Multi- and Single-Carrier Transmissions
OFDM for broadband transmission
Broadband signals received on a wireless medium are affected by reflections of surrounding objects that create replicas of the transmitted signals at different times and with different amplitudes and phases. In order to compensate for this phenomenon, equalization is needed at the receiver. Since the ‘90s, a preferred technique to compensate for dispersive channel has been the orthogonal frequency division multiplexing (OFDM), which can be seen as a way of dividing the available spectrum into small portions (subcarriers), each of which is affected by a flat channel. Equalization at the receiver boils down to a set of multipliers, one for each subcarrier. The transmitter and the receiver are implemented with efficient fast Fourier transform techniques, providing a very efficient solution. Examples of systems using OFDM are WiFi (IEEE 802.11x), DVB-T (T2) and long term evolution (LTE) of 3GPP (downlink).
Our group has studied OFDM since its origin and has continued to investigate new issues in OFDM. For example, the use OFDM in DVB-T poses problems when the receiver is moving (e.g. in a in-car reception), since OFDM is particularly sensitive to Doppler. Special signal processing techniques have been developed to address this issue.
OFDM is a particular example of multicarrier modulation, where in general spectrum is divided into fraction of different shapes: another important example of multicarrier transmission is filtered multitone modulation (FMT), which allow a very sharp separation in the frequency among the different subcarriers. Relevant contributions have been done by our group on the study of FMT.
Single carrier modulation
More recently with respect to OFDM, an alternative technique in the flavor of OFDM has been proposed by our group. In a pioneering work in 2002, we have proposed the non-linear equalization of single carrier transmission. In this case, a conventional single carrier (e.g. QAM) transmission is performed, with a suitable block format. At the receiver, equalization is performed in the frequency domain, again with the use of efficient fast Fourier transforms. It can be shown that this technique is able to achieve the same performance of OFDM, provided that equalization is performed in a non-linear fashion, i.e. by using past decisions on received samples to remove interference and improve detection. The resulting technique has been included in the LTE of 3GPP (uplink) and was considered for WiMan (WiMax) networks. The main advantage of single carrier modulation with frequency domain equalization are a reduced peak on average power ratio of the transmitted signal with respect to OFDM. Moreover, complexity is shifted from the transmitter to the receiver, and this is particularly useful in the uplink of a cellular system.
Students of the classes of Ingegneria dell’Informazione and Telecomunicazioni are welcome to join the group for thesis activity of these and other topics. Please contact dr. Stefano Tomasin for further information.