Page 34 - ITU Journal, Future and evolving technologies - Volume 1 (2020), Issue 1, Inaugural issue

ITU Journal on Future and Evolving Technologies, Volume 1 (2020), Issue 1

2. BENEFITSAND WEAKNESSESOF THE CO‑ signal and low‑frequency noise when a sinusoidal signal
HERENT AND NON‑COHERENT DEMOD‑ plus noise is applied to several types of detector. It is
ULATION SCHEMES shown that a considerable gain may be obtained by using
the (new at that time) coherent detector as compared to
2.1 Coherent demodulation schemes (CDS) the non‑coherent square‑law detector when the input sig‑
nal to noise ratio (SNR) is low. In [8], a complete theory
Well‑known coherent detection requires a replica of the of detection is presented for threshold reception, which
carrier at the receiver, with frequency and phase synchro‑ requires either a suitably weighted cross‑correlation of
nized, with the transmitted one, and an estimation of the the received data with the a priori known signal (CDS), or
channel attenuation and phase. Then, the received signal a suitably weighted autocorrelation of the received data
and a replica of the received version of all possible trans‑ with itself (NCDS). The Kineplex system developed by
mitted signals can be cross‑correlated to make a decision. Collins Radio Company introduced the technique of dif‑
CDS are widely used by many communication systems. In ferential phase shift keying (DPSK), as described in [9].
particular, they are used in 5G [1], where the advantages Today, NCDS have been re‑proposed as an alternative to
of MIMO‑OFDM are exploited, providing a high through‑ the traditional CDS due to the fact that they are able to
put through the use of the well‑known ‑ary quadrature recover the transmitted symbols without any CSI, that is,
amplitude modulation (QAM). With this modulation for‑ knowledge of the amplitude and phase of the carrier is not
mat, the information is transported in both the amplitude required. Hence, reference signals are no longer needed,
and phase of the carrier, making an ef icient use of the reducing the undesirable signalling overhead. This effect
transmission channel. However, these bene its come at is more relevant for high mobility and/or very frequency‑
the expense of transmitting some reference signals in or‑ selective scenarios. Additionally, the complexity of the
der to obtain accurate enough CSI, so that the effects pro‑ transceivers is signi icantly reduced. Typical approaches
duced by the propagation channel to the received symbols involve the detection of the signal energy of phase dif‑
can be equalized before a decision. When the channel is ferences. Despite its simplicity, non‑coherent detection
frequency‑selective, OFDM facilitates the implementation usually implies a 3 dB loss in SNR as compared to CDS.
of CDS due to the fact that each subcarrier can be consid‑ For this reason, it has just been used in a few communi‑
ered as having an independent lat‑fading channel, reduc‑ cation systems where low complexity was a primary re‑
ing the complexity of the equalization. quirement. Examples of application are Bluetooth [25],
The need to obtain accurate enough CSI is accepted in with a non‑coherent frequency shift keying (FSK) receiver
most communication systems, in particular when the or Zigbee [26], using DPSK.
channel impulse response remains quasi‑static for a cer‑ Differential modulation is one of the most frequently used
tain period of time and the number of antennas is not techniques for NCDS [12] ‑ [19]. In this case the infor‑
very large. Under these conditions, a reduced amount mation is encoded by computing the phase difference be‑
of reference signals are used in order to track the chan‑ tween the current complex data symbol and the previ‑
nel variations in time, frequency and space dimensions. ously transmitted symbol. At the receiver, a simple dif‑
On the other hand, if we would like to provide commu‑ ferential decoder is required, detecting the phase differ‑
nications in new challenging environments, such as high ence betweentwo contiguoussymbols. To applythis tech‑
speed trains, autonomous vehicles, etc. these are mainly nique, the constellation is constrained to have a constant
characterized by a signi icant Doppler spread due to the modulus, such as DPSK, and a single reference symbol is
high mobility. In these situations, the traditional CDS re‑ needed at the beginning of each stream to have an ini‑
quires an enormous amount of reference signals in order tial phase reference. This means a negligible overhead to
to continuously and accurately track the variations of the the system. It is also required that the channel response
channel, reducing considerably the overall ef iciency of of every two contiguous differential symbols should be
the system, as pointed out in [10], [11], [20]. Otherwise, if very similar, otherwise the differential decoder is not able
the CSI is not properly estimated, the performance of the to successfully recover the transmitted information data.
CDS is also seriously compromised. Moreover, when mas‑ This condition is usually met, even in fast varying chan‑
sive MIMO is considered, the procedures of channel esti‑ nels.
mation and the computation of the pre/post‑coding ma‑ Recently, the combination of NCDS with massive MIMO
trices may increase the complexity of the system. For ex‑ has been proposed in order to improve its performance
ample, the channel inversion of large dimension matrices leveraging the high number of antennas. In the context
for each subcarrier may be prohibitive for some real‑time of UL, [10], [11] showed that asymptotically NCDS can
applications when a zero‑forcing (ZF) criterion is chosen. achieve the same performance as CDS. Nevertheless, the
proposed technique that illustrated this idea required a
2.2 Non‑Coherent demodulation schemes very large number of antennas to get an acceptable per‑
(NCDS) formance. Then, [12] ‑ [16] proposed the use of DPSK to‑
gether with an averaging process performed at the base
Looking back in history, NCDS are older than CDS. In [7], station (BS) over the spatial domain after non‑coherent
a comparison is made of the output spectra comprising demodulation, in order to mitigate the effects of the chan‑

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