Multiple-antenna systems, denoted as multiple-input multiple-output (MIMO), promise huge performance gains over conventional single-antenna systems. However, it is not possible to employ multiple antennas in size limited scenarios. Recently new schemes called cooperative diversity and cooperative spatial multiplexing have been proposed. In these schemes, various users cooperate among themselves to form virtual antenna array and thus emulate multiple transmit antenna scenario. An extension of cooperative spatial multiplexing in which source and relays use different modulation techniques, called as cooperative spatial multiplexing with mixed modulations is investigated in this thesis. Detection algorithms at relays and destination are outlined. It is shown that this scheme performs better than the cooperative spatial multiplexing scheme which uses same modulation techniques at source and relays. Also, this scheme outperforms cooperative diversity scheme for high spectral efficiency regimes. Some of the significant findings of this thesis are as follows. In a cooperative spatial multiplexing scheme using mixed modulations, it is advantageous to place relays close to source and the system performs best if more relays are used and each relay employs modulations with smaller constellation size, thereby reducing the complexity and power requirements at relays. Extensive simulations are performed to compare this scheme in several scenarios.