In the following, the hardware and software structure of the test-bed is described. **5.1. Hardware setup**

structure of four-multi test-bed is illustrated in Fig. 3.

*A. Hardware Setup* The current version of the test-bed consists of six nodes where three of them are fixed and take the role of transmitting sources while the other three are movable receiving destinations. All the nodes The current version of the test-bed consists of six nodes where three of them are fixed and take the role of transmitting sources while the other three are movable receiving destinations. All the nodes are equipped with two vertically polarized dipole antennas spaced 20 cm apart which is 1.6 times of the carrier's wavelength. Twelve Ettus Research USRP N210 (see www.ettus.com) are used to govern the twelve antennas in the network. The source USRPs are equipped with the standard Ettus XCVR2450 RF dautherboards while the destination USRPs use custom boards to achieve sufficient noise figure and dynamic range. The output signal of each source USRP is amplified by a ZRL-2400LN power amplifier. Two Linux

are equipped with two vertically polarized dipole antennas spaced 20 cm apart which is 1.6 times

of the carrier's wavelength. Twelve Ettus Research USRP N210 (see www.ettus.com) are used to

govern the twelve antennas in the network. The source USRPs are equipped with the standard Ettus

XCVR2450 RF dautherboards while the destination USRPs use custom boards to achieve sufficient

noise figure and dynamic range. The output signal of each source USRP is amplified by a ZRL-

2400LN power amplifier. Two Linux computers control all the USRPs in the network. The network

The network is designed to work at 2.49 GHz center frequency with 12 MHz bandwidth. Syn-

chronization of the network is performed in three levels, namely time, frequency and transmit-receive

synchronizations. The time and transmit-receive synchronizations are done by means of a pulse-

per-second (PPS) signal (0-5 V, 1 Hz square wave) and a national marine electronics association

(NMEA) signal (an ASCII protocol that provides hour-minute-second time), respectively. Both signals

October 14, 2014 DRAFT

computers control all the USRPs in the network. The network structure of KTH four-multi testbed is illustrated in Fig. 3.

The network is designed to work at 2.49 GHz center frequency with 12 MHz bandwidth. Synchronization of the network is performed in three levels, namely time, frequency and transmit-receive synchronizations. The time and transmit-receive synchronizations are done by means of a pulse-per-second (PPS) signal (0-5 V, 1 Hz square wave) and a national marine electronics association (NMEA) signal (an ASCII protocol that provides hour-minute-second time), respectively. Both signals are generated by an EM406A GPS module and distributed through the network. The frequency synchronization is also performed by helps of 10 MHz reference clocks (CLK). All the source's local oscillators are locked to the same clock while a separate clock is provided for each of the destinations. In a real implementation the same synchronization would be achieved using common control and synchronization channels (cellular systems) or from the burst preambles (wireless local area networks). In a system with interference alignment, transmitter will in any case need some kind of back-haul to provide a common time reference and disperse scheduling decisions.
