Providing deeper insights into dynamic BTS/UE signals with wireless link analysis software

1. Introduction
2. Sketching the Dynamics of Link Operation
3. Understanding Wireless Link Analysis
4. Visualizing an Effective Approach to Analysis
5. Making Dynamic Measurements
6. Conclusion
7. Related Information

For system-integration engineers and verification engineers, the troubleshooting of new BTS and UE designs will only get tougher as wireless standards become more advanced. Within standards such as LTE, the biggest challenges stem from the complex interactions between the physical (PHY) and media access control (MAC) layers during signaling operations.

Engineers who typically work at the PHY layer tend to rely on two tools of choice: RF signal analyzers such as the Agilent PXA and vector signal analysis software such as the Agilent 89600 VSA. As a complement to the 89600 VSA, Agilent has created the 89600 wireless link analysis (WLA) software to help PHY-focused engineers understand the autonomous MAC-layer messaging that occurs between devices. In both system integration and verification the ultimate benefit is a deeper level of insight that accelerates day-to-day troubleshooting and ultimately improves time to market.

In LTE, at least six different control loops are used to manage the radio link between a base station and a mobile device. These loops often interact as they deal with power, timing, modulation, transmission mode and retransmissions in both the downlink and uplink channels. The information needed to manage these control loops is stored in the downlink signal, starting when a connection is established between a BTS (eNB) and a UE, and continuing while data is transferred.

During integration and verification, simultaneous testing of the RF, baseband and control functions can be challenging due to the inherently dynamic nature of advanced wireless signals. If the processes are not fully evaluated and analyzed, the performance in a live system may be affected, resulting in unexpected variations between devices from different vendors. Additional time and money will be expended while rapidly developing workarounds to problems that are discovered in the field.

As implemented in the 89600 WLA, wireless link analysis decodes control messages and correlates them with the PHY-layer signals they manage (Figure 1). This provides greater visibility into link-layer communication and leads to greater insight into unexpected behavior.

The key benefit is the ability to view and interpret RF measurements—power, modulation format, timing, etc.—in a MAC-message context, and to view and interpret MAC messages in an RF context. For example, it’s one thing to know that the hardware is transmitting one watt of power; it’s another thing altogether to match this against MAC-layer commands and then discover that the device was told to transmit at only one-half watt. This type of low-layer control tends to be embedded deeply in the system and it operates with a high degree of autonomy. As a result, one of the only ways to observe and monitor link behavior is through combined PHY/MAC analysis.

The ideal way to assess dynamic problems is to represent information graphically. This approach enables fast identification of anomalies, even if the engineer is not expecting to see a problem—and especially if they are not an expert in signaling protocols. As an example, display markers that are coupled across separate measurement traces allow immediate correlation between the timing of a problem and its location in a captured signal.

Once a problem is identified visually, detailed analysis will help reveal the cause of the suspect behavior. The debugging process is faster and more effective when team members—co-located or geographically dispersed—can share the original I/Q capture and high-level data (in smaller file sizes). Issues can be logged and included in future verification tests.

Testing starts by verifying the operation of control processes: These should be set up and running as expected when the channel is fixed and unimpaired. It’s important to verify this state because the many options in link configuration and their dynamic nature can mean operating states are not reset or modified as expected.

In LTE, the radio system is designed to operate with high levels of packet error, relying on the use of multiple simultaneous low-latency retransmission processes known as hybrid automatic repeat requests or HARQ. If the system is error-free, then the processes should cycle around without interruption.

Built-in tracking algorithms in the 89600 WLA allow immediate assessment of the operation of HARQ retransmission processes. Figure 2 shows data transfer per HARQ process on a time (frame number) axis. The measured signal was assumed to be good; however, the various lines represent individual HARQ processes and the deviations in several of the lines immediately alert the user to an unsuspected problem.

These capabilities are also relevant in 3GPP certification testing, which involves the use of randomly faded channels that approximate the various scenarios end-user customers may encounter in actual operation. Testing at this level conceals much of what is happening in the radio. Simpler tests to assess and correct the response of individual control loops will build confidence that the measured performance accurately represents what a mobile device or base station is capable of when deployed.

Specific to LTE, the 89600 WLA includes several additional capabilities that enhance the analysis of new devices:

• Extends the existing LTE FDD and TDD analysis capabilities provided by the 89600 VSA.
• Performs multi-layer decoding, which reveals information carried in upper-layer messages that determine the detailed format of RF-level signals.
• Produces graphical displays of control loop activity, including user-configurable plotting of changes in the 20 or more downlink control parameters.
• Provides easy and convenient display and location of cyclic redundancy check (CRC) failures.

Advanced wireless systems depend on efficient operation of autonomous control loops that manage BTS/UE interaction. The extraction of useful information from complex link activity is greatly simplified by the combined capabilities of the 89600 VSA and the 89600 WLA software. With its graphical approach to testing and analysis, the 89600 WLA provides views that reveal the details of link operation. The net result is a quick and effective way to enhance device evaluation earlier in the development process, ultimately ensuring the creation of robust, efficient designs that get to market faster.