1 Power Efficient Broadcasting with Cooperative Diversity in Ad hoc Networks Gentian Jakllari +, Srikanth V. Krishnamurthy + , Michalis Faloutsos + and Prashant Krishnamurthy † + Department of Computer Science and Engineering University of California, Riverside † Department of Information Sciences and Telecommunications, University of Pittsburgh {jakllari, krish, michalis}@cs.ucr.edu, [email protected] Abstract—Cooperative diversity entails the simultaneous at the moment, is simply denoted as SISO 1. The deploy- transmission ofinformation by multiple nodes that are in ment of antenna arrays on small mobile nodes, however, is the proximity of each other; this process emulates the func- infeasible due to the required size of these antennas. More tionality of an antenna array. The use of cooperative com- specifically, the space between two elements of a multi- munications can help combat interference and fading and ple element antenna array must be at least of the order of hence, significantly improve the reliability of the wireless λ channel without the physical deployment of cumbersome 2 , λ being the wavelength used for transmissions. For antenna arrays on mobile terminals. The increased reliabil- the commonly used 2.4 GHz frequency band, the required ity allows for nodes to use lower transmission power levels inter-element distance is 6.125 cm. Therefore, even an an- and yet, achieve the transmission range possible with tra- tenna with four elements can be too big to be mounted on ditional single-input single-output (SISO) communications. a laptop and even more so on a PDA or a low cost sensor In this paper, we focus on saving energy while performing node. network-wide broadcasting in ad hoc networks by using co- 2 operative diversity. We develop a new broadcast protocol Cooperative diversity is a recent breakthrough at the that exploits cooperative diversity links to disseminate infor- physical layer, which exploits the broadcast nature of the mation to all the nodes in the network with much lower con- wireless channel to emulate a MISO system. Nodes par- sumed power. Our simulation results demonstrate that our ticipating in an cooperative diversity scheme, broadcast protocol maintains the coverage that is possible with broad- the same packet simultaneously. By providing the receiver cast schemes based on traditional SISO transmissions but with multiple replicas of the same signal, one can achieve with a significantly higher power efficiency. the same benefits of an antenna system mounted on a sin- Index Item— Cooperative Diversity, Broadcasting, Ad gle node. hoc Networks Laneman [4] was the first to introduce cooperative di- versity and showed that it can achieve full diversity. For the case of two nodes, he proved that the outage proba- I. INTRODUCTION bility decays in proportion to the inverse of the square of the signal-to-noise-ratio (1/SNR2), rather then 1/SNR The use of antenna arrays in conjunction with space- without cooperative diversity. Sendoranis et al. [8], in- time codes leads to significant diversity and/or multiplex- troduced the cooperative diversity as a mean to increase ing gain that can revolutionize the quality of communi- capacity of the uplink in cellular networks. The size of cations over the wireless channel. Depending on whether mobile units is too small to accommodate an array of an- multiple transmitting antennas (inputs) and/or multiple re- tennas, therefore the authors contend that the only way to ceiving antennas (outputs) are used, one could have a enable the diversity gain is by user cooperation. Multi-Input Single-Output (MISO) system, a Single-Input Multi-Output (SIMO) system or a Multi-Input Multi- The research at the physical layer has proved the sig- Output (MIMO) system. The classic point-to-point com- nificant potential of cooperative diversity. The question munication, employed by most of the wireless standards raised to the protocol designer, however, is: How can we exploit this new physical layer technology to build upper This work is supported in part by, the U. S. Army Research Of- fice under the Multi-University Research Initiative (MURI) grant 1In the rest of the paper we use the terms SISO link and point-to- No: W911NF-04-1-0224 and in part by NSF CAREER grant from ANI point interchangeably No: 0237920. 2aka virtual antenna arrays layer protocols with higher performance? In this paper we the reliability of received information at the destination focus on the problem of the network-wide broadcasting, a node. The symbols are replicated in space and time in a key component that aids both routing and dissemination specific manner that enables the destination node to com- services in ad hoc networks. Broadcasting by its nature, is bine the received symbols in a simple manner (linear) to a transmission intense operation and hence, results in the reap the benefits of diversity. Such a replication is per- consumption of significant power. For this reason, there formed in blocks of k symbols and is hence referred to have been efforts [9][6][2] toward designing energy effi- as space-time block coding. In the presence of indepen- cient broadcast protocols for ad hoc networks. However, dently flat Rayleigh fading channels between the many most of the previous work assumes SISO links to be de- transmitters and the receiver, space-time block coding can ployed at the underlying physical layer. provide large diversity gains. In this paper we design of a distributed broadcast pro- On a virtual MISO link, there are N transmitters that ∗ tocol that exploits cooperative diversity to disseminate in- transmit m complex symbols ±si, ±si over kTs seconds; ∗ formation to the whole network, while achieving signif- here, si is simply the complex conjugate of the symbol icant decrease in power consumption over protocols that si and m ≤ k. In a SISO system, the single transmitter use only SISO links. Through simulations, we show that would send m symbols in mTs seconds for a symbol rate our protocol can perform network-wide broadcasting with of 1/Ts. In the virtual MISO case, the symbol rate will up to eight times lower power consumption than a corre- be m 1 . The measure of bandwidth utilization is the rate k Ts sponding protocol that uses SISO links. The savings in of the space-time block code R = m/k. If m = k, then, power are achieved without compromising the coverage R = 1 and the bandwidth is completely utilized; codes achieved by the broadcast. In fact, our simulations show that facilitate this are referred to as full-rate space-time that we are able to also improve the coverage, especially block codes. in sparse networks. Space-time block codes are characterized by a k × N The rest of the paper is organized as follows. In Section matrix S that specifies the pattern as per which symbols II, we describe the relevant physical layer background. In must be transmitted by the N antennas in each of the k Section III we describe our protocol for broadcasting and time units of duration Ts. The rows correspond to time some preliminary simulation results. The results from our (the times at which the symbols are transmitted) and the simulations form Section IV. Our conclusions and future columns to space (the antenna elements on which they thoughts make up Section V. are transmitted). The well known Alamouti Code which is a 2 × 2 space-time block code [1]. This code has a II. VIRTUAL MISO utilization (rate) R = m/k = 1. The matrix S2 (the subscript 2 indicates that it is for a 2 × 2 space-time block In this section, we provide a brief discussion on space- code) corresponding this code is given by : time codes and the physical layer properties of links formed with cooperative diversity. Since multiple trans- s0 s1 S2 = ∗ ∗ (1) mitters cooperate to communicate with a single receiver, −s1 s0 we refer to such links as virtual MISO links. Virtual MISO links and Space Time Codes: In a With this coding scheme, two symbols are transmitted by SISO system, at the physical layer, a group of l bits of a two transmitters over 2Ts time units (Tx0 transmits the ∗ packet that are received from the MAC layer are converted symbols s0 and −s1 in (0,Ts) and (Ts, 2Ts), respectively ∗ to one of M = 2l symbols as per a chosen M-ary modula- and, Tx1 transmits the symbols s1 and s0 in the same two tion scheme (note that there are 2l possible l-bit groups). time units). We briefly describe how diversity is provisioned on a If the symbol duration is Ts, the symbol rate is 1/Ts sym- virtual MISO link using Alamouti codes3. Given that the bols/second and the bit rate is l/Ts bps. If M = 2, we have a binary modulation scheme where each symbol has Alamouti code is a 2 × 2 space-time block code, two co- one bit (e.g., binary phase shift keying or BPSK). In the operating transmitters are needed. Let us suppose that the case of Quadriphase PSK (QPSK), 8-PSK and 16-QAM channels between the two transmitters and the receiver are (quadrature amplitude modulation), 2, 3 and 4 bits are re- represented (in baseband) by the complex coefficients h0 jφ spectively mapped on to one symbol. and h1 respectively. In general hi = αie i where αi is On a virtual MISO link, multiple transmitters will trans- the magnitude and φi is the phase response of the channel. mit the same symbols to a common destination; this joint 3A similar approach is possible with higher order space-time codes. transmission improves the signal quality and therefore, However, a discussion on this is beyond the scope of this paper. 2 The magnitude αi is Rayleigh distributed.