Global Sources
EE Times-India
Stay in touch with EE Times India
EE Times-India > Networks

Grasping coherency, synchronisation in MIMO systems

Posted: 22 Jul 2015     Print Version  Bookmark and Share

Keywords:multiple-input multiple-output  MIMO  Beamforming  Multi-antenna  signal-to-noise 

These timing, processing and data collection challenges make prototyping vital. For researchers to validate theory, this means progressing from theoretical work to testbeds. Using real-world waveforms in real-world scenarios, researchers can develop prototypes to determine the feasibility and commercial viability of massive MIMO. As with any new wireless standard or technology, the transition from concept to prototype impacts the time to actual deployment and commercialisation. And the faster researchers can build prototypes, the sooner society can benefit from the innovations.

An example of this comes from Lund University, in Sweden, where Professors Ove Edfors and Fredrik Tufvesson worked with NI to develop the world's largest MIMO system. Their system uses 50 USRP RIO software-defined radios (SDRs) to create a 100-antenna configuration for the massive MIMO BTS.

Figure 5: The massive MIMO testbed at Lund University in Sweden is based on USRP RIO with a custom cross-polarized patch antenna array.

Just like other cellular communications systems, Massive MIMO systems consist of BTS and user equipment (UE) or mobile users. It differs, however, from the traditional approach by assigning a large number of BTS antennas to communicate with multiple UEs simultaneously. In the system that NI and Lund University developed, the BTS uses a system design factor of 10 base station antenna elements per UE, providing 10 users with simultaneous, full-bandwidth access to the 100-antenna base station. This design factor of 10 base station antennas per UE has been shown to allow for most theoretical gains to be harvested.

In a massive MIMO system, a set of UEs concurrently transmits an orthogonal pilot set to the BTS. These known uplink pilots can then be used to perform channel estimation. In the downlink time slot, this channel estimate is used to compute a precoder for the downlink signals. Ideally, this results in each mobile user receiving an interference-free channel with the message intended for them. Precoder design is an open area of research and can be tailored to various system design objectives. For instance, precoders can be designed to null interference at other users, minimise total radiated power, or reduce the peak-to-average power ratio of transmitted RF signals. Although many configurations are possible with this architecture, the Massive MIMO Application Framework from NI supports up to 20MHz of instantaneous real-time bandwidth that scales from 64 to 128 antennas and can be used with multiple independent UEs.

Just like in test and measurement, clock synchronisation remains a serious concern in prototyping communications systems. In Lund University's massive MIMO, they used an Ettus Research OctoClock 8-channel clock distribution module. This uses matched-length traces to distribute a signal from either an internal GPS-disciplined oscillator (GPSDO) or an amplified externally-supplied reference clock. In order to provide an accurate and stable reference, Lund opted to use multiple OctoClock modules, with a common clock generated from an NI PXIe-6674T timing and synchronisation module, which uses an oven-controlled crystal oscillator (OCXO) capable of 50 ppb accuracy. The BTS system shares this common 10MHz reference clock, as well as a master digital trigger to start generation or analysis on each radio, ensuring synchronisation across the entire system.

Massive MIMO, massive data
Beyond the topic of synchronisation, a further consideration with massive MIMO is how to cope with the increased volume of data. Naturally, more transceivers means more data being sent, received and digitised, so adequate platforms for data aggregation and processing must be considered. Firstly, the bus that transports data from the radios to the system controller cannot be a bottleneck – it must be capable of streaming with sufficient throughput to cope with the large data rates from multiple radios. Commonly used buses like GPIB or USB will struggle to provide the required throughput, so instead a bus like MXI-Express should be used. MXI-Express provides a cabled PCI Express Generation 2 link to allow data streaming back to a host PXI system at up to 3.2 GB/s.

Once the data has been transferred to the host, the burden is on the processor to run decoding and processing algorithms to make sense of the received signals. To help meet this demand, NI offers the PXIe-8880 controller, which features an Intel Xeon 2.3GHz eight-core processor. Additionally, by including FPGA-based processing nodes like FlexRIO, data can be processed inline and in real-time with up to 3 GB/s throughput and less than 5µs of latency from radio to processing node.

Whether in the communications systems deployed today or those we're developing for tomorrow, MIMO plays a huge part in enhancing the capability of our networks. Whilst the consumer will experience a more reliable network with higher data rates, it's engineers who will need to face the challenges of testing today's and designing tomorrow's MIMO communications systems. MIMO will continue to drive the need for measurement and prototyping platforms to offer high degrees of synchronisation and coherency. MIMO is truly not just a phase.

About the author
Jeremy Twaits is a Senior RF Marketing Engineer for National Instruments, Northern Europe.

 First Page Previous Page 1 • 2 • 3

Comment on "Grasping coherency, synchronisation ..."
*  You can enter [0] more charecters.
*Verify code:


Visit Asia Webinars to learn about the latest in technology and get practical design tips.


Go to top             Connect on Facebook      Follow us on Twitter      Follow us on Orkut

Back to Top