional path exists between the mobile station and the base station. This bi-directional channel enables the normal communication that takes place between these device pairs. While these channels can be half duplex, as in WiMAX time division duplexing (TDD), they can also be full duplex, as in WiMAX frequency division duplexing (FDD). Often described as downlink and uplink relative to the base station/access point, these bi-directional channels undergo reciprocal fading and multipath conditions.
Additionally, the real channel from the base station/access point to mobile station is balanced and in cases such as beamforming, the radio relies on this reciprocity principal to work properly - to be identical or calibrated. As a result, channel emulators must not only offer bi-directional channels with fading and multipath in both the downlink and uplink directions, but the emulated channels must be reciprocal and the reciprocal channels must be calibrated to have balanced phase and amplitude in order to accurately test AAS techniques.
True Device Performance Assessment
WiMAX and MIMO-enabled Wi-Fi communications technologies present very demanding requirements on RF dynamic range and fidelity. Modern radio systems employ advanced digital modulation technologies to increase capacity or the bits per symbol. For example, WiMAX 64QAM (quadrature amplitude modulation) offers a capacity of 6 bits per symbol. High order modulations demand wide dynamic range and excellent signal linearity. To operate properly, an OFDM 64QAM signal is capable of a high signal-to-noise ratio (S/N) (>26dB) and a peak-to-average power ratio (PAPR) of 13dB.
Advanced modulation products will have some dynamic rate adaptations allowing the device to change to a less aggressive modulation and coding scheme when conditions do not support a more aggressive scheme. The implementation of rate adaptation, combined with transmit power control, results in a signal power that can change over a significant range (>10dB) during normal radio operation.
The summation of all these factors requires a test device that has a very wide dynamic range of operation. For OFDM 64QAM, 26dB (to maintain adequate S/N) +13dB (PAPR) +20dB (rate adaptation and power control) equals a dynamic range for the expected input signal of at least 59dB.
To provide a true assessment of device performance, test equipment should also minimize the distortion of signals it passes to the DUT. To avoid the introduction of unwanted signal distortions a channel emulator must offer significantly better RF fidelity than the system under test. The WiMAX standard IEEE 802.16 specifies that a WiMAX transmitter should have output fidelity of no worse than -31dB, as described by error vector magnitude (EVM). The emulator must pass the waveform with little significant degradation. EVM is often treated as a noise power quantity. If the emulator EVM is equal to that of the DUT, the overall EVM at the emulator output will be 3dB worse than the DUT (noise is twice as big). However, with an emulator EVM 10dB better than the DUT, the overall EVM is -30.6dB, much closer to the originally transmitted signal.
Another consideration for emulator performance is the output noise floor. If this floor is too high, it can cause DUT receiver circuitry to falsely detect the presence of a signal. A high noise floor can be reduced using an attenuator, but this lowers the signal power and reduces the effective output dynamic range. Ideally, a channel emulator's noise floor should be low enough without the attenuator to prevent false triggering.
Advanced Channel Control
Effective WiMAX and Wi-Fi testing will incorporate running long samples or selecting random points in time to begin the channel model sampling and so full control of the channel model operation will be highly beneficial to the user and troubleshooter.
Simple control to start, pause, resume and stop the channel is not sufficient. In addition, the ability to skip forward or backward, select a portion of the model and loop through that, as well as single step through the model allows the user to identify, hone in on and find exact conditions of the channel.
Channel control commands enable engineers to start the emulator and immediately fast forward to the time period of interest. Once the channel conditions under investigation are being run, engineers may need to debug the radio by looping on a point in time, single stepping the channel and observing the actual channel parameters during those steps. At such a point the channel information should be readily available to allow users to analyze the scenario and channel conditions.
Enhanced Channel Emulation
The need to run long statistically significant tests with a channel emulator has been outlined above. That coupled with the need to run several models as well as adjust parameters like range and motion results in many hours or days of testing. Programmatic control of all the advanced functionality allows complete automation of the test and eliminates the need for engineers to be physically present or log in frequently to the test set-up to make all the necessary changes. A channel emulator that is completely coupled to a test automation environment, which not only controls the emulator, but can also offer automation of the base station/access point and mobile stations/clients can enhance testing by batching the test to run without human intervention.
The Table shows critical engineering and management requirements for effective WiMAX and MIMO-enabled Wi-Fi channel emulation and corresponding channel emulator or test system features that address each requirement.
Channel emulation enables engineers to test WiMAX and Wi-Fi devices using recreated real-world channel conditions and avoid the time and expense of testing in the "actual" real world.
by Graham Celine,
Senior Director, Marketing,
Azimuth Systems Inc
Nikkei Electronics Asia -- March 2008