OFDM Pulse-Shaped Waveforms for Dynamic Spectrum Access Networks Paul Sutton, Barıs ¸ ¨ Ozg¨ ul, Irene Macaluso, and Linda Doyle Abstract—In dynamic spectrum access networks (DySPANs), users share access to available spectrum while minimizing the likelihood of harmful interference. In this demonstration we present a dynamic spectrum access network which employs a re- configurable orthogonal frequency-division multiplexing (OFDM) based waveform. In order to avoid the creation of harmful interference, the out-of-band (OOB) emissions of the waveform are dynamically tailored to the properties of spectrum neighbours through the use of OFDM pulse shaping. The demonstration network is built upon the highly reconfigurable Iris 2.0 software radio platform and illustrates the capabilities of this platform as well as the utility of OFDM pulse shaping in the context of dynamic spectrum access networks. I. OFDM PULSE SHAPING Orthogonal frequency-division multiplexing (OFDM) is a highly popular multicarrier modulation scheme due to its robustness against frequency-selective fading, use of a cyclic prefix to eliminate ISI and computationally-efficient imple- mentation. In the context of dynamic spectrum access, OFDM proves an attractive modulation scheme due to the ability to perform spectrum sculpting through parametric reconfigura- tion. For this demonstration, we focus on the ability to sculpt the spectrum of the waveform through dynamic pulse-shaping. An OFDM signal can be represented in a generic form as: x(t)=  ℓ J -1  j=0  P j γ j,ℓ e i2πfj t q(t - ℓ(1 + β)T s ) (1) where x(t) is the complex envelope of an OFDM signal with a cyclic prefix, γ j,ℓ is the independent and identically distributed message symbol which has unit amplitude and is transmitted upon subcarrier j during OFDM symbol ℓ, P j is the power level for the j th subcarrier, f j is the center frequency for the j th subcarrier, where f j - f j-1 =1/T is the subcarrier spacing, J is the total number of subcarriers and q(t) is a shaping pulse of duration (1 + 2β)T s as shown in Fig. 1. T is the source symbol length and T g is the cyclic prefix length such that T s = T g + T . In traditional OFDM systems, q(t) is a rectangular pulse of duration T s which leads to β =0. Although this approach reduces the effective symbol duration, it also results in sharp transitions between successive OFDM symbols such that the The authors are with CTVR Telecommunications Research Group, University of Dublin, Trinity College, Ireland. (Email: suttonpd@tcd.ie, baris.ozgul@cs.tcd.ie, macalusi@cs.tcd.ie and ledoyle@tcd.ie) This material is based upon work supported by Science Foundation Ireland under Grant No. 03/CE3/I405 as part of the Centre for Telecommunications Value-Chain Research (CTVR) at University of Dublin, Trinity College, Ireland. power spectral density (PSD) of the waveform can be repre- sented as: P (f )= J -1  j=0 P j T s sinc 2 ((f - f j )T s ), (2) and out-of-band (OOB) emissions become significant due to large sidelobes of sinc pulses [1], [2]. Shaped OFDM is an alternative to avoid sinc pulses in the frequency domain, which results in smoother temporal transitions between the successive OFDM symbols through the extension of symbol duration and the modification of q(t) to apply raised cosine windowing to OFDM symbols. As shown in Fig. 1, effective symbol duration is increased to (1+β)T s by using OFDM symbols overlapping in the roll-off region [1], [2]. As a result of symbol shaping, each subcarrier is characterized by a raised cosine pulse with a main lobe width of 2/ ( (1 + β)T s ) and a roll-off factor of β/(1 + β). Fig. 1. Generation of a shaped OFDM symbol. A number of authors have questioned the value of OFDM pulse shaping in order to reduce the side-lobe power levels of OFDM-based signals [1], [2]. This is largely due to the fact that for a single carrier, a large roll-off factor (and hence overhead) is required to achieve significant side-lobe attenua- tion in the adjacent subcarriers. However, we have found that a considerable reduction in the overall OOB emission levels can be achieved for a low level of overhead when a large number of subcarriers are employed in the waveform, as is typically the case in an OFDM-based system. This is due to the aggregate effect of the pulse shaping over many subcarriers. Fig. 2 compares the roll-off in power spectrum which can be achieved through pulse shaping and subcarrier nulling, an alternative approach to spectrum shaping. In this figure, the OFDM signal consists of 256 subcarriers and a cyclic prefix of length T/8 is used. For the subcarrier-nulling approach, 64 subcarriers are nulled, while for the pulse-shaping approach, an extension factor of β =1/4 is employed. In this way, This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE DySPAN 2010 proceedings 978-1-4244-5188-3/10/$26.00 ©2010 IEEE