The ultimate test for a developer or quality test engineer is one that perfectly captures the user experience in a controlled, repeatable, and cost-effective way, where devices and systems are guaranteed to work as designed when delivered to the consumer. In wireless product testing, this is a huge challenge. As a user device moves throughout a network, over-the-air, real-world conditions change frequently. In addition, the device may be affected by many factors that seem impossible to replicate.

“Recreating air” or modeling the effects of typical over-the-air transmission phenomena has long been performed using channel emulation and modeling tools. Now, there’s another option that combines channel modeling with data collected in the field in real-world deployments. It has proven effective in both pre-deployment device and infrastructure performance testing as well as in post-deployment troubleshooting and field problem resoluiton.

Armed with drive test data collected in the field, this field-to-lab test methodology can more accurately recreate additional elements that are important in lab-based network testing and that have never before factored into the testing. These variables include rapid variation of signal strength; the fact that multiple sectors are visible to the user in most real deployments; and the fluctuating speed of the device or the rate of change of air-effect phenomena. Until now, lab testing typically ignored these parameters.

Combining this important information from drive test logs with an effective channel emulation environment creates a test solution that’s closer than ever to the “real world” for existing 2G, 3G, and emerging 4G radio technologies.

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Emulating Channel Conditions And Propagation Effects

Channel conditions and propagation effects influence the performance of a signal as it moves from transmitter to receiver. Some effects, such as fading, multipath, and correlation, are easily recreated in the lab using a channel emulator and statistical modeling tools:

• Fading: As wireless moves from transmitter to receiver , the over-the-air transmission effects and obstructions in its path will cause the signal strength to vary rapidly. Fading is frequency-selective. Therefore, while the signals may momentarily fade high at one point in the band, a nulling effect (down fade) may simultaneously occur in another part of the signal bandwidth. Since the signal in broadband wireless consists of multiple subcarriers over the band, the frequency-selective fading may have a negative effect one carrier and no effect on another. Known as “fast fading,” it’s typically influenced by the type of environment and speed of the mobile device .

• Multipath: The signal may pass directly, via line-of-sight, from the transmitting antenna(s) to the receiving antenna(s) . However, it will also reflect off buildings, vehicles, terrain, and other objects, resulting in multiple copies of the signal with different signal strengths and delays. Additionally, it will cause some phase difference because of the longer path that is travelled to arrive at the destination antenna at the same time. Multipath is a result of the physical environment; urban and rural environments may both have multipath, but of very different delays and strengths.

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• Spatial correlation or antenna correlation: Since the antennas are relatively close to one another in a user device and relatively far apart in the basestation, the proximity and orientation of antennas will define the similarity of the signals when received by one or more antennas. Or, in other words, they define the correlation of the paths. The more similar—or diverse—the signals, the greater the effect on the ability to decode, or sum, or differentiate between the signals at each receive antenna.

Field Conditions To Take T o The Lab

As stated earlier, the channel effects are critical when testing the system, but other varying effects will significantly impact the performance of 2G/3G/4G systems that aren’t recreated in standard channel modeling. Furthermore, if added to the test, they will significantly enhance the test conditions. These include:

• Variation of signal strength: The signal strength of the transmission from basestation to device will define many different things for the device. For instance, it can define the quality of the signal received and the ability to decode the signal. It also defines factors such as CQI (call quality indicator), which is reported back to the transmitter, and other parameters that will dynamically determine the devices’ transmission speed (encoding mechanism). Standard tests define the signal strength to be used and, in rare cases, may define the variation of the signal over time.

However, in the real world, the signal strength change is highly dynamic, affected by multiple factors in the environment. Therefore, when a device in the field is subjected to rapidly changing conditions, its behavior may be very different. The drive test logs capture the signal strength data seen at the device, and the field-to- lab solution utilizes this very same data to vary signal strength in real time in the laboratory network testbed—exactly as it’s seen in the field.

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• Varying speed of the device: Most tests are performed in static or simulated motion conditions (Doppler effects) and channel modeling varies based on the test’s defined “velocity.” Despite their benefits, once again they do not map to real-world driving conditions (i.e., stops, starts, acceleration, and deceleration). Using the GPS coordinates collected as the drive test log is recorded, the field-to- l ab solution can dynamically vary the propagation conditions in parallel with the other effects (like varying signal strength above) as well as evaluate the effects of such conditions on the device when testing in the lab.

• Multiple basestations: Most lab tests will involve one, two, and in rare cases, multiple basestations. In reality, most user devices will “see” between three and tens of basestations or sectors at any time. The conditions and proximity of the device to the different sectors will change over time as well. Using the Azimuth field-to- lab solution , users can map up to 16 sectors at a time, varying the relationship to each sector based on the data collected in the drive test. Subsequently, the test can evaluate how the device behaves when it’s subjected to multiple base station signals and the network needs to quickly decide on roaming and transmission conditions.

Combining the effects of straightforward fading, multipath, and antenna-correlation propagation conditions with multiple basestations, varying conditions per basestation, and real-time velocity changes—just as the device sees in the field—significantly increases the complexity of the test environment. And, more importantly, closely matches real-world field conditions.

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Additional Considerations For MIMO Systems

No doubt, the fundamental advantage wrought by the emerging 4G wireless world is enhanced throughput. Two key technology advances make this happen: multiple antenna transmission or multiple input/multiple output (MIMO) and enhanced data encoding or orthogonal frequency division multiplexing (OFDM).

MIMO uses channel conditions to deliver more, or improved, data transmission with techniques such as:

Spatial multiplexing: It transmits different data from each antenna, but on the same frequency and time. Since the conditions for transmission on each antenna path are different, spatial multiplexing allows the receiving antennas to split them out to discrete data streams at the receiving end. This technique effectively doubles the throughput on the same RF space.

Diversity: Used in 2G/3G as well as 4G systems, diversity is effective in combating over-the-air propagation effects like fading. Diversity comes in two flavors. TX Diversity transmits the same data on multiple antennas at the same time, increasing the reliability of received data or error detection and data correction. RX Diversity captures the data transmitted from a single source on two receive chains. Each chain has an antenna that’s positioned differently; therefore, as conditions change and the two receive signals are added together, it increases the chance of the correct error-free data being received.

Beamforming: This more advanced technique steers the antenna array to literally direct the transmission strength and reliability. Beamforming can be used to enhance range, reduce interference, and improve transmission reliability.

OFDM encoding is more resilient and delivers more reliable data transmission than legacy encoding schemes. One of the many advantages of OFDM, for example, is that it divides the channel into narrowband sub-channels, making it less susceptible to the frequency selective fading that affects broadband transmission. So, while it significantly improves the reliability of transmissions in real-world propagation conditions (and hence the throughput), OFDM can only be realistically evaluated for throughput if it’s tested in a fading channel.

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The net result is that 4G delivers much higher throughput to the mobile users as well as more reliable transmissions than previous cellular technologies.

In short, to achieve the desired improvements, MIMO and OFDM take advantage of, and depend on, the propagation conditions of the real-world scenario in which they operate. As a result, recreation of such channel conditions in the lab becomes critical. The beauty of field-to- lab solutions is their recreation of channel conditions so they’re as close to the real-world conditions as possible.

What Field To What Lab?

Operators perform thousands of drive tests with different objectives, most for the purpose of collecting field data for coverage planning and validation. Often they will pick representative drive routes to use for testing before mobile devices are approved for distribution to subscribers. Also, when subscribers report problems, such as call drops, operators will troubleshoot by actively heading into the field and driving certain segments of the network.

Driving these segments is time-consuming and expensive. Moreover, it must be performed at specific times of the day and requires the same level of cost of mobilization every time it’s repeated. Because drive test logs are typically archived, the vendor or service provider can repurpose these logs in the field-to-lab tool.

Azimuth’s field-to- lab solution uses data from most commercial drive test tools. However, over time, the data collected by the drive test tools will be enhanced, especially in regards to MIMO and emerging technologies like Long-Term Evolution (LTE) and WiMAX. Once the data is collected, it can be conditioned, saved, and reused as many times as required with many different devices. This capability is where the value really emerges.

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Field-to-lab playback in the lab offers a number of practical uses:

Repeatable test conditions: Field-to-lab provides a repeatable test environment for testing devices. While field testing may follow the same route each time, traffic and other conditions may significantly change from drive to drive. With Azimuth’s field-to- lab solution , one set of real-world conditions can be replayed anytime, anyplace, and multiple times. For example, traffic may be slower at one time of day, resulting in slower velocity and a higher number of users on the network.

Device qualification: Major providers may qualify tens of new handsets each month—each one demanding a rigorous set of field trials. Using a large representative set of drive logs in a field-to-lab solution makes it possible to perform fully automated, unmanned test runs during nights and over weekends, when test engineers aren’t around. It provides easily comparable test results to evaluate against reference handsets, as well as all other devices or even infrastructure from the same and other vendors.

Network troubleshooting: Dissatisfied customers are quick to report their dissatisfaction with network performance and providers often can quickly home in on “problem areas.” The solution, however, may not be so simple. Network issues like reduced performance and dropped sessions could be caused by coverage limitations or, perhaps, by network load, configuration issues or bad performance of specific devices under certain conditions. Using field-to- lab testing, engineers can go directly to the scene of the problem, even if it’s in another country. They can replay the scenario and evaluate the issue with their sophisticated analysis tools—without stepping out of the lab. The same data can later be used to evaluate new devices under the same problematic conditions.

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These examples represent just a sampling of the many test cases experienced by engineers and network analysts, as well as how field-to- lab capability helps speed the recovery process by replaying real-world conditions in the lab.

Drive Test Logs To Enhance Wireless Products

The most indispensable part of the field-to-lab test methodology involves analyzing the drive test logs and conditioning the data for playback. A good tool allows the engineer to easily visualize the data collected in the field, which is in the log file. The log contains lots of valuable information, from the duration of the drive test and where it took place, to how many basestations or sectors are visible at any time. This data will help the user select what parameters were tested and how the data will be used when played back in the lab through a process called “mapping.” In addition, the log includes the actual data collected on signal strength, velocity, noise levels, and propagation conditions, based on the selected mapping parameters (Fig. 1).

The mapping process needs to pluck the relevant data for the selected sectors from the log file and place it in a playback file for reuse in the channel emulator. Since the drive test tools were designed for different purposes, the detail and accuracy of the data may not be perfectly suited to real-time playback. Thus, the mapping software may also smooth data transitions or fill in blanks if data is missing. All of these conditioning steps are user-configurable.

Playback In The Channel Emulator

Once all of the data is collected, conditioned, mapped, and analyzed, the result characterizes the channel conditions between the device under test (DUT) or handset and the multiple basestations or sectors in the network. This is based on real network conditions seen at a specific place in a real-world deployment. The data can be streamed in real time onto the channel, which is typically a channel emulator situated between the DUT and a representative number of basestations in the lab.

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Figure 2 shows the mapping of field-collected data into channel emulators that are part of a network test bed. In this scenario, the network test bed consists of multiple 2x2 MIMO basestations transmitting/receiving data from a single mobile device . The network test bed and the field-to-lab solution, which are configured to support the most typical conditions, support single input/single output (SISO) and/or MIMO, as well as 2G, 3G, and/or 4G radio technologies.

Proving The Validity And Value Of The Solution

In real-world drive tests, the metrics of a specific device can be recorded and measured. The field-to-lab solution’s proof of value comes in the form of measured field results that can be repeated consistently in the lab (Fig. 3). Once the accuracy and reliability of the system are proven, the benefits of the field-to-lab solution become evident.

Summary

Field-to-lab testing offers the ability to characterize expected performance, reproduce and verify field issues, benchmark interoperability with other devices, and optimize performance algorithms. As a result, research and development engineers, as well as quality assurance engineers, can more comprehensively test wireless devices before they are ever deployed in the field as well as improve post-deployment trouble shooting and resolution.