  We present the uplink and downlink of a time-division duplex distributed
multiple-input multiple-output (D-MIMO) testbed, based on a 1-bit
radio-over-fiber architecture, which is low-cost and scalable. The proposed
architecture involves a central unit (CU) that is equipped with 1-bit
digital-to-analog and analog-to-digital converters, operating at 10 GS/s. The
CU is connected to multiple single-antenna remote radio heads (RRHs) via
optical fibers, over which a binary RF waveform is transmitted. In the uplink,
a binary RF waveform is generated at the RRHs by a comparator, whose inputs are
the received RF signal and a suitably designed dither signal. In the downlink,
a binary RF waveform is generated at the CU via bandpass sigma-delta
modulation. Our measurement results show that low error-vector magnitude (EVM)
can be achieved in both the uplink and the downlink, despite 1-bit sampling at
the CU. Specifically, for point-to-point over-cable transmission between a
single user equipment (UE) and a CU equipped with a single RRH, we report, for
a 10 MBd signal using single-carrier 16QAM modulation, an EVM of 3.3% in the
downlink, and of 4.5% in the uplink. We then consider a CU connected to 3 RRHs
serving over the air 2 UEs, and show that, after over-the-air reciprocity
calibration, a downlink zero-forcing precoder designed on the basis of uplink
channel estimates at the CU, achieves an EVM of 6.4% and 10.9% at UE 1 and UE
2, respectively. Finally, we investigate the ability of the proposed
architecture to support orthogonal frequency-division multiplexing (OFDM)
waveforms, and its robustness against both in-band and out-of-band
interference.
