Bandwidth Efficient System , define bandwidth efficiency in digital communication

, define bandwidth efficiency in digital communication , what is  Bandwidth efficiency ?
Bandwidth Efficient System
All the basic modulation and demodulation techniques try to achieve more and more power and bandwidth efficiency in a stationary additive white Gaussain noise channel. Because bandwidth is a limited and natural resource, one of the primary design objective of all the basic modulation schemes is to minimize the required transmission bandwidth. Special spectrum techniques, on the other hand, employ a transmission bandwidth that is several orders of magnitude greate than the minimum required signal bandwidth. While this system is very bandwidth inefficient for a single user, the advantage of spread-spectrumis that many users can simultaneously use the same bandwidth without significantly interfering with one another. In a multiple user, multiple access interference (MAI) environment, spread spectrum system become very bandwidth efficient.
The primary advantage of a spread- spectrum communication system is its ability to reject interference whether it be the unintentional interference by another user simultaneously attempting to transmit through the channel, or the intentional interference by a hostile transmitter attempting to jam the transmission.
The definition of spread-spectrum modulation may be stated in following two parts:
(i)         Spread spectrum is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it.
(ii)        The spectrum spreading is accomplished before transmission through the use of a code that is independent of the data sequence. The same code is used in the receiver (operating in synchronism with the transmitter) to despread the received signal so that the original data sequence may be recovered.
Although standard modulation techniques such as frequency modulation and pulse-code modulation do satisfy part (i) of this definition, they are not spread-spectrum techniques because they do not satisfy part (ii) of the definition.
Spread-spectrum modulation was originally developed for military applications, where resistance to jamming (interference) is of major concern. However, there are civilian applications that also benefit from the unique characteristics of spread-spectrum modulation. For example, it can be used to provide multipath rejection in a ground-based mobile radio environment. Yet another application is in multiple-access communications in which a number of independent users are required to share a common channel without an external synchronizing mechanism. Here, for example, we may mention a ground-based radio environment involving mobile vehicles that must communicate with a central station.
(ii) Pseudo Noise (PN) Sequence or Pseudo Noise Code
            Apart from occupying a very large bandwidth, spread spectrum m signals are pseudorandom and have noise like properties when compared with the digital information data. The spreading Waveform is controlled by a pseudo-noise (PN) sequence in pseudo-noise code, which is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers. Spread spectrum signals are demodulated at the receiver through cross-correlation with a locally-generated version of the pseudorandom carrier. Cross-correlation with the correct PN sequence despreads the spread spectrum signal and restores the modulated message in the same narrow band as the original data, where Cross-correlating the signal from an undesired user results in a very small amount of wideband noise at the receiver output.
(iii)       Special Properties
Spread spectrum modulation has several properties that make it particularly well-suited for the mobile radio environment.
The most important advantage is its inherent interference rejection capability. Since each user is assigned a unique PN code which is approximately orthogonal to the codes of other users, the receiver can separate each user based on their codes, even though they occupy the same spectrum at all times.

This means that up to a certain number of users, interference between spread spectrum signals using the same frequency is negligible.
Not only can a particular spread spectrum signal be recovered from a number of other spread spectrum signals, it is also possible to completely recover a spread spectrum signal even when it is jammed by a narrowband interferer.
Since narrowband interference effects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information.
Since all users are able to share the same spectrum, spread spectrum may eliminate frequency planning, since all cells can use the same channels.
Resistance to multipath fading is another fundamental reason for considering spread spectrum systems for wireless communications. We know that wideband signals are frequency selective. Since spread spectrum signals have uniform energy over a very large bandwidth, at any given time only a small portion of the spectrum will undergo fading.
Viewed in the time domain, the multipath resistance properties are due to the fact that the delayed versions of the transmitted PN signal will have poor correlation with the original PN sequence, and will thus appear as another uncorrelated user which is ignored by the receiver. This means that long as the multipath channel induces at least one chip of delay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal.
The correlation properties of PN sequences are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, so that the multipath contributions appear invisible to the desired received signal.
Spread spectrum systems are not only resistant to multipath fading, but they can also exploit the decayed multipath components to improve the performance of the system. This can be done using a RAKE receiver which anticipates multipath propagation delays of the transmitted spread spectrum signal and combines the information obtained from several resolvable multipath components to form a stronger version of the signal.
A RAKE receiver consists of a bank of correlators, each of which correlate to a particular multipath component of the desired signal. The correlator outputs may be weighted according to their relative strengths and summed to obtain the final signal estimate.
In this chapter, we discuss principles of spread-spectrum modulation, with emphasis on direct-sequence and frequency-hopping techniques. In a direct-sequence spread-spectrum technique, two stages of modulation are used. First, the incoming date sequence is used to modulate a wideband code. This code transforms the narrowband data sequence into a noiselike wideband signal. The resulting wideband signal undergoes a second modulation using a phase-shift keying technique. In a frequency-hop spread-spectrum technique, on the other hand, the spectrum of a data modulated carrier is widened by changing the carrier frequency in a pseudo-random manner. For their operation, both of these techniques rely on the availability of a noiselike spreading code called a pseudo-random or pseudo-noise sequence. Since such a sequence is basic to the operation of spread-spectrum modulation, it is logical that we begin our study by describing the generation and properties of pseudo-noise sequences.