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The multiple-input multiple-output (MIMO) techniques are anticipated to be widely employed in future wireless networks to address the ever-increasing capacity demands. A MIMO channel, typically modeled as a matrix with independent and identically distributed (i.i.d.) complex Gaussian entries, provides multiple spatial dimensions for communications. At high signal-to-noise-ratio (SNR), Shannon capacity can increase linearlywith the minimum number of transmit and receive antennas. However, achieving these dramatic capacity gains in practice, especially for outdoor deployment, could be problematic. One severe problem is the rank deficiency and ill-conditionness of the MIMO channel matrix H. This is mainly caused by the spatial correlation due to the scattering environment and the antenna configurations, and sometimes by the "keyhole" effect even though the fading is essentially uncorrelated on each end of the channel. Therefore, the MIMO capacity may be greatly reduced and adding more (co-located) antennas only wastes resources. The distributed antenna is an efficient way to solve this problem. In the downlink of a distributed antenna system, antennas allocated at several base stations (BS) constitute an antenna pool and can be used with a suitable MIMO transmission mode. Compared with a conventional centralized MIMO system, the distributed (de-centralized) MIMO system possesses many advantages, such as higher system capacity, better handoff performance, lower outage probability and higher data rates, especially at cell boundaries. However, it is also easier to be affected by phase noise (PN). Oscillator noise stems from oscillator inaccuracies in both the transmitter and the receiver, and manifests itself in the baseband as additional phase and amplitudemodulation of the received samples. The influence of oscillator noise on the signal depends on the noise characteristics of the oscillators in the system and on the signal bandwidth. It is generally split in amplitude noise and phase noise (PN). The influence of the amplitude noise on the data samples is often neglected (Robins, 1982). The phase noise contribution of both the transmitter and receiver can be viewed as an additional multiplicative effect of the radio channel, like fast and slow fading.

Analysis and Mitigation of Phase Noise in Centralized/De-centralized MIMO Systems