Signal propagation in free space travels in a straight line, with receiving power declining proportional to the square of the distance between sender and receiver. In real environments, additional factors like fading, shadowing, reflection, refraction, scattering, and diffraction can affect the receiving power. Multiplexing allows for the multiple use of a shared medium by separating signals using space, time, frequency, or code. Examples of multiplexing techniques include frequency division multiplexing, which separates the spectrum into frequency bands; time division multiplexing, which allocates the full spectrum to each channel for short time periods; and code division multiplexing, which assigns unique codes to each channel to share the full spectrum simultaneously.

2. SIGNAL PROPAGATION
Propagation in free space always like light (straight line)
Receiving power declines proportional to 1/d2 in vacuum
(d = distance between sender and receiver)
Receiving power in real environments additionally
affected by
• fading due to the atmosphere and large distances
• shadowing
• reflection at large obstacles
• refraction depending on the density of a medium
• scattering at small obstacles
• diffraction at edges

3. Transmission range
• communication possible in both directions
• relatively low error rate
Detection range
• detection of the signalpossible no communication
possible
(error rate too high)
Interference range
• signal may not be
detected
• signal adds to the
• background noise
• and may interfere
• other transmissions

4. Multipath propagation
Signals can take many different paths between sender
and receiver arriving at
different times with different signal strength at the
receiver
Run time dispersion (delay spread): signal is dispersed
over time
• signal is divided into weaker pulses
• interference with “neighbor” symbols, Inter Symbol
Interference (ISI)
• reduction of effective bandwidth

5. MULTIPLEXING
Goal:
multiple use of a shared medium realized
on the physical layer (or on the MAC layer as in the
wired case)
Multiplexing can be done regarding
4 dimensions (parameters):
• space (si)
• time (t)
• frequency (f)
• code (c)

6. Example for Space Division Multiplexing (SDM):

7. Frequency Division Multiplexing (FDM):
Separation of the whole available spectrum into smaller
frequency bands
A channel gets a certain band of the spectrum exclusively
for the whole time
Advantages:
• no complex coordination necessary between sender and
receiver

8. Disadvantages:
• waste of bandwidth if channels are used not
permanently, at different times
• inflexible
• heavy restriction on #of potential senders

9. Time Division Multiplex (TDM):
A channel gets the whole frequency spectrum for a
certain amount of time
Advantages:
• only one carrier in the medium at any time
• throughput high even for many users.
Disadvantages:
• Precise clock synchronization necessary

11. Combination of FDM and TDM:
A channel gets a certain frequency band for a
certain amount of time
Example: GSM for communication with base
station
Advantages:
• better protection against frequency and time
selective interference
• better protection against tapping (security aspect)
Disadvantage:
• complex coordination necessary between sender
and receiver.

13. Code Division Multiplex (CDM):
Each channel has a unique code
All channels use the same spectrumat the same time
Big Advantage:
• good protection against interference and tapping
Disadvantages:
• more complex signal regeneration in the receiver
(decoding)
• signals from the communication partner should
arrive with highest signal strength
• lower user data rates