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Journal of Communications and Information Networks
4
2.3 System architecture
Considering the unique characteristics of the
mmWave frequency, an advanced RF transceiver
architecture is needed to improve the system speed,
energy efciency, and spectral efciency. With the high
integration level in the chip and very compact antenna
array, it is suitable to realize a mmWave system using
MIMO and beamforming techniques
[15-18]
.
As shown in Fig.2 and Fig.3, for mmWave comm-
unication, there are two typical transceiver
architectures, namely direct conversion and dual-
conversion
[9,19,20]
; both of these have their advantages
and disadvantages. Direct conversion, which is
popular in GHz RF radio, can achieve compact and
low power implementation, as well as operation
exibilities. However, for mmWave applications, its
high-frequency quadrature modulation/demodulation
typically gives rise to severe distortion, such as I/
Q imbalance and LO (Low Oscillator) feedthrough.
Moreover, the indispensable quadrature PLL (Phase-
Locked Loop) has introduced several implementation
issues. These are the trade-off between the phase
noise, tuning range, and phase error, as well as the
frequency pulling/pushing caused by the power
amplier
[20]
.
To overcome the above-mentioned design issues
and improve the SNR, an alternative solution is to
use the dual-conversion architecture (also referred to
as sliding-IF), as shown in Fig.3. In this architecture,
the quadrature modulation and demodulation working
frequency is significantly reduced. Moreover, the
tuning range and operating frequency of the LO are
reduced, simplifying the system design. Therefore,
the sliding-IF architecture is very popular in mmWave
systems
[19, 21-23]
.
Combined with the beamforming technique, based
on the above transceiver architecture, we can realize
the phased-array system, thus achieving beam-
scanning capability, which is good for LOS and
NLOS communication scenarios. With the above
features, mmWave wireless access and wireless
backhaul can be simultaneously supported, increasing
the system exibility
[2-4]
. Moreover, with a large-scale
antenna array, we can realize a large antenna array
gain, compensating the large path loss associated
with high frequency and increasing the coverage
Accordingly, we can establish a stable wireless link.
To be more specific, from an implementation
perspective, mmWave can achieve a high directivity
using a large array antenna, and the actual power
that is required to deliver a certain EIRP (Equivalent
Isotropic Radiated Power) is significantly reduced.
Accordingly, the requirements on each RF front-
end component, such as the antenna gain and PA
output power, can be relaxed, thereby increasing the
system energy efciency. On the other hand, we can
achieve a small beam, increasing the signal spatial
isolation and simultaneously accommodating many
co-channel users. In other words, a kind of spatial
ltering on the co-channel signal is achieved, and the
spectrum efciency can be improved. Combined with
the mmWave large path loss feature, the interference
can be relaxed and the frequency can be reused by
using small-cell techniques, as well as increasing the
spectrum efficiency and communication capacity.
Figure 2 Direct-conversion architecture
Figure 3 Dual-conversion architecture