SMASHER: A Strategy-Proof Combinatorial Auction Mechanism for Heterogeneous Channel Redistribution∗

(1)

SMASHER: A Strategy-Proof Combinatorial Auction Mechanism for Heterogeneous Channel Redistribution

^∗

Zhenzhe Zheng, Fan Wu

^†

, and Guihai Chen

Shanghai Key Laboratory of Scalable Computing and Systems Department of Computer Science and Engineering

Shanghai Jiao Tong University, China

[email protected], {fwu,gchen}@cs.sjtu.edu.cn ABSTRACT

Auction is believed to be an effective way to solve or re- lieve the problem of radio spectrum shortage, by dynamically redistributing idle wireless channels of primary users to secondary users. However, to design a practical channel auction mechanism, we have to consider five challenges, in- cluding strategy-proofness, channel spatial reusability, channel heterogeneity, bid diversity, and social welfare maxi- mization. Unfortunately, none of the existing works fully considered the five design challenges. In this paper, we present the first in-depth study on the problem of dynamic channel redistribution by jointly considering the five design challenges, and present SMASHER, which is a Strategy- proof coMbinatorial Auction mechaniSm for HEterogeneous channel Redistribution. Our analyses show that SMASHER achieves both strategy-proofness and approximately efficient social welfare.

Categories and Subject Descriptors

C.2.1 [Computer-Communication Networks]: Network Architecture and Design – Wireless Communication

General Terms

Algorithm, Design, Economic

Keywords

Channel Allocation; Combinatorial Auction

†F. Wu is the corresponding author.

∗This work was supported in part by the State Key De- velopment Program for Basic Research of China (Grant No. 2012CB316201), in part by China NSF grant 61272443, 61133006, 61073152, and in part by Shanghai Science and Technology fund 12PJ1404900 and 12ZR1414900. The opin- ions, findings, conclusions, and recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the funding agencies or the government.

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full cita- tion on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or re- publish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected].

MobiHoc’13,July 29–August 1, 2013, Bangalore, India.

1. INTRODUCTION

The last two decades have witnessed a rapid development of wireless communication technology. Unfortunately, natu- rally limited radio spectrum is becoming a more and more serious bottleneck of the ongoing growth of wireless applications and services. Most of the countries have specific departments to regulate spectrum usage, e.g., Federal Com- munications Commission (FCC) in the US and Radio Ad- ministration Bureau (RAB) in China. They statically allo- cate spectrum to wireless application service providers on a long term basis for large geographical regions. Such static management leads to low spectrum utilization in the spatial and temporal dimensions. Large chunks of radio spectrum are left idle most of the time at a lot of places, while new wireless applications are starving for the radio spectrum.

Therefore, an open and market-based framework is highly needed to dynamically redistribute the radio spectrum, and thus improve the utilization of the radio spectrum.

Auctions are the most well-known market-based mechanisms to redistribute resources [3]. Since 1994, FCC has conducted a series of auctions for the licenses of radio spectrum. While FCC auctions target at large wireless service providers, our focus is on small wireless applications, such as community wireless networks or home wireless networks.

There exist many challenges in designing a practical channel auction mechanism. We list five major challenges:

• Strategy-Proofness: In strategy-proof auction mechanisms, simply submitting truthful channel demands (e.g., valuation of the channels) maximizes each par- ticipant’s utility. Since the participants are normally rational and selfish, they always tend to strategically manipulate the auction, if doing so can increase their utilities. Therefore, it discourages truthfully behav- ing participants from joining the auction, if strategy- proofness is not guaranteed.

• Spatial Reusability: Spatial reusability differentiates the wireless channels from conventional goods. Two wireless users can use the same wireless channel simultaneously, if they are well-separated.

• Channel Heterogeneity: Channel heterogeneity comes from both spatial heterogeneity and frequency heterogeneity. On one hand, the availability and quality of a channel vary at different locations. On the other hand, channels with different central frequency may have different propagation and penetration characteristics.

• Bid Diversity: Wireless devices may be equipped with multiple radios, each of which can work on a distin- guished channel at the same time. Consequently, a

305

(2)

wireless user may request multiple bundles of channels, according to her quality of service requirement.

Buyers have higher opportunities to obtain channels by submitting diverse bids, which makes the channel redistribution more flexible. Therefore, it is necessary to allow users to express diverse demands for channels.

• Social Welfare: The objective of any auction is to maximize social welfare, which is the sum of the auction winners’ valuations of the allocated goods.

A number of related works (e.g., [1, 2, 4–6, 8–10]) exist in the literature. Unfortunately, none of these works fully consider the five design challenges. Some of strategy-proof channel auction mechanisms (e.g., VEARITAS [8], TRUST [9], SMALL [5]) consider channel spatial reusability, but only work when the trading channels are homogenous. Two re- cent works TAHES [2] and CRWDP [1] consider channels’

heterogeneity, but TAHES restricts each user to bid for a single channel while CRWDP ignores the spatial reusability of channels.

In this paper, we conduct an in-depth study on the problem of dynamic channel redistribution by jointly considering the five design challenges, and present SMASHER, which is a Strategy-proof coMbinatorial Auction mechaniSm for HEterogeneous channel Redistribution. SMASHER is a novel combinatorial auction mechanism for indivisible heterogeneous channel redistribution, and achieves both strategy- proofness and approximately efficient social welfare.

We make the following contributions in this paper:

• First, we present a general model of combinatorial auction for heterogeneous channel redistribution. The auction model is powerful enough to express channel spatial reusability and heterogeneity, as well as bid diversity.

• Second, we introduce the concept of virtual channel to capture the conflict of channel usage among different auction participants. By using virtual channels, we transform the problem of heterogeneous channel allocation to a classic multi-unit combinatorial auction.

• Third, we propose SMASHER, which is a combinatorial auction mechanism for heterogeneous channel redistribution, achieving both strategy-proofness and approximately efficient social welfare.

2. PRELIMINARIES AND PROBLEM FOR- MULATION

In this section, we present the auction model for the problem of heterogeneous channel allocation.

2.1 Auction Model

We consider a static scenario, in which there is a primary spectrum user, called “seller”, who wants to lease out her temporarily unused wireless channels, and some secondary users (e.g., WiFi access points), called “buyers”, who want to lease channels to provide services to their customers at certain quality of service (QoS). We consider that the channels for leasing are heterogeneous, and thus the buyers have their own preference over the channels due to spatial vari- ance (e.g., background noise, temperature, and landform).

Since wireless devices can be equipped with multiple radios, the buyers may request more than one channel according to their requirements of QoS. Considering the diversity of

QoS demand and heterogeneity of the channels, we allow the buyers to submit multiple channel requests, among which one of the requests can be granted. We assume that the buyers have uniform valuation over any of their channel requests, because the buyers’ requirement of QoS can be satisfied if one of their requested bundles is allocated. Different from the allocation of traditional goods, wireless channels can be spatially reused, meaning that well-separated buyers can work on the same channel simultaneously, if they do not have interference between each other.

We model the process of heterogeneous channel redistribution as a sealed-bid combinatorial auction, in which buyers simultaneously submit their demands for channels to a trustworthy auctioneer, such that no buyer can know other participants’ information. The auctioneer makes the deci- sion on channel allocation and the charge to each winner.

We denote the set of orthogonal and heterogeneous channels for leasing by C , {c1, c2, . . . , cm}, and the set of buyers by N , {1, 2, . . . , n}. We list useful notations in our model of combinatorial channel auction as follows:

Channel Request Ri: Each buyer i ∈ N submits a vector of requested channel bundles Ri, Si¹, S²_i, . . . , S_i^K to the auctioneer. Any channel bundle S^j_i ⊆ C, 1 ≤ j ≤ K can satisfy her QoS. We assume that buyer’s request is strict, meaning that the buyer is only interested in winning a whole bundle S^j_i in her request vector. We call a buyer, who submits a request vector of K channel bundles, and is interested in winning one of the bundles, as K-minded buyer. If K = 1, then the buyer is single-minded. Note that our auction model is a generalization of existing models with single-minded buyers (e.g., [1, 2]). We denote the channel request vector ~R of all the buyers as ~R , (R1, R2, . . . , Rn) .

Valuation vi: Each buyer i ∈ N has an uniform valuation viover any requested channel bundles in Ri. Here, viis the private information of the buyer i. This is also known as type in mechanism design. We denote the valuation vector V of all the buyers as~ ~V , (v1, v2, . . . , vn).

Bid bi: Each buyer i ∈ N submits a bid bito the auctioneer, meaning that if she wins any channel bundle S_i^j, she would like to pay no more than bi for it. Here, the bid bi

may not necessarily be equal to her valuation vi. Let vector

~B represent the bids of all the buyers~B , (b1, b2, . . . , bn).

Clearing price pi: The auctioneer charges each winning buyer i ∈ N a clearing price pⁱ. The loser in the auction is free of any charge. We use vector ~P , (p1, p2, . . . , pn) to represent the clearing prices of all the buyers.

Utility ui: The utility of a buyer i ∈ N in the auction is defined as the difference between her valuation on the bundle of channels she wins and her clearing price pi:

ui, vⁱ− pi. (1)

We consider that the buyers are rational and selfish, thus their goals are to maximize their own utilities. In contrast to the buyers, the auctioneer’s objective is to maximize social welfare. Here social welfare is defined as follows:

Definition 1 (Social Welfare). The social welfare in a channel auction is the sum of winning buyers’ valuations on their allocated bundles of channels, i.e.,

SW , X

i∈W

vi, (2)

where W is the set of winners.

306

(3)

In this paper, we assume that buyers do not collude with each other and do not cheat about their channel bundles, while leaving these problems to our future works.

3. MULTI-UNIT COMBINATORIAL CHAN- NEL AUCTION

Different from existing works on strategy-proof channel allocation, we introduce a novel concept of virtual channel to represent the conflict of channel usage among the buyers.

By introducing virtual channels, we transform the problem of heterogeneous channel allocation to a classic multi-unit combinatorial auction.

3.1 Virtual Channel

We introduce virtual channel to capture the interference among the buyers on different channels. Specifically, a virtual channel vc^k_i,j denotes that the buyer i and the buyer j may cause interference between each other on channel ck, and thus they cannot work on channel ck simultaneously.

Since virtual channel vc^ki,j represents the exclusive usage of channel ck between the buyer i and j, its quantity is set to 1. When virtual channel vc^k_i,jis added to the requested bundle(s) that contains channel ck from the buyer i and j, at most one of the requests containing channel ckfrom the two buyers can be granted. Consequently, the exclusive usage of channel ck between the buyer i and j is guaranteed. We present the definition of virtual channel as follows.

Definition 2 (Virtual Channel). There is a virtual channel vc^ki,j, if the buyer i and buyer j are within the interference range of each other on channel ck.

In most of existing works on channel auction, a single conflict graph is used to represent the interference among buyers. However, in the case of heterogeneous channels, each channel may have a distinctive conflict graph. Let Gk , (Ok, Ek) denote the conflict graph on channel ck, where Ok ⊆ N is the set of buyers who can access channel ck, and each edge (i, j) ∈ Ek represents the interference between the buyer i and j on channel ck.

Since conflict graph is commonly assumed to be available in wireless networks, we construct the virtual channel from conflict graph. We create a virtual channel vc^ki,j, if there is an edge between the buyer i and j in conflict graph Gk, and append vc^k_i,j to the requested bundle(s) containing channel ckfrom the buyer i and j, while remaining the corresponding bids unchanged. Let VC be the set of virtual channels and

~R⁰ be the vector of updated requests with virtual channels.

3.2 Multi-Unit Combinatorial Auction

Given the virtual channel introduced in last section, we are ready to transform the problem of heterogenous channel allocation to a classic multi-unit combinatorial auction.

The goods in the multi-unit combinatorial auction are the channels and virtual channels. The quantities of each channel ck ∈ C and virtual channel vc^ki,j ∈ VC are n and 1, respectively. Let x i, S_i^0j = 1 denote that the channel set S_i^0j is granted to the buyer i; otherwise, x i, S_i^0j = 0. The process of winner determination can be modeled as a binary program. The objective is to maximize the social welfare.

We use bi, instead of vi, because the strategy-proof mechanism shown in later sections will guarantee that bidding truthfully is the dominate strategy of each buyer i ∈ N.

Objective:

M aximize X

i∈N K

X

j=1

x(i, S_i^0j) × bi

Subject to:

X

i∈N

X

S_i^0j∈R⁰_i,S^0j_i3c_k

x i, S_i^0j

≤ n ∀ck∈ C (3)

X

i∈N

X

S_i^0j∈R⁰_i,S^0j_i3vc_k

x i, S_i^0j

≤ 1 ∀vck∈ VC (4)

K

X

j=1

x i, S_i^0j

≤ 1 ∀i ∈ N (5)

x i, S_i^0j

∈ {0, 1} ∀i ∈ N, 1 ≤ j ≤ K (6) If the optimal social welfare can be achieved by solving the above binary program, then the celebrated VCG mechanism can be applied to calculate the clearing price that can ensure the strategy-proofness of the auction mechanism. Un- fortunately, the above winner determination problem can be proven to be NP-hard by reducing to the exact cover problem. Considering the computational intractability of the winner determination problem, we integrate a greedy allocation algorithm with a novel pricing mechanism to provide a strategy-proof and approximately efficient combinatorial auction mechanism for heterogeneous channel redistribution in next section.

4. HETEROGENEOUS CHANNEL REDIS- TRIBUTION

As shown in Section 3.2, finding the optimal auction de- cision is computationally intractable. In this section, we present SMASHER, which is a strategy-proof and approximately efficient combinatorial auction mechanism for heterogenous channel redistribution.

4.1 Design of SMASHER

SMASHER consists of the following three major components: virtual channel generation, winner determination, and clearing price calculation.

4.1.1 Virtual Channel Generation

The process of virtual channel generation is the same as the method discussed in Section 3.1, except that we add one more virtual channel vciwith unit quantity to each requested bundle of buyer i ∈ N . Virtual channel vci is used to ensure that at most one of the requested bundles from the buyer i can be granted.

S_i^0j= S_i^0j∪ {vci}, i ∈ N, 1 ≤ j ≤ K, where S^0j_i is updated bundle with virtual channels.

4.1.2 Winner Determination

Before presenting the approximation algorithm for winner determination, we introduce virtual bid. The uniform virtual bid ˜bi over any of requested bundles from the buyer i is defined as

˜bi, bi 1≤l≤Kmax

q S_i^0l

. (7)

307

(4)

Algorithm 1: Approximation Algorithm for Winner Determination

Input: Vector of updated channel requests ~R⁰, vector of bids ~B.

Output: A pair of sets of winning buyers and allocated bundles of channels (W, S).

(W, S) ← (∅, ∅); V ← ∅;

1

foreach i ∈ N do

2

˜bi← bi/ max1≤l≤K

q S_i^0l

3 ; end

4

Sort ˜biin non-increasing order: L¹: ˜b1≥ ˜b2≥ . . . ≥ ˜bn;

5

for i = 1, . . . , n do

6

Sort S_i^0j in non-decreasing order of bundle size:

7

L²: S⁰¹i

≤

Si⁰²

≤ . . . ≤ Si^0K

; for j = 1, . . . , K do

8

if S_i^0j∩ V = ∅ then

9

Add virtual channels in S^0j_i to V;

10

(W, S) ← W ∪ {i}, S ∪S_i^0j ;

11

break;

12

end

13

end

14

end

15

return (W, S);

16

SMASHER sorts all the buyers according to their virtual bids in non-increasing order:

L¹: ˜b1≥ ˜b2≥ . . . ≥ ˜bn.

In case of a tie, SMASHER breaks the tie following a bid- independent rule, such as lexicographic order of buyers’ ID and channel number.

Following the order in L1, SMASHER greedily grants the smallest channel bundle, in which no virtual channel has already been allocated, to each buyer.

Algorithm 1 shows the pseudo-code of above winner determination process. In practice, the number of buyers n is much larger than K, thus the time complexity of Algo- rithm 1 is O(n log n).

4.1.3 Clearing Price Calculation

The clearing price is calculated based on critical virtual bid.

Definition 3 (Critical Virtual Bid). The critical virtual bid cr(i) ∈ L1 of buyer i ∈ N is the minimum virtual bid that the buyer i must exceed to be allocated one of her channel bundles.

We note that according to the definition of critical virtual bid, no matter which request of the buyer i is granted in the auction, the critical virtual bid cr(i) is always the same.

The critical virtual bid of the buyer i ∈ N can be calculated by the following procedure. Given other buyers’ requests and bids~R⁰−i, ~B⁻ⁱ

, we greedily select virtual bid by rerunning Algorithm 1 until none of the buyer i’s requests can be satisfied. The threshold virtual bid cr(i) we select finally is regarded as the critical virtual bid of the buyer i.

We now show the method of calculating the clearing price of the buyer i by distinguishing two cases:

1. If the buyer i loses the auction or cr(i) does not exist (denoted by cr(i) = 0), then her clearing price is 0.

2. If the buyer i is granted channel bundle ˆS_i^0jand there exists a critical virtual bid cr(i), the clearing price pi

of the buyer i is set to pi, cr(i) × max

1≤l≤K

q S_i^0l

. (8)

4.2 Analysis

We prove the strategy-proofness and analyze the approximation ratio of SMASHER in this section.

Theorem 1. SMASHER is a strategy-proof combinatorial auction mechanism for heterogeneous indivisible channel redistribution.

Theorem 2. The approximation ratio of SMASHER is O(n√

m), where n is the number of buyers, m is the number of channels.

We leave the detailed proofs in our technical report [7].

5. CONCLUSION

In this paper, we have made an in-depth study on channel redistribution problem by jointly considering the five design challenges. We have presented a strategy-proof combinatorial auction mechanism for dynamic heterogeneous channel redistribution, namely SMASHER. Our analyses show that SMASHER achieves strategy-proofness and approximately efficient social welfare.

6. REFERENCES

[1] M. Dong, G. Sun, X. Wang, and Q. Zhang.

Combinatorial auction with time-frequency flexibility in cognitive radio networks. In INFOCOM, 2012.

[2] X. Feng, Y. Chen, J. Zhang, Q. Zhang, and B. Li.

TAHES: Truthful double auction for heterogeneous spectrums. In INFOCOM, 2012.

[3] J. Huang, R. A. Berry, and M. L. Honig.

Auction-based spectrum sharing. Mobile Networks and Applications, 11(3):405–418, 2006.

[4] J. Jia, Q. Zhang, Q. Zhang, and M. Liu. Revenue generation for truthful spectrum auction in dynamic spectrum access. In MobiHoc, 2009.

[5] F. Wu and N. Vaidya. SMALL: A strategy-proof mechanism for radio spectrum allocation. In INFOCOM, 2011.

[6] P. Xu, S. Wang, and X.-Y. Li. SALSA: Strategyproof online spectrum admissions for wireless networks.

IEEE Transactions on Computers, 59(12):1691 –1702, 2010.

[7] Z. Zheng, F. Wu, and G. Chen. SMASHER:

Strategy-proof combinatorial auction mechanisms for heterogeneous channel redistribution. Technical report, available at http://www.cs.sjtu.edu.cn/

~fwu/res/Paper/ZWC12TRSMASHER.pdf, 2012.

[8] X. Zhou, S. Gandhi, S. Suri, and H. Zheng. eBay in the sky: Strategy-proof wireless spectrum auctions. In MobiCom, 2008.

[9] X. Zhou and H. Zheng. TRUST: A general framework for truthful double spectrum auctions. In INFOCOM, 2009.

[10] X. Zhou and H. Zheng. Breaking bidder collusion in large-scale spectrum auctions. In MobiHoc, 2010.

308