Telecom
Last Updated: Apr 9th, 2008 - 15:00:00
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Services utilizing next-generation mobile wireless technology (like WiMAX or 802.11n Wi-Fi and, in the future, LTE, UMB, and 802.16m) represent an important next step in the drive to broaden end-user access to high-speed wireless communication, information and advanced data, voice and video services.
Multiple-input multiple-output (MIMO) RF technology is the foundation of this next generation of mobile wireless broadband products. By leveraging multiple transmit and receive antennas to employ techniques such as spatial multiplexing, and adaptive antenna systems (AAS) such as beamforming, MIMO-enabled WiMAX and Wi-Fi products deliver the greater throughput and range that drives cost-effective, high-speed wireless products and services.
For engineers, the pressure is on to design, develop and test products and services that deliver the robust quality of communications (performance and interoperability) necessary to drive sustained consumer demand.
As both the mobile WiMAX and Wi-Fi standards are in the midst of a MIMO technology transition, great incentive exists to find design and test tools that can accelerate the delivery of high performance products. In-lab controlled channel emulation, using a channel emulator, is central to accurately characterize 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.
This article discusses the critical test requirements of MIMO-enabled WiMAX and Wi-Fi devices, and identifies channel emulator features that are required for effective and efficient testing of MIMO-enabled devices.
Using Channel Emulation for Accurate Measurement of Over-the-Air Conditions
The role of the channel emulator in wireless test is to recreate the complex, constantly changing, over-the-air conditions (often called impairments), so that the transmitting and receiving sides of the communication can be subjected to the real life signal quality of many different open air locations. The channel emulator uses statistical models to recreate the effects of radio transmissions in real-world setups, and can realistically reproduce the effects of disturbances, interference, reflections, noise, motion and more.
To achieve this effectively, the channel emulator must have:
- Dynamic emulation to replicate the constantly changing over-the-air channel conditions;
- A bidirectional real-time path to precisely represent the inherent bi-directional nature of the client device and base station path.
Channel emulation technology utilizes sophisticated channel models to recreate conditions that occur in a real-world wireless transmission. These channel models serve as statistical characterizations of specific environments in which the wireless devices may be used. The conditions provided by the channel model are based on random processes that create specific instances of a channel condition caused by signal fading, signal multipath and antenna correlations.
The models are dynamic in the sense that the conditions (phase of reflections and signal strength) are always changing. To accurately represent the dynamic channel conditions, the emulator must also be able to run endless dynamic models in order to change the channel conditions in time. To provide a reasonable statistical representation similar to a real-world channel, a channel emulator must provide long intervals of non-repeating channel conditions. This provides the devices under test with a very large number of unique channel instantiations similar to real-world conditions, resulting in adequate test coverage.
Standards bodies and industry forums define channel models to represent certain classes of channel conditions typical to their usage scenarios. There are a number of channels defined by standards organizations to create a baseline for testing; current baseline testing of WiMAX devices use the ITU M.1225 Pedestrian B and Vehicular A channel models, while IEEE 802.11n channel models A through F provide the baseline for testing Wi-Fi devices.
As an example, Table 1 provides key parameters of the ITU M.1225 Pedestrian B and Vehicular A channel models, including changes by the WiMAX Forum for spatial considerations to make these models relevant for multi-antenna configurations. Table 2 provides key parameters of IEEE 802.11n channel models A through F.
| Parameters | Models | ITU
Pedestrian B | ITU
Vehicular A | ITU
Vehicular A (long tap) | Max Doppler speed (km/h)
| 3 | 60 | 120 | | RMS Delay Spread (ns) | 375 | 595 | 530 | | Maximum Delay (ns) | 3700
| 2510 | 10000 | | Number of Taps | 6 | 6 | 6 | | Total Angular Spread (BS) | 5° | 5° | 5° | | Total Angular Spread (MS) | 68° | 70° | 70° | | Antenna Configurations | BS 4l, MS l/2 (High Correlation)
BS, MS Cross Polarized (Med. Correlation)
BS 4l, MS l/2 Cross Polarized (Low Correlation)
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Table 1: Parameters of ITU M.1225 Pedestrian B and Vehicular A channel models
Parameters
| Models | | A | B | C | D | E | F | | Avg 1st Wall Distance (m) | 5 | 5 | 5 | 10 | 20 | 30 | RMS Delay Spread (ns)
| 0 | 15 | 30 | 50 | 100 | 150 | | Maximum Delay (ns) | 0 | 80 | 200 | 390 | 730 | 1050 | | Number of Taps | 1 | 9 | 14 | 18 | 18 | 18 | | Number of Clusters | N/A | 2 | 2 | 3
| 4 | 6 | | Rx and Tx Antenna Spacing | 1/2λ, 1λ, 4λ |
Table 2: Parameters of IEEE 802.11n channel models
Many test organizations, product development labs and others may have their own models or permutations of these standard 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 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. Sometimes these channels are half duplex, as in the case with WiMAX time division duplexing (TDD), and sometimes they can be full duplex, as in the case of WiMAX frequency division duplexing (FDD). These channels are most often described as downlink and uplink relative to the base station/access point. The bi-directional downlink and uplink channels undergo fading and multipath conditions and, for a system test to closely represent actual channel conditions, the channel emulator must offer bi-directional channels, with fading and multipath in both the downlink and uplink directions.
The real channel from the base station/access point to mobile station is reciprocal, that is, the paths traversed by the signal from an access point transmit antenna to a mobile station receive antenna are identical, but reversed to the signal launched at the mobile station and received at the access point. In some cases, such as beamforming, the radio relies on this reciprocity principal to work properly.
For a channel emulator to accurately reproduce real-world channel conditions, the emulated downlink and uplink channels must also adhere to the reciprocity principal. This further implies an inherent “balance” between these channels. An emulator that does not provide such balance cannot accurately test AAS techniques. Beamforming requires not only reciprocity but, since its operation is based on reporting received signal conditions, the reciprocal channels must be calibrated to have balanced phase, amplitude and delay.
Multiple Antenna Connection Support is a Key Component
Next-generation wireless broadband systems make use of multiple antenna technologies to improve range, performance and network capacity. Examples of these technologies include spatial multiplexing, and AAS schemes like beamforming, 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).
Figure 1 shows SISO, MISO, SIMO and MIMO antenna configurations, and indicates the performance enhancing techniques that each configuration may enable.
Figure 1: SISO, MISO, SIMO and MIMO antenna configurations
Spatial multiplexing provides performance improvements by increasing the capacity of the system, defined effectively as bits per second per hertz (bps/Hz). Beamforming 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 channel impairments.
Proper system testing of any of these techniques can only be accomplished using multiple antenna connections over a real or emulated channel. Furthermore, as the antennas have some correlation on the devices under test, a test system that accurately considers correlation factors like line of site components, angle of arrival and departure, angular speed and cross correlation is essential.
WiMAX systems will utilize AAS techniques and many antennas at the base station to maximize the range of the system and reduce the number of base stations required to cover a given area. A WiMAX MISO system with four antennas, and MIMO-enabled Wi-Fi systems with three or more antennas, are common today. At mobile stations/clients, STC (MISO) helps improve performance and mobility without impacting device battery life.
Effective channel emulation needs to offer scalable system support to be effective, which can mean up to four antennas (4x4) to handle the many modes that are being deployed today, and to be ready for technologies deployed in future WiMAX and MIMO-enabled Wi-Fi devices.
True Device Performance Assessment Drives Exceptional Specifications from the Test Equipment
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 six bits per symbol. High order modulations, such as WiMAX 64QAM, demand wide dynamic range and excellent signal linearity. To operate properly, an OFDM 64QAM signal is capable of a high signal-to-noise ratio (SNR) (> 26 dB), and a peak-to-average power ratio (PAPR) of 13 dB.
Products that employ advanced modulation will have some dynamic rate adaptation that allows the device to change to a less aggressive modulation and coding scheme when the 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 (> 10 dB) 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, 26 dB (to maintain adequate SNR) + 13 dB (PAPR) + 20 dB (rate adaptation and power control) equals a dynamic range for the expected input signal of at least 59 dB.
To provide a true assessment of device performance, test equipment should minimize the distortion of signals that it passes to the device under test. As a signal is “processed” and passed by the channel emulator to the device under test, avoiding the introduction of unwanted signal distortions requires a channel emulator that offers significantly better RF fidelity than the system under test.
IEEE 802.16, the standard for WiMAX, specifies that a WiMAX transmitter should have an output fidelity of not worse than -31 dB, 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 3 dB worse than the DUT (noise is twice as big). However, with an emulator EVM 10 dB better than the DUT, the overall EVM is -30.6 dB, 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 also lowers the signal power and reduces the effective output dynamic range.
Ideally, the noise floor of a channel emulator should be low enough without the attenuator to prevent false triggering. The inherent thermal noise (at room temperature) in a 20 MHz wide channel (20 MHz is the maximum defined WiMAX channel bandwidth) is given by -174 dBm/Hz * 20 MHz = 101 dBm. 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. As such, the channel emulator should have a noise floor of -91 dBm (or better) at 20 MHz.
Advanced Channel Control Simplifies Problem Troubleshooting
As previously discussed, effective WiMAX and Wi-Fi testing requires running channel models for long periods of time, often several hours or even days. Effective 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 to provide this flexibility, as well as for the troubleshooter who may want to review a sample of the model and evaluate results a second time or multiple times to understand the behavior in a test.
Simple control to start, pause, resume and stop the channel is not sufficient; in addition the ability to skip forward or backward in the model, to select a portion of the model and loop through that, and then finally to single step through the model will assist the user in identifying, honing in on, and finding exact conditions of the channel which may be good or bad for their unit under test.
Channel control commands enable the engineer to start the emulator and immediately fast forward to the time period of interest (which may have occurred hours or days earlier in a test run). Once the channel conditions under investigation are being run, the engineer 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 the user to take that scenario and analyze the channel conditions.
Figure 2 shows the results of a throughput vs. time performance test conducted on a SISO device. The test plot identifies significant throughput reductions at multiple points throughout the test. Using advanced channel emulator controls, the user “plays” the specific time periods of interest to help diagnose the cause(s) of the throughput reductions.
Figure 2: Throughput vs. Time plot showing throughput reduction conditions of interest
Automation Enhances Channel Emulation Test Systems
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 the need to adjust parameters like range (of the client device from the base station/access point) and motion, can mean that many hours or days of testing effort is required to achieve full test coverage.
Programmatic control of all the advanced functionality allows complete automation of the test, and eliminates the need for engineers either to be physically present or to frequently log in 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 automate the base station/access point and mobile stations/clients, can enhance testing by batching tests to run without human intervention.
Summary
Table 3 documents the critical engineering and management requirements for effective WiMAX and MIMO-enabled Wi-Fi channel emulation, as well as the corresponding channel emulator or test system feature that addresses each requirement.
Effective Channel Emulation Requirement
| Key Channel Emulator/Test System Feature(s) | | Accurate representation of over-the-air conditions | Dynamic with very long intervals of statistically non-repeating channel conditions
Standard channel models as well as custom, user-defined models
Bi-directional and reciprocal channel emulation | | Test flexibility | Accommodate devices with many antennas
Up to 4x4 MIMO channel emulation | | True assessment of device performance | Wide dynamic range
High EVM
Low noise floor | | Simplify control and troubleshooting | Sophisticated channel model play/pause/resume stop/forward/rewind/loop/step control | | Full automation (No human intervention) | Full remote command and control API
Integration with system level test automation |
Table 3
The performance and interoperability demands of the data, voice and video applications that will run over WiMAX and Wi-Fi networks, as well as the sheer complexity MIMO technology, is driving the need for extensive testing of WiMAX and Wi-Fi devices. Channel emulation enables engineers to test WiMAX and Wi-Fi devices using recreated, real-world channel conditions, and to avoid the time and expense of testing in the “actual” real world.
A test solution needs to provide the flexibility and functionality to recreate test setups and scenarios typical to the real world. This demands broad support for channel models and flexible topologies up to 4x4 MIMO.
The technical specifications of the test tool are a critical consideration in choosing a channel emulator. To accurately recreate real-world over-the-air conditions, a channel emulator must be dynamic and bi-directional, with reciprocal channel calibration. The performance of an emulator must be better than the device under test, and the noise floor must be low.
In selecting a channel emulator, engineers should look for the combination of technical specifications and features/functionality that meet the demanding requirements of their WiMAX and Wi-Fi product or service design and test program. n
Graham Celine is a senior director of marketing with Azimuth Systems, and can be reached at graham_celine@azimuthsystems.com.
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