When it comes to Long Term Evolution (LTE), it’s all about the user experience and therefore, user equipment testing is extremely important. LTE is cutting edge in terms of speed and network performance, but according to Aeroflex, in any real-life mobile communications network there will be considerable degradation of the signal transmitted between the base station and the handset or other user equipment (UE).
“This is because there are an infinite number of paths that the signal can take, each with different values of attenuation and phase difference, and the received signal is the sum of all these multi-path signals. The varying, constructive and destructive combination of these multiple signals results in an effect known as fading, and the degradation in the signal path is characterized by the Rayleigh coefficient.”
What Equipment to Use
As the world’s cellular network operators work towards adopting LTE, the demand is growing to meet all of the LTE
requirements, including fading performance. To make sure there are regulations in place, The Third Generation Partnership Project (3GPP) has specified a range off-loading profiles in its standard TS 36.521-1. These are required to be used for the measurement of fading performance in LTE handsets. Traditional methods of testing fading performance involve using an external fading simulator and noise generator to modify the signal from the normal radio test equipment. “This equipment can add tens of thousands of dollars in cost to the overall test setup, as well as needing additional software to control and coordinate the individual instruments. Real systems will carry paths between the transmitter and receiver. Paths that have significantly greater loss than the ‘best’ path will have a negligible effect on the received signal quality and can therefore be ignored. Similarly paths that have a large delay will also suffer large losses, and do not need to be emulated.” Further simplifications can be made for the purposes of radio channel emulation are as follows:
• Transformations at RF may be equivalently applied to baseband signals.
• Paths with a similar delay (within a delay range of ~0.1/Fs) where Fs is the bandwidth of the transmitted signal, can be treated as carrying the same baseband data but having different carrier phase which changes as the receiver moves. If the carrier frequency Fc >> Fs, combining these paths gives a random path gain which is time variable with Rayleigh statistics.
• The time variation of the Rayleigh path gain is related to relative movement between transmitter and receiver. This leads to Doppler (frequency shift) effects. If the Doppler frequency Fd << Fs, then the time variation of the path gain can be applied at a low sample rate compared to Fs, and will have negligible effect on the bandwidth of the time varying signal. [1]
How to Test
“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.” [2]
[1] Aeroflex, White Paper: Radio Channel Emulation for LTE User Equipment Testing, http://www.aeroflex.com/ats/products/prodfiles/wpapers/7100_LTE_Test_wp.pdf
[2] ECN, Testing LTE: What you need to know, http://www.ecnmag.com/articles/2010/04/testing-lte-what-you-need-know
