This section describes how to set up your USRP-2920/2921/2922/2930/2932 devices for multiple input, multiple output (MIMO) synchronization. NI-USRP devices support MIMO synchronization from 2 × 2 to 8 × 8 antenna configurations. You can combine multiple USRP devices to form a phase-coherent antenna array for transmission and reception.

A MIMO communication testbed requires all receive (Rx) channels to act as a single receiver, and all transmit (Tx) channels to act as a single transmitter. The MIMO testbed achieves synchronization by sharing a 10 MHz Reference Clock and a PPS timebase among the NI-USRP devices.

Setting up your Devices

MIMO synchronization requires the following fundamental parameters:

  • Device Names—The unique IP address of each USRP being controlled. The default IP address for new USRP hardware is 192.168.10.2.
  • IQ Rate—The sample rate of the baseband I/Q waveform determines the real time bandwidth being acquired or synthesized.
  • Carrier Frequency—The mid-point in the spectrum you are analyzing. The carrier frequency becomes the center or DC component when converted to baseband I/Q.
  • Gain—Adjustable gain, typically 0 dB to 30 dB, that you apply to the analog RF signal.
  • Active Antenna—Switch setting that determines which Rx or Tx SMA port to use for acquisition or transmission.
  • Number of Samples—The number of samples to acquire or transmit from the device with each driver call to the niUSRP Fetch Rx Data or Write Tx VI.

Set up your block diagram so that it is similar to the following figure:

Figure 5. MIMO Block Diagram


Configuring Multiple Devices

You can connect multiple devices in the following ways:

Table 1. Device Configurations
Interface Option Description
Multiple Ethernet Interfaces One device for each interface.
Single Ethernet Interface One device connected to the interface, additional device connected using an optional MIMO cable.
Multiple devices connected to an unmanaged switch.
Note Sharing a single gigabit Ethernet interface among devices may reduce overall signal throughput. For maximum signal throughput, NI recommends you connect no more than one device per Ethernet interface.

Set up your synchronization testbed to resemble the following figure. Refer to the getting started guide for your device for more information about setting up the network.

Note The following figure shows an 8 × 8 device setup. Your setup may vary depending on the number of devices you want to synchronize.
Figure 6. Synchronization Testbed


The testbed requires the following components:

Note The following table provides component quantities for an 8 × 8 device setup. Your part quantities may vary depending on the number of devices you want to synchronize.
Table 2. MIMO Parts List
Part Quantity
SMA-connected magnetic mount antenna Minimum: 4 Maximum:16
USRP device kit Minimum: 4 Maximum:16
USRP MIMO synchronization and data cable 1 per 2 USRP-2920/2921/2922/2930/2932 devices
Trimble Thunderbolt (REF and PPS) 2
Category 5 Ethernet cables 1 per USRP-2920/2921/2922/2930/2932 device
0.3 m BNC-to-BNC cables 4
3-way female BNC "T" 4
3-way male-female-female BNC "T" 8
BNC (m)-to-SMA (m) cable 16
10 port Gigabit Ethernet switch 1
PCIe Gigabit Ethernet card (Intel chipset) 1

The coherent LO is derived from the 10 MHz Reference Clock. An oven-controlled oscillator (OCXO) supplies the 10 MHz Reference Clock output and digital pulse per second (PPS) output. The OCXO provides an accurate frequency source that operates without a GPS antenna. You can add a GPS antenna to improve synchronization and to provide a global concept of time.

MIMO Synchronization with the NI-USRP Driver

Perform the following steps to configure and synchronize the MIMO system:

  1. Open the Rx session by specifying the IP address of each USRP. Separate each IP address with a comma.
  2. Set the Start Trigger time, in seconds and fractional seconds, to indicate when all USRP devices should begin fetching data.
  3. Set the parameters shared by all eight devices.
  4. Specify which devices use the external reference or MIMO cable.
  5. Use the niUSRP Set Time VI to set the time of the onboard device timer. Set this time to be earlier than the Start Trigger time you set in step 2. Set the onboard device timer according to your device configuration:
    • If the device uses an external PPS signal (PPS In or GPS), set the apply timestamp parameter to Next Timebase Edge.
    • If the device uses a PPS signal from the MIMO connector, do not call the niUSRP Set Time VI. The onboard device timer automatically synchronizes to the master device and drives the timestamp over the MIMO connector.
    • If the device does not use an external PPS signal (Internal), set the apply timestamp parameter to Now.
    Note The channel list specifies which devices to set to 0.
  6. Fetch the data using a single driver call, returned as a 2D array of complex samples where each row contains the complex I/Q waveform from a single antenna.
  7. Repeat steps 1-6 to configure a Tx session.