On Channel State Inference and Prediction Using Observable Variables in 802.11b Networks Shirish Karande, Syed Ali Khayam, Yongju Cho, Kiran Misra, Hayder Radha, Jaegon Kim and Jin-Woo Hong Abstract- Performance of cross-layer protocols that recommend the relay of corrupted packets to higher layers can be improved significantly by accurately inferring/predicting the bit error rate (BER) in the packets. Here, inference refers to estimating the BER in an already received packet, while prediction specifically refers to anticipating the BER in a future packet. This paper presents a measurement based study of 802.11b WLANs that analyzes the utility of observable variables in channel state inference (CSI) and prediction (CSP). The first part of the paper investigates the utility of SSR and background traffic (BT) intensity ρ as observable side-information for CSI. Our results show significant utility for SSR indications and the feasibility of utilizing ρ for additional improvements. The second part of the paper utilizes the proposed CSI mechanism for BER prediction (i.e., the CSP aspect). We observe that the BER of temporally adjacent packets are correlated and thus BER of a current packet, by itself or on the basis of link-specific temporal correlation model, can provide efficient prediction. However since BER of a packet is not an observable variable, the above prediction mechanism cannot be realized practically without significant processing overheads. Thus we propose to estimate the channel state for the current packet using observable side- information and this estimate is subsequently used as an input to a BER based predictor. Our analysis and simulations based on an extensive set of actual 802.11 traces show that the proposed methods can provide accurate CSI/CSP under a variety of realistic channel conditions Index Terms— Channel State Estimation, Wireless Local Area Networks, Cross-layer Protocols I. INTRODUCTION HIS paper presents a study based on an extensive set of actual 802.11b link-level bit error traces and associated actual measurements of Signal to Silence Ratio (SSR) and Background Traffic (BT). The motivating applications for this work are the recently proposed cross-layer protocols for delivery of multimedia, in particular video [1]-[9]. It is crucial to elaborate upon this motivation: Unlike the traditional wired Internet based communication, the number of packet drops due to bit errors can be substantial in wireless networks. Bandwidth hungry multimedia systems can be adversely affected by such packet drops. Therefore many recent multimedia related studies [1]-[9] have recommended the development of cross-layer protocols that do not discard partially damaged packets. Relay of corrupted packets to higher layers can lead to significant improvements in the video throughput. However, the efficacy of information recovery from a corrupted packet is a function of the bit error rate (BER) in the packet. Hence, accurate channel state inference (CSI) and prediction (CSP) can provide substantial capacity gains. Utility of channel awareness in improving capacity is a well researched area and has been demonstrated by many theoretical- and simulation-based studies, most of which focus on the physical layer. However unlike the physical layer, the channel observed at the link/MAC layer is discrete, and each individual bit does not have a signal strength associated with it. This makes the task of practically providing channel state information (CHSI) about the corruption levels in a packet significantly harder. Therefore, the authors in a prior work started identifying practical mechanisms that can provide CHSI at the link-layer [4], [14]. The work in this paper is a continuation of [4], [14]. We defer detailed discussion on how the present work relates with our previous studies to the related work section. Our experimental methodology allows us to associate three link quality measures with each individual packet: ρ represents the BT overheard between two received packets in terms of packets/sec, SSR measures the Signal to Silence Ratio (SSR) for the first few microseconds ( μ s ) and we assume that a link-level checksum can provide us with a binary indication (Z) of whether a received packet is corruption free or not; which is a viable assumption in cross-layer protocols based on an underlying 802.11 MAC that supports CRC checksum. We refer to ρ , SSR and Z as T Manuscript received September 28, 2006. S. Karande is with the Electrical and Computer Engineering Department, Michigan State University, East Lansing, MI 48824 USA. (phone: 517-355- 3769; fax: 517-353-1980; e-mail: karandes@ egr.msu.edu). S. Khayam, K. Misra and H. Radha are with the Electrical and Computer Engineering Department, Michigan State University, East Lansing, MI 48824 USA. (e-mail: {khaymsy, misrakir, radha)@ egr.msu.edu). Y. Cho was with Electronics and Telecommunications Research Institute, Daejeon, Korea, 305-350. He is now with the Electrical and Computer Engineering Department, Michigan State University, East Lansing, MI 48824 USA. (e-mail: yongjucho@etri.re.kr}. J. Kim and J. Hong are with Electronics and Telecommunications Research Institute, Daejeon, Korea, 305-350. (e-mail: {jgkim,jwhong}@etri.re).