Abstract
The performance of multicarrier systems can be enhanced by the water filling strategy, in which different rates and powers are allocated to subcarriers. However, the induced large signalling overhead leads to less transmission efficiency. To suppress this effect this article proposes an alternative strategy by dynamically assigning the same rate to each subcarrier per user. First, we quantify the asymptotic limits of its instantaneous persymbol performance loss compared to water filling. Then, we apply this strategy to weighted sum rate maximization subject to minimum required rates and limited transmission power. Due to the simplicity of the proposed strategy, a lowcomplexity method is given, which can be used for other resource allocation problems in multicarrier systems with small modifications. Simulations demonstrate that the instantaneous persymbol performance loss of our method compared to the water filling strategy becomes insignificant if the number of users is large. The proposed method has even better performance for fast timevarying channels with respect to the signalling overhead. Moreover, given the subcarrier assignment from the proposed method, water filling can be performed and the output is close to the primal optimum.
Keywords:
OFDM; Resource allocation; Rate maximizationIntroduction
Orthogonal frequency division multiplexing (OFDM) divides the whole transmission band into multiple subcarriers to combat the intersymbol interference. This allows for allocating different rates and powers to subcarriers according to channel characteristics so that the system performance is enhanced. There are two basic resource allocation problems [1]. One is the marginadaptive (MA) problem, where the transmission power is minimized subject to a fixed rate. The other aims at maximizing the data rate subject to limited transmission power, so called the rateadaptive (RA) problem. Both can be solved by the wellknown water filling approach. As a result different rates and powers are allocated to subcarriers.
The resource allocation is executed at the transmitter, while channel state information is usually measured at receivers. Receivers must be notified about the employed allocation and coding scheme, thus, inducing a significant signalling overhead is required [2]. The faster channels change, the more frequently the resource allocation scheme alters. The temporal channel variation highly depends on the velocity of receivers and reflectors between transmitters and receivers. For multiuser resource allocation, even though the channel rapidly changes for only one user, the resource allocation scheme has to be updated frequently. Therefore, water filling may degrade in fast timevarying environments.
In [3,4], the same power is assigned to subcarriers achieving certain data rates. Since each data rate corresponds to a certain mapping and coding scheme, dynamic mapping and coding schemes cannot be identified from the allocated power. Thus, the signalling overhead still remains large. To reduce the signalling overhead, [5] suggests to cluster subcarriers into blocks. If the number of subcarriers in one block is large, the performance impairs severely. In contrast, the overhead is not significantly reduced. The same power and rate are statically allocated to a fixed number of subcarriers, which have greater channel gaintonoise ratios (CNRs) [6]. This fixed number is known to the receiver, which implies that the subcarrier assignment does not adapt to the channel conditions. The same signaltonoise ratios are obtained over all subcarriers in [7]. However, the resulting performance loss may be very large, when the channel frequency selectivity is severe.
Different from previous studies, this article proposes to dynamically allocate to the same rate to all subcarriers assigned to a user, while the rates to subcarriers of different users may differ. In doing so, the signalling overhead is drastically reduced, such that the overall performance is improved. It follows that the resource allocation scheme can be more efficiently updated after changing the subcarrier assignment than for water filling. Consequently, an easily implementable and lowcost heuristic can be designed. Compared to the conventional multiuser resource allocation [8], minimum rate constraints [9] are included to become closer to practical requirements. This results in additional challenges for the heuristic design, since the rate constraints fulfilled at equality and inequality have to be separated. The proposed method has linear complexity in the number of users and subcarriers and only small performance loss compared to the dual optimum [10,11], which has the closest performance to exhaustive search. Moreover, a constant rate is allocated to subcarriers in wireless local area networks (WLAN) [12]. In our proposed strategy, the same rate but different powers are allocated.
Thus, we only change the scale of the symbol mapper according to the resource allocation scheme. In this sense, only minor modifications are needed and the proposed strategy can be applied with simple modifications.
The remainder of this article is organized as follows. In Section ‘Preliminaries’, the considered multiuser resource allocation problem is formulated including signalling overhead, when water filling and the proposed strategies are applied. Water filling achieves the best instantaneous persymbol performance, when the signalling overhead is not considered. Theoretical asymptotic limits are given for the instantaneous persymbol performance loss of the proposed strategy applied to singleuser resource allocation in Section ‘Singleuser resource allocation’. The equalrate resource allocation is specified for a fixed subcarrier assignment in Section ‘Multiuser resource allocation given subcarrier assignment’. Subsequently, a heuristic is designed to determine the multiuser subcarrier assignment in Section ‘Heuristic subcarrier assignment’. Simulation results are presented in Section ‘Simulation results’.
Preliminaries
We consider a multiuser OFDM system with one transmitter, K users and N subcarriers. Perfect channel knowledge is assumed at the transmitter, where resource allocation is executed. A resource allocation scheme is effective for L OFDM symbols, while L is determined by temporal channel variation and the time duration of one OFDM symbol. In a fast timevarying environment L cannot be large. For example, one frame in the worldwide interoperability for microwave access (WiMAX) [13] is composed of 48 OFDM symbols. The resource allocation scheme is updated for every L OFDM symbols via the signalling overhead. There are 2^{M}data rates that may be allocated to each subcarrier. It means that M bits are necessary to identify one discrete rate. According to [2], if the water filling strategy is used, NM bits are required for expressing one resource allocation scheme and sent at first. Thereafter, data symbols follow. Note that N⌈ log_{2}(K)⌉ bits are always needed to notify each receiver of which subcarriers are assigned to it. This amount is constant in this study.
We aim at maximizing the weighted sum rate subject to limited transmission power and individual minimum rate required by users. When the water filling strategy is adopted, it is stated as
The nonnegative power and rate allocated to subcarrier n for user k is denoted by p_{k,n} and r_{k,n}, respectively. They are related by the equality constraint, where g_{k,n}is the CNR of subcarrier n of user k. Once subcarrier n is assigned to a user, M bits must be sent at first for every L OFDM symbols. The average rate for signalling over each subcarrier is M/L. The achieved rate for user k is weighted by positive w_{k}, which is given by the system for the purpose of fairness control among users. Each
user requires a minimum rate R_{k}, expressed by the second constraint. The transmission power is limited to P, illustrated by the third constraint. The set of users is referred to as
The dual optimum of (1) can be obtained by the dual method [10]. The KarushKuhnTucker conditions [14] are applied to (1), where K + 1 dual variables appear. Then, the ellipsoid method is used to let these dual variables
iteratively converge. The number of iterations is related to
To notify receivers of the employed resource allocation scheme with a smaller signalling overhead, the same rate r_{k} may be allocated to the subcarriers assigned to user k=1,…,K. Then, only KM bits are sufficient to distinguish data rates for all subcarriers. The signalling overhead is reduced by (N−K)M bits. The resource allocation problem (1) can be rewritten as
In (1), M bits must be sent for each subcarrier to identify the employed rate in the current L OFDM symbols. Thus, on average M/L bits/OFDM symbols must be additionally achieved over each subcarrier. In (2), only M bits of signaling overhead is necessary for each user. Hence, on average M/Lbits/OFDM symbols must be additionally achieved for each user. Thus, each minimum required rate is increased by M/L for signalling. In the following, we first extract two classical singleuser resource
allocation problems to quantify the instantaneous persymbol performance loss of the
proposed strategy. Thereafter, a heuristic method is designed to solve (2). The variables
of problem (1) are r_{k},p_{k,n} and
Equal rate resource allocation
In this section, singleuser and multiuser equal rate resource allocations are investigated. A heuristic solution is given for the considered problem (2).
Singleuser resource allocation
We first focus on resource allocation for a single user k. The signalling overhead is not considered here to investigate the instantaneous
persymbol performance loss of the proposed strategy. If the subcarrier assignment
where the transmission power is minimized while satisfying the required rate. Obviously,
the allocated rate and power are r_{k}=R_{k}/s_{k} and
where H_{k} is the harmonic average of the CNRs
where G_{k} is the geometric average of the CNRs
The equation above implies that the performance loss is limited, when either the required rate or the number of users is large. This may be satisfied in large scale systems, where many users are served and each of them demands a large rate.
Similarly, the new strategy is applied to a singleuser RA resource allocation problem
[1] given
where the data rate is maximized subject to the transmission power limit. The power
and rate allocation can be easily derived by taking
where the associated water level is μ_{k}=P_{k}/s_{k} + 1/H_{k}. Compared to the water filling solution s_{k} log_{2}(μ_{k}G_{k}) from [1], the instantaneous persymbol performance loss is
The performance loss (9) tends to
Similar to the previous asymptotic limit of performance loss for the singleuser MA problem, the performance loss for the singleuser RA problem is limited when a small number of subcarriers is assigned to user k or the transmission power is large.
Without considering the signalling overhead, Figure 1 shows the instantaneous persymbol performance loss by the proposed strategy compared to water filling for the singleuser MA problem (3). It approaches the limit (6) as the required rate increases. For the singleuser RA problem (7), the performance loss by the proposed strategy is shown in Figure 2. It asymptotically reduces to zero (10). The maximum loss appears, when these two strategies employ different numbers of subcarriers. The asymptotic limit is 27.89 % in Figure 1, while the loss goes up to 7.4 % in Figure 2, since the CSI variation is very large with respect to the example subcarriers. Note that we want to give an example for these theoretical asymptotic limits, which can simply be repeated by readers. The performance loss with random CNRs is smaller than the special case. This can be explicitly seen from the simulation result for the multiuser case.
Figure 1. Singleuser equal rate resource allocation 1. Instantaneous persymbol performance loss of singleuser MA equal rate resource allocation compared to water filling versus the increasing required rate with N = 8 subcarriers g_{k,n} = 9 − n, n = 1,…,8.
Figure 2. RA singleuser equal rate resource allocation 2. Instantaneous persymbol performance loss of singleuser RA equal rate resource allocation compared to water filling versus increasing power limit with N=8 subcarriers g_{k,n} = 9 − n, n = 1,…,8.
In water filling, different CNRs over subcarriers result in different allocated rates
and powers. Compared to that, the singleuser equalrate resource allocation can be
viewed as water filling over subcarriers having the equal CNR H_{k}, shown by (4) and (8). Thus, both strategies have the same water level given the
same subcarrier assignment. When subcarrier assignment
Multiuser resource allocation given subcarrier assignment
The above interpretation can be used to solve (2) while relaxing the rate constraints,
which will be met by an iterative processing later. Let
where the power constraint must be met at equality due to the complementary slackness
condition [14]. Then, the dual problem is to maximize
where the sharing factor νis
and νw_{k} is the water level for user k. However, from (12), it can be seen that the power allocated to user k may be zero or not adequate to achieve the minimum required rate
Heuristic subcarrier assignment
Given the subcarrier assignment, the solution of (1) can be viewed as twodimensional
water filling over users and subcarriers, while the solution of (2) reduces to a onedimensional
water filling only over users by allocating an equal rate to subcarriers of each user.
In [16], for the single user k, the sum power (4) can be depicted as an inverse unimodal function of s_{k} with subcarriers in
Initialization for subcarrier assignment
We use the idea from [18], where the initialization has two steps. First, the cardinality, i.e., the number of subcarriers assigned to each user, is evaluated according to the average CNR over subcarriers and the rate and power constraints. After that, specific subcarriers are assigned to users according to these evaluated numbers. To fit the idea to our problem, we propose the cardinality evaluation, summarized in Algorithm 1. In each iteration, only one cardinality increases by one. If the sum power for reaching the minimum required rates is beyond the power limit, we add one to the cardinality that induces the largest power decrement. Otherwise, we add one to the cardinality, which results in the largest weighted sum rate to (11) without the minimum rate constraints considered.
Algorithm 1. Cardinality evaluation
1:
2: s_{k}←1, k=1,…,K
3: fori=1,…,N−Kdo
4: if
5:
6: else
7:
8: end if
9:
10: end for
Let
Algorithm 2. Initialization of subcarrier assignments
1:
2: repeat
3: for each k=1,…,Kdo
4:
5:
6:
7: end for
8: until
Subcarrier adjustment
As before,
In the inner loop of Algorithm 3, each subcarrier is successively adjusted among users
to improve the objective of problem (2). This procedure repeats I times iteratively by the outer loop. Hence, we name our method iterative successive
subcarrier adjustment (ISSA). Before each inner loop, the users are divided into sets
Table 1. Complexity comparison
Simulation results
In this section, simulations are performed to compare the proposed strategy to the dual optimum of (1). The simulation system consists of 128 subcarriers. Each minimum rate and each weight are uniformly distributed within [10,20] bits per OFDM symbol and [1, 10], respectively. The frequency selective channel is modeled as consisting of 16 independently Rayleigh distributed paths with an exponential decaying profile. The expected CNR of each subcarrier is normalized to −5 dB. The transmission power is limited to 20 dBW. Here, we show the performance loss of the proposed method in percent compared to the dual optimum.
For the considered multiuser resource allocation (1) and (2), Figure 3 plots the instantaneous persymbol performance loss of the proposed heuristic solution
of problem (2) compared to the dual optimum of problem (1), as
Figure 3. Multiuser equal rate resource allocation 1. Instantaneous persymbol performance loss of the equal rate resource allocation compared to the dual optimum versus the number of users.
Algorithm 3. Iterative successive subcarrier adjustment
1: fori ∈ {1,…,I}
2:
3: for each n∈{1,…,N}do
4: if
5: adjust subcarrier n among MA users in
6: else
7: adjust subcarrier n among RA users in
8: end if
9: end for
10: end for
In [2], NM bits represent one resource water filling allocation scheme, where M bits are used to distinguish different available rates. There are LR
which is equivalent to
If the resource allocation is updated per frame, we define
Figure 4. Multiuser equal rate resource allocation 2. Minimum number of OFDM symbols for the case that water filling has better performance versus the number of users.
Finally, the output subcarrier assignment of the proposed heuristic method can be used to provide a solution to (1). Water filling, i.e., different powers and rates allocated to subcarriers, can be performed over the output subcarrier assignment. Then, a heuristic solution is obtained for (1). It is very close to the dual optimum of multiuser water filling (1), as shown in Figure 5. The performance loss becomes insignificant as K increases. The curves are the performance loss in percent compared to water filling, expressed as the ratio
Figure 5. Multiuser equal rate resource allocation 3. Instantaneous persymbol performance loss of water filling over the subcarrier assignment given by the proposed method compared to the dual optimum versus the number of users.
It is equal to
At first, as the number of users increases, the user diversity becomes bigger and the sum rate by water filling grows significantly. Hence, the ratio above gets bigger and bigger. Then, as the number of users continues increasing, even though the user diversity still increases, the minimum require rates are more. To satisfy those minimum required rates, the sum rate becomes smaller. Thus, the ratio decreases in this range. When a subcarrier is assigned to an inappropriate user, the performance becomes worse. This probability becomes higher as the number of users increases.
It can be seen from Figures 3 and 5 that the gap between two neighboring numbers of iterations becomes smaller as I increases. However, it may occur that some occasional cases do not converge as I increases. In such a case we can set a maximum value for I.
Conclusion
This article investigated resource allocation for multiuser OFDM. It proposed an equal rate allocation to the subcarriers assigned to one user. This resulted in a small signalling overhead and a lowcomplexity method of multiuser resource allocation. Asymptotic limits were given for the instantaneous persymbol performance loss of the proposed strategy in the case of singleuser resource allocation. With this strategy, a heuristic method was designed to maximize the weighted sum rate subject to the minimum rate constraint and the power limit. Simulations demonstrated that the proposed strategy has a better performance than water filling when channels vary rapidly. This method gave a near optimal subcarrier assignment to resource allocation problems with water filling applied. This implies that our method can be used for other resource allocation problems in multicarrier systems with small modifications.
Competing interests
The authors declare that they have no competing interests.
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