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Journal of Electrical and Computer Engineering / 2010 / Article

Research Article | Open Access

Volume 2010 |Article ID 419430 | https://doi.org/10.1155/2010/419430

Tao Qin, Cyril Leung, Chunyan Miao, Zhiqi Shen, "Resource Allocation in a Cognitive Radio System with Imperfect Channel State Estimation", Journal of Electrical and Computer Engineering, vol. 2010, Article ID 419430, 5 pages, 2010. https://doi.org/10.1155/2010/419430

Resource Allocation in a Cognitive Radio System with Imperfect Channel State Estimation

Tao Qin,1 Cyril Leung,2 Chunyan Miao,1 and Zhiqi Shen3

1School of Computer Engineering, Nanyang Technological University, Singapore

2Department of Electrical and Computer Engineering, The University of British Columbia, Vancouver, B.C., Canada

3School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore

Academic Editor: Adam Panagos
Received17 Oct 2009
Accepted23 Feb 2010
Published23 May 2010

Abstract

Cognitive radio (CR) is a promising concept for improving the utilization of scarce radio spectrum resources. Orthogonal frequency division multiplexing (OFDM) is regarded as a technology which is well matched for CR systems. It is shown that channel estimation errors can result in a severe performance degradation in a multiuser OFDM CR system. A simple back-off scheme is proposed, and simulation results are provided which show that the proposed scheme is very effective in mitigating the negative impact of channel estimation errors.

1. Introduction

It is believed that the cognitive radio (CR) concept can be used to greatly improve spectral utilization by allowing secondary (unlicensed) users (SUs) to use frequency bands not currently being used by the primary (licensed) users (PUs) in a certain location [1, 2]. Orthogonal frequency division multiplexing (OFDM) is regarded as a good modulation scheme for CR systems due to its flexibility in allocating resources among SUs [3]. In CR systems, it is important to manage mutual interference problems effectively since primary and secondary users will often simultaneously use adjacent bands.

The problem of power, bit, and subchannel loading for multiuser OFDM CR systems has been studied in [4], in which it is assumed that perfect channel state information is available. In practice, this assumption is often unrealistic, and it is therefore important to study the performance degradation due to imperfect channel state estimation. In [5], the effect of partial channel information in a non-CR multiuser MIMO-OFDM system is discussed. However, the effect of mutual interference which would arise between primary and secondary users in a CR system is not addressed.

A reduced complexity (RC) resource allocation (RA) scheme is proposed for a multiuser OFDM CR system in [4], and it is shown that the scheme provides good performance when perfect channel estimates are available. In this paper, the performance degradation due to imperfect channel state estimation for the RC RA scheme is shown to be quite severe. A simple back-off scheme is proposed and found to be very effective in reducing this degradation.

2. System Model

The system model used in this paper is the same as in [4] and is summarized here for the convenience of the reader. We consider the problem of allocating resources on the downlink of an OFDM CR system in which a CR base station (CRBS) serves one primary and 𝑀 secondary users. The PU band is π‘Šπ‘β€‰Hz wide. On each side of the PU band, there are 𝐾/2 OFDM subchannels, each of width π‘Šπ‘ β€‰Hz. As the CRBS can transmit simultaneously to the PU and SUs, the PU signal can cause interference to the SUs and vice versa.

The baseband power spectral density (PSD) of the π‘˜th subchannel SU signal is modeled as [6] Ξ¦π‘˜(𝑓)=π‘ƒπ‘˜π‘‡π‘ ξ‚΅sinπœ‹π‘“π‘‡π‘ πœ‹π‘“π‘‡π‘ ξ‚Ά2,(1) where π‘ƒπ‘˜ is the transmit power of the π‘˜th subchannel signal and 𝑇𝑠 is the symbol duration. The interference power introduced by this signal into the PU band is πΌπ‘˜ξ€·π‘‘π‘˜,π‘ƒπ‘˜ξ€Έ=π‘ƒπ‘˜πΌπΉπ‘˜,(2) where πΌπΉπ‘˜=ξ€œπ‘‘π‘˜+π‘Šπ‘π‘‘/2π‘˜βˆ’π‘Šπ‘/2||π‘”π‘˜||2𝑇𝑠sinπœ‹π‘“π‘‡π‘ πœ‹π‘“π‘‡π‘ ξ‚Ά2𝑑𝑓(3) is the interference factor for the π‘˜th subchannel. In (2) and (3), π‘”π‘˜ is the channel gain from the CRBS to the PU for the π‘˜th subchannel, and π‘‘π‘˜ is the spectral distance between the π‘˜th subchannel and the center frequency of the PU band.

The interference power introduced by the PU signal into the π‘˜th subchannel band at SU π‘š isπ‘†π‘šπ‘˜ξ€·π‘‘π‘˜ξ€Έ=ξ€œπ‘‘π‘˜+π‘Šπ‘ π‘‘/2π‘˜βˆ’π‘Šπ‘ /2||β„Žπ‘šπ‘˜||2Ξ¦PU(𝑓)𝑑𝑓,(4) where β„Žπ‘šπ‘˜ is the subchannel π‘˜ gain from the CRBS to SU π‘š, and Ξ¦PU(𝑓) is the PSD of the signal destined for the PU.

It is assumed that each subchannel can be used for transmission to at most one SU at any given time. Let π‘ƒπ‘šπ‘˜ denote the transmit power allocated to subchannel π‘˜ of SU π‘š. From [7], the maximum number of bits in a symbol transmitted on this subchannel is set toπ‘π‘šπ‘˜=log2||β„Ž1+π‘šπ‘˜||2π‘ƒπ‘šπ‘˜Ξ“ξ€·π‘0π‘Šπ‘ +π‘†π‘šπ‘˜ξ€Έ,ξƒͺ(5) where βŒŠβ‹…βŒ‹ denotes the floor function, 𝑁0 is the one-sided noise PSD, and π‘†π‘šπ‘˜ is given by (4). For convenience, the parameter Ξ“ is set to unity in the remainder of this paper.

Let π‘Žπ‘šπ‘˜βˆˆ{0,1} be a subchannel allocation indicator, that is, π‘Žπ‘šπ‘˜=1 if and only if subchannel π‘˜ is allocated to SU π‘š. Our objective is to maximize the total bit rate for all SUs subject to total transmit power, fairness, and PU interference constraints. Specifically, the optimization problem is expressed as follows: maxπ‘Šπ‘ π‘€ξ“π‘š=10π‘₯0200π‘‘πΎξ“π‘˜=10π‘₯0200π‘‘π‘Žπ‘šπ‘˜π‘π‘šπ‘˜(6) subject to π‘Žπ‘šπ‘˜βˆˆ{0,1},βˆ€π‘š,π‘˜(7)π‘€ξ“π‘š=1π‘Žπ‘šπ‘˜π‘ƒβ‰€1,βˆ€π‘˜,(8)π‘šπ‘˜β‰₯0,βˆ€π‘š,π‘˜,(9)π‘€ξ“π‘š=10π‘₯0200π‘‘πΎξ“π‘˜=10π‘₯0200π‘‘π‘Žπ‘šπ‘˜π‘ƒπ‘šπ‘˜β‰€π‘ƒtotal,(10)π‘€ξ“πΎπ‘š=1ξ“π‘˜=10π‘₯0200π‘‘π‘Žπ‘šπ‘˜π‘ƒπ‘šπ‘˜πΌπΉπ‘˜β‰€πΌth,(11) where 𝑃total is the total SU power budget, and 𝐼th is the PU's maximum tolerable interference power. Inequality (8) follows from the assumption that a subchannel can be allocated to at most one SU. Inequalities (10) and (11) correspond to the power and interference constraints, respectively. The nominal bit rate weight (NBRW) for SU π‘š is denoted by πœ†π‘š so that πœ†π‘š/βˆ‘π‘€π‘–=1πœ†π‘– is the fraction of the total number of SU bits loaded that is to be fairly allocated to SU π‘š. It is also convenient to denote the total number of bits per symbol period allocated to SU π‘š by π΅π‘šβ‰œβˆ‘πΎπ‘˜=1π‘π‘šπ‘˜ and define the total bit rate, π‘…π‘š, of SU π‘š as π‘…π‘šβ‰œπ‘Šπ‘ π΅π‘š. The total bit rate for all SUs is π‘…π‘ β‰œβˆ‘π‘€π‘š=1π‘…π‘š.

Channel state estimation errors are modeled as follows: let β„Ž denote the actual (complex) gain of a channel. This channel could correspond to the π‘˜th OFDM subchannel from the CRBS to SU π‘š or to the PU. For simplicity, all channels are assumed to be independently Rayleigh faded, that is, their complex gains are drawn from circularly symmetric, complex Gaussian distributions. The estimated (complex) channel gain is given by ξβ„Ž=β„Ž+𝑒,(12) where 𝑒 is the channel estimation error. For the simulation results presented below, 𝑒 is assumed to be the outcome of an independent, circularly symmetric, complex Gaussian random variable.

The impact of channel estimation errors on the total SU bit rate, 𝑅𝑠, can be described as follows. The maximum bit rate, π‘…π‘šπ‘˜β‰œπ‘Šπ‘ π‘π‘šπ‘˜, that can be achieved for SU π‘š on subchannel π‘˜ depends on the channel gain, β„Žπ‘šπ‘˜, the transmit power, π‘ƒπ‘šπ‘˜, and the total interference-plus-noise power, as shown in (5). However, the CRBS knows only ξβ„Žπ‘šπ‘˜ and not β„Žπ‘šπ‘˜. It thus calculates an estimated maximum transmit bit rate, ξπ‘…π‘šπ‘˜. If ξπ‘…π‘šπ‘˜<π‘…π‘šπ‘˜, then the opportunity for a higher transmit bit rate is lost. On the other hand, if ξπ‘…π‘šπ‘˜>π‘…π‘šπ‘˜, then ξπ‘…π‘šπ‘˜ exceeds the channel capacity and the actual achieved transmit bit rate is zero. Therefore, the channel estimation errors, if not carefully taken into account in the design of the RA scheme, can result in a severe throughput degradation.

3. A Scheme for Mitigating Throughput Degradation

In order to reduce the overall throughput degradation caused by the use of inaccurate channel gain values, we introduce a back-off factor, 𝐡𝐺,0≀𝐡𝐺≀1, such that the RA algorithm uses π΅πΊξβ„ŽΓ—|π‘šπ‘˜|2 instead of |ξβ„Žπ‘šπ‘˜|2 as the channel power gain in calculating the estimated maximum transmit bit rate, ξπ‘…π‘šπ‘˜. Therefore, (4) and (5) are modified accordingly as ξπ‘†π‘šπ‘˜ξ€·π‘‘π‘˜ξ€Έ=π΅πΊξ€œπ‘‘π‘˜+π‘Šπ‘ π‘‘/2π‘˜βˆ’π‘Šπ‘ /2|||ξβ„Žπ‘šπ‘˜|||2Ξ¦PÛ𝑏(𝑓)𝑑𝑓,(13)π‘šπ‘˜=⎒⎒⎒⎒⎣log2βŽ›βŽœβŽœβŽœβŽπ΅1+𝐺|||ξβ„Žπ‘šπ‘˜|||2π‘ƒπ‘šπ‘˜Ξ“ξ‚€π‘0π‘Šπ‘ +ξπ‘†π‘šπ‘˜ξ‚βŽžβŽŸβŽŸβŽŸβŽ βŽ₯βŽ₯βŽ₯βŽ₯⎦.(14)

Let 𝐼PUβ‰œβˆ‘πΎπ‘˜=1π‘ƒπ‘˜πΌπΉπ‘˜ be the total interference power introduced into the PU band by SU signals. To control the probability, π‘ƒπ‘œ, that 𝐼PU exceeds 𝐼th, the proposed scheme uses a second back-off factor, 𝐡𝐼, such that the RA algorithm uses 𝐡𝐼×𝐼th instead of 𝐼th as the target PU interference power threshold value. A lower value of π‘ƒπ‘œ generally requires a lower 𝐡𝐼 value.

From (14), the incremental power required for transmitting one bit to SU π‘š on subchannel π‘˜ is given by Ξ”π‘ƒπ‘šπ‘˜=𝑁0π‘Šπ‘ +ξπ‘†π‘šπ‘˜π΅πΊ|||ξβ„Žπ‘šπ‘˜|||22Μ‚π‘π‘šπ‘˜.(15) From (2) and (15), the incremental interference power generated by such a transmission to the primary user is Ξ”πΌπ‘šπ‘˜=Ξ”π‘ƒπ‘šπ‘˜πΌπΉπ‘˜.(16)

The MP, MI, and RC RA algorithms were proposed in [4] to improve the throughput in a multiuser OFDM-based CR system. In this paper, modified versions, referred to as m-MP, m-MI, and m-RC, are proposed to mitigate the negative impact of channel estimation errors. Pseudocode listings of these three algorithms are provided below. In the algorithms, ξπ΅π‘š is the estimated total number of bits allocated to SU π‘š and 𝑃SU is the total transmit power of SUs.

The m-MP algorithm is used to determine the interference power, 𝐼MP, introduced into the PU band if, at each bit loading, we choose the subchannel which minimizes the incremental power needed for the selected SU.

Algorithm m-MP
(1) Step 1β€”Initialization (a)Set 𝑃SU=0,𝐼MP=0. (b)Set ξπ΅π‘š=0 for π‘šβˆˆ{1,2,…,𝑀}. (c)Set Μ‚π‘π‘šπ‘˜=0, and calculate Ξ”π‘ƒπ‘šπ‘˜ as in (15), for π‘šβˆˆ{1,2,…,𝑀} and π‘˜βˆˆ{1,2,…,𝐾}.
(2) Step 2(a)Determine π‘šβˆ—=argminπ‘šξπ΅π‘š/πœ†π‘š; ties are first broken in decreasing order of πœ† then randomly.(b)Determine π‘˜π‘ƒ=argminπ‘˜Ξ”π‘ƒπ‘šβˆ—π‘˜.(c)If 𝑃SU+Ξ”π‘ƒπ‘šβˆ—π‘˜π‘ƒβ‰€π‘ƒtotal, perform the following updates: ξπ΅π‘šβˆ—=ξπ΅π‘šβˆ—+1,𝑃SU=𝑃SU+Ξ”π‘ƒπ‘šβˆ—π‘˜π‘ƒ, Μ‚π‘π‘šβˆ—π‘˜π‘ƒ=Μ‚π‘π‘šβˆ—π‘˜π‘ƒ+1, calculate Ξ”π‘ƒπ‘šβˆ—π‘˜π‘ƒ as in (15), Ξ”π‘ƒπ‘šπ‘˜π‘ƒ=∞,βˆ€π‘šβ‰ π‘šβˆ—, and go to step 2(a).(d)If 𝑃SU+Ξ”π‘ƒπ‘šβˆ—π‘˜π‘ƒ>𝑃total, then set π‘šβˆ— to be the user with the next higher value of ξπ΅π‘š/πœ†π‘š and go to step 2(b). Stop if all users have been considered.

Similarly, the m-MI algorithm is used to determine the total power, 𝑃MI, required for transmitting to the SUs if, at each bit loading, we choose the subchannel which minimizes the incremental interference power introduced into the PU band.

Algorithm m-MI
(1) Step 1β€”Initialization (a)Set 𝑃MI=0,𝐼PU=0. (b)Set ξπ΅π‘š=0 for π‘šβˆˆ{1,2,…,𝑀}. (c)Set Μ‚π‘π‘šπ‘˜=0 and calculate Ξ”πΌπ‘šπ‘˜ as in (16), for π‘šβˆˆ{1,2,…,𝑀} and π‘˜βˆˆ{1,2,…,𝐾}.
(2) Step 2(a)Determine π‘šβˆ—=argminπ‘šξπ΅π‘š/πœ†π‘š; ties are first broken in decreasing order of πœ† then randomly.(b)Determine π‘˜πΌ=argminπ‘˜Ξ”πΌπ‘šβˆ—π‘˜.(c)If 𝐼PU+Ξ”πΌπ‘šβˆ—π‘˜πΌβ‰€π΅πΌπΌth, perform the following updates: ξπ΅π‘šβˆ—=ξπ΅π‘šβˆ—+1, 𝐼PU=𝐼PU+Ξ”πΌπ‘šβˆ—π‘˜πΌ,𝑃MI=𝑃MI+Ξ”πΌπ‘šβˆ—π‘˜πΌ/πΌπΉπ‘˜πΌ, Μ‚π‘π‘šβˆ—π‘˜πΌ=Μ‚π‘π‘šβˆ—π‘˜πΌ+1, calculate Ξ”πΌπ‘šβˆ—π‘˜πΌ as in (16), set Ξ”πΌπ‘šπ‘˜πΌ=∞,βˆ€π‘šβ‰ π‘šβˆ—, and go to step 2(a).(d)If𝐼PU+Ξ”πΌπ‘šβˆ—π‘˜πΌ>𝐡𝐼𝐼th, then setπ‘šβˆ— to be the user with the next higher value ofξπ΅π‘š/πœ†π‘š and go to step 2(b). Stop if all users have been considered.

The relative importance of the SU power and PU interference is measured using 𝑃𝑉𝑃=MIβˆ’π‘ƒtotal𝑃total𝐼,𝑉𝐼=MPβˆ’π΅πΌπΌth𝐡𝐼𝐼th,(17) respectively. Note that 𝑉𝑃 is negative when 𝑃total>𝑃MI, and 𝑉𝐼 is negative when 𝐡𝐼𝐼th>𝐼MP.

The m-RC algorithm uses 𝑉𝐼 and 𝑉𝑃 as follows.

Algorithm m-RC
(1) Step 1β€”Initialization(a)Set 𝑃SU=0,𝐼PU=0. (b)Set ξπ΅π‘š=0 for π‘šβˆˆ{1,2,…,𝑀}. (c)Set Μ‚π‘π‘šπ‘˜=0, and calculate Ξ”π‘ƒπ‘šπ‘˜ as in (15) and Ξ”πΌπ‘šπ‘˜ as in (16), for π‘šβˆˆ{1,2,…,𝑀} and π‘˜βˆˆ{1,2,…,𝐾}.
(2) Step 2(a)Determine π‘šβˆ—=argminπ‘šξπ΅π‘š/πœ†π‘š; ties are first broken in decreasing order of πœ† then randomly.(b)Determine π‘˜π‘ƒ=argminπ‘˜Ξ”π‘ƒπ‘šβˆ—π‘˜.(c)Determine π‘˜πΌ=argminπ‘˜Ξ”πΌπ‘šβˆ—π‘˜.(d)Compute 𝑋=𝑉𝐼(Ξ”π‘ƒπ‘šβˆ—π‘˜π‘ƒπΌπΉπ‘˜π‘ƒβˆ’Ξ”πΌπ‘šβˆ—π‘˜πΌ)/Ξ”πΌπ‘šβˆ—π‘˜πΌ and π‘Œ=𝑉𝑃(Ξ”πΌπ‘šβˆ—π‘˜πΌ/πΌπΉπ‘˜πΌβˆ’Ξ”π‘ƒπ‘šβˆ—π‘˜π‘ƒ)/Ξ”π‘ƒπ‘šβˆ—π‘˜π‘ƒ(e)If 𝑋β‰₯π‘Œ, set π‘˜βˆ—=π‘˜πΌ; otherwise set π‘˜βˆ—=π‘˜π‘ƒ.(f)If 𝑃SU+Ξ”π‘ƒπ‘šβˆ—π‘˜βˆ—β‰€π‘ƒtotal and 𝐼PU+Ξ”πΌπ‘šβˆ—π‘˜βˆ—β‰€π΅πΌπΌth, perform the following updates: ξπ΅π‘šβˆ—=ξπ΅π‘šβˆ—Μ‚π‘+1,π‘šβˆ—π‘˜βˆ—=Μ‚π‘π‘šβˆ—π‘˜βˆ—+1,𝐼PU=𝐼PU+Ξ”πΌπ‘šβˆ—π‘˜βˆ—,𝑃SU=𝑃SU+Ξ”π‘ƒπ‘šβˆ—π‘˜βˆ—, calculate Ξ”π‘ƒπ‘šβˆ—π‘˜βˆ— as in (15) and Ξ”πΌπ‘šβˆ—π‘˜βˆ— as in (16), set Ξ”π‘ƒπ‘šπ‘˜βˆ—=∞,Ξ”πΌπ‘šπ‘˜βˆ—=∞,andβˆ€π‘šβ‰ π‘šβˆ—, and go to step 2(a).(g)If 𝑃SU+Ξ”π‘ƒπ‘šβˆ—π‘˜βˆ—>𝑃total or 𝐼PU+Ξ”πΌπ‘šβˆ—π‘˜βˆ—>𝐡𝐼𝐼th, then set π‘šβˆ— to be the user with the next higher value of ξπ΅π‘š/πœ†π‘š, and go to step 2(b). Stop if all users have been considered.(h)Calculate π‘π‘šπ‘˜ as in (5),If Μ‚π‘π‘šπ‘˜>π‘π‘šπ‘˜, then set π‘π‘šπ‘˜=0. Otherwise, set π‘π‘šπ‘˜=Μ‚π‘π‘šπ‘˜. Update π΅π‘š.

Good values for the back-off factors, 𝐡𝐺 and 𝐡𝐼, are chosen as follows (1)For given values of 𝑃total and 𝐼th, both π‘ƒπ‘œ and the throughput, 𝑅𝑠, increase with 𝐡𝐼, whereas 𝐡𝐺 has little effect on π‘ƒπ‘œ. We therefore choose to use the largest value of 𝐡𝐼 which can satisfy the π‘ƒπ‘œ requirement.(2)Once the value of 𝐡𝐼 is chosen, we determine the throughput for different values of 𝐡𝐺 and select the 𝐡𝐺 value which yields the highest throughput.

4. Results

Computer simulations were run with the proposed m-RC RA algorithm assuming that the PU signal is an elliptically filtered white noise process and using the same parameter values as in [4]. In addition, let 𝑉𝑒 denote the variance of the real (or imaginary) part of the channel estimation error in (12).

Figure 1 shows the total SU bit rate, 𝑅𝑠, as a function of 𝑃total with 𝐼th=6Γ—10βˆ’6 W for two different values of 𝑉𝑒 and three cases: (1) perfect channel estimation, (2) channel estimation errors with the RC RA algorithm in [4], and (3) channel estimation errors with the proposed m-RC RA algorithm. It can be seen that in the presence of channel estimation errors, the RC RA algorithm in [4] shows a big drop in 𝑅𝑠 compared to the perfect channel estimation case. Furthermore, π‘ƒπ‘œ for the RC RA algorithm in [4] can be as high as 6%. Using the m-RC RA algorithm with 𝐡𝐺=0.84 and 𝐡𝐼=0.815, the loss relative to the perfect channel estimation case is small, and π‘ƒπ‘œ is negligibly small. The results show that the proposed m-RC RA is quite robust against channel estimation errors.


Figure 1 
Total bit rate, 𝑅 𝑠 , as a function of 𝑃 t o t a l with 𝐼 t h = 6 Γ— 1 0 βˆ’ 6 W .

Figure 2 shows 𝑅𝑠 for the m-RC RA algorithm as a function of 𝑃total for different values of 𝐼th. It was found from the simulation results that a 𝐡𝐺 value of 0.84 is near-optimal for maximizing 𝑅𝑠. In this figure, 𝐡𝐼 was set to 0.905 to achieve a π‘ƒπ‘œ of 10βˆ’3. As expected, 𝑅𝑠 increases with 𝐼th. For comparison, the perfect channel estimation curves are also included in Figure 2. It can be observed that the 𝑅𝑠 value for m-RC RA is less than 5% lower.


Figure 2 
Total bit rate, 𝑅 𝑠 , as a function of 𝑃 t o t a l for the m-RC RA algorithm with 𝑃 π‘œ = 1 0 βˆ’ 3 .

5. Conclusion

A simple back-off scheme was proposed to counter the deleterious effect of channel estimation errors in a multiuser OFDM CR system. Simulation results show that the proposed scheme can greatly reduce the loss in the total bit rate to SUs.

Acknowledgment

This research was supported in part by the Singapore Ministry of Education Grant no. RGM24/06.

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Copyright © 2010 Tao Qin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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