Security Embedding on UWB-IR Physical Layer Ahmed Benfarah *† , Benoit Miscopein * , and Jean-Marie Gorce † * Orange LABS, F-38240 Meylan, France. Email: name.surname@orange.com † Universit´ e de Lyon, INRIA, INSA-Lyon, CITI Laboratory F-69621 Villeurbanne, France. Email: jean-marie.gorce@insa-lyon.fr Abstract—The main goal of this work is to incorporate security in an existing ultra wideband (UWB) network. We present an embedding method where a tag is added at the physical layer and superimposed to the UWB-impulse radio signal. The tag should be added in a transparent way so that guaranteeing compatibility with existing receivers ignoring the presence of the tag. We discuss technical details of the new embedding method. In addition, we discuss embedding strength and we analyze robustness performance. We demonstrate that the proposed embedding technique meets all the system design constraints. Index Terms—Embedding, UWB-IR, security. I. I NTRODUCTION Wireless communications and security cannot be dissociated due to the intrinsic nature of the channel: an adversary can easily eavesdrop, jam or tamper with radio signals. Despite this observation, a lot of radio standards have been designed with- out security. It is only added as an extra feature on the upper layers of the protocols stack. The result is that many wireless networks with different security layers have to coexist: hotspot, WEP, WPA or WPA2...Compatibility may be an issue here because existing users without security must still live in the same network. This problem was first tackled in the context of wireless networks by Kleider et al. [1]. They proposed secu- rity embedding in order to guarantee backward compatibility while including security features at the physical layer level. Embedding is a modification of an existing physical layer where an extra signal is superimposed to a primary signal and sent concurrently with the data payload. This superimposing must be transparent to an insecure receiver. On the opposite, a secure receiver can extract some security feature from the modified signal. Embedding has been proposed for many radio technologies ranging from single carrier [2] to multicarrier [3] transmissions. In this work, we are interested by the embedding problem in ultra wideband-impulse radio (UWB-IR) systems. This technology is characterized by the emission of very short temporal pulses (ns) with low duty cycle. It has received a great attention over the last decade thanks to attractive characteristics such as the robustness to multipath fading, the possibility of low consumption architectures and the ability of accurate ranging in indoor conditions. Our approach is based on the superposition of two orthogonal waveforms and is new compared to existing physical layer embedding methods. In the design of an embedding technique, two constraints must be satisfied: transparency and robustness. Embedding degrades the performance of insecure receivers. However, this degradation must be kept as low as possible. Our solution is designed to limit this degradation to only 1dB. Then, the security features added to the signal must be robust to errors. This problem is faced in our scheme by using a BCH error- correcting code. The paper is organized as follows. In Section II, the context of the work is introduced and the problem is formulated. In Section III, the principle of the new embedding technique is detailed. The proposed method is analyzed in Section IV. Finally, Section V deals with the related work and Section VI concludes the paper. II. CONTEXT AND PROBLEM STATEMENT In this section, we first describe the reference system using UWB-IR physical layer. Then, we introduce the problem and constraints of the work. A. Context We consider an UWB-IR system based on packet transmis- sion. The packet is composed of two parts: preamble and pay- load (data). The preamble is notably used for synchronization and channel estimation. An error-correcting code is applied to the payload. We assume that payload length is equal to a practical value of 1024 bits after channel coding. One data symbol is composed of N f frames each of duration T f . The frame is also divided into N c slots and only one slot per frame is active, and contains a pulse. The position of the pulse is defined by a time-hopping code {c j }. The role of time-hopping is to avoid the spectral lines due to the periodic emission of pulses. Consequently, N f pulses are repeated per symbol which are modulated in BPSK. The expression of the transmitted signal s(t) for one symbol time can be described as: s(t)= N f −1  j=0  E p · b · p ( t − jT f − c j T c ) , (1) where p(t) is the pulse waveform, E p is the pulse energy and b ∈ {−1, 1} is the information symbol. The first pulse waveform proposed in UWB systems was a Gaussian monocycle [4]. However, it does not respect the spectrum mask specified by the FCC [5]. To offer a flexibility in terms of frequency band, the Gaussian monocycle 978-1-4673-0921-9/12/$31.00 ©2012 IEEE Globecom 2012 - Communication and Information System Security Symposium 807