22-01-2013, 01:06 PM
An Overview of MIMO Systems in Wireless Communications
An Overview of MIMO.pdf (Size: 2.64 MB / Downloads: 227)
Future Broadband Wireless Systems
• Desired attributes
– Significant increase in spectral efficiency and data rates
– High Quality–of–Service (QoS) — bit error rate, outage, . . .
– Wide coverage
– Low deployment, maintenance and operation costs
• The wireless channel is very hostile
– Severe fluctuations in signal level (fading)
– Co–channel interference
– Signal power falls off with distance (path loss)
– Scarce available bandwidth
Diversity Gain
• Intuitively, the more independently fading, identical copies of a
given signal the receiver is provided with, the faster the bit error rate
(BER) decreases as a function of the per signal SNR.
Capacity of Fading Channels
• Rayleigh fading: the capacity grows linearly with min(N,M)
• Ricean channels: Increasing the line–of–sight (LOS) strength at fixed
SNR reduces the capacity
• If the gains in H become highly correlated, there is a capacity loss
• Waterfilling (WF) capacity gains over Equal Power (EP) capacity
are significant at low SNR but converge to zero as the SNR increases
=) Question: Is it beneficial to feed the channel state back to the
transmitter ?
• Many exact capacity results are known for i.i.d. Rayleigh channels.
For other channels (Rice, etc.), we have many limiting results
Conclusions
• MIMO channels offer multiplexing gain, diversity gain, array gain
and a co–channel interference cancellation gain
• Careful balancing between those gains is required
• MIMO systems offer a promising solution for future generation
wireless networks
• Ongoing research
– Space–time coding (orthogonal designs, etc.)
– Receiver design (ML receiver is too complex)
– Channel modeling
– Capacity of non–ideal MIMO channels