17-08-2012, 04:24 PM
Selected Topics in DSP for Wireless
[attachment=31912]
Antenna (or micro) diversity.
at the mobile
Covariance of received signal amplitude
J02(2πfDτ) = J02(2πd/λ).
antenna spacing of λ/2 is enough
at the base station
All signal come from approximately the same direction
received signals are highly correlated
Larger antenna separation needed
Relevant parameter:
distance between scattering objects antenna (typically, a is 10 .. 100 meters), and
distance between mobile and base station.
Site (or macro) diversity
Receiving antennas are located at different sites.
Example: at the different corners of hexagonal cell.
Advantage: multipath fading, shadowing, path loss and interference all become "independent"
Angle diversity
Waves from different angles of arrival are combined optimally, rather than with random phase
Directional antennas receive only a fraction of all scattered energy.
Time diversity
Each message is transmitted more than once.
Useful for moving terminals
Similar concept: Slow frequency hopping (SFH):
blocks of bits are transmitted at different carrier frequencies.
Selection Methods
Selection Diversity
Equal Gain Combining
Maximum Ratio Combining
Advanced filtering
if interference is present
wiener filtering (MMSE), smart antenna’s, adaptive beam steering, space-time coding
Post-detection combining:
Signals in all branches are detected separately
Baseband signals are combined.
Pure selection diversity
Select only the strongest signal
In practice: select the highest signal + interference + noise power.
Use delay and hysteresis to avoid ping-pong effects (excessive switching back and forth)
Simple implementation: Threshold Diversity
Switch when current power drops below a threshold
This avoids the necessity of separate receivers for each diversity branch.
[attachment=31912]
Antenna (or micro) diversity.
at the mobile
Covariance of received signal amplitude
J02(2πfDτ) = J02(2πd/λ).
antenna spacing of λ/2 is enough
at the base station
All signal come from approximately the same direction
received signals are highly correlated
Larger antenna separation needed
Relevant parameter:
distance between scattering objects antenna (typically, a is 10 .. 100 meters), and
distance between mobile and base station.
Site (or macro) diversity
Receiving antennas are located at different sites.
Example: at the different corners of hexagonal cell.
Advantage: multipath fading, shadowing, path loss and interference all become "independent"
Angle diversity
Waves from different angles of arrival are combined optimally, rather than with random phase
Directional antennas receive only a fraction of all scattered energy.
Time diversity
Each message is transmitted more than once.
Useful for moving terminals
Similar concept: Slow frequency hopping (SFH):
blocks of bits are transmitted at different carrier frequencies.
Selection Methods
Selection Diversity
Equal Gain Combining
Maximum Ratio Combining
Advanced filtering
if interference is present
wiener filtering (MMSE), smart antenna’s, adaptive beam steering, space-time coding
Post-detection combining:
Signals in all branches are detected separately
Baseband signals are combined.
Pure selection diversity
Select only the strongest signal
In practice: select the highest signal + interference + noise power.
Use delay and hysteresis to avoid ping-pong effects (excessive switching back and forth)
Simple implementation: Threshold Diversity
Switch when current power drops below a threshold
This avoids the necessity of separate receivers for each diversity branch.