1. A subchannel allocation apparatus for allocating subchannels of a MIMO radio channel to a plurality of users, the MIMO radio channel comprising a first number of subchannels, a subchannel comprising a transmission capacity, comprising: a subspace determiner for determining a subspace of the MIMO radio channel, the subspace comprising a second number of subchannels, the second number being lower than the first number and each subchannel of the second number of subchannels comprising a higher transmission capacity than the subchannel with the lowest transmission capacity of the first number of subchannels; and a subchannel allocator responsive to the subspace determiner for allocating a subchannel from the second number of subchannels of the subspace to a user based on an allocation metric.