Communications Engineering 1 Missed bits and pieces

Code division multiplexing CDM Code division multiplexing (CDM) allows signals from a series of independent sources to be transmitted at the same time over the same frequency band. This is accomplished by using orthogonal codes to spread each signal over a large, common frequency band. At the receiver, the appropriate orthogonal code is then used again to recover the particular signal intended for a particular user. The key principle of CDM is spread spectrum. Spread spectrum is a means of communication with the following features: 1. Each information-bearing signal is transmitted with a bandwidth in excess of the minimum bandwidth necessary to send the information. 2. The bandwidth is increased by using a spreading code that is independent of the information. 3. The receiver has advance knowledge of the spreading code and uses this knowledge to recover the information from the received, spread-out signal. Spread spectrum seems incredibly counterintuitive. We’ve spent studying ways to transmit information using a minimum of bandwidth. Why should we now study ways to intentionally increase the amount of bandwidth required to transmit a signal? Spread spectrum is a good technique for providing secure, reliable, private communication in an environment with multiple transmitters and receivers. In fact, spread spectrum and CDM are currently being used in an ever-increasing number of commercial cellular telephone systems

Spreading The spreading code has a bit rate 3 times the bit rate of the source => Output has a higher bit rate => Hence the output has a higher bandwidth compared to the source. At the receiver we XOR the received signal with the same spreading code.

Spreading

Spreading

Spreading

Overview of transmission media Copper: Twin lines Coaxial lines Refer to Introduction to ICT notes. Optical fiber: Refer http://www.explainthatstuff.com/fiberoptics.html

Waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound waves. They enable a signal to propagate with minimal loss of energy by restricting expansion to one dimension or two. Without the physical constraint of a waveguide, signals will typically be radiated and decreased according to the inverse square law as they expand into three dimensional space. There are different types of waveguides for each type of wave. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves.

Waveguide

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