Radio wave attribute:
- Amplitude
- Frequency
- Phase
RF Transceiver consist of transmitter and receiver.
RF Modulation converting the digital signal representing digital data to analog signal.
Main forms of modulation:
- Amplitude Shift-Keying (ASK)
- Frequency Shift-Keying (FSK)
- Phase Shift-Keying (PSK)
Quadrature Amplitude Modulation (QAM)
Spread Spectrum
Spread spectrum radio components use either direct sequence or frequency hopping for spreading the signal. Direct sequence modulates a radio carrier by a digital code with a bit rate much higher than the information signal bandwidth. Frequency hopping quickly hops the radio carrier from one frequency to another within a specific range.
Orthogonal Frequency Division Multiplexing
OFDM divides a signal modulated with FSK, PSK, or QAM across multiple sub-carriers
occupying a specific channel. OFDM is extremely efficient, which enables it to provide
the higher data rates and minimize multi-path propagation problems. OFDM has also been around for a while
supporting the global standard for asymmetric digital subscriber line (ADSL), a high speed wired telephony
standard.
RF Signal Propagation
RF Signal Propagation has some problem:
- Attenuation
- Multipath Propagation
- Noise
Attenuation (Затухание)
1.Free Space Loss (FSL)
A large part of the decrease in amplitude with attenuation results from what’s known as free space loss (FSL). The atmosphere causes the modulated signal to attenuate exponentially as the signal propagates farther away from the antenna. The amplitude of a radio wave is proportional to the inverse of the square of the distance from the source.
You can calculate the attenuation (in dB) of a radio signal in free space (line of sight with no obstructions)
using the following formula:
FSL (dB) = 20Log10(d) + 20Log10(f) – 147.56
where d is the distance from the transmitter in meters, and f is the f is the frequency in Hertz. For example, you can calculate the FSL for an 802.11n (2.4 GHz signal) propagating over 100 meters as follows:
FSL (dB) = 20Log10(100) + 20Log10(2,400,000,000) – 147.56 = 80 dB
If using 5 GHz frequencies for the 802.11n implementation, the free space loss at 100 meters would be as
follows:
FSL (dB) = 20Log10(100) + 20Log10(5,000,000,000) – 147.56 = 86 dB
Thus, the attenuation of radio waves based on free space loss of a typical 802.11n system would is 80 to 86 dB over 100 meters.
2. Obstacles
As radio waves have travel through physical obstacles, such as walls, and ceilings, they decrease much more compared to when traveling through open air. The amount of attention varies significantly depending on the material, but a typical radio wave used in wireless LANs at signal amplitude of -60 dBm, for example, will decrease to approximately -63 dBm after going through an interior wall of a building
Multipath Propagation
Multipath propagation occurs when portions of radio wave take different paths when propagating from source to destination. A portion of the signal may go directly to the destination, and another part may bounce from a desk to the ceiling, and then to the destination. As a result, some of the signal will encounter delay and travel longer paths to the receiver.
Noise and SNR
The ability of the receiver to make sense of the radio wave it receives depends on the presence of other nearby radio waves, referred to as interfering signals or noise. This noise can distort the communications, making it difficult for the receiver to correctly understand the data being sent.
