Spectrum Shaping Using Guard Carriers in OFDM based Cognitive Radios

摘要

The increasing demand for wireless communication systems to deliver high data speeds has resulted in the need for flexible and efficient use of the spectrum resource, which is a scarce commodity. Cognitive radio has emerged as a promising technology to maximize the use of limited radio bandwidth while adjusting to the growth in the number of services and applications in wireless networks. A cognitive radio (CR) transceiver is capable of adapting to the dynamic radio environment and network settings to maximize utilization of scarce radio resources while offering wireless access flexibility. Orthogonal frequency division multiplexing (OFDM) is a promising candidate for designing the physical layer of any spectrally multiband agile radio because it can achieve highspeed data communications by collectively using a number of orthogonally spaced frequency bands that are modulated by many slower data streams. As OFDM uses rectangular type pulses to represent the symbols transmitted the subcarriers during one time instant, large side lobes that occur can potentially interfere with signal transmissions from nearby existing systems or with transmissions from other rental users. There are various techniques to reduce the out of band radiations (OOBR) and to enable mutual co-existence of cognitive-OFDM systems with primary license owner systems. In this paper, we use guard subcarriers as spectrum shaping method i.e. dynamic deactivation or nulling of certain subcarriers which results in decrease of data carriers and consequently increases distance between primary user's band and secondary user's band, hence OOBR decreases. This method is the easiest to implement. We study spectrum shaping techniques based on zero, four and eight guard subcarriers. We show through simulations that eight guard subcarriers gives appreciable improvement over the four or zero guard subcarrier.