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The use of return-to-zero (RZ) formats is proposed to enhance the dispersion tolerance of intensity-modulated carrier-unsuppressed (IM-CUs) optical single sideband (oSSB) systems. The performance of these systems, employing electrical dispersion compensation (EDC), is accessed using unipolar and polar RZ signalling formats, and compared with common non-return-to-zero (NRZ). Simulation results at 10.7 Gb/s reveal that oSSB systems with EDC using both NRZ and unipolar RZ signalling formats are limited to low extinction ratio (ER) values. However, the use of polar RZ allows increased ER and transmission distances, because of higher inter-symbol interference tolerance and the absence of discrete spectral tones. Considering a minimum ER of 9 dB and resort to forward error correction schemes, polar RZ allows single-channel error free transmission over 1400 km of standard single mode fibre, substantially improving the 980 km obtained for NRZ with ER of 6 dB. Dense wavelength division multiplexing transmission was also assessed, retrieving improved resilience of IM-CUs oSSB polar RZ.