Javad Abdoli

Filtered OFDM: A new waveform for future wireless systems

A spectrally-localized waveform is proposed based on filtered orthogonal frequency division multiplexing (f-OFDM). By allowing the filter length to exceed the cyclic prefix (CP) length of OFDM and designing the filter appropriately, the proposed f-OFDM waveform can achieve a desirable frequency localization for bandwidths as narrow as a few tens of subcarriers, while keeping the inter-symbol interference/inter-carrier interference (ISI/ICI) within an acceptable limit. Enabled by the proposed f-OFDM, an asynchronous filtered orthogonal frequency division multiple access (f-OFDMA)/filtered discrete-Fourier transform-spread OFDMA (f-DFT-S-OFDMA) scheme is introduced, which uses the spectrum shaping filter at each transmitter for side lobe leakage elimination and a bank of filters at the receiver for inter-user interference rejection. Per-user downsampling and short fast Fourier transform (FFT) are used at the receiver to ensure a reasonable complexity of implementation. The proposed scheme removes the inter-user time-synchronization overhead required in the synchronous OFDMA/DFT-S-OFDMA. The performance of the asynchronous f-OFDMA is evaluated and compared with that of the universal-filtered OFDM (UF-OFDM), proposed in [1], [2].

Interference and X Networks With Noisy Cooperation and Feedback

The Gaussian K-user interference and M × K X channels are investigated with no instantaneous channel state information at transmitters (CSIT). First, it is assumed that the CSI is fed back to all nodes after a finite delay (delayed CSIT), and furthermore, the transmitters operate in full-duplex mode, i.e, they can transmit and receive simultaneously. Achievable results on the degrees of freedom (DoFs) of these channels under the above assumption are obtained. It is observed that, in contrast with no CSIT and full CSIT models, when CSIT is delayed, the achievable DoFs for both channels with the full-duplex transmitter cooperation are greater than the available achievable results on their DoF without transmitter cooperation. Then, K-user interference and K × K X channels are considered with output feedback, wherein the channel output of each receiver is causally fed back to its corresponding transmitter. Our achievable results with output feedback demonstrate strict DoF improvements over those with the full-duplex delayed CSIT when K 5 in the K-user interference channel and K > 2 in the K × K X channel. Next, the combination of delayed CSIT and output feedback, known as Shannon feedback, is studied and strictly higher DoFs compared with the output feedback model are achieved in the K-user interference channel when K = 5 or K 6, and in the K × K X channel when K > 2.

Universal Receiver for Weighted Circularly Convolved OFDM/OQAM

The weighted circularly convolved OFDM/OQAM (WCC-OFDM/OQAM), a variant of OFDM/OQAM without time-domain tail overhead, is investigated in a doubly dispersive fading environment. A new receiver is proposed that resolves the self-interference present at a regular WCC-OFDM/OQAM receiver when the channel variations are too fast compared to the signal burst duration. This is achieved by exploiting the channel state information (CSI) available at the receiver to compensate for the channel discontinuity that appears in the receivers circular operations. The proposed receiver is universal in two regards: first, it performs equally well for a wide range of speeds and modulation and coding schemes (MCSs), and second, it does not require any change to the WCC-OFDM/OQAM signal structure, and thus, is transparent to the transmitter.