jueves, 7 de julio de 2011

Measuring LTE Modulation Quality Over-The-Air

LTE is a step toward the 4th generation (4G) of radio technologies designed to increase the capacity and speed of mobile telephone networks. The main advantages of LTE are high throughputlow latencyplug and play, an improved end-user experience and a simple architecture resulting in low operating costs. A variety of multi-antenna techniques play a key role in delivering these performance improvements.

Transmit diversity uses signals that originate from two or more transmitters with identical data streams, but different coding; this helps overcome the effects of fading, which is one of the major limitations of wireless systems—especially at the cell edge where the signal strength is low. Spatial multiplexing uses multiple input multiple output (MIMO) wireless communications to transmit independent and separately encoded data signals from each of several transmit antennas. Receivers use matrix mathematics to separate the two data streams and demodulate the data. Transmitting data in multiple streams in parallel improves bandwidth but requires a relatively high signal-to-noise ratio. Beamforming uses patterns of constructive and destructive interference on the wavefront to increase and decrease the signal in specific areas, thus improving the SNR at the receiver while decreasing interference.

Multi-antenna methods also increase the difficulty of basic operational and troubleshooting techniques that were much easier with the previous generation of network technology. Spatial multiplexing and beamforming provide the biggest challenges for over-the-air measurements; the dynamic nature of which multi-antenna technique is used at any given moment adds even greater complexity. With spatial multiplexing the different antennas appear to be co-channel interference to a single receiver, thus necessitating a very expensive and heavy measurement device with multiple receivers. Beamforming can also present problems because it increases or decreases the amount of power received in particular areas on a continually changing basis—making reliable measurements of the signal impossible for a passive measurement device. 

While transmit diversity does not present a measurement problem (because multiple antennas’ signals can be recovered with a single receive channel), each Physical Downlink 
Shared Channel (PDSCH) used to transmit LTE data can change multi-antenna mode dynamically based on signal conditions per user. When looking at a captured signal with a measuring instrument, it’s impossible to know if each resource block uses spatial multiplexing, beamforming, or transmit diversity. Of course, you can avoid these complications by directly connecting the instrument to the transmitter. This approach provides the most thorough and accurate measurements of modulation quality so it is essential in many cases. However, there are a number of limitations associated with direct-connect measurements. It takes time to open up a shelter or building. If the transmitter has a test port then connecting to the instrument is not a problem. If there is no test port, then you have to disconnect the transmitter from the antenna which is usually difficult and time-consuming. If the site uses a Remote Radio Head (RRH) or Remote Radio Unit (RRU), then you need to gain physical access to the RF signal. This may not be difficult if the RRH/RRU is mounted inside a building or on a roof with reasonable access, but if the RRH/RRU is mounted on a tower or inaccessible roof, then you need to climb the tower or 
otherwise get access to the transmitter—usually a difficult and expensive process.

Making measurements over the air is much easier and faster which is always important for technicians and engineers with many responsibilities and limited time. Speed is particularly important when troubleshooting a reported problem

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