Optimal Spectrum Fragmentation in NC-OFDMA based Multi-hop Networks Muhammad Nazmul Islam, Narayan B. Mandayam, Sastry Kompella * and Ivan Seskar WINLAB, Rutgers University, Email: {mnislam,narayan,seskar}@winlab.rutgers.edu * Information Technology Division, Naval Research Laboratory, Email: sk@ieee.org Abstract—Wireless transmission using non-contiguous chunks of spectrum is becoming increasingly necessary due to: incum- bent users in TV white space, anticipated spectrum sharing between commercial and military systems, and uncoordinated interference in unlicensed bands. Multi-Channel Multi-Radio (MC-MR) platforms and Non-Contiguous Orthogonal Frequency Division Multiple Access (NC-OFDMA) technology are the two commercially viable transmission choices to access these non- contiguous spectrum chunks. Fixed MC-MR’s do not scale with increasing number of non-contiguous spectrum chunks due to their fixed set of supporting radio front ends. NC-OFDMA allows nodes to access these non-contiguous spectrum chunks and put null sub-carriers in the remaining chunks. However, nulling sub- carriers increases the sampling rate (spectrum span) which, in turn, increases the power consumption of radio front ends. Our work characterizes this trade-off from a cross-layer perspective. Specifically, we perform joint power control, spectrum span selection, scheduling and routing to minimize system power of multi-hop NC-OFDMA networks. Numerical simulations suggest that our approach reduces system power by 4 - 12 dB over classical water-filling based cross-layer algorithms. I. I NTRODUCTION Demand for wireless services is becoming much greater than the currently available spectrum. Some experts predict a 1000 fold increase in data traffic by 2020 [1]. FCC has already opened up 300 MHz in TV bands [2] and plans to open up an additional 500 MHz by 2020 [3] to meet this demand. These channels will be license-by-rule; i.e., any radio can use these channels if it abides by the FCC specifications [3]. If uncoordinated networks (e.g. different broadband wireless service providers) use these channels, they will adjust spectrum usage according to their individual traffic demands. As a result, the available spectrum will become partitioned into a set of non-contiguous segments. For some bands, like white space [2], the available spectrum itself is non-contiguous. Multi-Channel Multi-Radio (MC-MR) technology allows nodes to simultaneously access multiple fragmented spectrum chunks [4], [5]. Fixed MC-MR uses traditional hardware based technology. Therefore, the number of non-contiguous bandwidth slots that fixed MC-MR can access is limited by the number of available radio front ends. Software defined radio based Non-Contiguous Orthogonal Frequency Division Multiple Access (NC-OFDMA) technology allows nodes to transmit in these non-contiguous bandwidth slots with a single radio front end. Nodes can null interference-limited channels and select better channels in NC-OFDMA enabled networks. Fig. 1. Advantages and drawbacks of NC-OFDMA in multi-hop networks. Node A transmits to the AP via node B. Node B transmits directly to the AP. There are three channels: 1, 2 and 3. Node X, an external interferer, transmits in channel 2. Node B can use NC-OFDMA, transmit in channel 1 and 3 (node A transmits in channel 2) and place null subcarrier in interference limited channel 2. In this scenario, Node B spans 3 channels, instead of 2. Hence, NC-OFDMA has grabbed a lot of attention in re- source allocation [6]–[8] and cooperative forwarding [9], [10]. However, nulling unwanted subcarriers increases the spectrum span and the sampling rate of nodes since the sampling rate should be at least twice the spectrum span. Fig. 1 illustrates the benefits and this inherent challenge of NC-OFDMA in a two-hop network. It is well known that the circuit power consumptions of ADC and DAC increase linearly and exponentially with sampling rate and the number of quantization bits respec- tively [11], [12]. As software defined radios continue to go for higher quantization resolution, the ADC and DAC that are used in the radio circuits will dominate the amount of power consumed. A comparison between Table I and Table II shows that power consumption of some commercial ADCs is more than 10 dB higher than the maximum allowed trans- mission power for portable devices in the 802.22 standard. On the one side, NC-OFDMA reduces transmission power by selecting channels with better link gains, while on the other side, this increased spectrum span increases circuit power consumptions of the transceiver. We investigate this trade- off between transmission power reduction and circuit power increase in the context of cross-layer optimization of NC- OFDMA based wireless networks. Specifically, by jointly performing power control, spectrum span selection, channel scheduling and routing, we minimize the total system power arXiv:1309.0861v1 [cs.NI] 3 Sep 2013