利用D2D链路支持低速率MTC：优化网络协作与效率

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本文主要探讨了在无线蜂窝网络中，两个关键的技术趋势——直接设备对设备（Device-to-Device, D2D）通信和机器类型通信（Machine-Type Communications, MTC）的融合。D2D通信通过利用无线设备之间的直接连接，重新利用现有的蜂窝频谱和无线电接口，旨在提高网络效率并降低数据传输的延迟。而MTC则聚焦于大量低速率、低功耗设备（Machine-Type Devices, MTDs）的接入，这些设备对网络提出了新的挑战，如边缘设备可能因为功率限制而面临通信中断（outage problem），以及大规模接入可能导致无线电接口过载（massive access issue）。在处理这些挑战时，作者提出了一种利用D2D链接支持MTC的新策略。传统上，MTC流量通常通过直接连接到基站传输，但通过D2D作为中继，可以间接减轻基站的压力。MTC被模型化为固定速率的流量，并对数据传输的可靠性（outage requirement）有着严格的要求。为了实现这一目标，文中提出了两种网络辅助的D2D合作方案。首先，采用了机会性干扰取消（Opportunistic Interference Cancellation）技术，通过智能地管理不同设备间的通信，避免或减少对其他用户的干扰，从而确保MTC服务的稳定性和高可靠性。其次，借鉴认知无线电（Cognitive Radio）的底层原理，允许MTDs在不引起严重干扰的前提下共享频谱资源，进一步提升了通信效率。通过理论分析和数值结果，研究者展示了这两种D2D方案如何有效地缓解MTC带来的问题，同时保持了其他宽带服务的速率最大化。这表明，通过巧妙地集成D2D通信，可以优化无线网络资源分配，满足MTC的需求，提升整体网络性能。因此，D2D通信技术对于解决MTC中的挑战，以及实现无线网络的高效和可持续发展具有重要意义。

U2U1

M1-U1

U1-B B-U2

U2-M2

M1-U1 U1-B

M1-U1 + U1-B

U1-B

U1-B + U2-M2

U2-M2

R.1

U.1

R.2

U.2

(a) (b)

TS1 TS2

Fig. 2. System model. (a) the four possible information ﬂows; (b) The

coupling and respective reference schemes duration in number of time slots.

III. SYSTEM MODEL

Figure 2(a) shows the network topology that captures the

quintessential example of the transmission schemes proposed

in this paper. The links will be denoted by X-Y, where it is

understood that X is the transmitter and Y is the receiver. All

nodes are half-duplex, i. e. a node cannot transmit and receive

at the same time. These network nodes are characterized as

follows:

• A Base Station (B) capable of adapting the transmission

rate and multipacket reception through Opportunistic

Interference Cancellation (OIC) [5];

• Two normal devices (U1 and U2) capable of adapting the

transmission rate and multipacket reception through OIC;

and

• Two MTDs M1, M2 with low complexity, unable to per-

form advanced processing, such as multipacket reception.

They use low power and a low, ﬁxed rate R

The main ingredient of the proposed scheme is the underlay-

ing i. e. simultaneous activation of the links M1-U1 and B-U2.

Clearly, the underlaying would work regardless whether the

transmission B-U1 carries data for U1 only or it also contains

data that needs to be relayed over the link U2-M2 at a future

point. Another underlaying occurs between the links U1-B and

U2-M2. If the data sent from U2 is only relayed data that has

been previously received from B, then the signal transmitted by

U2 is completely known by B and can be canceled, such that B

can receive the transmission from U1 over a “clean channel”.

Such schemes have been treated in [44]–[49]. However, here

we leave the opportunity that U2 sends to M2 data that may

have originated at U2 or at another MTD connected to U2,

such that the signal sent by U2 cannot be assumed a priori

known by B. Therefore, B should try to decode the signal from

U2 if it needs to cancel this interference.

We propose a network-assisted underlaying solution that,

based on the collected Channel State Information (CSI) and

machine link outage requirements, selects the transmission

power and maximal rate of the transmitters at cellular links

U1-B and B-U2. The links M1-U1 and U2-M2 are where

the MTC occur. Any transmission to/from MTD is done over

a D2D link and at a ﬁxed rate R

, with a requirement on

the maximal outage probability of P

Out

. On the other hand,

the links U1-B and B-U2 adapt the rate/power to the channel

conditions.

Fig. 2(b) shows the proposed underlayed transmissions and

the respective reference schemes, along with the required num-

ber of Time Slots (TS). The total duration in time slots of each

of these schemes is denoted as a scheduling epoch. In the ref-

erence schemes the underlaying of a link with another does not

take place. All the transmissions have normalized bandwidth

of 1 Hz, therefore the duration of a time slot can be measured

in terms of the number of symbols N . The analysis here is

done by considering that all communications are performed

using capacity-achieving Gaussian codebooks. Furthermore, at

most two simultaneous transmissions are considered over the

multiple access channel, the other transmissions are modeled

as noise. The receivers that apply OIC are aware of the

codebooks used by the transmitters. We note that for MTC,

the assumptions of sufﬁcient large N and Gaussian codebooks

may not hold, since the MTC messages can be very short and

therefore ﬁnite block length theory should be used instead in

their characterization [50]. In that sense, the results shown in

this paper are optimistic, but the approach is valid and the

follow-up work can rely on the ﬁnite block length theory.

The underlaying (U.x) and respective reference schemes

(R.x) are differentiated as follows:

• (R.1): Due to the absence of underlaying, outage in the

link M1-U1 occur only due to deep fades that render R

undecodeable;

• (U.1): The link M1-U1 underlays B-U2. An outage can

occur both due to a deep fade on M1-U1 as well as due

to the interference from B’s transmission to U2. Device

U1 can use OIC to cancel out the interference from B-U2

and thus decode M1-U1;

• (R.2) - similar to (R.1), outage occurs only due to a deep

fade on U2-M2 and undecodeability of R

;

• (U.2) - the link U2-M2 underlays the link U1-B. An

outage can occur both due to deep fade on U2-M2 as well

as due to the interference from the transmission from U1

to B. The Base Station B can use OIC to cancel out the

interference from U2-M2 and afterward decode U1-B;

The rate/power applied on the links U1-B and B-U2 are

decided at the network infrastructure side; therefore the term

network-assisted. In this particular example, we consider that

the decision is made at B and communicated to the network

nodes using the appropriate signaling, but the details of it

are outside the scope of this work. Therefore B needs to

know the CSI of possibly all communication links. This

information can be available in the form of instantaneous CSI

or statistical/average CSI. Here we consider two different cases

regarding the CSI availability. Fig. 3(a) denotes full channel

state information (F-CSI), where B is aware of the instan-

taneous channel realizations for all links. Fig. 3(b) denotes

partial CSI (P-CSI), where B is aware of the instantaneous

realization of the channel for the links between B and U1 and

U2, while for each of the links between Mi-Ui, Mi-Uj, B it

knows the average SNRs

We note that the implementation of a signaling procedure that enables

F-CSI is expensive in terms of the required signaling overhead. Nevertheless,

we do consider it here as it provides a stepping stone to obtain the P-CSI

underlaying procedure.

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