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8960 in Wireless Device Design News

Issue 5 Series 2

Real world inside

Six Steps to Better Performance of HSPA Mobile Phones

High Speed Packet Access (HSPA) - the combination of High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) - benefits mobile phone users by providing faster uplink and downlink data rates as well as the ability to run applications such as mobile gaming and two-way Voice Over IP (VOIP) with lower latency. The technology also is expected to enable larger coverage areas with reasonable data rates. For operators, HSPA offers a more flexible way to allocate bandwidth and the ability to sell new services.  However, this increased capability comes with a price. HSPA places greater demands on the mobile devices that now have to carry voice along with high speed data while dealing with handovers, stricter power control, and RF impairments.

In the lab, integration and validation engineers are tasked with verifying that HSPA wireless devices work as designed, meet 3GPP standards, and fulfill service provider requirements. This process is iterative and begins when the core components of the device - RF and baseband chipsets, operating system, protocol stack, and applications - are first integrated into what is intended to be a working device. The device then must be tested and validated, with the goal of having a quality product that can be manufactured for as little cost as possible.

A six-step method using the Agilent 8960 wireless communications test set and Lab Application software is an effective way to find and resolve many of the issues that can occur in HSPA mobile devices. This approach exercises the different protocol layers from the bottom up for the purpose of isolating errors and validating designs. The first four steps in this method are aimed at the crucial task of getting data flowing through the layers. The fifth and sixth steps, however, push the mobile device to the limits of its performance by exposing it to scenarios that could be expected in a live network or by stressing it to find the breaking points. The following paragraphs examine each of these steps in detail.

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Step 1 - Validate RF and MAC performance

Testing at the physical layer is the first step in evaluating the performance of a mobile device or application. For example, we can determine the maximum throughput of the physical layer data by setting up a radio-bearer (RB) test mode call in "user-defined" mode, as shown in Figure 1. This setup forces the HSPA device to use complex 16QAM modulation and demodulation schemes. Physical layer data throughput gives an initial indication of how well the mobile device is performing but doesn’t point directly to the root cause of any errors that may occur.

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Other measurements such as reported channel quality indicator (CQI) or block error ratio (BLER) may be useful to further understand the performance of the device. The reported CQI, for example, should map to a well-behaved curve. If it doesn’t, a problem may exist with the CQI algorithm that could eventually interfere with the proper assignment of system resources and negatively affect the entire network. In the case of HSDPA BLER, the test setup described above should report the maximum data rate with relatively few "not acknowledged" (NACK) messages.

The HSPA Lab Application for the 8960 has tools for investigating the HSUPA physical layer. Additionally, nearly every Layer 1 configuration parameter for HSDPA is made available on the 8960 front panel and can be activated or changed directly. In contrast, many other test solutions require test scripts that must be edited and recompiled if the configuration changes. Complementing the Lab Application is the 8960’s Wireless Test Manager software, which is also an excellent tool for physical layer testing. It is especially useful for running multiple iterations of RF parametric tests.

Step 2 - Validate the radio link control, driver, and IP

Building on the results of the physical layer testing in Step 1, the next step is to validate the radio link control (RLC) protocol, driver, and IP. This begins to tell us how the HSPA device will perform on a live network. Application layer data throughput tests should also be carried out at this time to see how well the mobile device performs from an end user’s perspective.

To keep testing simple at this stage, we can initiate a packet-switched call and "flood" the device with User Datagram Protocol (UDP) packets. Recall that UDP transmissions do not depend on acknowledgements by the device. Thus some of the TCP parameter requirements (such as window size) are taken out of the measurement picture for now. A specific data rate can be sent via UDP and the results measured to see how well the device tolerates the flood of data. For example, Figure 2 shows a device receiving a flood of UDP data at 7.2 Mbps while the device simultaneously sends UDP data to the network at 2 Mbps. As the figures shows, the performance of the device is fairly stable in this case.

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Step 3 - Validate the UDP application

Once testing confirms that the mobile device can handle UDP data appropriately, Step 3 adds an application that places a medium load on the device’s processors. In this example, we set up a packet-switched call with a video streaming server. Video streaming is a good application to use for this test because it is an on-demand or time-sensitive application, and it requests just the resources it needs, typically without tying up all the system resources. Also, video streaming applications often use UDP to avoid extra overhead.
 
Everyone has seen examples of good quality and poor quality video performance. As the data throughput screen in Figure 3 shows, when channel quality degrades during a video transmission, more data must be transferred over the air (OTA) to maintain the IP data rate needed for the streaming video application.

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Step 4 - Validate TCP and the system

To fully stress the system from a data throughput perspective, in Step 4 we set up a packet-switched call and use File Transfer Protocol (FTP) to send and receive a data file. The FTP simulates running an application that requires acknowledgment of the packets received - for example, downloading a file from the Internet. On average, because of the acknowledgements and other overhead associated with FTP, the FTP data rates will be somewhat lower than the UDP data rates. This test scenario is shown in Figure 4.

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Step 5 - Perform real-world/user-experience tests

Steps 1 through 4 help get the maximum data flowing through the HSPA mobile device. Now in Step 5 we create an environment that emulates a user’s real network experience. First a "user-defined" packet-data call is established, and then noise and fading are added. The downlink rate can be changed using the active CQI response. The uplink rate can also be changed by altering the serving grant or power level.

To test the mobile device, we have to perform repetitive events - such as receiving 10 text messages at one time - and multi-thread activities - such as streaming video while receiving an SMS, or making a video call while surfing the web - just as a "real user" would. Agilent’s new Interactive Functional Test (IFT) software is designed to simplify exactly these tasks, with the goal of experiencing how the mobile device performs just as a subscriber would in a real, operational network.

Step 6 - Perform stress tests

In Step 6, the same types of activities and tests are performed except this time the goal of testing is to subject the mobile device to the most stressful usage scenarios to find weak points in the design. In other words, Step 6 attempts to "break" the device and then find ways to fix it or improve the margins of error so that the device will not fail in the field at some later time.

Figure 5 shows some of the many features of the IFT software that make it possible to execute user-experience and stress tests in the lab. By incorporating Steps 5 and 6 into the integration and validation process, engineers can release their designs with a higher degree of confidence that the product will pass final conformance testing and operator acceptance tests without requiring extensive and expensive rework.

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Agilent solutions for HSPA

The six steps for testing outlined here are aimed at getting better performance from HSPA mobile designs. Testing requires the right tools, however. As the examples have shown, Agilent’s 8960 Lab Application tools are ideal for HSPA testing. Some of the highlights of the 8960 solution are summarized here:

Wireless Protocol Advisor (WPA) is powerful, PC-based diagnostic software that comes with each 8960 Lab Application. WPA performs real-time protocol logging and post capture analysis, allowing engineers to quickly collect and interpret wireless protocol messaging, verify functionality, and isolate and resolve protocol problems associated with designing new mobile devices or applications. A connection trace feature helps identify TCP/IP data throughput inefficiencies in a device. WPA can capture all data in one log with multilayer decodes that provide information from IP to bits.

Data Throughput Monitor is a quick and effective tool to view a device’s data throughput performance, especially under stressful or changing conditions. It has the ability to test to the most extreme data rates, helping inspire confidence in a device’s overall ability to manage data properly. It also provides coupling data throughput measurements with RF impairments, which further prove a device’s performance. Impairments typically include fading, changing the CQI value, adding AWGN, and changing code channel power levels. The Data Throughput Monitor offers an easy way to demonstrate high-speed data operation of a mobile device or application.

Two different PC-based automation software tools are available for use with the 8960 HSPA Lab Application. The first is Wireless Test Manager (WTM), which is used primarily for testing a device’s RF parametric performance.

The second is Interactive Functional Test (IFT) software, which is especially helpful for meeting test needs beyond those defined in the industry standards. IFT software makes it easy to set up many different real-world and stress-test scenarios. Phone control via standard AT commands is provided as well as the ability to create customer-specific phone control commands.

To learn more about Agilent’s tools used in the Six Steps, visit our pages:

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