單種技術下單一細胞之短時段動態頻譜接取拍賣機制設計

Abstract

行動通訊技術的進步、頻譜管理的革新以及終端用戶對多元服務的需求，促成動態頻譜接取技術成為增加頻譜使用效率與孕育創新服務的主流技術。我們將研究重點放在合作式頻譜共享，也就是次級使用者(如行動虛擬網路運營商的用戶)必須獲得頻譜權擁有者(如行動網路運營商)的同意才能接取其頻帶。合作式共享可望促進許多新穎的商務模型或交易機制設計，也會產生例如虛擬運營商所提供的針對商務或即時體育通訊服務等新的應用。 根據許多實驗報告，頻帶使用時間空檔常為數十秒到數十分鐘的短時段，因此本論文研究短時段合作式共享市場的設計。拍賣被公認為是一種分配稀有資源很有效率的機制[GBC07]，且已廣泛使用於頻譜市場，所以我們設計了一個拍賣市場，主要分為行動網路運營商(MNO)，頻譜仲介者(Spectrum broker)與行動虛擬網路運營商(MVNO)三造。終端用戶接取MNO的基地台並使用MVNO透過頻譜仲介者向MNO租用的頻譜。市場設計的目標是提供MNO(賣方)與MVNO(買方)足夠動機來參與交易，以提高整體頻譜使用效率並促成創新服務的產生。 我們研究一個存在單一MNO與多個MVNO的市場。假設頻譜以頻道為單位來交易，每個頻道都是同質的，且容許交易的時間很短，所以我們參考Engelbrecht與Kahn 在1998年所提出的拍賣:報價支付，單回合密封標，競標者有Ji個需求就出Ji個單位標價，選標方式為將K個目標物分配給所有標價中前K高的標金。這種拍賣不適用在對未提供承諾服務品質有賠償金的頻譜拍賣。因為DSA市場中頻道的價值與獲得的頻道數有關，所以在基礎環境方面，我們設計了一種能供MVNO對不同頻道數下不同標金的投標格式，稱為需求反映投標格式，好處是不會有頻譜價值與得標金額不對稱的情形。此外，底標與標金增額也被採用以提高拍賣效率並保障MNO的利潤。我們也設計了頻譜交易市場其相對應的交易協定與各造間的訊息傳遞。 我們建立仲介者選標問題的數學模型，頻譜仲介者依銷售收益最大化原則來分配頻道給MVNO，受限於拍賣進入限制，標金限制與頻道總數限制。也建立MVNO的投標決策數學模型，MVNO將其他MVNO的投標行為整合視如單一對手並依此決定最佳投標策略來最大化預期利潤，受限於拍賣進入限制，標金限制與利潤限制。解決拍賣問題的挑戰在於計算不同標金組合之獲勝機率非常複雜。 有了選標機制與最佳化投標模型，為要分析所設計的拍賣市場特性，我們進而藉由一個簡單的1-MNO-2-MVNO拍賣例子來探討MVNO競標策略的性質。在此例中，一個商務MVNO需求為3個頻道，另一個低價MVNO需求為4個頻道，但MNO只有5個頻道要賣。我們假設一個啟發式策略稱為「集中投標策略」，考慮頻譜仲介者的選標政策，MVNO在所需頻道數標其可行範圍內的最高價，而在小於需求頻道數都以可行範圍內的最低價來標，這是為要提高獲得所需頻道數的機率而得出的策略。本例分析結果如下: 1. 採用投標決策數學模型相較啟發式策略能使MVNO獲得較高預期利潤 2. 競標策略對未提供承諾服務品質的賠償金敏感 高賠償費率的MVNO會在所需頻道標最高價以獲取所需頻道數;低賠償費率的MVNO會在小於需求頻道數標最低價以獲取最高利潤。 3. MNO與兩MVNO皆可獲利 本例中，市場需求大於市場供給且頻道全部賣出。MNO的利潤是底價的16倍，且兩MVNO都可藉由拍賣獲利。 關鍵字： 動態頻譜接取、次級市場、合作式共享、行動網路運營商、虛擬運營商、頻譜仲介者、價值鏈、商務模型、市場架構、拍賣、短時段、市場機制

With the technological advances in mobile communications and spectrum management and the divergent service demands of end-users, Dynamic Spectrum Access (DSA) has been recently proposed to increase bandwidth usage efficiency and provide innovative or specialized services such as low-cost telecommunication services with low QoS guarantee through spectrum sharing and/or trading. In the proposed DSA paradigm, we focus on the cooperative sharing of access rights where a secondary user (ex: end-user of a Mobile Virtual Network Operator, MVNO) has access to a licensed band with the permission from its primary right holder (ex: a Mobile Network Operator, MNO). Such cooperative sharing may facilitate many innovative business models or trading mechanism designs and new applications such as business-specific or real-time sports telecommunication services provided by the MVNO. In this thesis, we investigate the design of a short-interval market because the under-utilized spectrum bands often appear in short duration ranging from tens of seconds to tens of minutes according to many empirical reports. Because auction is widely viewed as an efficient way of allocating scarce resources [GBC07]and has been widely adopted in spectrum market, an auction market is designed, which consists of three parties: MNO, Spectrum Broker, and MVNO. In this design of short-interval auction market, the End-users access the MNO’s base station (BS) by using the spectrum that MVNO leases from MNO through the brokerage of a SB. The market design objective is to design a platform which provides MNO and MVNOs with sufficient incentives to trade and thus increase the spectrum utilization and stimulate the growth of innovative services. We study a 1-MNO-N-MVNO market. In view that there are multiple units of homogeneous objects to trade and little time for trading, we refer to the auction proposed by Engelbrecht and Kahn in 1998: a pay-as-bid, single-round sealed-bid auction, bidder submits Ji bids if he has Ji demands, and the bid selection policy is allocating K objects to the most highest K bids. This auction is not suitable for DSA market because in DSA market the value of channels partly depend on the amount of channels allocated. Instead, a demand responsive bidding form where MVNO has to represent the bid price of each number of channels is used. The advantage of it is there will not be asymmetry of the channel resource value and the payment. Furthermore, the reserve price and bid increment are also set to enhance auction efficiency and secure MNO’s profit. We then design the trading protocol and message passing for the market. We first mathematically formulate Spectrum broker’s bid selection problem as the SB maximizes the sales revenue by allocating channels under the entrance, bid and total channel availability constraints. The bid decision problem is formulated as a problem where each MVNO view other MVNOs’ bidding behaviors as a single opponent’s and also aggregate their bids to decide the bid offer which maximizes the overall expected profit under the constraints of entrance, bid and profit. The challenge of solving the auction problem lies in the complicated computation of winning probabilities of different bid offers. We then explore the properties of MVNO’s bidding strategy and the auction outcome by considering a simple example of N=2. There are a business MVNO with a 3-channel demand and a discount MVNO with a 4-channel demand, and the MNO offers 5 channels for the two to bid. A heuristic MVNO behavior assumption of “Concentrated bidding strategy” is assumed, where considering the SB’s total sales maximization strategy, an MVNO would bid high on getting the demanded number of channels and bid minimum on number of channels lower than demanded to increase the probability of winning the demanded. The analyses of this example are as follows: 1. BDPi model gives MVNO a higher expected profit than that of the heuristic strategy. 2. Bidding strategy is sensitive to the penalty of unsatisfied guaranteed QoS. MVNO with high penalty rate will bid the highest on the demanded to acquire all demanded channels while MVNO with low penalty rate will bid the lowest on channels lower than the demanded to obtain the highest profit. 3. Both MNO and MVNOs gain profits. In this example, demand is higher than supply and the proposed auction design sells out all channels. Also, MNO’s profit is 16-fold the reserve price, and the two MVNOs could earn profit via auction. Key words - DSA, secondary market, cooperative sharing, MNO, MVNO, spectrum broker, value chain, business model, market structure, auction, short-interval, market mechanism