ADAPTIVE POWER ALLOCATION IN COOPERATIVE OFDM
TRANSMISSION WITH THREE-HOP DF RELAYING
Jun Wang
1
, Xiaomeng Chai
2
, Xu Zhang
3
, Zhongshan Zhang
4
, Keping Long
5
Institute of Advanced Network Tech. and New Services (ANTS) and Beijing Key Laboratory of Knowledge Engineering for
Materials Science, USTB Beijing China
power allocation, adaptive, cooperative, relays,
decode and forward.
Adaptive power allocation subject to the total power
constraint in an end-to-end cooperative orthogonal frequency
Division multiplexing (OFDM) system is studied, with the
optimal power allocation in terms of maximizing the sum
transmission Rate being derived. The conventional two-hop
system model is extended to a three-hop structure in this
paper to improve the Wireless coverage. Independent and
identically distributed (i.i.d.) sub-channels are assumed in
each Decode-and-Forward (DF) relay without considering the
effect of inter-relay interference. Relay selection is also
performed in the proposed cooperative transmission, and each
relay itself decides whether to forward data for the source
depending on the quality of the sub-channels. The
transmission bottleneck in the conventional two-hop
cooperative system can be overcome effectively, especially
when the source-to-destination and first-hop relay-to-
destination links are both in a deep fading. The proposed
theoretical analysis is validated by using simulation, and it’s
shown that the sum rate of the proposed algorithm is the
monotonically increasing function of both the sub-channel
numbers and total system power. Numerical results also prove
the superiority of the proposed three-hop model over the
conventional two-hop system in terms of end-to-end sum data
rate.
Cooperative transmission, regarded as a virtual Multiple-
Input Multiple-Output (MIMO) system, has been very
promising in recent years [1]. Several technical advantages,
such as the ability to boost system performance by improving
the channel capacity and the overall quality, the potential of
extending the coverage area and the network lifetime, etc,
have been announced in cooperative communications [2].
Among the existing cooperation protocols which have been
proposed, amplify-and-forward (AF) and decode-and-forward
(DF) are the most well-known relaying modes [3]-[5].
Besides, some other well-known relaying mode, such as
compress-and-forward (CF), has also been studied
considerably to obtain a performance gain by performing
signal compression at the relays [6].
In order to combat the severe frequency-selective fading in
cooperative channels, orthogonal frequency division
multiplexing (OFDM) modulation has already been employed
in the relay-assisted system to provide a more reliable and
efficient data transmission [7], [8]. To adequately improve the
benefits of the relay-assisted OFDM system, adaptive power
allocation, which is a critical but still challenging work, is
required. As compared to the conventional single-hop OFDM
system, an efficient per sub-channel power allocation
algorithm should be performed in each relay to optimize the
overall relay-assisted transmission [9], [10].
Recently, many studies have been carried out to deal with the
resource allocation in two-hop cooperative OFDM systems.
Resource allocation with relay selection in cooperative
system has been addressed in [11], [12]. Resource allocation
for a multi-relay DF cooperative OFDM system is
investigated in [13]-[15], with the total power constraint
being considered. As compared to it, power allocation in a
point-to-point two-hop DF cooperative OFDM transmission
has been studied in [16], with both the total power and
individual power constraints being considered. Resource
allocation in the two-hop relay-assisted OFDM transmission
for a cognitive radio (CR) environment is studied in [17], [18],
with both the AF and DF relaying modes being considered.
Fairness issues in resource allocation in cooperative OFDM
system have been taken into consideration in [19], [20].
Besides, a joint resource allocation method in a two-hop AF
cooperative OFDM system is studied in [21], with some
critical parameters, including power, subcarrier and relay
nodes, being considered simultaneously.
However, all the studies aforementioned focused only on the
conventional two-hop relaying scenarios and without paying
attention to the scenarios of multi-hop relay transmission,
which are expected to provide a broader wireless coverage.
Power allocation in multi-hop relay-assisted OFDM system is
Studied in [22], [23], with both the AF and DF relaying
modes being analyzed. However, the direct source-to-
destination link is not utilized in those two algorithms, and
this operation will consequently degrade the spatial diversity
gain of the whole system.
In this paper, a daptive power allocation subject to the total
power constraint for the multi-hop DF cooperative OFDM
system will be investigated, with relay selection being
considered. Our objective is to maximize the end-to-end
transmission rate under the total power constraint. As shown
in [24], as the number of sub-carriers increases, the duality
gap for the capacity maximization problem vanishes.
In this