Wireless: End of Chapter 2

Went over audio stuff, sample size and rate.  Not sure what this has to do with chapter 2….

Digital Modulation Cont.

-Binary signals are digital data transmissions are typically sent in bursts of bits.  There are three types of binary signaling techniques that can be used:

Return-to-zero (RZ) calls for the signal to rise to represent 1 bit.  It must return to zero in its allotted time.

Non-return-to-zero (NRZ) the voltage signal remains positive, or high, for the entire length of the bit period.

Polar non-return-to-zero (polar NRZ) raises the signal to represent a 1 bit, but drops to a negative amount to represent a 0 bit.  Works well with light by using different colors.

A variation on a non-return-to-zero-level is a non-return-to-zero, invert-on-ones (NRZ-I).  A change in voltage level represents a 1 bit, whereas no change in voltage level indicates that the next bit is a 0. 

 The rising edge is on the left of the pulse, the falling edge is on the right.  The top is the level edge of the pulse.  

More After the Break

 

-Amplitude Shift Keying (ASK) is a binary modulation technique similar to amplitude modulation.  The height of the carrier signal can be changed to represent a 1 bit or a 0 bit.  ASK uses the NRZ coding. 

-Frequency Shift Keying (FSK) is a binary modulation technique that changes the frequency of the carrier signal.  More wave cycles are needed to represent a 1 bit. 

 

-Phase Shift Keying (PSK) is a binary modulation technique similar to phase modulation.  The transmitter varies the starting point of the wave.  The PSK signal starts and stops because it is a binary signal.  Quadrature amplitude modulation (QAM) is a technique of combining amplitude and phase modulation. Receivers can detect phase changes much more reliably than a frequency or amplitude change in the presence of noise.  PSK-based systems are more attractive for high-speed wireless communications.  Quadrature phase shift keying (QPSK) combines amplitude modulation with PSK. 
 

Ps- Phase Shift.  All of the points are exactly the same distance from the origin. On figure 2-31, you can now get 16 vs just 8 from 2-30.  0 is the first 0degrees, the second 0degrees on the  line will be 8 and so on.   

Spread Spectrum

-Narrow-band transmissions signals transmit on one radio frequency or a very narrow range of frequencies.  They are vulnerable to outside interference from another signal.  Radio signal transmissions are considered narrow-band.

-Spread spectrum transmission takes a narrow band signal and spreads it over a broader portion of the radio frequency band.  It results in less interference and fewer errors.  There are two common methods, frequency hopping and direct sequence.  

-Frequency Hopping Spread Spectrum uses a range of frequencies and changes several times during transmission.  Its hopping code is the sequence of changing frequencies.  The reciving station must also know the hopping code.  Multiple radios can each use a different sequence of frequencies within the same area and never interfere with each other.  If interference is encountered on a frequency only a small part of the message is lost. 

-Direct Sequence Spread Spectrum uses an expanded redundant code to transmit each data bit and then a modulation technique such as QPSK.  A DSSS signal is effectively modulated twice.  A Barker code (aka chipping code) is a particular sequence of 1’s and 0’s.  It is ideal for modulating radio waves as well as for being detected correctly by the receiver.  It is also called a pseudo-random code.  Before transmission, add the original data bit to the chipping code.  DSSS system transmits combinations of multiple chips.  11 chips are transmitted at a rate 11 times faster than the data rate. 

 

-The characteristics are as follows:

The frequency of the digital component of the signal is much higher than that of the original data (chip rate). 

          A plot of the frequency spectrum of this signal would look similar to random noise. 

          All of the information contained in the original signal is still there. 

-Advantages are as follows:

DSSS signal appears to be an unintended narrow-band receiver to be low-powered noise. 

Noise can cause some of the chips to change value.  The receiver can recover the original data bit using statistical techniques and mathematical algorithms, thus avoiding the need for retransmission.

-DSSS devices are typically higher-end products because they are more expensive to manufacture than FHSS systems.