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5G air interfaces require channel measurements

Posted: 18 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:5G? air interface? algorithms? channel impulse response? measurement systems?

5G wireless communications should bring increased network capacity, higher peak data rates, and more reliable service in mobile communications systems. Many of the goals are 10x, 100x or 1000x today's performance but aren't achievable in the currently available spectrum below 6GHz. Therefore, new air interfaces are being investigated in Centimeter (cm) and Millimeter-wave (mmWave) frequencies up to 100GHz. Characterisation of the radio channel at mmWave frequencies presents many new challenges for engineers. Here are some of these challenges and some considerations.

To define new air interface standards, researchers will need to characterise the radio channel so they can understand how the signal will propagate. Researchers are using channel sounding techniques to collect the channel impulse response (CIR) data so they can extract channel parameters by using channel parameter estimation algorithms. The extracted data are then used for developing new channel models as shown in figure 1. Sounds straightforward and easy, right?

Well, not quite.

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Figure 1: The model of a wireless transmission channel consists of channel sounding, estimating of channel parameters, and statistics.

Channel sounding measurement systems can range from simple to complex depending on parameters being estimated. When measuring time-varying channels with multi-path propagation, you need to understand the complex impulse response with time and phase information. In addition, one of the key challenges is being able to duplicate or validate the measurements with different measurement systems in similar conditions.

Key technical challenges include:
???Signal generation and analysis at mmWave frequencies with greater than 500MHz bandwidth and with multi-channel support
???Data capture and storage
???Channel parameter estimations
???Calibration and synchronisation
???Now let's discuss considerations to help you address these challenges.

Signal generation and analysis
To meet the high demands for 5G, the air interface standards will likely include mmWave frequencies up to 100GHz, with 500MHz to 2GHz bandwidth, and with multi-channel support. That's a lot to consider. The requirements will place great demand on the channel sounding measurement system. The measurement system needs to support these core requirements and provide repeatable measurements. Key components for this measurement system will be a wideband digital to analogue converter (DAC) in the form of a base band AWG (arbitrary waveform generator) and an ADC (analogue-to-digital converter) that will take the form of a wideband digitizer or oscilloscope to support the needed bandwidth and provide enough resolution to support the dynamic range needed to capture the signal. Also, because 5G is not yet defined, the test equipment should be flexible so that it can be configured and reconfigured as the test requirements and standards evolve.

Data capture and storage
When you consider the raw data that needs to be collected with a wideband measurement system that also has multi-channel capability, an eight-channel, 1GHz bandwidth measurement can consume gigabytes of data in just one second, quickly filling disc drives. In addition, consider how to get this data from the ADC to a storage device. It's nearly impossible for the data to be captured and streamed in real-time. Disc drive manufacturers might like this because they will sell more storage, but it's just not practical. Instead, there are two other data capture-methods to consider that can reduce the amount of collected data:
???If the sounding signal is less than one transmitting period, you can capture only the effective data or only the data needed to perform the CIR calculations. This method can greatly reduce the data collection.
???Taking this one step further, you can perform the wideband measurement with real-time autocorrelation and signal processing with an onboard FPGA to produce the effective CIR data within the measurement system, now only the CIR results need to be saved. Thus, significantly saving storage space and providing the CIR results much faster.

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