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 Satish Mohanram, Technical Marketing Manager, National Instruments India

Wireless communication is the next jump in the PC/Computing devices. Connectivity over the air has added a lot of convenience in interfacing external devices, connecting and sharing information to other devices and being able to remain connected to work/personal life. As the standards evolved the throughput of these communication standards increased and has opened up new possibilities.
High throughput/bandwidth wireless communications mean:
- Streaming High quality Audio, - Streaming HD Video  Video calls, - Better signal quality through diversity reception,- High-speed data streaming, - Mobile gaming over the air
These applications will mean that the over the air communication protocol provides capabilities such as:
- High Bandwidth  Sustained high data throughput, - Reliable connectivity, - Good connectivity on the move
Technologically this means that these standards will involve:
- High Bandwidth in the spectrum, - Better Modulation techniques, - Better encoding techniques, - MIMO  Diversity transmission and reception

The traditional approaches to test wire-less systems don't scale to deliver the required throughput to these advanced technological tests. These systems need a reconfigurable, modular approach that can be upgraded as these standards evolve. Also being able to combine tests that involve functional verification of the audio, video quality is a key.

The NI PXIe-5645R Vector Signal Trans-ceiver (VST) is a PXI express module that has a vector signal analyzer (VSA), vector signal generator (VSG), Digital I/O, baseband IQ interface and integrated LabVIEW-programmable FPGA. Its highend instru-mentation quality RF input and output, tightly integrated with a powerful FPGA, makes the VST the best Software Designed instrument that is capable of delivering the best throughput in testing these next-generation communication systems.

The multiple standards operating parallel with LTE are expected to create complex technical issues, making interconnectivity with legacy networks a challenge
Wireless communication channel models, often called radio propagation models, describe variations of electromagnetic waves from the transmitter to the receiver in the time, frequency and space domains . The radio channel is highly random and is very hard to model due to various reasons such as shape and location of obstacles in the transmission path, carrier frequency, environment (indoor/outdoor) antenna heights, weather, movements of transmitter and/or receivers, existence of other signals in these bands etc.  Therefore, statistical channel models are used for research and development in many cases. The wireless channel can be modeled in two ways: large-scale propagation model and small-scale or fast fading. The large-scale model characterizes the mean received signal strength that is affected by the path-loss and scattering (log-normal shadowing) over large transmitter-receiver separation distances. The small scale fading models rapid fluctuation of the received signal strength and phase in a few wavelengths range or short time durations. In most of cases, the receiver performance is dominated by small-scale fading because the received signal power may vary by 30 or 40 dB. One of the most unique features of the wireless channel is that the electromagnetic waves travel in several directions called multipath. The channel may or may not have line-of-sight (LOS), and each ray experiences different delay and attenuation (fading), which generates frequency selectivity in the channel. The power delay profile (PDP) is a commonly used measure of relative strength of the received signal associated with a given multipath delay

The power delay profile and delay spread represent the time dispersive characteristics of the channel regardless of the movement of the transmitter and/or receiver. However, the freedom of movement is one of the most distinctive features of the wireless communication services. Doppler spread and its time domain dual, coherence time, describe the variation of channel due to the movements, which cannot be disregarded in fading channel models.

As an example application of the VST beyond traditional RF parametric test, National Instruments presents reference architecture for a wireless channel emulator developed on the VST with the LabVIEW System Design software. The wireless channel emulator is an instrument that emulates the real-world electromagnetic propagation environment by applying real-time channel impairments on incoming RF signal(s), before sending the impaired signal(s) out.  It is widely used in early R&D, verification and validation, and even conformance testing of wireless communication devices.  More than one device is required to support MIMO channel emulation. For example, to support 2x1 (2 Tx antenna and 1 Rx antenna), 1x2, or 2x2 MIMO configuration, two VSTs are needed. Single input single output (SISO) channel can be emulated on one VST with a controller. Figure illustrates four VST configurations installed on the NI PXIe-1085 chassis. It has an embedded controller NI PXIe-8135, and four NI PXIe-5644Rs. Note that different models of controller and chassis can be used depending on the computational requirements. Also note that supported MIMO configurations (number of TX and RX antenna) are dependent on various parameters such as sampling rate, system bandwidth, latency, chassis slot count, and so on.


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