The document discusses multipath propagation, RAKE receivers, and Gaussian Minimum Shift Keying (GMSK). It provides the following key points: 1) RAKE receivers were designed to equalize the effects of multipath propagation by using correlators, delays, and combining to separate multipath signals. 2) RAKE receivers combine symbols from different propagation paths using channel information and combining schemes like maximum ratio combining. 3) GMSK uses Gaussian filtering of minimum shift keying signals to achieve smooth phase transitions, reducing bandwidth requirements compared to minimum shift keying. However, it increases intersymbol interference.

1. RAKE Receiver
Marcel Bautista
February 12, 2004

2. Propagation of Tx Signal

3. Multipath
 Multipath occurs when RF
signals arrive at a location
via different transmission
paths due to the reflection of
the transmitted signal from
fixed and moving objects.
 The combination of the
direct and reflected signals
most often leads to
significant signal loss due to
mutual cancellation.

4. RAKE Receiver: Basic Idea
 The RAKE receiver was designed to
equalize the effects of multipath.
 It uses a combination of correlators,
code generators, and delays, or
“fingers”, to spread out the individual
echo signals of the multipath.
 Each signal is then delayed according to
peaks found in the received signal.

5. Impulse Response
Measurement

6. RAKE Receiver Continued
 The same symbols obtained via different
paths are then combined together using the
corresponding channel information using a
combining scheme like maximum ratio
combining (MRC).
 The combined outputs are then sent to a
simple decision device to decide on the
transmitted bits.

7. RAKE Receiver Block Diagram

8. Another Block Diagram

9. Maximum Ratio Combining of
Symbols
 MRC corrects channel
phase rotation and weighs
components with channel
amplitude estimate.
 The correlator outputs are
weighted so that the
correlators responding to
strong paths in the
multipath environment
have their contributions
accented, while the
correlators not
synchronizing with any
significant path are
suppressed.

10. End Result of RAKE Receiver
 By simulating a multipath environment
through a parallel combination of
correlators and delays, the output
behaves as if there existed a single
propogation path between the
transmitter and receiver.

11. Gaussian Minimum Shift
Keying

12. Gaussian Minimum Shift
Keying
 GMSK is based on minimum shift keying which is a
special form of frequency shift keying.
 Minimum shift keying (MSK) is generated as
follows:

13. Gaussian Minimum Shift
Keying
 GMSK is similar to MSK except it
incorporates a premodulation Gaussian
LPF
 Used extensively in 2nd generation
digital cellular and cordless telephone
apps. such as GSM

14. GMSK Block Diagram
 h( ): Gaussian impulse response
 b( ): rectangular pulse train
 p( ): smoothed (Gaussian filtered)
pulse train

15. GMSK: Impulse Response,
Pulse Width
 B: -3dB bandwidth of the Gaussian filter
 Pulse shape characterized by –3dB
bandwidth times the bit period, BTb
 Pulse width increases as BTb decreases

16. GMSK Example

17. GMSK Improvement
 Achieves smooth phase transitions
between signal states which can
significantly reduce bandwidth
requirements

18. GMSK Tradeoffs
 There are no well-defined phase
transitions to detect for bit
synchronization at the receiving end.
 With smoother phase transitions, there
is an increased chance in intersymbol
interference which increases the
complexity of the receiver.

19. GMSK Tradeoffs Continued
 A compromise between spectral
efficiency and time-domain
performance must be made.