In this blog post, the first in a series on massive multiple-input/multiple-output (MIMO) mechanics, I explain how the advantages of the MicroTCA architecture can be used to scale massive MIMO systems. As the system increases in size, so do the real-world concerns of the implementation. The first such concerns I will address is how to provide the system with synchronized clocks and how to initialize the individual subsystems.
In a massive MIMO system, all receive (RX) and transmit (TX) signals must be sampled by the same clock so that they are synchronized in phase and frequency. To achieve this, a single clock source must be used that distributes the clock to all of the system's analog-digital and digital-analog converters (ADC/DAC). Special care must be taken to have the same length for each clock path (from source to ADC/DAC) in order to maintain phase alignment on all the channels. If there is a small length difference between two channels, the phase difference will be deterministic and therefore circumventable by a phase adjustment.
In the Nutaq system, all the Radio420m modules receive the disciplined clock. A clock distribution unit sends the synchronized clock to each system chassis, which in turn distributes it to all the Perseus/Radio420m pairs within the chassis. The pulse per second (PPS) signal is also distributed to each FPGA, enabling the system to share a common time base.
In a massive MIMO system, another real-world concern is the control channel used to initialize all of the system's radio heads. A typical massive MIMO system will have one central processing unit controlling all the peripheral units. In a MicroTCA-based system, the control is done through Gigabit Ethernet or PCI Express. Gigabit Ethernet is the most convenient way to control the system from the central processing PC as you can link all the MicroTCA carrier hubs (MCH) with Ethernet cables. PCI Express, on the other hand, adds complexity to the design but offers a very low latency when controlling the radio heads.
The following initializations must occur in the system:
- The radio ADC and DAC phase-locked loop (PLL): As mentioned early, all of the radios in the massive MIMO system must receive a clock from a single source. Therefore, all radios are configured to use an external clock (from the chassis clock fabric) and set their PLL in bypass in order to maintain the synchronization in frequency and phase.
- Local oscillators (LO): In a massive MIMO system, all the RX and TX local oscillators are configured at the exact same frequency. The LO PLL initialization must therefore be done with the exact same configuration for all radios. The GPS disciplined clock ensures the LO frequency is at the exact required frequency.
- Transceiver calibration: The side-band and local oscillator leakage suppression calibration must be performed on each radio independently (as they are influenced by each individual analog RF paths).
In Nutaq's massive MIMO system, the radio initialization is done in parallel for each system node, which considerably reduces the required setup time.
In the next installment in this series on massive MIMO mechanics, I will discuss mass waveform recording and playback.