Application Note Synchronization and MIMO ... - Ettus Research

Application Note
Synchronization and MIMO Capability with USRP Devices
Ettus Research
Introduction
Some applications require synchronization between multiple USRP (Universal Software Radio
Peripheral) devices. To meet these requirements, the Ettus Research USRP N200 and USRP N210
kits provide a MIMO cable that allows the user to easily build a 2X2 system. External 10 MHz and 1
PPS signals can also be used to synchronize all Ettus Research USRP models, except for the USRP1.
Some USRP models provide the option to integrate an internal GPS Disciplined Oscillator, which
enables timing synchronization over wider geographic areas. This document will introduce and explain
the synchronization features of the Universal Software Radio Peripheral (USRP) product family.
MIMO Requirements – Time and Frequency Synchronization
For a transceiver to be considered MIMO capable, each channel in the radio must meet two
requirements:
1. The sample clocks must be synchronized.
2. The sample times must be aligned when any DSP operations occur.
The Ettus Research USRP N200/N210 and USRP1 meet both of these requirements. In the case of the
USRP N200/N210, this is achieved by sharing a common 10 MHz reference and 1 PPS signal to two or
more devices. A MIMO system can be built with the USRP N200/N210 MIMO cable, or by using an
external reference. The USRP1 contains slots for two daughterboards, and uses a common clock
source for all operations. 10 MHz reference and 1 PPS inputs are not provided on the USRP1, but are
not required for MIMO operation since the USRP1 has two daughterboard slots.
Beamforming and Direction Finding Requirements
Some MIMO systems are not ideal for applications requiring beamforming or direction finding (DF)
capability. These application areas require a prior knowledge of the phase relationships between
multiple channels. Due to phase ambiguities caused by phased-locked loops, it is possible a software
defined radio (SDR) can have sample clock and time alignment, but not phase alignment. In this
context, phase alignment implies a minimal difference between multiple transmit/receive channels, or
that the phase differences can be measured and compensated for.
Other Variables That Effect Phase Alignment
As mentioned, other variables aside from LO phase offsets contribute to channel-to-channel phase
error. Filters, mixers, amplifiers and other components produce phase variations that vary with time,
temperature, mechanical conditions, etc. These types of errors can generally be calibrated out with
intermittent, low-rate routines detecting the channel-to-channel phase with a common signal generator.
These errors will not change with each PLL retune but may change over time and temperature
variation.

MIMO Capability vs. Phase Alignment
While applications such as direction finding and beamforming require knowledge of the phase offsets
between channels, many MIMO implementations do not require a prior knowledge of the phase
relationships between channels. In these cases, the phase differences between channels can be
measured with a training sequence during a frame preamble. Many wireless devices, such as WiFi,
routers work on this concept. Applications such as direction finding or adaptive beamforming do
require a prior knowledge of the phase relationships between channels. In these cases, a combination of
the resynchronization methods described above and calibration can be used to derive these
relationships.
USRP B100 and E100 Synchronization – Not MIMO Capable
Like the USRP N200/N210, the USRP B100 and E100/110 can be synchronized with an external
reference and PPS source. However, note this does not imply the USRP B100 and E100/E110 are
MIMO capable in all cases. The flexible frequency clocking architecture used in the USRP B100 and
E100/110 produces phase ambiguity in ADC/DAC sample clock. Therefore, sample edges are no
longer aligned and the requirements for MIMO are not met.
Despite the sample edge misalignment, it is still possible to produce samples with accurate time stamps
for each sample. This is useful for time-difference-of-arrival or similar algorithms.
One exception to these statements is when you make use of a daughterboard containing more than one
channel in a single slot. For example, LFRX/TX, BasicRX/TX, and TVRX2 can all be used to achieve
MIMO capability with a USRP B100 and E100/110. This is because the same clock is used for both
channels.
Synchronization with GPS Disciplined Oscillator
It is also possible to provide synchronization over a wider geographic area with a GPS disciplined
oscillator (GPSDO). A GPSDO derives a 10 MHz and 1 PPS reference from received GPS signals.
The GPSDO is accurate to approximately +/-50 ns across the globe. Ettus Research provides an
optional GPSDO module with the USRP N200/N210. An optional GPSDO module can be integrated
into the USRP N200/N210, and this module can be purchased from Ettus Research.
Figure 1 - GPSDO
MIMO Capability of Ettus Research USRP Devices

Table 1 provides an overview of the synchronization and MIMO capabilities of the USRP product line.
Note the USRP B100 and E100/110 are not shown as MIMO capable. However, some daughterboards
with multiple channels can provide MIMO capability in a single slot.
USRP Model
BW Capability
(MSPS w/
16-bit)
System
Clock
Input
MIMO
Capable
Ext Ref.
Input
1 PPS Input
Internal
GPSDO
Option
Plug and
Play MIMO
USRP1 8 X X
N200 25 X X X X X
N210 25 X X X X X
E100 4 X X X
E110 4 X X X
B100 8 X X
Table 1- USRP Synchronization Capabilities
In order to provide additional translation between the USRP synchronization mechanisms summarized
summarized in Table 1 and the characteristics provided by the daughterboards, Table 2 is presented. In
this matrix, “MIMO” means that both time and sample clock alignment are possible with multiple
USRP devices. “Time” means samples can be provided with known sample times, while “phase”
means the daughterboard local oscillators can generate a phase aligned LO or, in the case of the Basic
and LF boards, there is no local oscillator.
USRP N200/N210 USRP USRP
USRP1
B100 E100/110
SBX MIMO, Time, Phase* Time Time MIMO, Time
WBX MIMO, Time Time Time MIMO, Time
XCVR2450 MIMO, Time Time Time MIMO, Time
DBSRX2 MIMO, Time Time Time MIMO, Time
TVRX2 MIMO, Time Time Time MIMO, Time
BasicRx/TX MIMO, Time, Phase Time Time MIMO, Time,
Phase
LFRX/LFTX MIMO, Time, Phase Time Time MIMO, Time,
Phase
Table 2 - Synchronization Capability vs. Daughterboard and USRP
Table 3 shows options for assembling MIMO systems of various sizes with a USRP device. When
“external” is stated, you should synchronize multiple devices, or pairs of devices, with a distributed
copy of a 10 MHz and 1 PPS signal. This is accomplished with passive or active splitters. This
implementation is discussed in more detail in latter sections.
USRP Model 2x2 MIMO 4x4 MIMO M x N MIMO
USRP1 2x Daughterboard Not Recommended Not Recommended
N200/N210 MIMO Cable MIMO Cable + External External
Table 3- MIMO Implementation Options

N200/N210 – Plug and Play 2x2 MIMO System
The easiest method to implement a high-performance 2x2 MIMO system is to utilize two N200/N210s
synchronized with a single MIMO cable. In this configuration, a single Gigabit Ethernet (GigE)
interface is used to access both USRP devices. The USRP connected to the GigE acts as a switch and
provide data/to from both USRP devices. It will also handle time synchronization of the data so the
synchronization process is transparent.
Consider the throughput capability of the N200/N210s GigE interface in this example. The total
sample rate for USRP devices connected to a single GigE port on the host computer cannot exceed 25
MS/s in 16-bit mode, or 50 MS/s in 8-bit mode.
The UHD API allows you to select synchronization settings. These settings are exposed to the GNU
Radio Companion (GRC) blocks. This allows for a quick, illustrative example of how to use the
N200/N210 MIMO system. GRC is used for the basic illustration of the N200/N210s MIMO
capability.
Figure 2 shows a system receiving data from two unsynchronized USRP devices accessed via two
separate GigE interfaces via external switch. The flowgraph reads the streams from the independent
UHD sources, and plots the real component on a single scope. A 400.01 MHz signal is injected into
both USRP devices, and the real component in shown on a WX GUI Scope. Under ideal conditions,
this yields a constant 10 kHz tone on both channels. Notice in Figure 3 there are obvious phase and
frequency differences between the two signals. This is a result of variations in the two unsynchronized
reference clocks. The frequency difference is caused primarily between differences in the
daughterboard LO. Heat is applied to one of the USRP devices to accentuate the frequency variation
between the two units.
Figure 2- GRC - Un-Synchronized Reception with Two USRP N200/210s

Figure 3- Phase and Frequency Differences in Time Domain Display w/ Asynchronous USRP devices
The plug and play 2X2 system is illustrated in the block diagram below. Notice the relative simplicity
of the system. A MIMO system is created simply by connecting two N200/N210s with a MIMO cable
sharing both the Ethernet connectivity and a single 10 MHz reference. A signal generator drives the
receive inputs of both inputs via a two-way splitter to assist with a demonstration of this system. A
flowgraph showing the GNU Radio connectivity is shown in Figure 5.
RF Signal Generator
2-Way Splitter
USRP N210
MIMO
Cable
USRP N210
1x
GigE
Host PC
Figure 4 - N200/N210 2x2 Reference Design and Test

Figure 5 - GRC Flowgraph of w/ 2X2 MIMO System
A single UHD block is used in the GRC flowgraph. The block parameters are configured to set up a
2x2 MIMO system. The settings of interest are:
Device addr: addr0=192.168.10.2,addr1=192.168.10.3
Sync = don't sync
Num Mboards = 2
Mb0 Clk Src = Default
Mb0 Time Src = Default
…
Mb1 Clk Src = MIMO Cable
Mb1 Time Src = MIMO Cable
These settings configure the first USRP device, Mb0, which is the first device occurring in the address
string to use its default reference for clocking and timing. The second USRP (Mb1) is configured to
accept its frequency and timing reference from the MIMO cable. The signals are provided by Mb0.
All other standard settings such as center frequency and gain assignments apply as well.
Like the first flowgraph shown, the real part of each USRP channel is displayed on a scope. A phase
correction hier block is included to compensate for the random, but constant, phase offset discussed
earlier in this paper. A reference signal of 400.010 MHz is injected into both USRP devices through a
splitter and cables equal length. Figure 6 includes a screenshot of the resultant display both before
phase adjustment. Note the phase is constant and can be easily tuned by hand with the flowgraph
shown.

Figure 6- Time Domain Display Showing Received Signal before Phase Compensation
Figure 7 shows signals from the two synchronized N210s with phase correction applied. In this plot, it
is clear the MIMO connection has enabled the frequency and time references to be synchronized. The
random, but constant, phase offset discussed above is corrected with a complex phase shift in the
flowgraph. In real applications, this phase correction would be implicitly generated with algorithms
such as MRC, or periodic calibration.
Figure 7- Time Domain Display Showing Received Signal with Phase Compensation
This is a simple illustration of the MIMO capability provided by the USRP N200/N210. Keep in mind
in most applications the phase compensation is implemented with some automated process. However,

calibrating the devices with this type of manual correction and a calibration source is certainly an
option. All principles here apply equally to operations in the transmit direction.
Larger MIMO Systems
Utilizing the external reference and 1 PPS inputs, it is possible to build a system larger than 2X2 with
an N200/N210. It is possible to reduce the overall cost of a system by utilizing the plug and play
MIMO capability to create pairs of USRP devices. For example, if you want to build a 4X4 MIMO
system, two sets of N200/N210 pairs can be used. A distribution amplifier can be used to split a
common 10 MHz reference and 1 PPS signal. An illustration of this setup is shown in Figure 8.
USRP N200
MIMO
Cable
USRP N200
USRP N200
MIMO
Cable
USRP N200
Splitter
Splitter
1x GigE
1x GigE
10 MHz
Clock
Source
1 PPS
Source
Gigabit
Ethernet
Switch
Host PC
Figure 8- 4X4 MIMO Configuration
In this case, a pair of signal splitters/amplifiers is used to distribute the reference and 1 PPS signals to
two pairs of N200/N210s. In this testing, Ettus Research used the TADD series from TAPR, which is
no longer produced and not universally compatible to USRP devices other than the N200/N210.
Alternative options are presented later in this document. A low-cost alternative can use a passive RF
splitter for the 10 MHz reference and 74AC series logic for the 1 PPS signal. A GigE switch is used to
communicate with the two pairs of N200/N210s. The constraints of GigE effectively limit the total
sample bandwidth to 25 MSPS in each direction when 16-bit samples are used. This translates to 6.25

MSPS or less per channel. Ettus Research has had success using the EXPI9404PTLBLK 4-Port GigE
card to provide full-rate access to four USRP device simultaneously.
Figure 9 - Ettus Research 4X4 Reference Design
External Distribution Options and Other Notes
There are several options to provide the external distribution required for large MIMO system. The
quality of the distribution component will determine the cost, and ultimately the performance, of the
synchronization. The performance parameters of interest are peak-to-peak timing error, and or/ phase
noise in the case of the frequency reference. Note all cables from the external distribution devices
should connected to their respective USRP devices with matched length cable of the same type and
connectorization.
Possible distribution devices can be seen in Table 4. Other devices can be used, but care should be
taken to meet the input requirements for the associated USRP radio. These requirements are seen in
Table 5. Remember any time a signal pass through a two-way splitter the resultant signals are -3 dB
relative to the original reference.

1 PPS Distribution* Clock Distribution*
Passive 2-Way RF Splitter
Part: ZFSC-2-1W
Cost: ~$50
Manufacturer: Mini-Circuits
74 Series TTL/CMOS
Part: 74AC04
Cost: < $5
Manufacturer: n/a
1:4 TTL Fanout Buffer
Part: PRL-414B
Cost: $415
Manufacturer: Pulse Research
Active, 4-Way 10 MHz Splitter
Part: DEM 10-4
Cost: $75 (assembled w/ enclosure)
Manufacturer: Down East Microwave
Table 4 – Potential Clock and PPS Distribution Options
*Solutions not warranted by Ettus Research. Final selection must be based on users requirements.
USRP Model 1 PPS Specification External Ref. Specification*
N200 3.3-5V 0 to 18dBm
N210 3.3-5V 0 to 18dBm
USRP2 5V 5 to 18dBm
E100 3.3-5V 5 to 15 dBm
E110 3.3-5V 5 to 15 dBm
B100 3.3-5V 5 to 15 dBm
USRP1 – Low-Cost 2X2 MIMO System
Table 5- External Reference and 1PPS Input Specifications
*50 ohm Terminated
It is possible to use the USRP1 as a 2X1 MIMO transceiver in applications requiring a more costsensitive
solution. The USRP1 contains two receive DSP chains and one transmit DSP chain. The
aggregate host bandwidth of the USRP1 is 8 MS/s in 16-bit mode. This limits the practical use of a full
duplex, symmetric, 2x1 MIMO transceiver to 2 MHz of usable bandwidth per channel. The GRC
flowgraph in Figure 10 shows how the USRP1 can be accessed in a MIMO configuration. The key
parameters that need correct settings are:
Num Mboards: 1
Num Channels: 2
Clock Source: Default
Mb0: Subdev Spec: A:0 B:0
The Subdev Spec must contain enough terms to specify the input for each channel. As you access two
channels, you must include to Subdec Spec terms, “A:0” and “B:0.”. This specifies the receive
connection of each of the SBXs used in this particular setup.

Figure 10 - Two Channel MIMO Flowgraph with USRP1
While the USRP1 does provide MIMO capability, it does not have an external reference or 1 PPS input.
Thus, it cannot be easily expanded to a system larger than 2X2. If a system larger than 2X2, or more
than 2 MHz of bandwidth per channel is required, the N200/N210 is the recommended solution.
Conclusion
This application note discusses some of the basic principles associated with MIMO system
implementation using an Ettus Research USRP SDR. GRC flowgraphs are used for a conceptual
demonstration of phase coherency versus phase alignment capabilities. Reference designs for 2X2 and
4X4 systems, and potential distribution elements, are shown. Additional information on how to use the
UHD API to manage synchronization in software can be found at:
http://files.ettus.com/uhd_docs/manual/html/sync.html
We invite and appreciate your feedback, which can be provided to support@ettus.com
References
Mini-Circuits. (n.d.). Mini-Circuits. Retrieved February 15, 2012, from http://www.minicircuits.com/