EDA & Development
Testing LTE: What you need to know
Beyond technical issues there are some broader aspects of the LTE service offering that still require resolution. Phil Medd looks at some of these issues in the context of the challenges they present to the test engineer
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? Before looking at some of the technical issues related to the design of user equipment (UE), a wider view reveals other potential pitfalls:
N
■ 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
34 July/August 2010
ow that the first LTE networks have gone live, the race is on to provide mobile broadband
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.
Components in Electronics
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 characterised 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.
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, power control, and receiver sensitivity, will be familiar from earlier technologies, but due to the transmission schemes used (OFDMA in the downlink, SC-FDMA in the uplink), new measurement equipment will be needed to support these tests.
Other measurements are specific to
LTE. With its OFDMA transmission scheme, for example, error vector magnitude (EVM) per sub-carrier becomes an essential test of modulator performance. With the availability of the 700-MHz analogue TV spectrum, LTE will be deployed at lower frequencies than GSM or WCDMA, resulting in much broader bandwidths: 20 MHz/700 MHz = 2.8%, compared with 5 MHz/2100 MHz = 0.24% for typical WCDMA devices. This can pose a challenge with some modulator architectures as it results in a higher EVM at the band edges, so it needs special attention at the design stage. Due to the dynamic nature of some
of the tests, such as power control, the measurement conditions need to be established using the signaling protocol. This makes it essential for the test equipment to include the protocol stack, simulating the evolved Node B (eNB) base station. Since these measurements are usually performed by RF engineers rather than protocol specialists, the test equipment used must be simple to configure, allowing engineers to focus on the measurement being made.
Protocol testing
One of the main challenges for the protocol stack developer will be to ensure that the state change response requirements are met. Although the LTE specifications have reduced the number of states that a terminal can be in to RRC¬_IDLE and
RRC_CONNECTED, the time it takes to change from one to the other will be a major part of the latency budget when data needs to be sent.
In RRC_IDLE mode, as much as possible of the device electronics will be in a low-power consumption state to ensure good battery life, with the receiver activated periodically to check for paging messages. When data transmission is scheduled, the device must wake up and rapidly synchronise its uplink.
Protocol testing can often involve expending as much effort on generating test cases as on creating the protocol stack, so access to comprehensive and efficient test facilities is vital. In order to be able to break down the testing, it is important to be able to test each sub-layer in both the User Plane and Control Plane. Protocol test diagnostic features are essential when tracing faults. Typically,
www.cieonline.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44