LTE测试：你需要知道的那些事

Now that the first LTE networks have gone live, the race is on to provide mobile broadband services on a large scale. However, have all the technical issues been addressed, or are there problems still to be overcome before subscribers gain the full benefit of next-generation cellular technology? Beyond the technical issues, there are also some broader aspects of the LTE service offering that still require resolution. This article takes a look at some of these issues in the context of the challenges they present to the test engineer.

Before looking at some of the technical issues related to the design of user equipment (UE), a wider view reveals other potential pitfalls:

At the time of writing, formal certification of LTE devices has not yet begun. The major certification bodies (GCF [Ref.1], PTCRB [Ref.2]) are working to introduce conformance testing schemes for protocol, RF and radio resource management, with a target of December 2010. However, with devices already on sale in some markets, this leads to the question: will these devices be capable of passing the conformance tests once they are introduced? With LTE Category 3 devices capable of supporting high data rates (100Mbps in the downlink, 50Mbps uplink), will the backhaul capacity be sufficient to cope? In the longer term, as the number of LTE users rises, sharing of bandwidth on the radio network between all users in the cell will become a significant factor, with crowded cells not performing so well. Also, as the number of active users rises, cell-edge performance will suffer due to a higher SNR. With a potentially global mobile data network, the expectation of global roaming for data users needs to be addressed. Although it is technically possible, the cost of roaming data services for consumers needs to be addressed. On the other hand, flat-rate data plans are already an issue for network operators, with effectively fixed revenues being earned for providing variable,and almost certainly increasing, data volumes. With LTE being the next-generation technology choice for CDMA2000 network operators (eg Verizon Wireless), interworking with 3GPP2 CDMA2000 high-rate packet data services is a definite requirement. Merging the 3GPP and 3GPP2 network topologies at the LTE radio network interface is an interesting development that will require careful testing to ensure it operates as expected. Maintaining voice services using an IP network in parallel with the circuit-switched legacy networks will be a challenge for the network operators. The agreement announced at Mobile World Congress 2010 by the major network operators to standardize on VoLTE (Voice over LTE) will largely address this issue. However, this technology, which uses 3GPP’s IMS (IP Multimedia Subsystem) still needs to be deployed on a large scale.

Meeting the Challenge

To match the demanding requirements of LTE terminal devices, it is essential to break the design down into subsystems and to build a test plan that allows each part of the design to be characterized thoroughly before testing the complete device. Without this modular approach, the diagnosis of problems can occur so late in the program that it becomes difficult to manage the final release stages, including field trials and compliance testing.

Measurement Needs

Regardless of whether the device design is begun from scratch, evolved from an earlier design, or uses third-party component integration, several key performance measurements need to be made. Some of these, such as maximum output power, pow