5G与未来网络：HetNets中上下行解耦的关键技术进展

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随着5G及未来网络时代的到来，满足爆炸性增长的数据流量需求成为关键挑战之一。在异构网络（HetNets）中，通过密集部署小型基站（Small Base Stations, SBSs），可以显著提高蜂窝系统的频谱效率和能源效率，从而支撑日益增长的通信需求。然而，HetNets的复杂网络结构对传统的蜂窝系统提出了新的课题，其中用户关联（User Association）问题尤为突出。传统的用户关联策略通常采用耦合接入（Coupled Access, CA），在这种模式下，用户在上行链路（Uplink, UL）和下行链路（Downlink, DL）的接入点通常是相同的，这可能导致资源分配的不均衡，特别是在SBS和宏站（Macro Base Station, MBS）之间。为了克服这一局限，分离接入（Decoupled Access, DA）策略应运而生。DA允许用户在UL和DL时选择不同的接入点，这样不仅可以提升UL性能，还可以根据用户行为动态调整资源分配，实现更有效的网络资源利用。这篇综述论文《5G及未来网络中的下行和上行解耦在异构网络中的研究》由Xin Liu、Ran Li、Kai Luo和Tao Jiang共同撰写，于2018年发表在《通信与信息网络》期刊，DOI号为10.1007/s41650-018-0023-4。文章首先概述了异构网络中DA应用的不同典型场景，包括城市热点区域、郊区和农村等不同地理环境下的用户分布和行为特点。接着，论文深入探讨了DA技术如何实现UL性能的优化，例如通过灵活的功率控制、多级调度以及负载平衡策略来提升信号质量、减少干扰和提高吞吐量。此外，作者还讨论了DA在提升网络覆盖概率、降低功耗和节能减排方面的作用，以及与传统用户关联策略如最大信干比（Maximal Ratio Combining, MRC）和最小路径损耗（Minimum Path Loss, MPL）方法的比较。论文最后总结了当前DA技术的研究进展，并对未来可能的研究方向和技术挑战进行了展望，包括如何更好地处理网络中的移动性和多址接入问题，以及如何在保证服务质量的同时，实现网络的可扩展性和经济性。这篇论文深入剖析了5G及未来网络中下行和上行解耦在HetNets中的核心作用，为优化用户关联策略、提升系统性能和应对未来网络需求提供了宝贵的理论支持和实践指导。

Downlink and Uplink Decoupling in Heterogeneous Networks for 5G and Beyond 3

ε = 1, the pathloss is completely compensated by the power

control and if ε = 0, no channel compensation is applied and

all users transmit with the same power. The symbol X

is the

distance between a user and its associated vBS. The vector

∈ C

×1

is the Rayleigh fading channel between vBS and

a user associated to it. s

is the normalized transmitted signal

of the user. The ﬁrst term represents the received signal while

the second and third terms are the intra-cell interference and

inter-cell interference, respectively. Φ

and Φ

represent the

point processes formed by the thinned PPP of the scheduled

users of the vBSs and the wBSs tier, respectively, n ∈ C

×1

is a vector of the complex additive white Gaussian noise at the

tagged BS.

Nonetheless, the inter-cell interference could also be ig-

nored reasonably for the sake of convenient analysis

[17,18]

UL. Since the MBS tier and the SBS tier operate on differ-

ent frequency bands, inter-tier interference can be eliminated.

Besides, adjacent small cells are assumed to use different op-

erating frequencies, which leads to the fact that no inter-cell

interference exists at SBSs. Moreover, inter-cell interference

at MBSs can be neglected due to the large coverage area of

MBSs and the low transmit power level of users. Therefore,

by neglecting the inter-cell interference in Eq. (2), a simpliﬁed

signal model for MISO is given by

−α

+ n, (3)

where the ﬁrst term

−α

represents the received sig-

nal of the vBS from the user associated to it. Then, the re-

ceived signals will go through the detectors such as maximal

ratio combining (MRC) and zero-forcing (ZF), which is dif-

ferent from that in SISO

[11,19]

C. MIMO

In two-tier multiuser MIMO HetNets, vBSs deploy L

an-

tennas, which is the same as the assumption in MISO HetNets

(see Fig. 2). Nevertheless, instead of serving single user in one

resource block (i.e., MISO), BSs are assumed to serve mul-

tiple users simultaneously in each resource block in MIMO

scenario. Note that each user still associates with one BS in

UL/DL.

Let s

∈ C

(·)

×1

be the normalized transmit signal across

the K

(·)

users, where K

(·)

6 L

) is the number of users

associated with vBSs in UL with (·) = (DA) for DA scenario

and (·) = (CA) for CA scenario. Besides, the set of K

(·)

users

is denoted by ϕ

(·)

. Fast fading is considered here while the

shadowing is neglected for simplicity. Then, the received sig-

nal at vBS, i.e., y

∈ C

×1

, is given as

= H

1/2

∑

j∈Φ

\ϕ

(·)

−α

v, j

+ n. (4)

The term H

1/2

represents the received signal of vBS

from users associated to it where H

∈ C

×K

(·)

is the channel

matrix with each component obeying the Rayleigh distribu-

tion. Moreover, the diagonal matrix Q

with [Q

]

= QX

−α

v,k

is the received signal power of vBS from users associated to

it. The second term on the right hand side is the interference

from all the users associated to other BSs, where X

v, j

is the

distance from vBS to the jth user which is not associated to it.

Besides, the channel vector h

v, j

is an i.i.d. complex Gaussian

distributed vector, representing the channel between vBS and

the jth user. Furthermore, the transmit signal of the jth user

is denoted as s

Since most of the works in DL and UL decoupling in Het-

Nets are based on the system models described above, two

most important UL user association policies for DA will be

introduced.

III. PATHLOSS-BASED

ASSOCIATION POLICIES

In UL, users are associated to the BS which achieves the

highest UL average power. Due to the fact that the fading-

induced variations can lead to ping-pong effects in the asso-

ciation process, the average power is obtained by averaging

over the received signals in Eq. (1) with respect to the fading,

which is given as

v,U

] = QX

−α

. (5)

Then, a user associates with the BS that offers the highest av-

erage power, i.e., the user is associated to MBS in UL if

−α

> QX

−α

. (6)

Otherwise, the user is associated to SBS. The association rule

described in Eq. (6) is called minimum pathloss association

policy.

According to the PDF of X

[20]

and the minimum pathloss

association policy described in Eq. (6), the association proba-

bility of associating to vBS under the minimum pathloss pol-

icy, represented by A

, can be calculated as

+ λ

. (7)

It can be seen that the association probability A

under the

minimum pathloss policy is only related to the densities of

vBSs and wBSs.

Next, with the biased factor considered in the association

policy, i.e., the user is associated to MBS in UL if

Qη

−α

> Qη

−α

, (8)

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