基于OFDMA的中继网络分布式功率优化与用户公平性提升

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本文主要探讨了"中继功率分配"在OFDMA（正交频分多址）基干的接力网络中的应用，由IEEE Transactions on Vehicular Technology, Vol. 60, No. 3, March 2011发表。作者Yuwen Pan、Andrew Nix和Mark Beach（IEEE成员）提出了几种分布式资源分配方案，旨在解决基站边缘小区用户信号覆盖的问题。在现代无线通信系统中，中继节点作为增强网络覆盖和容量的关键组件，其功率分配策略对系统的性能至关重要。该研究针对固定中继站，利用认知无线电技术来优化资源利用，通过迭代的水填充（Iterative Waterfilling）算法进行功率分配，这有助于提高频谱效率并实现用户公平性。结果显示，通过这些迭代方法，可以达到在研究应用场景下的最优功率分配。此外，为了进一步提升频谱利用率，文中引入了功率重分配算法。针对基站到中继链路质量较差导致的数据速率限制，提出了一种新颖的迭代屏障约束水填充（Iterative Barrier-Constrained Waterfilling）算法，它不仅解决了速率瓶颈，而且具有快速收敛特性。这种算法设计考虑了分布式网络的实施，有效降低了计算复杂度，使得整个系统的运行更为高效。文章还涉及了分布式子载波和功率分配的优化，强调了在实际网络环境中，通过网络层面的协同工作，能够实现更灵活且高效的资源管理。这种分布式方法使得网络能够根据实时条件动态调整，提高了整体的通信质量和可靠性。这篇论文为OFDMA基干的中继网络提供了一套完整的、高效的功率分配策略，对于提升无线通信网络的性能和扩展边缘覆盖具有重要意义。通过理解和应用这些理论，电信运营商和工程师们可以更好地设计和优化未来的无线通信系统，以满足日益增长的带宽需求和用户体验。

PAN et al.: DISTRIBUTED RESOURCE ALLOCATION FOR OFDMA-BASED RELAY NETWORKS 921

allocation. Section VII evaluates the performance of the

suggested algorithms. Finally, conclusions are presented in

Section VIII.

II. S

YSTEM MODEL

This paper investigates the problem of resource allocation in

an OFDMA-based relaying network. In particular, the OFDMA

downlink transmission is studied for a single BS cell. The

system consists of a single BS, K mobile users or UE terminals,

and L RSs. Each radio unit is equipped with a single antenna

[see Fig. 1(b)].

The relaying scenario operates as follows. The BS broadcasts

OFDMA s ymbols, and the RSs decode the symbols and retrans-

mit them to the UEs (using a DF relaying strategy) in a different

frequency band from the BS broadcast channel. Two-hop trans-

mission is considered, as shown in Fig. 1(b). The layout of the

transceivers in this ﬁgure is only for illustration. The RSs select

and allocate different subcarriers and power levels for each UE

to utilize the multiuser diversity present in the network. Perfect

knowledge of the channel-state information (CSI) is assumed at

the receivers, and this information is fed back to the RSs. The

RSs communicate with the UEs after the subcarrier allocation

has occurred. This condition ensures that the UEs can combine

the relayed symbols and the symbol that was s ent by the BS

using maximum-ratio combining (MRC). This paper focuses

on distributive resource allocation schemes for the RSs. The

subcarrier allocation decision at the BS is assumed not to be

affected. Therefore, the RSs can join or leave the system at

any time and improve the system performance without causing

instability at the BS.

TheBSisassumedtohaveN data subcarriers numbered

from 1 to N in the frequency domain. There are L RSs, and

each RS l has N

data subcarriers, where l ∈ [1,L], and N

≤

N. The subcarrier allocation results are mapped by Ω

(n),

where l ∈ [1,L], n ∈ [1,N], and Ω

(n) ∈ [1,N

]. For example,

(n)=n

means that the n

th subcarrier at the lth RS is used

to relay the data symbol carried by the nth subcarrier of the

BS. l =0 represents the BS, and consequently, Ω

(n)=n.

For a multiuser system, the subcarrier allocation at the BS

for multiple UEs is mapped by U(n), where n ∈ [1,N], and

U(n) ∈ [1,K]. U(n)=k means that the nth subcarrier of the

BS is used to transmit data for UE k. Ω

(n)=0represents the

situation where RS l does not relay for subcarrier n of the BS,

perhaps because the link quality of subcarrier n from the BS to

RS l is low, or the total number of subcarriers at RS l is less

than N.

One example is shown in Fig. 2. The BS has eight subcarriers

numbered from 1 to 8. Ω

(3) = 1 means that the ﬁrst subcarrier

of RS 1 is allocated to relay for the third BS subcarrier, and so

on. Because the third BS subcarrier transmits data for UE 2, i.e.,

U(3) = 2, the ﬁrst subcarrier at RS 1 will relay data f or UE 2. In

this example, for RS 1, the values of Ω

(4), Ω

(6), and Ω

(8)

are all 0, because RS 1 does not relay for these subcarriers.

)

denotes the power allocation at the n

th subcarrier of

RS l, where l ∈ [1,L] and n

∈ [1,N

]. The channel gain from

the n

th subcarrier of RS l to UE k is represented by h

)

l,k

where l ∈ [1,L], k ∈ [1,K], and n

∈ [1,N

]. To simplify the

Fig. 2. Example of subcarrier allocation. The horizontal direction of tiles

lists the active subcarriers in sequence. Note that the alignment of tiles in the

horizontal direction does not refer to spectrum overlapping.

notation, p

(n)

and h

(n)

0,k

are used to represent the power al-

location and the channel gain at the BS, respectively, where

n ∈ [1,N]. The total bandwidth for N subcarriers is assumed to

be B, and the bandwidth for each subcarrier is B/N. Noise at

each subcarrier is modeled as a zero-mean circular symmetric

complex Gaussian sample with variance N

B/N.

III. I

TERATIVE WATERFILLING FOR OPTIMAL

POW E R ALLOCATION

The resource allocation problem deﬁned in the previous

section is a mixed integer and continuous variable optimization

problem that is generally difﬁcult to solve. In fact, even the

subcarrier allocation problem alone (with ﬁxed-power alloca-

tion, e.g., equal) can be viewed as a multidimensional weighted

matching problem and is NP-complete [20]. To simplify the

problem, in this section, we ﬁrst investigate the power alloca-

tion problem with the following assumptions.

1) The frequency spectrum utilized at each RS does not

overlap with that used by the other RSs. With this as-

sumption, the UEs can separately detect the signals from

the BS and all RSs and then combine them using MRC.

In Section IV, it will be shown that the optimal power

allocation can still be approximated when this assumption

is removed.

2) If Ω

(n) =0, the SNR from the BS to RS l at subcarrier n

is assumed to be high enough such that RS l can correctly

decode the symbol, because the geographical placement

of the RSs can ensure that this condition is met. This

assumption ignores the channel constraints from the BS

to the RSs. This case is valid in several situations where

the ﬁxed RSs are normally in the line of sight (LoS) of the

BS, and it is possible to deploy multiple receive antennas

at the ﬁxed RSs to improve the SNR. Alternatively, the

RS can simply choose not to relay if the SNR between

the BS and the RS is low. Conditions for removing this

assumption are investigated in Section V.

3) The subcarriers are assumed to have been allocated, and

the objective now is to ﬁnd the optimal power alloca-

tion for the RSs based on the given subcarrier alloca-

tion. The subcarrier allocation problem is investigated in

Section VI.

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