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.
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.
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.
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.
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.
GMSK is based on minimum shift keying which is a special form of frequency shift keying. Minimum shift keying (MSK) is generated as follows:
Gaussian Minimum Shift Keying GMSK is similar to MSK except it incorporates a premodulation Gaussian LPF Used extensively in 2 nd generation digital cellular and cordless telephone apps. such as GSM
GMSK Improvement Achieves smooth phase transitions between signal states which can significantly reduce bandwidth requirements
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.
GMSK Tradeoffs Continued A compromise between spectral efficiency and time-domain performance must be made.