A method comprising: transmitting within a first sub-band (84, 150, 152, 300, 302, 500, 502, 550, 552) using a plurality of sub-carriers (568, 580) with a first sub-carrier spacing (556, 558); transmitting within a second sub-band (88, 152, 154, 302, 304, 502, 504, 550, 552, 554) using a plurality of sub-carriers (570, 582) with a second sub-carrier spacing (558, 560), the first sub-band being (84, 150, 152, 300, 302, 500, 502, 550, 552) lower in frequency than the second sub-band (88, 152, 154, 302, 304, 502, 504, 550, 552); wherein a highest frequency sub-carrier used in the first sub-band is spaced from a lowest frequency sub-carrier used in the second sub-band by a spacing equal to an integer multiple of the smaller of the first and second sub-carrier spacings (556, 568, 560), and equal to a non-integer multiple of the larger of the first and second sub-carrier spacings (556, 568, 560); wherein sub-bands (84, 88, 150, 152, 154, 300, 302, 304, 500, 502, 504, 550, 552, 554) with different sub-carrier spacings (556, 558, 560) have their sub-carriers (568, 570, 580, 582) in a common grid with a grid spacing based on the smallest sub-carrier spacing used by one of the first sub-band (84, 150, 152, 300, 302, 500, 502, 550, 552) and the second sub-band (88, 152, 154, 302, 304, 502, 504, 550, 552, 554); characterised by shifting the plurality of sub-carriers (570, 580, 582) of the first or second sub-band (550, 554) with the larger sub-carrier spacing (556, 560) by an integer multiple of the grid spacing, wherein a remaining bandwidth of said sub-band (550, 554) corresponding to a fraction of the larger of the first and second sub-carrier spacing (556, 558, 560) is allocated to one end of said sub-band (550, 554) and the shifting the plurality of sub-carriers (570, 580, 582) of said sub-band (550, 554) is applied from the other end of said sub-band (550, 554). transmitting within a first sub-band (84, 150, 152, 300, 302, 500, 502, 550, 552) using a plurality of sub-carriers (568, 580) with a first sub-carrier spacing (556, 558); transmitting within a second sub-band (88, 152, 154, 302, 304, 502, 504, 550, 552, 554) using a plurality of sub-carriers (570, 582) with a second sub-carrier spacing (558, 560), the first sub-band being (84, 150, 152, 300, 302, 500, 502, 550, 552) lower in frequency than the second sub-band (88, 152, 154, 302, 304, 502, 504, 550, 552); wherein a highest frequency sub-carrier used in the first sub-band is spaced from a lowest frequency sub-carrier used in the second sub-band by a spacing equal to an integer multiple of the smaller of the first and second sub-carrier spacings (556, 568, 560), and equal to a non-integer multiple of the larger of the first and second sub-carrier spacings (556, 568, 560); wherein sub-bands (84, 88, 150, 152, 154, 300, 302, 304, 500, 502, 504, 550, 552, 554) with different sub-carrier spacings (556, 558, 560) have their sub-carriers (568, 570, 580, 582) in a common grid with a grid spacing based on the smallest sub-carrier spacing used by one of the first sub-band (84, 150, 152, 300, 302, 500, 502, 550, 552) and the second sub-band (88, 152, 154, 302, 304, 502, 504, 550, 552, 554); characterised by shifting the plurality of sub-carriers (570, 580, 582) of the first or second sub-band (550, 554) with the larger sub-carrier spacing (556, 560) by an integer multiple of the grid spacing, wherein a remaining bandwidth of said sub-band (550, 554) corresponding to a fraction of the larger of the first and second sub-carrier spacing (556, 558, 560) is allocated to one end of said sub-band (550, 554) and the shifting the plurality of sub-carriers (570, 580, 582) of said sub-band (550, 554) is applied from the other end of said sub-band (550, 554).