D2D卸载效率优化：吞吐量保障与缓冲限制下的最大利用

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本文档探讨了"具有吞吐量保证和缓冲区约束的基于D2D的卸载效率最大化"这一关键主题，发表在2019年1月的IEEE Transactions on Vehicular Technology第68卷第1期上。D2D（Device-to-Device）通信作为一种有前景的技术，旨在缓解蜂窝网络的压力，通过设备间直接通信来提升数据传输效率。对于运营商和用户来说，一个理想的目标是实现高效的卸载率，同时确保用户的基本服务质量（QoS）。当前的解决方案往往侧重于单方面，要么优化卸载效率，要么保证QoS，但很少能两者兼顾。本文作者Yuben Qu、Chao Dong、Haipeng Dai和Qihui Wu针对这一问题提出了创新方法。他们关注的是在有限缓冲容量的情况下，如何最大化移动设备通过D2D进行内容下载的总卸载流量百分比，同时确保每个用户的最低数据传输速率。文章的核心挑战在于如何有效地整合所有潜在的D2D机会，以最大限度地提升整体卸载性能。为此，作者提出了一种空间-时间动态图(STDG)模型，这个模型能够捕捉到D2D通信中所有可能的交互和资源利用情况。通过STDG，研究者可以动态分析和规划D2D通信的节点分布、时间窗口以及数据传输策略，以找到最优的平衡点，使得在满足用户QoS需求的同时，最大程度地提高卸载效率。在这个模型中，作者可能会探讨如何设计有效的匹配算法，以实时找到合适的D2D通信对，避免竞争导致的缓冲溢出，并优化频谱资源分配。此外，可能还会涉及对网络中不同类型的用户（如高优先级、低优先级等）进行分类和差异化处理，以实现公平性和整体性能的最大化。总结来说，这篇研究论文深入剖析了在D2D通信环境中如何通过空间-时间动态图模型来解决吞吐量保证和缓冲区约束之间的权衡问题，这对于未来的无线网络设计和优化具有重要意义，尤其是在5G和物联网(IoT)的背景下，D2D通信将扮演更加关键的角色。

834 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 1, JANUARY 2019

assume that all UEs are rational and motivated as helpers in the

participation of mobile content downloading by using some ex-

isting micropayment schemes [14]–[16]. Also, we suppose that

the role of an user acting as a helper or receiver does not change

during the current content downloading, and it may shift in the

downloading of another content since some previous receivers

may not request the content while some helpers may request it

now. In Fig. 1, there are currently 4 receivers named as r

, r

and r

, and 6 helpers called h

, h

and h

To sum up, there exist three transmission modes in the envis-

aged D2D-based ofﬂoading as follows. 1) Direct cellular trans-

mission: a receiver directly obtains its requested content from

BSs, which also happens between BSs and helpers for relaying

as well as ofﬂoading. For example, in Fig. 1, both receiver r

and helper h

obtain the content from cellular BS b

directly

in frame l

. 2) Connected D2D transmission: the receiver ac-

quires its requested content through a connected path from BSs,

consisting of one or more helpers in its coverage. As depicted

in Fig. 1, in frame l

t+ 1

, r

obtains the content from b

through

a connected D2D path b

→ h

→ r

. 3) Opportunistic

D2D transmission: a helper stores the downloaded content in its

buffer, and then transmits the content to the receivers or other

helpers in future frames. In Fig. 1, h

receives the content from

BS b

in frame l

, and then receiver r

downloads the content

from h

in frame l

t+ 1

, which actually forms an opportunistic

D2D path b

→ h

→ r

. Note that according to our deﬁnition

of frame, any UE’s location and its access relationship is ﬁxed

within a frame, which implies that any established D2D link or

cellular link will not be interrupted during the frame. And it may

be disconnected or maintained at the next frame according to

both the status of the D2D UEs and the order of the cellular BSs.

The traditional D2D technologies are inadequate for the con-

sidered mobile content downloading system, since they are

based on WiFi and Bluetooth that work at 2.4 GHz unlicensed

band [17]. Therefore, following [17], [23], we focus on the

operator controlled D2D communication underlaying cellular

networks, where the D2D UEs use the same licensed band of

cellular BS under the full control of the cellular network. The

main advantage of this kind of D2D lies in that, the devices

can communicate directly with each other under the control of

the cellular network, including resource allocation, access au-

thentication, and connection control [17], [23]. In other words,

with this kind of D2D, the operator can control the communica-

tion process to provide better QoS and make proﬁt accordingly,

which suits for the problem of maximizing ofﬂoading efﬁciency

with throughput guarantee. Besides, since this kind of D2D

communications consumes part of the orthogonal spectrum re-

sources assigned to the cellular networks, it does not interfere

with the cellular communications. However, there may exist in-

terference between two D2D communication pairs, as all D2D

communications in the same cellular coverage work on the same

frequency, to reuse the spectrum resources. Following [23], [25],

we employ the IEEE 802.11 based MAC protocol to effectively

allocate the shared spectrum bandwidth for D2D communica-

tions. The main advantage of this kind of MAC protocol is

that, based on the IEEE 802.11 MAC, the nodes in the same

access domain are strictly not allowed to transmit simultane-

ously to avoid collision or interference. This perfectly matches

the “operator controlled D2D” underlaying cellular networks

we considered in this paper, as the establishment as well as

disconnection of all D2D links can be controlled by the cellular.

Thus, the interference between D2D communications can be

reasonably managed.

B. Problem Statement

In this study, we strive to maximize the ofﬂoading efﬁciency

via D2D while taking into account the minimum throughput re-

quirement of receivers, limited buffer size of helpers and other

system constraints. The system constraints mainly include the

constraints of ﬂow conservation, interference, and spectrum al-

location. To achieve the objective, UEs are expected to partici-

pate in more opportunistic D2D transmissions, which requires

us to capture all the potential D2D opportunities. In Section III,

based on the knowledge of UEs’ mobility trajectories and de-

ployment of BSs, we ﬁrst build a spatial-temporal dynamic

graph to model the transmission evolution in the dynamic

D2D-based ofﬂoading, which captures all the possible transmis-

sion opportunities during the content downloading duration. In

Section IV, based on the graph, we mathematically derive the

objective function as well as other involved system constraints.

III. S

PATIA L-TEMPORAL DYNAMIC GRAPH MODELING FOR

D2D-BASED OFFLOADING

Now, we employ the dynamic graph modeling method [18],

[19] to model the D2D-based ofﬂoading process in mobile con-

tent downloading. Speciﬁcally, we construct a spatial-temporal

dynamic graph (STDG) to capture all possible transmission op-

portunities of the aforementioned three different transmission

modes evolving in time. Assume that there are B cellular BSs

and U mobile UEs in the network. The sets of BSs and UEs are

denoted by B = {b

, ..., b

} and U = R



H, respectively,

where R = {r

, ..., r

} and H = {h

, ..., h

} represent

the receiver set and helper set, respectively. For convenience, U

is also denoted as {u

, ..., u

} to represent all UEs.

To describe the UEs’ time-varying communication opportu-

nities as well as their dynamic accessing relationships with the

BSs, we build the STDG, which is a time-expanded graph, based

on the UEs’ mobility trace and BSs’ deployment. To generate

the STDG, we ﬁrst need to identify the communication con-

tact events between any pair of nodes, including BS and UE,

helper and helper, and helper and receiver. The contact events

can be categorized into the four types as follows. 1) Cellular link

starts: the time when an UE establishes a cellular link with a BS;

2) cellular link ends: the time when an UE disconnects a cellular

link with a BS; 3) D2D link starts: the time when two neighbor-

ing UEs establish a D2D link; 4) D2D link ends: the time when

two UEs that have been in the D2D communication state break

the established D2D link.

According to the above communication contact events, which

sequentially occur at the different time τ

,τ

, ..., τ

, we parti-

tion the system time into several sequential frames l

, ..., l

where l

is deﬁned as the time duration between τ

t−1

and τ

t = 1, ..., T , and l

is deﬁned as the time before τ

. Based on the

deﬁnition of the frame, none of the four contact events occurs

during each frame. In other words, both the states of the cellular

BSs and UEs and the communication links remain unchanged

within each frame; otherwise, the system transforms into the

next frame. We also use t to denote the frame l

for brevity in

the rest of the paper.

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