analogue signal -> analogue modulation:
AM (Amplitude modulation)
FM (Frequency modulation)
PM (Phase modulation)
digital signal -> digital modulation
ASK (Amplitude Shift Keying) -> OOK (On Off Keying)
FSK -> MSK (Minimum Shift Keying) -> GMSK (Gaussian MSK) application in Europe GSM.
PSK -> PRK (Phase Reversal Keying, BPSK) -> QPSK (Quadrature PSK) application in CDMA.
APK (Amplitude Phase Keying) = ASK + PSK
PCM (Pulse Code Modulation) – sample -> quantization -> encode
DPCM (Differential PCM) two samples next each other are similar. So, the difference between two adjoining samples are coded instead of absolute singals, which reduce the bandwidth.
ADPCM (Adaptive DPCM) the quantising levels are adapted to the shape of the signal, which further reduce the bandwidth.
bandwidth – range of frequency
Nyquist-Shannon sampling theorem (Whittaker-Nyquist-Kotelnikov-Shannon sampling theorem) - when sampling a signal (e.g., converting from an analog signal to digital), the sampling frequency must be greater than twice the bandwidth of the input signal in order to be able to reconstruct the original perfectly from the sampled version. If B is the bandwidth and Fs is the sampling rate, then the theorem can be stated mathematically: Fs > 2B
Shannon-Hartley theorem - The maximum theoretical error free information capacity for a channel of bandwidth B (Hz) with Signal to ratio S/N in the presence of Gaussian noise is given by:
C =Blog2 (1+S/N) Bits/second
SNR (signal-to-noise ratio)
SNR = (Eb/No)* (D/B)
D/B, spectral efficiency;
energy per transmitted bit (Eb joules) required to achieve a specified probability of bit error (Pb) in the presence of gaussian noise (No watts/Hz).
QPSK application in Cable upstream
64-QAM/256-QAM application in Cable downstream
BPSK/DBPSK——application in WLAN，1Mbps；
QPSK/DQPSK——application in WLAN，2Mbps；
CCK/PBCC——application in WLAN，5.5/11Mbps；
PBCC-22——application in WLAN，802.11b+(802.11g),22Mbps/2.4GHz；
16-QAM/64-QAM——application in WLAN 802.11a，54Mbps？（802.11a，6、9、12、18、24、36、48、54Mbps）
时间: 2005-2-4 04:08
作者: sinca
03-02-05 Transmission line overview
There are three effects may occurs in transmission
•Delay Td = length / velocity, delay may cause dispersion (signals with different frequency arrives the destination with different delay)
•Attenuation (dB/m) is the loss of the transmission line, typical attenuation figures at about 1GHz are:
microstrip 10 dB/m
coaxial 0.04 dB/m
waveguide 0.004 dB/m
•Reflection. Different transmission lines with different line impedance (e.g. 50for general RF use; 75 for video and studio interconnections) while two transmissions are not completely matched, parts of the signals may reflected.
There are three ways to express the reflection:
1. Reflection coefficient = reflected voltage/ incident voltate = (Zload - Zo)/ (Zload + Zo)
where Zload = load impedance
Zo = line characteristic impedance
2. return loss (dB) = reflected signal/incident signal
(range from (dB) for a perfect match to 0 (dB) for a short or open circuit)
3. vswr (voltage standing wave ratio)
(range from 0 (dB) for a perfect match to (dB) for a short or open circuit)
时间: 2005-2-5 15:02
作者: sinca
04-02-05 Transmission lines
Telecommunication spectrum
Coaxial cable
Coaxial cable was invented in 1929 and first used commercially in 1941.
There are two kinds of widely used coaxial cables: 50Ω coaxial cable, which is often be referred to as baseband coaxial cable, is used to transmit digital signal; 75Ω coaxial cable, which is called as broad band coaxial cable, is used to transmit broad band analogue signal.
Twisted pair
Twisted - pair cables are consisted of copper wires surrounded by insulator, which twisted together to prevent interference. A pair could be used as a single circuit to transmit data.
UTP (unshielded twist-pair) is commonly used rather than STP (shielded twist-pair) that provides protection against cross talk.
The EIA/TIA defines the following categories of twist-pair:
Category 1 - Traditional telephone cable.
Category 2 - For data transmission up to 4Mbps.
Category 3 - For data transmission up to 16Mbps.
Category 4 - For data transmission up to 20Mbps.
Category 5 - For data transmission up to 100Mbps.
Category 5e/6 - For data transmission up to 350Mbps.
Category 7 - For data transmission up to 1Gbps.
Crosstalk exists in twisted-pair cables. There are two varieties of crosstalk: near-end crosstalk and far-end crosstalk.
Optical fiber
Optical fiber is made of silicon. It's cheap in cost, but good in performance.
Semiconductor light sources such as LED (light-emitting diode) and LD (laser diode) was used as the transmitter. Till now, the range of operable wavelength is from 650nm to 1550nm, with the key operating windows as 850nm, 1300nm and 1500nm.
PIN and APD (avalanche photodiode) are often used as receivers.
According to the transmission characteristics, there are single mode fiber and multi-mode fiber.
According to the refractive index, there are step index fiber and graded index fiber.
The adoption of WDM (wavelength-division multiplexing) enhances the performance of fiber.
Optical fiber cables are widely used as trunk links in the telecommunication networks. Gradually, with the development of optical manufacture and the increasing customer request, optical fiber cables are used in customer-access networks.
时间: 2005-2-8 05:33
作者: sinca
07-02-05 optical fiber
Transmission in optical fiber is realized by imply Snell’s law. Optical fiber has the core with high index material surrounded by low index cladding, which results total internal reflection.
According to the refractive index difference between the core and cladding, fiber could be divided into:
• Step index fiber which includes step index multi-mode (SIMM) and step index single mode (V < 2.405).
• Graded index fiber (graded index fiber supports less modes than step index fiber, with the same value V)
Dispersion in optical fiber:
• For multimode fiber, total dispersion = intermodal + intramodal (= material + waveguide (for mm, it is negligible))
• For single-mode fiber, total dispersion = material + waveguide
Intramoal dispersion = chromatic dispersion = material dispersion
Data rate has limitation due to dispersion,
Loss in optical fiber:
Loss = intrinsic [= absorption (part caused by UV, part caused by OH, windows between 1.3 and 1.55) + scattering (Rayleigh scattering, utilized by OTDR] + bending + (splices + connector) + coupling loss
Optical fiber connection
Fresnel reflection is caused by the light reflected at the joint point, though two fibers are perfectly connected. It can be reduced by using index matching fluids.
Fiber splice – permanent joint between fibers, includes fusion splicing and mechnical splicing.
Fibre connecters (between 0.2 to 3 dB)
Fibre coupler – distribute light from one fibre several branches, or combine light from several fibres to a single one.
Total system consideration
Pi = Po + (fc + j)L + cr + DL + Ma dB
fc, cable loss; j, splice loss; cr, connector loss; DL, dispersion-equalisation penalty or ISI penalty; Ma, safety margin; Pi, input power; Po, output power)
There are two kinds of light source for optical fibre system:
Incoherent sources – light emitting diodes (LED) 850nm, 1310nm -> multimode fibre
Coherent sources – laser(light amplification by stimulated emission of radiation) diodes, 1310nm, 1550nm
时间: 2005-2-11 04:50
作者: sinca
08-02-05 Antenna
Gain = Efficiency x Directivity = (Ae is the effective aperture)
Antenna types: wire (dipole), aperture (horn), array (yagi).
A half wave dipole has: impedance=73,
gain=2.1dB,
polarised parallel to axis,
no radiation directly off the tips
Monopoles provide:
twice the gain of a dipole, (ie 5.1dB for quarter wave)
half the radiation resistance of a dipole, (ie 36.5 for quarter wave)
twice the capacitance of a dipole, (electrically short)
Received power calculation:
Pd=E.H (Watts/m2) electric field strength E (volts/m) and magnetic field strength H (Amps/m).
