Beyond conformance testing

20 Mar 2007

Release 99 is largely built on the traditional CDMA principle of separating users in the code domain. Each user is assigned a dedicated physical channel (DPCH). The network operator manages the users in a cell by using a fast power-control technology to ensures that each user gets a fair share of resources and allows for maximum cell capacity.

While this configuration works well for moderate data rates, it doesn't do a good job of supporting high-data-rate users. The R99 network must allocate a large portion of the code space to the high-data-rate users, which diminishes the available cell capacity for other users.

Data traffic is often bursty. That is, you may download a video file at a very high data rate, but then not need the data link for a significant period of time after that. In other words, the physical-layer resources in a network may not be in use all the time. From the operator's point of view, this is an inefficient use of resources. Unfortunately, R99 doesn't do a good job of quickly reallocating resources to support the instantaneous demands of the users in a cell.

HSDPA changes the rules of the game within a cell. First, the network allocates one big, fat pipe for high-speed data and the remaining cell resources for voice and low-rate data traffic. The big, fat pipe is known as the HS-DSCH (high-speed downlink shared channel) and is shared among multiple users. This is different from the concept of each user assigned to a dedicated physical channel.

HSDPA then uses three new technologies to optimize the resources available in the big, fat pipe. These include adaptive modulation and coding (AMC), including a choice of two modulation schemes - 16QAM and QPSK; a protocol to handle retransmissions and to guarantee error-free data transmission; and a fast packet scheduling algorithm, which allocates the pipe's resources such as time slots and codes to different users.

For these three technologies to do their jobs, some of the functions needed to be moved to different locations within an R99 network. Specifically, some functions, previously found in the RLC protocol layer and the serving RNC are moved down into the MAC protocol layer as well as into the base station, node B. But because there's no standard to define how the node B implements scheduling, an effective test plan requires a certain amount of flexibility.

The 3GPP standards community has defined RF and protocol test cases for HSDPA, providing a basis for RF and protocol performance testing.. However, several key aspects of HSDPA are outside the scope of traditional conformance testing. To design a truly comprehensive HSDPA test plan, the real-world environment and network interoperability scenarios must be factored in as well.

And the resulting testing needs to move beyond conformance testing only. Network operators are nervous about putting new handsets on their networks. If services aren't delivered as promised, it's the operator that gets blamed, not the handset supplier.

On the other side of the fence, cdma2000 operators faced a similar scenario when introducing EV-DO services to their basic 3G networks. Two cdma2000 handsets may have both passed the conformance test, but they often varied greatly in performance and in the demands that they made on network resources. Engineers found that the test standards weren't as comprehensive as needed. The early revisions of the new standards were based on visible needs and were written without the benefit of real-world experience.

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