Index Modulated Circular Block Filtered Multicarrier System

Abstract

In cognitive radio (CR) based wireless sensor network (WSN), the transmitter sensor nodes need to sense the operational electromagnetic environment for available spectrum opportunities. Based on the results of the spectrum sensing algorithm, the CR-WSN nodes can decide on the interference temperature limit i.e. minimum tolerable interference to the primary users (PU) within the spectrum bands. The CR-WSN nodes can allocate those spectrum bands to the users' transmissions where interference temperature limit is not violated. It is assumed that the PU system is based on OFDM standard, whereas the CR-WSN system's waveform of choice is a variant of the conventional filtered multi-tone (FMT) waveform. More specifically, the CR-WSN system transmits subcarrier index modulation based circular block filtered multicarrier signal to avoid harmful interference to the PU sensor nodes. Unlike OFDM, FMT privileges frequency confinement rather than time confinement. The high sub-channel frequency confinement renders FMT a good candidate for multicarrier asynchronous communication. Circular block filtered multi-carrier is a refinement of the conventional FMT where linear convolutions in the synthesis and analysis filter banks are replaced by circular convolutions. Our proposed approach is to activate a sub-set of the total available subcarriers for signal transmission, a method known as subcarrier index modulation. The incoming bit-stream can be divided into equal sized blocks. Each such block can be further partitioned into sub-blocks. The bit pattern in the 1st sub-block can activate a set of subcarriers for transmission and the bit pattern in the 2nd sub-block can select a symbol form the signal constellation. The activated subcarriers span the spectrum bands where according to spectrum sensing result, the interference temperature threshold is not violated. Transmitter side processing steps involving the circular block filtered modulation is then performed on the subcarrier set. This kind of transmission strategy helps in achieving interference avoidance to the PU sensor nodes thereby leading to better spectral coexistence between the CR-WSN and the PU sensor nodes. Moreover the application of index modulation to circular block filtered multicarrier waveform benefits from the frequency selectivity of the channel by exploiting the subcarrier indices as sources of information. Key words: Circular Block Filtered Multicarrier, Index Modulation, Cognitive Radio