A method of allocating resources in a communication system which uses a plurality of sub-carriers arranged in a sequence of physical resource blocks wherein adjacent blocks of said sequence are arranged in groups having an associated resource block group, RBG, size, and wherein the RBG size is dependent on a system bandwidth, the method comprising, in a base station: determining at least one virtual resource block of a distributed type for allocation to a user device; signalling data identifying each virtual resource block of the distributed type to said user device; and identifying (S6, S7, S8, S9, S10) using mapping data, physical resource blocks, of each allocated virtual resource block of the distributed type, to be used for communications with the user device; wherein said mapping data provides a gap between the mapped physical resource blocks of each virtual resource block of the distributed type, for each of a plurality of different system bandwidths having a different respective RBG size, wherein the gap between the identified physical resource blocks is an integer multiple of the square of the number of physical resource blocks in an RBG for each of the different respective RBG sizes, and wherein the plurality of different respective RBG sizes comprises at least an RBG size in which the number of physical resource blocks in each RBG is two, an RBG size in which the number of physical resource blocks in each RBG is three, and an RBG size in which the number of physical resource blocks in each RBG is four. determining at least one virtual resource block of a distributed type for allocation to a user device; signalling data identifying each virtual resource block of the distributed type to said user device; and identifying (S6, S7, S8, S9, S10) using mapping data, physical resource blocks, of each allocated virtual resource block of the distributed type, to be used for communications with the user device; wherein said mapping data provides a gap between the mapped physical resource blocks of each virtual resource block of the distributed type, for each of a plurality of different system bandwidths having a different respective RBG size, wherein the gap between the identified physical resource blocks is an integer multiple of the square of the number of physical resource blocks in an RBG for each of the different respective RBG sizes, and wherein the plurality of different respective RBG sizes comprises at least an RBG size in which the number of physical resource blocks in each RBG is two, an RBG size in which the number of physical resource blocks in each RBG is three, and an RBG size in which the number of physical resource blocks in each RBG is four. wherein said mapping data provides a gap between the mapped physical resource blocks of each virtual resource block of the distributed type, for each of a plurality of different system bandwidths having a different respective RBG size, wherein the gap between the identified physical resource blocks is an integer multiple of the square of the number of physical resource blocks in an RBG for each of the different respective RBG sizes, and wherein the plurality of different respective RBG sizes comprises at least an RBG size in which the number of physical resource blocks in each RBG is two, an RBG size in which the number of physical resource blocks in each RBG is three, and an RBG size in which the number of physical resource blocks in each RBG is four. wherein the plurality of different respective RBG sizes comprises at least an RBG size in which the number of physical resource blocks in each RBG is two, an RBG size in which the number of physical resource blocks in each RBG is three, and an RBG size in which the number of physical resource blocks in each RBG is four.