  This paper sets up a framework for designing a massive multiple-input
multiple-output (MIMO) testbed by investigating hardware (HW) and system-level
requirements such as processing complexity, duplexing mode and frame structure.
Taking these into account, a generic system and processing partitioning is
proposed which allows flexible scaling and processing distribution onto a
multitude of physically separated devices. Based on the given HW constraints
such as maximum number of links and maximum throughput for peer-to-peer
interconnections combined with processing capabilities, the framework allows to
evaluate modular HW components. To verify our design approach, we present the
LuMaMi (Lund University Massive MIMO) testbed which constitutes the first
reconfigurable real-time HW platform for prototyping massive MIMO. Utilizing up
to 100 base station antennas and more than 50 Field Programmable Gate Arrays,
up to 12 user equipments are served on the same time/frequency resource using
an LTE-like Orthogonal Frequency Division Multiplexing time-division
duplex-based transmission scheme. Proof-of-concept tests with this system show
that massive MIMO can simultaneously serve a multitude of users in a static
indoor and static outdoor environment utilizing the same time/frequency
resource.
