PN (Pseudo-Noise) sequences

PN sequence is a noise-like spreading code which is basic to the operation of spread-spectrum modulation. A PN sequence is defined as a coded sequence of 1's and 0's with certain auto-correlation properties. PN sequence is periodic i.e., a sequence of 1's and 0's repeats itself exactly with a known period.
     The 'maximum-length sequence' represents a commonly used periodic PN sequence. Such sequences have long periods and require simple instrumentation in the form of a linear feedback shift register.
Maximum-length sequence generator for m=3 is shown in the figure below.                                                
                                                                                                                                                           
  

Spread Spectrum Systems

Introduction:

      
A major issue in digital communication is to provide the efficient utilization of bandwidth and power. For example, the system may be required to provide a form of secure communication in a hostile environment such that the transmitted signal is not easily detected or recognized by unwanted listeners. This requirement is catered to by a class of signalling techniques known collectively as Spread Spectrum Modulation. The primary advantage of a spread spectrum communication is its ability to reject interference whether it be the unintentional interference of another user or the intentional interference of a hostile transmitter attempting to jam the transmission. 

Definition:

1. Spread spectrum is a means of transmission in which the data of interest occupies a bandwidth in excess of the minimum bandwidth necessary to send the data.
2. The spectrum spreading is accomplished before transmission through the use of of a code that is independent of the data sequence. The same code is used in the receiver to de-spread the received signal to recover the original data.

Applications:

• Military applications where resistance to jamming is of major concern.
• Provides multipath rejection in a ground-based mobile radio environment.
• Multiple-access communication in which a number of independent users are required to share a common channel without an external synchronizing mechanism.
• Code-division multiple access (CDMA). 

        

Noise in communication systems

NOISE:

In every communication system, unwanted random signals or noise is present. Noise is defined as an unwanted form of electrical signal which tends to interfere with the desired signals. Many disturbances of an electrical nature produce noise in receivers, modifying the signal in an unwanted manner.

• In radio receiver noise may produce hiss in loud speaker output.
• In television receivers it may appear as snow or confetti(colored snow) superimposed on the picture.
• In pulse communication systems, noise may produce unwanted pulses which may cancel the wanted ones and lead to serious error in the detected signal.

Noise is thus seen as limiting the range of systems for a given transmitted power. It also affects the sensitivity(ability to amplify weak signals) of receivers.


Classification:

External noise:-   originating from sources outside the receiver.


Man-made noise: 
               This noise is strongest in industrial area. The frequency of man-made noise ranges between 1 to 600 MHz. The intensity of noise made by human easily outstrips than that created by any other source, internal or external to the receiver.

Atmospheric noise:
              This noise arises from lightning during thunderstorms and other electrical disturbances occurring in the atmosphere. This noise becomes less at frequencies above 30 MHz because the effect of noise components is very less in the VHF(Very High Frequency) range and above.

Extra-terrestrial noise:
              It arises from sun and other distant stars.

Solar noise:
              It arises from the sun. Under normal conditions, there is a constant noise radiated from the sun simply because it is a large body heated at a very high temperature(6000 •C on the surface).

Cosmic noise:
              This refers to noise coming from distant stars other than sun. These stars are also heated bodies with radiated signals spreading a wide range of frequency. This is the strongest component of noise in the range 20-120 MHz.


Internal noise:-   arising from the system itself.

Shot noise:
              Shot noise arises in electronic devices because of discrete nature of current flow in the device. When this noise is amplified and fed to loudspeaker it sounds like a shower of lead shots striking a metal plate and hence the name 'shot noise'.

Thermal noise:   
              This noise arises due to the random motion of free electrons in the conducting medium such as resistors. Each free electron inside a resistor is in motion due to its thermal energy. 
         



                                     

Bus Structures in Computer Organization

If you are interested in learning about computer organization and architecture, the basic thing you need to know is about a 'BUS'.

What is a bus?
When a word of data is transferred between units then all its bits are transferred in parallel i.e; the bits are transferred simultaneously over many wires or lines as one bit per line. A group of such lines that serves as connecting path for several devices is called a 'bus'.
Bus structures are of two types namely - 'Single bus structure' and 'Multiple bus structure'.

Single Bus Structure:
The simplest way to interconnect functional units is to use a single bus as shown below. 

                                  

All units are connected to this bus.The main advantage of the single-bus structure is its low cost and its flexibility for attaching peripheral devices. Memory and processor units operate at electronic speeds, making them the fastest parts of a computer. 

Multiple Bus Structure:
It is of two types namely- 'Traditional bus architecture' and 'High performance bus architecture'. 

                                     

                                


There is a local bus that connects the processor to a cache memory and a local i/o controller. The cache memory controller connects to the system bus also, which is attached to all the main memory modules.
It is possible to connect i/o controllers directly onto the system bus and the most efficient solution is to make use of one or more expansion buses.
           An expansion bus interface buffers data transfers between the system bus and the i/o controllers on the expansion bus.  Some of the devices attached to the expansion bus are:

• Network connections which include Local Area Network(LAN) and Wide Area Network(WAN).
• SCSI(Small Computer System Interface) is itself a type of bus used to support local disk drives and other peripherals.
• Modem(Modulator and Demodulator) is used to convert analog signals to digital signals and vice versa.
• A serial port could be used to support a printer or scanner.

                                

The traditional bus architecture is efficient but begins to breakdown as higher and higher performance is seen in the i/o devices. So we go for the 'High Performance Bus Architecture' in which a high-speed bus is used.