The typical linear processing technique used in MIMO is time division duplexing (TDD) where the uplink and downlink transmissions occur consecutively. Frequency division duplexing (FDD) is another lesser-used linear processing technique where uplinks and downlinks occur simultaneously at different frequency bands. But while FDD transmissions are faster, TDD systems also leverage channel reciprocity, a property that allows for an estimation of channel state information (CSI) at the transmitter. This property offers an elegant solution to improving signal quality in massive MIMO systems, and is thus becoming the preferred method for linear processing.
Having nearly solved their linearity issues, 5G engineers have more recently been working on their next biggest challenge: developing a number of precoding, or beamforming techniques, to successfully transmit and receive multiple signals from the same base station while still upholding performance and quality of service. The difficulty lying in developing beamforming techniques is the high number of antennas at each base station.
With a number of notable and successful installations of massive MIMO systems to date, offering vastly enhanced spectral efficiency over current 4G networks, the future of massive MIMO is bright. But there is still much room for advancement. A known disadvantage to massive MIMO systems, using TDD for instance, is pilot contamination caused when a quick pilot sequence is used to estimate CSI at the receiver (CSIR). This is an iterative signal degradation problem when used in co-channel cells in a set-up that includes many massive MIMO systems.
The race to establish 5G infrastructure as rapidly as possible comes with an increased need for robust and customizable test and measurement solutions that can handle various potential MIMO approaches. And as with any new and evolving technology, a common goal is also to do it all as inexpensively as possible. So keeping the testing solutions of these ever-changing MIMO technologies simple and accessible is crucial. But until recently, that’s been easier said than done. MIMO exploits a multipath scenario, and the testing for each individual path often needs digital attenuation to adjust signal amplitude and RF switching, to allow for going back and forth between transmitter and receiver paths.
The advent of USB-powered portable test equipment has allowed for a streamlined and customized approach to these MIMO testing challenges—even in the most challenging of massive MIMO situations where large testbeds with over 100 antennas can be stacked together. The antennas and their respective RF circuitry in these testbeds often cause non-symmetry in the communication channel, and this first needs to be calibrated out before leveraging channel reciprocity. There are straight-forward techniques to calibrate these maturing systems today, but there is research being done on more effective testbed methods which only require a limited amount of CSI. These testbed solutions include software defined radios (SDRs), which include a component for time and frequency synchronization, such as crystal oscillators, and a central controller, often a laptop. Research and field teams working in tandem with these new SDR-driven testbeds benefit from USB-connected RF switches that can lessen the number of RF ports required in these multichannel scenarios.
Because it’s a moving target, the common goal in 5G development is to keep the testing of MIMO and other technologies as simple and low cost as possible. When evaluating portable test devices, look for vendors that offer a variety of frequency ranges and performance levels to keep up with your needs. They should also offer easy-to-install-and-use GUI’s, and the ability to operate multiple devices directly from a PC or self-powered USB hub. This effectively creates an automated test equipment (ATE) stand on-the-fly. Compact packaging and robust construction will also allow your devices to stand-up over time to constant re-arrangement and transportation. They should also be easy to stack or place in a test rack, and have API, DLL and LabVIEW compatible drivers readily available.
In the race to 5G, the engineering teams who can not only design and develop, but test and deploy advanced MIMO systems effectively, will be the teams who come out on top.
To learn even more about 5G MIMO Networks, download our tech brief "Insights on Evolving 5G MIMO Networks and Test Methods" or read part 1 of this Blog series "3 MIMO Architectures Aiming to Solve 5G’s Challenges"