Page 39 - 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

the beamforming, any other additional overhead used 5.1 Simulation parameters
in the CDS, such as the demodulation‑reference signals
(DM‑RS), is avoided, increasing the spectral ef iciency. To show some illustrative results, the numerology of the
OFDM signal is chosen according to 5G [1]. The carrier
spacing is Δ = 30 KHz, which is the most frequent value
4.2 Frequency diversity
in different bands [35]. The bandwidth of the system is
Due to the usually limited number of antennas at the UE, = 100 MHz and the carrier frequency is = 3.5

averaging in any dimension other than space (e.g. in time GHz. The BS is equipped with a uniform linear array
or frequency) is proposed in [19] to provide an additional (ULA) of = 128 antenna elements, which are simul‑
source of diversity. This diversity is needed in order to taneously serving two UEs ( = 2) in the whole band‑
obtain the required SINR gain for a good performance of width. Their angular separation corresponds to 72° and
the NCDS [12]. It is particularly needed if we want to mul‑ the path loss is not considered, since the power control is
tiplex several UEs in the constellation domain or enable assumed to work perfectly. We adopt a geometric chan‑
services that are critical in terms of performance. The use nel model, which corresponds to a spatially correlated
of the frequency dimension is described in [17], with the channel, where the power delay pro ile corresponds to
advantage that each OFDM symbol can be independently the Type B given in [35], the delay spread is = 16
processed. The proposed scheme can be easily extended ns and the angular spread is = 5°. We assume that
to averaging either in time (processing several consecu‑ there is a Doppler shift of = 1.6 KHz which corre‑
tive OFDM symbols) or space (increasing the number of sponds approximately to a speed of = 500 km/h at the
receive antennas of the UE, when possible). mentioned carrier frequency. We assume a perfect time‑
The way to leverage frequency diversity consists in trans‑ frequency synchronization and power control at the re‑
mitting the same differential complex symbol in several ceiver. For the sake of space we do not provide any re‑
frequency resources. At the transmitter, after performing sults for a higher carrier frequency. However, the cho‑
the differential encoding, the differential symbols are sen delay and angular spread can be also representative
repeated as of the propagation at higher frequencies, and the same
Doppler frequency would correspond to a smaller speed.
= ∣ = mod ( − 1, ) + 1, = × , Hence, the conclusions obtained with these numerical re‑
,
,
(11) sults, in particular those including beamforming (which
would be mandatory to compensate the path loss), can
1 ≤ ≤ , 1 ≤ ≤ , 1 ≤ ≤ ,
be extrapolated to other higher frequency bands, such as
where is the frequency repetition/averaging factor. mm‑Wave [36]. The SNR is conventionally de ined as the
At the receiver, analogously to ((6)), the frequency diver‑ ratio of the received signal power over the noise power at
sity is exploited in the non‑coherent detection, where the each antenna of the receiver.
received data in the subcarriers that carry the same trans‑ For a baseline CDS system to compare the performance,
mitted data are averaged as we adopt the pilot con iguration speci ied for the demod‑
ulation reference signals (DM‑RS) in 5G [1]. In the time
1 −1 ∗ domain, due to the high mobility, we set four reference
, = ∑ ( ) , (12)
−1+ , + , symbolsforeachslot, whichcorrespondstothemaximum
=0
pilot density allowed by the standard. In the frequency
domain, we assume the con iguration Type‑1, where each
2 ≤ ≤ , 1 ≤ ≤ . half of the subcarriers are allocated to each UE: the even
subcarriers are used for the channel estimation of the UE
With this scheme there is a trade‑off between overhead 1
and robustness. We will see that for some particular sce‑ and the odd subcarriers are for UE . At the receiver, the
2
narios with high mobility, even with the added overhead, channel estimation is irstly obtained at the pilot symbol
this scheme will outperform the CDS in terms of through‑ resources by applying least squares (LS) [37]. Then, these
put. estimates are interpolated to the entire resource grid by
In Fig. 6 the block diagram of the system proposed in [19] using spline interpolation.
is shown, combining the beamforming with the NCDS. At Moreover, some hardware impairments are also consid‑
the receiver, assuming single‑antenna devices, only fre‑ ered, such as the effects of PN and the non‑linear HPA. The
quency diversity is exploited in order to reduce the noise effect of the PN is due to the instability of the local oscil‑
and MUI terms. lators, that can only be reduced by making a more expen‑
sive one. The PN is typically modelled according to a clas‑
sical Wienner random walk process given in [23], where
5. PERFORMANCE DISCUSSION
the system performance is related to the phase noise in‑
2
In this section we illustrate the performance of the com‑ crement variance over the sample period , where

bination of NCDS with a large number of antennas by dis‑ 2 = 2 , with equal to the 3‑dB bandwidth of

cussing some numerical results. A comparison with some the Lorentzian power density carrier spectrum. The neg‑
CDS counterparts is also provided. ative effect of the PN not only will degrade the received

© International Telecommunication Union, 2020 19