So many measurements, so little time. Make sure your receiver meets the Lee sampling criterion.
Early wireless equipment was used as a safety and navigation aid on large passenger ships but did not gain wide adoption by cargo vessels and smaller craft until after the RMS Titanic sank in 1912. Although there were predictions of radio telephones as early as the 1890s and Marconi himself published a ship's newspaper in 1899 based on content from transmitting stations on shore, development of the new technology took many years.2 The wireless receiver, as it initially was called, only became a common household appliance in the 1930s.
Today, the term wireless has again gained wide acceptance but with a less clear understanding of what is meant. In its broadest sense, the word means without wires. Fortunately, connotations have developed that distinguish different subsets of features within the wireless grouping.
For example, cordless has become a euphemism for battery-powered, satellite television requires a dish antenna and is strictly not portable, and infrared controllers change channels but do not help people communicate. Of all the meanings that wireless communications could have, generally we use the term to denote cell phones, WLANs, and sometimes two-way radios.
Because of the mobility inherent in the use of these things, it's important that related test equipment be portable. In addition, because portability often is an essential requirement, an instrument's capabilities must support accurate and thorough testing. It simply is not possible in many field applications to use a conventional full-featured benchtop lab instrument, so the portable test tool, in spite of its small size and low weight, must provide high-quality measurements.
What's Out There?
As you may expect, instruments delivering the most focused and detailed answers aren t very good general-purpose tools. On the other hand, they completely address their intended applications.
This classification is meant to cover instruments based on receivers. Ideally, the device should be invisible to the wireless network and cause no disruption to its normal operation. An example is the Berkeley Varitronics Systems• Yellowjacket• 802.11 a, b, or b/g Wi-Fi Receiver System that interfaces to the HP iPAQ PocketPC. Berkeley's Hive• Software runs on the iPAQ and allows you to conduct indoor site surveys.
Rather than requiring location information from a GPS, the program references an imported floor-plan bitmap. You can customize it to be as realistic as necessary with drag-and-drop doors, windows, and other icons representing possible obstructions to communications.
Coverage can be organized by RSSI, MAC address, and SSID. Data is output to an Excel spreadsheet for further analysis. In addition, the measured survey data can be viewed in multiple windows and plotted.
Willtek markets the Model 8300 Griffin Fast Measurement Receiver, also intended for site QoS surveying but capable of protocol analysis. An important and distinguishing parameter is measurement speed. Because this type of receiver may be used to investigate link characteristics while the mobile terminal is moving, the measurements must be taken quickly.Reception is affected by Rayleigh fading as well as terrain-based fading. By applying the well-accepted 40-λ metric proposed by Lee and Yeh in their 1974 IEEE paper On the Estimation of the Second-Order Statistics of Log Normal Fading in Mobile Radio Equipment, you can eliminate the Raleigh effects and deal only with the signal-strength variations caused by terrain changes. The sampling requirement proposed in the paper is commonly referred to as the Lee criterion.
The authors found that averaging measurements for the time taken to travel at least 40 wavelengths rejected Raleigh effects. Samples must be taken at least one per wavelength. For example, the sampling rate of a measurement receiver in a car traveling at 60 mph should be at least 88 S/s for an
800-MHz signal with a 1-ft wavelength.
Because Raleigh fading is spatial rather than temporal and receivers often sample at a rate faster than the minimum, more averaging usually is done beyond the Lee criterion. Typically, a grid size of a few hundred feet on a side would be appropriate in the New York City area.
According to Willtek's product manager, Kameel Mohamed, the measurement speed of the Model 8300 allows full-scale Lee-criteria drive testing on seven channels at 60 mph. The receiver supports surveying on up to 1,000 channels/s and 100,000 readings/s. Optional analysis software and compatible up- and down-converters help to extend the applications that can be addressed.
Fluke Networks• OptiView Series II is another tool that accomplishes site mapping. Michael Hadley, the company's wireless product marketing manager, explained, The system automatically scans the 2.5-GHz and
5-GHz wireless environment via a trimode 802.11 a/b/g NIC. A snapshot of the wireless networking environment is graphically presented together with lists of related access points and clients within range of the instrument.
A base-line reading can be established for site APs, including key metrics of signal strength, retries, errors, and utilization rate. This capability allows technicians to compare current wireless measurements with historical base-line data,• he continued. Changes in RF characteristics within the networking environment can be identified quickly.The Tektronix NetTek• Analyzer fitted with the YBT250 Base Station Transmitter and Interference Analyzer determines code-domain power, error vector magnitude ρ, and measurements of power vs. time. The exact mix of capabilities depends on the options chosen for the YBT250 module. Available formats include GSM, GSM and GSM/GPRS and EDGE, W-CDMA/UMTS, cdmaOne, CDMA2000, and analog.
The instrument's spectrogram display presents frequency, power level, and time information and allows you to quickly identify interfering signals. Having a modular construction and several software options allows you to purchase only the required functionality.
Active Test Sets
Devices in this category provide a stimulus as well as a receive function. The OptiView Series II qualifies because it can issue a PING command to other devices on the network. It also performs a throughput test using FTP transfers.
If an instrument has an independent signal source, it can perform VoIP quality testing. The Wireless VQT System from GL Communications is a good example. It uses notebook computers connected to wireless receivers at both the transmitting and receiving ends of a link and can determine the degree of degradation suffered by an audio message passing through the complete end-to-end network.
The GL VQT uses three widely accepted and ITU-standard algorithms to evaluate voice quality,• said Matt Yost, project leader for the company's wireless group. They are the PESQ LQ/LQO per Recommendation P.862/P.862.1, the PAMS per Rec. P.800, and the PSQM per Rec. P.861.
PESQ provides an objective measurement of subjective listening tests on telephony systems. PAMS predicts overall subjective listening quality without requiring actual human listening tests,• he explained. PSQM relates to the subjective quality of speech coders/decoders (codecs) without requiring human listening tests. All three algorithms are simultaneously performed by comparing the reference voice file and the degraded file. Results are presented in both graphical and tabular formats.
Comarco's Seven.Five Network Analyzer and Performance Tester measures end-to-end parameters such as access, setup time, throughput, delay, and connection failures. It accomplishes this via a set of built-in automated data test calls that can be applied to multiple phones using multiple technologies.
The instrument controls phones to make calls and logs the resulting data from the phone,• said Francis Sideco, product manager with the company. Everything the phone sees, including the layer 2 and 3 information, is time stamped with GPS information so that when hand-offs fail or calls cannot be made or are dropped, we can go back in time and investigate all aspects of the call to see exactly what happened, millisecond by millisecond. This phone data is coupled with RF scanning data that identifies nearby cell sites to hand off to so the reasons for any failures can be evaluated.
A third very broad product in this group is the Agilent E6474A Wireless Network Optimization Platform. As the name suggests, this instrument has basic functionality that can be augmented in a number of ways. Fundamentally, it is a Windows-based wireless network test solution that can be enhanced to enable site evaluation, base-station turn-on, system acceptance, and ongoing optimization.
The simplest configuration links a phone to a notebook PC and supports layer 1, 2, and 3 measurements including RLP, RLC/MAC, and LLC parameters. This type of testing allows you to view your network as subscribers perceive it.
The E6474A can link to a receiver instead of a phone, which gives you a technology-independent view of relevant RF activity in the area. This receiver-only configuration supports identification of interfering signals, investigation of CDMA pilot pollution, and uplink/downlink band-spectrum analysis. Both a receiver and a phone can be used simultaneously to provide correlation between a physical-layer artifact and a dropped call, for example.
The list of options includes many types of software applications related to drive testing, data network measurement, management of dial-up or LAN connections, and report generation together with appropriate graphical data presentations.
Veriwave has developed the WaveTest product targeted at WLAN troubleshooting and test. For example, an AP accommodates clients associating or disassociating from the network by adding or deleting them from its database. Following a brief power outage, the speed with which this can be accomplished for the many clients simultaneously vying for attention is an important characteristic of the AP. This is one of the parameters that the WaveTest can measure.
A series of preprogrammed tests is applied to determine performance metrics such as association database capacity, association rate, roaming delay, and power save capacity. Several WaveTest modules can be used in a single test setup to simulate interferers or other clients and to replicate the test setup in the lab if required. The WaveTest Traffic Generator/Performance Analyzer links to a portable PC.
Spectrum analyzers are in this category because they can function as a receiver with the appropriate antenna. The instruments also can be connected via coaxial cable to RF signals, such as via a directional coupler ahead of a transmitting antenna, in which case they are more intrusive.
The Anritsu MS2721A Spectrum Master• is an example of the functionality available in a portable spectrum analyzer. According to Aart Konynenberg, the company's MMD marketing manager, the MS2721A features a low -153-dBm DANL, which means that low-level signals don't get lost in the noise floor. In addition, the instrument's wide dynamic range allows small signals to be analyzed even in the presence of much larger nearby signals.
Specifications include a 100-kHz to 7.1-GHz bandwidth, +43-dBm maximum safe input power; -100 dBc/Hz phase noise at 10-kHz offset at 7.1 GHz; and independent control of RBW; VBW; input attenuation; and preamplifier on/off. Ease-of-use has been addressed through built-in demodulation, limit lines, multiple measurement markers as well as noise and frequency counter markers, and 64 MB of flash memory.
Standard measurement of occupied bandwidth, channel power, C/I, ACPR, and field strength makes a wireless network service technician's life less complicated. The dedicated software routines that calculate these values are presented as one-button measurements.
The Analyze-R• Model 2261 from XL Microwave is a wideband spectrum analyzer for the unlicensed 2-GHz and 5-GHz ISM and U-NII bands. It features the company's unattended datalogger function, Record-R , which records and logs a record of what the instrument sees including GPS derived position, date, and time information. Typically, this instrument would be used to differentiate between path-induced and equipment problems, confirm optimum antenna locations, and measure and document RF signatures to capture signal distortion or interfering sources.
Many power meters are intrusive because they terminate the line in which power is being measured. Readings from this type of power meter cannot account for interactions with the actual load because it must be disconnected to attach the meter.
So-called in-line power meters use directional couplers to tap off a small portion of the through-line power for the measuring element. Measurement of both the forward and the reflected power, for example, supports antenna match determination. The amount of RF power lost is very small, making this meter far less intrusive than the terminating type.
Bird Electronic has long produced in-line meters. Modern diode square-law elements now accurately cope with digital as well as traditional analog modulation. The recently introduced Model 5012 Directional Power Sensor can be used with the company's DPM 5000 Digital Power Meter or connected to a PC running an associated power measurement software application. Insertion loss is <0.05 dB to 1 GHz and <0.1 dB to 4 GHz with insertion VSWR of 1.05 to 2.5 GHz and 1.1 to 4 GHz.
Active Test Sets
Anritsu's MT8212B Cell Master is a single-box base-station test tool used for deploying, maintaining, and troubleshooting wireless base stations. It combines the functionality of a cable and an antenna analyzer, a spectrum analyzer, a power meter, a transmitter analyzer, an interference analyzer, and a channel scanner.
A large number of options allow the instrument to serve many specialized applications. In addition to measurement and analysis of on-air as well as wired signals, a built-in source can be provided to support gain and loss measurement of two-port devices such as filters, duplexers, and tower-mounted amplifiers.
As has been shown via many examples, a large number of portable wireless test products with varying capabilities are available. Your test needs may be special and only served by a general-purpose instrument that supports custom test development. On the other hand, a wide range of meaningful tests can be accomplished with application-specific products featuring built-in and easily applied test routines.
For a good discussion of the many factors that set WLAN test apart from wired LAN test, check out Veriwave's white paper The Testing Needs of WLAN Technology. Yes, you can accomplish the same data transfer operations over wired and wireless networks, but the relative unreliability of the WLAN RF link makes a big difference.
Wireless protocols are more sophisticated, and the range of products attached to networks and the rate of protocol evolution are much greater than for wired devices. These factors add to the test complications caused by network characteristics such as dynamic configuration, mobility, spatial considerations, and lack of isolation between networks.
Determining the root cause of a problem requires separation of RF link errors from data errors. Several test-instrument attributes have been shown to be beneficial:
• Repeatability Signal strength as well as precise timing must be controlled to ensure repeatable test results.
• Appropriate for Lab and Customer Site Some problems appear only at one test location.
• Spatial Awareness Drive testing or the simulation of a moving source relates to roaming and the hand-off activity taking place among adjacent APs.
• Flexibility to Handle Complex Protocols Visibility of performance at multiple protocol levels may be required to analyze cause-and-effect interactions such as found in VoIP testing.
FOR MORE INFORMATION
on the Berkeley Varitronics Systems Yellowjacket Receiver System
on the Willtek 8300 Griffin Fast Measurement Receiver
on the Fluke Networks OptiView Series II
on the Tektronix NetTek Analyzer
on the GL Communications Wireless VQT System
on Comarco's Seven.Five Network Analyzer and Performance Tester
on the Agilent E6474A Wireless Network Optimization Platform
on Veriwave's WaveTest product
on Anritsu's MS2721A Spectrum Master
on the Bird Electronic 5012 Directional Power Sensor
on Anritsu's Model MT8212B Cell Master
on the XL Microwave 2261 Spectrum Analyzer