Page 134 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 2

considered the same with our Design 2. TX digital BF is for P ≥ 15dBm, this design is incapable of guarantee‑

designed for SI minimization from the TX side, whereas ing residual SI power levels at any of the RX RF chains be‑
RX digital BF is given by U , as described in Section 5. low the required = −60dBm. Figs. 4a and 4b demon‑
A

Design 5: SotA with 0 taps/0 AUXTX. This is the SoftNull strate that the proposed multi‑tap‑based design ensures
method presented in [7] that does not adopt analog can‑ that the residual SI power satis ies the constraint for all
A
cellation, relying solely on TX digital BF to reduce SI at the considered TX powers for = 4 and = 8 taps, which
RX antennas of node . Any residual SI is handled by the translates to 25% and 50% less taps compared to the SotA
RX digital combiner. The combiner U used in the previ‑ requiring = 16 taps. In addition, Figs. 4c and 4d show‑

ous designs is used for the latter purpose. case that the proposed multi‑AUX TX solution with = 2
and = 3 AUX TXs is the only one based on AUX TXs that
6.2 Simulation parameters is capable of offering residual SI power below −60dBm
for all P values. Actually, the SotA design with = 4

We have assumed Rayleigh fading and a path loss of 110dB AUX TXs (i.e., with 50% and 25% more AUX TXs than the
for both the DL H , and UL H , channels. The SI chan‑ = 2 and = 3 AUX TXs cases) cannot meet the resid‑
nel H , is assumed to be subject to Ricean fading with ual constraint for P ≥ 15dBm.

‑factor equal to 35dB and path loss of 40dB [31]. All in‑ We now investigate in more detail how our proposed joint
volved wireless channels are assumed to be Independent analog cancellation and BF design adapts in order to meet
and Identically Distributed (IID), and perfectly estimated the constraint on residual SI, while providing spatial re‑
at the receivers (i.e., at the RXs of nodes and ). We have sources for DL and UL communications. Recall that
used 1000 independent channel realizations for all statis‑ used in the precoder solving 3 determines the effective
tical results. The DL transmit power P was set between number of TX antennas used for DL data transmission. An

10dBm and 40dBm, and the UL transmit power P was close to means that the TX BF of the FD node is us‑

set 20dB lower, hence, spanning a range from −10dBm ing more antenna resources for improving DL than for SI
to 20dBm [32]. The noise loor at node is −90dBm and reduction. Therefore, determines the trade‑off between
at node is −110dBm. The latter values are typical ones acceptable SI levels as well as DL and UL achievable rates.
for small cell base stations and mobile terminals. Follow‑ In Figs. 5a and 5b, we illustrate the average values of
ing the indings of [3], we consider a 14‑bit ADC at node chosen by our FD MIMO design as functions of the DL and
that renders digital SI mitigation of approximately 50dB UL TX powers for the case of the multi‑tap architecture
feasible. This means that for the noise loor of −110dBm (for = 4 and = 8 taps) and multi‑AUX TX architec‑
at node the residual SI after analog cancellation (i.e., at ture (for = 2 and = 3 AUX TXs), respectively, and
each RX RF chain’s input) must be less than −60dBm. In for = = 1 and = = 4. From these ig‑

Appendix A we present the two realistic models used for ures we observe that for a given and , the value of

simulating non‑ideal analog canceler hardware. The one increases as the number of taps (or AUX TXs) increases.
model concernsthe proposedmulti‑tap canceler architec‑ For example, in Fig. 5a for = = 1, the values of

ture and the other the multi‑AUX‑TX one. According to for = 8 taps are always larger than the values of for
these models, the multi‑tap canceler is capable of deliv‑ = 4 taps. The more the taps (or AUX TXs), the more
ering approximately 60dB of analog cancellation per tap, analog canceler resources are provided for SI mitigation,
whereas the multi‑AUX‑TX canceler offers approximately and hence, less antenna resources are required for this
35dB of cancellation per AUX TX RF chain. mitigationinordertomeettheresidualSIconstraint. This
is why our algorithm chooses a larger as the number of
6.3 Self‑interference mitigation capability taps (or AUX TXs) increases. Thus, the results in Fig. 5
verify that our FD MIMO design is capable of judiciously
We consider a 4 × 4 FD MIMO node (i.e., = = 4) dividing the burden of SI mitigation between the analog

and two different cases for the number of antennas at canceler and the TX digital BF by taking into account the
nodes and : the single‑antenna case (i.e., = = resources available for analog cancellation.

1) and the multi‑antenna with = = 4. We in‑ AnotherobservationfromtheresultsinFig.5isthatasthe

vestigate in Fig. 4 the probability that the residual SI af‑ number of RX antennas in DL and/or the number

ter analog cancellation meets the constraint of being less of TX antennas in UL increase, our FD MIMO design tends
than A = −60dBm. Results are shown for both pro‑ to be more conservative in the choice of since it chooses
posed multi‑tap and multi‑AUX TX architectures for vari‑ a smaller value for . For example, in Fig. 5a for the case
ous hardware complexity levels, as implicated by differ‑ of = 8 taps, the values of for = = 1 are larger

ent values of for the taps and AUX TXs, respectively. than those for = = 4. The reason for this behav‑

Within Fig. 4, we also sketch results for SotA designs with ior is as follows. Recall that the number of UL streams is
= 16 taps and with = 4 AUX TXs, as well as for the equal to ≤ min{ , }. Since = 4, then as

only digital SotA solution (i.e., 0 taps or 0 AUX TXs). For increases from 1 to 4, there will be more streams in the
the latter design, we have one DL stream for the precoder, UL communications. This increment of UL streams makes
since this was the con iguration yielding the largest SI re‑ the design of TX digital BF more demanding, since it has
duction, however, as shown from all sub igures of Fig. 4, to steer SI away from these several incoming UL streams

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