The 3GPP Long Term Evolution (LTE) standard moves beyond current 3G wireless communications capabilities to provide increased peak data rates, improved spectral efficiency, and reduced user plane and control plane latency. To realize these performance improvements, the standard relies on highly flexible and dynamic radio technologies, where the control messages are essential to understanding the RF signal. There are three key ingredients to the LTE downlink (DL): Orthogonal Frequency Division Multiple Access (OFDMA), Multiple-Input Multiple-Output (MIMO) and Hybrid Automatic Repeat Request (HARQ). The OFDMA signal format allows rapid adaptation of the signal format to the user’s data bandwidth requirements and channel conditions. MIMO is a smart antenna technology that can deliver increased spectral efficiency. Use of HARQ allows the optimization of overall system performance when many users are present and interference is a limiting factor in overall capacity.
While OFDMA, MIMO and HARQ are critical to enabling the LTE standard, attention to detail is needed to ensure their effective operation. The number and complexity of possible signal formats, coupled with the need to ensure interoperability between devices from different vendors, introduces a multitude of design and test challenges. One of these is to verify the data content and RF structure of the DL signal. Doing so is critical to ensuring LTE devices have been implemented according to the 3GPP Release 8 standard, and to help ensure all base stations and mobile devices work together in real networks.
Messages in the LTE DL signals provide all the information needed for the base station and mobiles to work together. Engineers require tools to examine their contents. This measurement information is necessary to independently verify baseband signals, find out what is in a third-party base station signal feeding into a User Equipment (UE) receiver, diagnose a low-level interoperability problem, or to just better understand the signal’s structure and its impact on system operation. DL signal information is also valuable in helping the engineer find out why throughput results are not as high as expected.
The construction of the LTE DL signal is the most complex used thus far in the implementation of a cellular system, making its analysis particularly challenging. The content of the transmitted signal is the result of a mixture of messages across several layers in the protocol stack, from the DL Control Information (DCI) to the Master Information Block (MIB) and System Information Blocks (SIBs). The messages are built up to create a highly dynamic signal that changes on every subframe. Verifying the resulting DL signal has been constructed according to the 3GPP Release 8 standard is often accomplished using multiple disparate measurement instruments and can be an error-prone process.