Wireless Oom » wireless

What was the Shannon’s key observation?

wirelessoom — Thu, 27 May 2010 00:49:25 +0000

The figure depicts the Shannon’s model of communication.

Shannon recognized that interference (noise) plays significant role in communication system.
The analysis of channel capacity is incomplete without including noise in the calculation.

What is dBm?

wirelessoom — Thu, 27 May 2010 00:33:44 +0000

dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form.

What is dB?

wirelessoom — Thu, 27 May 2010 00:27:13 +0000

It’s a jargon engineers or physicists are using.
It’s a promise.
It’s about describing a ratio between two quantities.
How big is the one compared to the other?

For example, Chan’s height is 40 inch and Han’s height is 80 inch.
Han is two times higher than Chan.

Using dB, we say

10 log (p2/p1) = 10 log 2 = 3 dB (where p2 is Han’s height and p1 is Chan’s height)

Note here that base is 10.

Q: Then, the real question is why we are using this complicated thing ?
Logarithm is being used to express too small or too large number in an easy manner.

For example, one million bigger, then
10 log 1000000 = 60 dB

Why is bandpass modulation needed for radio transmission?

wirelessoom — Wed, 24 Jun 2009 23:58:35 +0000

In other words, why is it necessary to carry a message waveform onto a carrier wave of much larger frequency than that of the message?

Is it possible to configure a radio with only a microphone, power amplifier and antenna?

wavelenth = light speed / frequency

Thus, for human voice (whose frequency is around 3000Hz),

wavelength = 3*10^8 / 3000Hz=100000m = 100km

The antenna size should be really large.

If carrier frequency is 3*10^9, then the wavelength is 0.1 meter which is a feasible and reasonable antenna size.

Furthermore, how you can differentiate your local voice and far away voice?

What is CRC?

wirelessoom — Wed, 24 Jun 2009 05:50:35 +0000

CRC: Cyclic Redundancy Check

Some redundant bits are attached to each message to detect the existence of errors at receiving side

Basic concept:

Given a k-bit block of bits (or messages), the transmitter generates an (n-k) bit sequence, known as a frame check sequence (FCS),

such that the resulting frame (consisting of n bits) is exactly divisible by some predetermined number.

The receiver then divides the incoming frame by that number and, if there is no remainder, receiver can assume there was no error.

What is for interleaving ?

wirelessoom — Tue, 23 Jun 2009 16:43:40 +0000

If there are consecutive 4 bit errors in a byte, it is difficult to reconstruct original byte even using channel coding.

Interleaving provides time diversity, which spreads signal in time domain.

Note that time delay will occur.

Does a receiver keep sending and receiving at the same time in CDMA ?

wirelessoom — Tue, 23 Jun 2009 06:58:31 +0000

YES!

Why use dB in communication?

wirelessoom — Sun, 21 Jun 2009 05:42:30 +0000

Because many signals have a very wide dynamic range, SNRs are usually expressed in terms of the logarithmic decibel scale.

In decibels, the SNR is, by definition, 10 times the logarithm of the power ratio.

10log10(Preceived/Pnoise)

What for Eb/N0 vs. BER (Bit error probability) ? That is, how to use it?

wirelessoom — Sun, 21 Jun 2009 05:39:05 +0000

Different communication systems will have different curve of Eb/No vs. BER

Two ways to compare– vertical line and horizontal line:

1. horizontal line:

(for a given BER) some system needs less energy, which is good.

2. vertical line:

(for the same amount of Eb/No) some system provides less BER, which is good.

Furthermore, based on the graph, transmit power can be calculated.

Why is Eb/N0 an important metric for digital communication?

wirelessoom — Sun, 21 Jun 2009 05:37:08 +0000

Q: What is Eb/N0?

Eb/No is S/N (Signal to Noise ratio) divided by spectral efficiency.

“SNR per bit”

Q: Why is Eb/N0 important in digital communication?

Motivation:

- analog waveform is a power signal (infinite energy, finite average power)

- digital waveform is an energy signal (zero average power, non-zero finite energy) since a waveform is transmitted only during each symbol duration.

- the digital waveform represents a digital message. The message may contain 1 bit(binary) 2 bit(4-ary)…

- suppose we are told that the waveform S/N=20…

- if the waveform had 1 bit meaning, then all 20 units of S/N ratio are allocated to that 1 bit

- if the waveform had 10 bit meaning, then each bit has 2 unit of S/N.