The Eurecom MIMO Openair Sounder (EMOS) is a part of the openairinterface platform which allows real-time MIMO channel measurements synchronously over multiple users moving at vehicular speed. EMOS consists of a base station (BS) that continously sends out a signalling frame, and one or more users that receive the frames to estimate the channel. Currently, for the BS a workstation with four PLATON data acquisition cards is employed. The PLATON boards are used along with a Powerwave 3G broadband antenna (part no. 7760.00) composed of four elements which are arranged in two cross-polarized pairs. The UEs consist of a laptop computer with a CardBus MIMO I data acquisition card and two clip-on 3G Panorama Antennas (part no. TCLIP-DE3G). Sounding Signal The EMOS is using an OFDM modulated sounding sequence. The duration of one transmit frame is 2.667 ms and it consists of a synchronization symbol (SCH), a broadcast data channel (BCH) comprising 7 OFDM symbols, a guard interval, and 48 pilot symbols used for channel estimation (see Fig. 5). The pilot symbols are taken from a pseudo-random QPSK sequence defined in the frequency domain. The subcarriers of the pilot symbols are multiplexed over the four transmit antennas to ensure orthogonality in the spatial domain. The BCH contains the frame number of the transmitted frame that is used for synchronization among the UEs. Fig. 5: Frame structure of the OFDM Sounding Sequence. Channel Estimation Procedure
Each UE first synchronizes to the BS using the SCH. It then tries to decode the data in the BCH. If the BCH can be decoded successfully, the channel estimation procedure is started. The channel estimation procedure consists of two steps. Firstly, the pilot symbols are derotated with respect to the first pilot symbol to reduce the phase-shift noise generated by the dual-RF CardBus/PCMCIA card. Secondly, the pilot symbols are averaged to increase the measurement SNR. The estimated MIMO channel is finally stored to disk. Multi-user Measurement Procedure In order to conduct multi-user measurements, all the UEs need to be frame-synchronized to the BS. This is achieved by storing the frame number encoded in the BCH along with the measured channel at the UEs. This way, the measured channels can be aligned for later evaluations. The frame number is also used to synchronize the data acquisition between UEs. One measurement run (file) starts every 22.500 frames (60 sec) and is exactly 18.750 frames (50 sec) long. Picture Gallery Fig. 1: Server PC with PLATON boards Fig. 2: Powerwave Antenna on their pylone Fig. 3: Dual-RF CardBus/PCMCIA Card 
Fig. 4: Panorama Antennas | 
