Signal Processing - January 2017 - 41

that add up constructively on the spots where the terminals are
terminal is deterministic and frequency- (subcarrier-) indelocated and destructively almost everywhere else (Figure 2).
pendent, which greatly simplifies resource allocation
As the ultimately most useful and scalable form of multiuser
problems, and facilitates simple closed-form solutions for
MIMO, massive MIMO departs from conventional MIMO
power control.
technology in several ways:
■ Different base stations do not cooperate, other than for
"slow" tasks such as power control and pilot sequence
■ The acquisition of accurate instantaneous channel state
assignment. Macrodiversity against shadow fading is
information (CSI) at the base station is facilitated through
accomplished by appropriate terminal-to-base station assotime-division duplexing operation and the transmission of
ciation; the large number of spatial degrees of freedom
pilot waveforms by the terminals. On uplink, terminals
guarantees that every base station has room to accommotransmit pilots and payload; subsequently on downlink,
date additional terminals with very high probability.
the base station beamforms to the terminals. Reciprocity
of uplink-downlink propagation is essential to the use
■ By virtue of the large array gain and the ability to null out
of uplink CSI for downlink precoding, and it is achieved
interference, max-min fairness power control is feasible and
in practice through calibration of the
can be exploited to yield uniform quality of
radio-frequency (RF) chains [4]. The
service within each cell.
Massive multiple-input,
reliance on reciprocity permits accurate
multiple-output (MiMO)
channel training even in highway-speed
Communications laboratory exercises:
is currently the most
mobility scenarios.
rf versus acoustic
compelling wireless
■ Extraordinary spectral efficiencies are
Laboratory work that uses communications
achieved [5], although no attempt is made
physical layer technology, over RF in general requires substantial
to operate at the Shannon limit. The base
equipment investments: the availability of a
and a key component
station uses linear signal processing, and
sufficiently interference-free environment
of fifth-generation
the terminal uses almost no signal pro(alternatively spectrum licenses for experi(5g) systems.
cessing at all. CSI is acquired only at the
mental operation); the use of high sampling
base station, obviating the need for
rates, which, in turn, generates large quantidownlink pilots. This renders massive MIMO entirely scalties of baseband data; and the access to high-performance
able with respect to the number of base station antennas.
measurement equipment for calibration and debugging purposes. Owing to precise requirements on the phase reference
■ Each terminal is assigned the full bandwidth. Thanks
distribution, these difficulties are significantly accentuated
to channel hardening, the effective channel gain for each

Figure 1. The massive MIMO laboratory setup at Linköping University. (Photo courtesy of Mikael Olofsson.)
IEEE SIgnal ProcESSIng MagazInE

January 2017

Table of Contents for the Digital Edition of Signal Processing - January 2017