Category: wireless Communication

Wireless Communication – Quick Guide Wireless Communication – Overview Wireless communication involves the transmission of information over a distance without the help of wires, cables or any other forms of electrical conductors. Wireless communication is a broad term that incorporates all procedures and forms of connecting and communicating between two or more devices using a wireless signal through wireless communication technologies and devices. Features of Wireless Communication The evolution of wireless technology has brought many advancements with its effective features. The transmitted distance can be anywhere between a few meters (for example, a television”s remote control) and thousands of kilometers (for example, radio communication). Wireless communication can be used for cellular telephony, wireless access to the internet, wireless home networking, and so on. Other examples of applications of radio wireless technology include GPS units, garage door openers, wireless computer mice, keyboards and headsets, headphones, radio receivers, satellite television, broadcast television and cordless telephones. Wireless – Advantages Wireless communication involves transfer of information without any physical connection between two or more points. Because of this absence of any ”physical infrastructure”, wireless communication has certain advantages. This would often include collapsing distance or space. Wireless communication has several advantages; the most important ones are discussed below − Cost effectiveness Wired communication entails the use of connection wires. In wireless networks, communication does not require elaborate physical infrastructure or maintenance practices. Hence the cost is reduced. Example − Any company providing wireless communication services does not incur a lot of costs, and as a result, it is able to charge cheaply with regard to its customer fees. Flexibility Wireless communication enables people to communicate regardless of their location. It is not necessary to be in an office or some telephone booth in order to pass and receive messages. Miners in the outback can rely on satellite phones to call their loved ones, and thus, help improve their general welfare by keeping them in touch with the people who mean the most to them. Convenience Wireless communication devices like mobile phones are quite simple and therefore allow anyone to use them, wherever they may be. There is no need to physically connect anything in order to receive or pass messages. Example − Wireless communications services can also be seen in Internet technologies such as Wi-Fi. With no network cables hampering movement, we can now connect with almost anyone, anywhere, anytime. Speed Improvements can also be seen in speed. The network connectivity or the accessibility were much improved in accuracy and speed. Example − A wireless remote can operate a system faster than a wired one. The wireless control of a machine can easily stop its working if something goes wrong, whereas direct operation can’t act so fast. Accessibility The wireless technology helps easy accessibility as the remote areas where ground lines can’t be properly laid, are being easily connected to the network. Example − In rural regions, online education is now possible. Educators no longer need to travel to far-flung areas to teach their lessons. Thanks to live streaming of their educational modules. Constant connectivity Constant connectivity also ensures that people can respond to emergencies relatively quickly. Example − A wireless mobile can ensure you a constant connectivity though you move from place to place or while you travel, whereas a wired land line can’t. Terms in Mobile Telephony Among the various terms used in Mobile telephony, the most used ones will be discussed here. Mobile Station (MS) − The Mobile Station (MS) communicates the information with the user and modifies it to the transmission protocols of the air interface to communicate with the BSS. The user information communicates with the MS through a microphone and speaker for the speech, keyboard and display for short messaging and the cable connection for other data terminals. The mobile station has two elements Mobile Equipment (ME) and Subscriber Identity Module (SIM). Mobile Equipment (ME) − ME is a piece of hardware that the customer purchases from the equipment manufacturer. The hardware piece contains all the components needed for the implementation of the protocols to interface with the user and the air-interface to the base stations. Subscriber Identity Module (SIM) − This is a smart card issued at the subscription to identify the specifications of a user such as address and type of service. The calls in the GSM are directed to the SIM rather than the terminal. SMS are also stored in the SIM card. It carries every user”s personal information which enables a number of useful applications. Base Station (BS) − A base station transmits and receives user data. When a mobile is only responsible for its user”s data transmission and reception, a base station is capable to handle the calls of several subscribers simultaneously. Base Transceiver Station (BTS) − The user data transmission takes place between the mobile phone and the base station (BS) through the base transceiver station. A transceiver is a circuit which transmits and receives, i.e., does both. Mobile Switching Center (MSC) − MSC is the hardware part of the wireless switch that can communicate with PSTN switches using the Signaling System 7 (SS7) protocol as well as other MSCs in the coverage area of a service provider. The MSC also provides for communication with other wired and wireless networks as well as support for registration and maintenance of the connection with the mobile stations. The following image illustrates the parts of different sub-systems. HLR, VLR, EIR and AuC are the sub-systems of Network sub-system. Channels − It is a range of frequency allotted to particular service or systems. Control Channel − Radio channel used for transmission of call setup, call request, call initiation and other beacon or control purposes. Forward Control Channel(FCC) − Radio channel used for transmission of information from the base station to the mobile Reverse Channel(RC) − Radio channel used for transmission of information from the mobile to base station. Voice Channel(VC) − Radio channel used for voice or data transmission. Handoff − It is defined as

Wireless Communication – Techniques In some cases, there is a scope of performance deterioration, which affects the output. The major cause for this might be the mobile channel impairments. To resolve this, there are three popular techniques − Equalizer An equalizer within a receiver compensates for the average range of expected channel amplitude and delay characteristics. In other words, an equalizer is a filter at the mobile receiver whose impulse response is inverse of the channel impulse response. Such equalizers find their use in frequency selective fading channels. Diversity Diversity is another technique used to compensate fast fading and is usually implemented using two or more receiving antennas. It is usually employed to reduce the depths and duration of the fades experienced by a receiver in a flat fading channel. Channel Coding Channel coding improves mobile communication link performance by adding redundant data bits in the transmitted message. At the baseband portion of the transmitter, a channel coder maps a digital message sequence in to another specific code sequence containing greater number of bits than original contained in the message. Channel Coding is used to correct deep fading or spectral null. Equalization ISI (Inter Symbol Interference) has been identified as one of the major obstacles to high speed data transmission over mobile radio channels. If the modulation bandwidth exceeds the coherence bandwidth of the radio channel (i.e., frequency selective fading), modulation pulses are spread in time, causing ISI. An equalizer at the front end of a receiver compensates for the average range of expected channel amplitude and delay characteristics. As the mobile fading channels are random and time varying, equalizers must track the time-varying characteristics of the mobile channel and therefore should be time varying or adaptive. An adaptive equalizer has two phases of operation: training and tracking. Training Mode Initially a known, fixed length training sequence is sent by the transmitter so that the receiver equalizer may average to a proper setting. Training sequence is typically a pseudo-random binary signal or a fixed, of prescribed bit pattern. The training sequence is designed to permit an equalizer at the receiver to acquire the proper filter coefficient in the worst possible channel condition. An adaptive filter at the receiver thus uses a recursive algorithm to evaluate the channel and estimate filter coefficients to compensate for the channel. Tracking Mode When the training sequence is finished the filter coefficients are near optimal. Immediately following the training sequence, user data is sent. When the data of the users are received, the adaptive algorithms of the equalizer tracks the changing channel. As a result, the adaptive equalizer continuously changes the filter characteristics over time. Diversity Diversity is a powerful communication receiver technique that provides wireless link improvement at a relatively low cost. Diversity techniques are used in wireless communications systems to primarily to improve performance over a fading radio channel. In such a system, the receiver is provided with multiple copies of the same information signal which are transmitted over two or more real or virtual communication channels. Thus the basic idea of diversity is repetition or redundancy of information. In virtually all the applications, the diversity decisions are made by the receiver and are unknown to the transmitter. Types of Diversity Fading can be classified into small scale and large scale fading. Small-scale fades are characterized by deep and rapid amplitude fluctuations which occur as the mobile moves over distances of just a few wavelengths. For narrow-band signals, this typically results in a Rayleigh faded envelope. In order to prevent deep fades from occurring, microscopic diversity techniques can exploit the rapidly changing signal. If the antenna elements of the receiver are separated by a fraction of the transmitted wavelength, then the various copies of the information signal or generically termed as branches, can be combined suitably or the strongest of them can be chosen as the received signal. Such a diversity technique is termed as Antenna or Space diversity. Frequency Diversity The same information signal is transmitted on different carriers, the frequency separation between them being at least the coherence bandwidth. Time Diversity The information signal is transmitted repeatedly in time at regularly intervals. The separation between the transmit times should be greater than the coherence time, Tc. The time interval depends on the fading rate, and increases with the decrease in the rate of fading. Polarization diversity Here, the electric and magnetic fields of the signal carrying the information are modified and many such signals are used to send the same information. Thus orthogonal type of polarization is obtained. Angle Diversity Here, directional antennas are used to create independent copies of the transmitted signal over multiple paths. Space Diversity In Space diversity, there are multiple receiving antennas placed at different spatial locations, resulting in different (possibly independent) received signals. The difference between the diversity schemes lies in the fact that in the first two schemes, there is wastage of bandwidth due to duplication of the information signal to be sent. Thus problem is avoided in the remaining three schemes, but with the cost of increased antenna complexity. The correlation between signals as a function of distance between the antenna elements is given by the relation − $$rho = J_0^2 lgroupfrac{2Pi d}{lambda}rgroup$$ Where, J0 = Bessel function of zero order and first kind d = distance of separation in space of antenna elements λ = carrier wavelength. Learning working make money

Propagation Losses Antenna and Wave propagation plays a vital role in wireless communication networks. An antenna is an electrical conductor or a system of conductors that radiates/collects (transmits or receives) electromagnetic energy into/from space. An idealized isotropic antenna radiates equally in all directions. Propagation Mechanisms Wireless transmissions propagate in three modes. They are − Ground-wave propagation Sky-wave propagation Line-of-sight propagation Ground wave propagation follows the contour of the earth, while sky wave propagation uses reflection by both earth and ionosphere. Line of sight propagation requires the transmitting and receiving antennas to be within the line of sight of each other. Depending upon the frequency of the underlying signal, the particular mode of propagation is followed. Examples of ground wave and sky wave communication are AM radio and international broadcasts such as BBC. Above 30 MHz, neither ground wave nor sky wave propagation operates and the communication is through line of sight. Transmission Limitations In this section, we will discuss the various limitations that affect electromagnetic wave transmissions. Let us start with attenuation. Attenuation The strength of signal falls with distance over transmission medium. The extent of attenuation is a function of distance, transmission medium, as well as the frequency of the underlying transmission. Distortion Since signals at different frequencies attenuate to different extents, a signal comprising of components over a range of frequencies gets distorted, i.e., the shape of the received signal changes. A standard method of resolving this problem (and recovering the original shape) is to amplify higher frequencies and thus equalize attenuation over a band of frequencies. Dispersion Dispersion is the phenomenon of spreading of a burst of electromagnetic energy during propagation. Bursts of data sent in rapid succession tend to merge due to dispersion. Noise The most pervasive form of noise is thermal noise, which is often modeled using an additive Gaussian model. Thermal noise is due to thermal agitation of electrons and is uniformly distributed across the frequency spectrum. Other forms of noise include − Inter modulation noise (caused by signals produced at frequencies that are sums or differences of carrier frequencies) Crosstalk (interference between two signals) Impulse noise (irregular pulses of high energy caused by external electromagnetic disturbances). While an impulse noise may not have a significant impact on analog data, it has a noticeable effect on digital data, causing burst errors. The above figure clearly illustrates how the noise signal overlaps the original signal and tries to change its characteristics. Fading Fading refers to the variation of the signal strength with respect to time/distance and is widely prevalent in wireless transmissions. The most common causes of fading in the wireless environment are multipath propagation and mobility (of objects as well as the communicating devices). Multipath propagation In wireless media, signals propagate using three principles, which are reflection, scattering, and diffraction. Reflection occurs when the signal encounters a large solid surface, whose size is much larger than the wavelength of the signal, e.g., a solid wall. Diffraction occurs when the signal encounters an edge or a corner, whose size is larger than the wavelength of the signal, e.g., an edge of a wall. Scattering occurs when the signal encounters small objects of size smaller than the wavelength of the signal. One consequence of multipath propagation is that multiple copies of a signal propagation along multiple different paths, arrive at any point at different times. So the signal received at a point is not only affected by the inherent noise, distortion, attenuation, and dispersion in the channel but also the interaction of signals propagated along multiple paths. Delay spread Suppose we transmit a probing pulse from a location and measure the received signal at the recipient location as a function of time. The signal power of the received signal spreads over time due to multipath propagation. The delay spread is determined by the density function of the resulting spread of the delay over time. Average delay spread and root mean square delay spread are the two parameters that can be calculated. Doppler spread This is a measure of spectral broadening caused by the rate of change of the mobile radio channel. It is caused by either relative motion between the mobile and base station or by the movement of objects in the channel. When the velocity of the mobile is high, the Doppler spread is high, and the resulting channel variations are faster than that of the baseband signal, this is referred to as fast fading. When channel variations are slower than the baseband signal variations, then the resulting fading is referred to as slow fading. Learning working make money

Wireless Communication – Useful Resources The following resources contain additional information on Wireless Communication. Please use them to get more in-depth knowledge on this. Useful Video Courses 17 Lectures 57 mins 14 Lectures 50 mins 17 Lectures 1.5 hours 94 Lectures 11.5 hours 9 Lectures 1 hours 8 Lectures 1 hours Learning working make money

Channel Characteristics The wireless channel is susceptible to a variety of transmission impediments such as path loss, interference and blockage. These factors restrict the range, data rate, and the reliability of the wireless transmission. Types of Paths The extent to which these factors affect the transmission depends upon the environmental conditions and the mobility of the transmitter and receiver. The path followed by the signals to get to the receiver, are two types, such as − Direct-path The transmitted signal, when reaches the receiver directly, can be termed as a directpath and the components presents that are present in the signal are called as directpath components. Multi-path The transmitted signal when reaches the receiver, through different directions undergoing different phenomenon, such a path is termed as multi-path and the components of the transmitted signal are called as multi-path components. They are reflected, diffracted and scattered by the environment, and arrive at the receiver shifted in amplitude, frequency and phase with respect to the direct path component. Characteristics of Wireless Channel The most important characteristics of wireless channel are − Path loss Fading Interference Doppler shift In the following sections, we will discuss these channel characteristics one by one. Path Loss Path loss can be expressed as the ratio of the power of the transmitted signal to the power of the same signal received by the receiver, on a given path. It is a function of the propagation distance. Estimation of path loss is very important for designing and deploying wireless communication networks Path loss is dependent on a number of factors such as the radio frequency used and the nature of the terrain. The free space propagation model is the simplest path loss model in which there is a direct-path signal between the transmitter and the receiver, with no atmosphere attenuation or multipath components. In this model, the relationship between the transmitted power Pt and the received power Pr is given by $$P_{r} = P_{t}G_{t}G_{r}(frac{lambda}{4Pi d})^2$$ Where Gt is the transmitter antenna gain Gr is the receiver antenna gain d is the distance between the transmitter and receiver λ is the wavelength of the signal Two-way model also called as two path models is widely used path loss model. The free space model described above assumes that there is only one single path from the transmitter to the receiver. In reality, the signal reaches the receiver through multiple paths. The two path model tries to capture this phenomenon. The model assumes that the signal reaches the receiver through two paths, one a line-of-sight and the other the path through which the reflected wave is received. According to the two-path model, the received power is given by $$P_{r} = P_{t}G_{t}G_{r}(frac{h_{t}h_{r}}{d^2})^2$$ Where pt is the transmitted power Gt represent the antenna gain at the transmitter Gr represent the antenna gain at the receiver d is the distance between the transmitter and receiver ht is the height of the transmitter hr are the height of the receiver Fading Fading refers to the fluctuations in signal strength when received at the receiver. Fading can be classified in to two types − Fast fading/small scale fading and Slow fading/large scale fading Fast fading refers to the rapid fluctuations in the amplitude, phase or multipath delays of the received signal, due to the interference between multiple versions of the same transmitted signal arriving at the receiver at slightly different times. The time between the reception of the first version of the signal and the last echoed signal is called delay spread. The multipath propagation of the transmitted signal, which causes fast fading, is because of the three propagation mechanisms, namely − Reflection Diffraction Scattering The multiple signal paths may sometimes add constructively or sometimes destructively at the receiver causing a variation in the power level of the received signal. The received single envelope of a fast fading signal is said to follow a Rayleigh distribution to see if there is no line-of-sight path between the transmitter and the receiver. Slow Fading The name Slow Fading itself implies that the signal fades away slowly. The features of slow fading are as given below. Slow fading occurs when objects that partially absorb the transmission lie between the transmitter and receiver. Slow fading is so called because the duration of the fade may last for multiple seconds or minutes. Slow fading may occur when the receiver is inside a building and the radio wave must pass through the walls of a building, or when the receiver is temporarily shielded from the transmitter by a building. The obstructing objects cause a random variation in the received signal power. Slow fading may cause the received signal power to vary, though the distance between the transmitter and receiver remains the same. Slow fading is also referred to as shadow fading since the objects that cause the fade, which may be large buildings or other structures, block the direct transmission path from the transmitter to the receiver. Interference Wireless transmissions have to counter interference from a wide variety of sources. Two main forms of interference are − Adjacent channel interference and Co-channel interference. In Adjacent channel interference case, signals in nearby frequencies have components outside their allocated ranges, and these components may interfere with on-going transmission in the adjacent frequencies. It can be avoided by carefully introducing guard bands between the allocated frequency ranges. Co-channel interference, sometimes also referred to as narrow band interference, is due to other nearby systems using the same transmission frequency. Inter-symbol interference is another type of interference, where distortion in the received signal is caused by the temporal spreading and the consequent overlapping of individual pulses in the signal. Adaptive equalization is a commonly used technique for combating inter symbol interference. It involves gathering the dispersed symbol energy into its original time interval. Complex digital processing algorithms are used in the equalization process. Learning working make money

Wireless Communication – TCP/IP The original TCP/IP protocol was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model with the layers named similar to the ones in the OSI model. Comparison between OSI and TCP/IP Suite When we compare the two models, we find that two layers, session and presentation, are missing from the TCP/IP protocol. The application layer in the suite is usually considered to be the combination of three layers in the OSI model. The OSI model specifies which functions belong to each of its layers but the layers of the TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched, depending on the needs of the system. The term hierarchical means that each upper level protocol is supported by one or more lower level protocols. Layers in the TCP/IP Suite The four layers of the TCP/IP model are the host-to-network layer, internet/network layer, transport layer and the application layer. The purpose of each layer in the TCP/IP protocol suite is detailed below. The above image represents the layers of TCP/IP protocol suite. Physical Layer TCP/IP does not define any specific protocol for the physical layer. It supports all of the standard and proprietary protocols. At this level, the communication is between two hops or nodes, either a computer or router. The unit of communication is a single bit. When the connection is established between the two nodes, a stream of bits is flowing between them. The physical layer, however, treats each bit individually. The responsibility of the physical layer, in addition to delivery of bits, matches with what mentioned for the physical layer of the OSI model, but it mostly depends on the underlying technologies that provide links. Data Link Layer TCP/IP does not define any specific protocol for the data link layer either. It supports all of the standard and proprietary protocols. At this level also, the communication is between two hops or nodes. The unit of communication however, is a packet called a frame. A frame is a packet that encapsulates the data received from the network layer with an added header and sometimes a trailer. The head, among other communication information, includes the source and destination of frame. The destination address is needed to define the right recipient of the frame because many nodes may have been connected to the link. The source address is needed for possible response or acknowledgment as may be required by some protocols. LAN, Packet Radio and Point-to-Point protocols are supported in this layer Network Layer At the network layer, TCP/IP supports the Internet Protocol (IP). The Internet Protocol (IP) is the transmission mechanism used by the TCP/IP protocols. IP transports data in packets called datagrams, each of which is transported separately. Datagrams can travel along different routes and can arrive out of sequence or be duplicated. IP does not keep track of the routes and has no facility for reordering datagrams once they arrive at their destination. Transport Layer There is a main difference between the transport layer and the network layer. Although all nodes in a network need to have the network layer, only the two end computers need to have the transport layer. The network layer is responsible for sending individual datagrams from computer A to computer B; the transport layer is responsible for delivering the whole message, which is called a segment, from A to B. A segment may consist of a few or tens of datagrams. The segments need to be broken into datagrams and each datagram has to be delivered to the network layer for transmission. Since the Internet defines a different route for each datagram, the datagrams may arrive out of order and may be lost. The transport layer at computer B needs to wait until all of these datagrams to arrive, assemble them and make a segment out of them. Traditionally, the transport layer was represented in the TCP/IP suite by two protocols: User Datagram Protocol (UDP) and Transmission Control Protocol (TCP). A new protocol called Stream Control Transmission Protocol (SCTP) has been introduced in the last few years. Application Layer The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model. The application layer allows a user to access the services of our private internet or the global Internet. Many protocols are defined at this layer to provide services such as electronic mail file transfer, accessing the World Wide Web, and so on. The protocols supported in this layer are TELNET, FTP and HTTP. Learning working make money

Wireless Communication – Multiple Access Multiple access schemes are used to allow many mobile users to share simultaneously a finite amount of radio spectrum. Multiple Access Techniques In wireless communication systems, it is often desirable to allow the subscriber to send information simultaneously from the mobile station to the base station while receiving information from the base station to the mobile station. A cellular system divides any given area into cells where a mobile unit in each cell communicates with a base station. The main aim in the cellular system design is to be able to increase the capacity of the channel, i.e., to handle as many calls as possible in a given bandwidth with a sufficient level of quality of service. There are several different ways to allow access to the channel. These includes mainly the following − Frequency division multiple-access (FDMA) Time division multiple-access (TDMA) Code division multiple-access (CDMA) Space division multiple access (SDMA) Depending on how the available bandwidth is allocated to the users, these techniques can be classified as narrowband and wideband systems. Narrowband Systems Systems operating with channels substantially narrower than the coherence bandwidth are called as Narrow band systems. Narrow band TDMA allows users to use the same channel but allocates a unique time slot to each user on the channel, thus separating a small number of users in time on a single channel. Wideband Systems In wideband systems, the transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel. Thus, multipath fading doesn’t greatly affect the received signal within a wideband channel, and frequency selective fades occur only in a small fraction of the signal bandwidth. Frequency Division Multiple Access (FDMA) FDMA is the basic technology for advanced mobile phone services. The features of FDMA are as follows. FDMA allots a different sub-band of frequency to each different user to access the network. If FDMA is not in use, the channel is left idle instead of allotting to the other users. FDMA is implemented in Narrowband systems and it is less complex than TDMA. Tight filtering is done here to reduce adjacent channel interference. The base station BS and mobile station MS, transmit and receive simultaneously and continuously in FDMA. Time Division Multiple Access (TDMA) In the cases where continuous transmission is not required, there TDMA is used instead of FDMA. The features of TDMA include the following. TDMA shares a single carrier frequency with several users where each users makes use of non-overlapping time slots. Data transmission in TDMA is not continuous, but occurs in bursts. Hence handsoff process is simpler. TDMA uses different time slots for transmission and reception thus duplexers are not required. TDMA has an advantage that is possible to allocate different numbers of time slots per frame to different users. Bandwidth can be supplied on demand to different users by concatenating or reassigning time slot based on priority. Code Division Multiple Access (CDMA) Code division multiple access technique is an example of multiple access where several transmitters use a single channel to send information simultaneously. Its features are as follows. In CDMA every user uses the full available spectrum instead of getting allotted by separate frequency. CDMA is much recommended for voice and data communications. While multiple codes occupy the same channel in CDMA, the users having same code can communicate with each other. CDMA offers more air-space capacity than TDMA. The hands-off between base stations is very well handled by CDMA. Space Division Multiple Access (SDMA) Space division multiple access or spatial division multiple access is a technique which is MIMO (multiple-input multiple-output) architecture and used mostly in wireless and satellite communication. It has the following features. All users can communicate at the same time using the same channel. SDMA is completely free from interference. A single satellite can communicate with more satellites receivers of the same frequency. The directional spot-beam antennas are used and hence the base station in SDMA, can track a moving user. Controls the radiated energy for each user in space. Spread Spectrum Multiple Access Spread spectrum multiple access (SSMA) uses signals which have a transmission bandwidth whose magnitude is greater than the minimum required RF bandwidth. There are two main types of spread spectrum multiple access techniques − Frequency hopped spread spectrum (FHSS) Direct sequence spread spectrum (DSSS) Frequency Hopped Spread Spectrum (FHSS) This is a digital multiple access system in which the carrier frequencies of the individual users are varied in a pseudo random fashion within a wideband channel. The digital data is broken into uniform sized bursts which is then transmitted on different carrier frequencies. Direct Sequence Spread Spectrum (DSSS) This is the most commonly used technology for CDMA. In DS-SS, the message signal is multiplied by a Pseudo Random Noise Code. Each user is given his own code word which is orthogonal to the codes of other users and in order to detect the user, the receiver must know the code word used by the transmitter. The combinational sequences called as hybrid are also used as another type of spread spectrum. Time hopping is also another type which is rarely mentioned. Since many users can share the same spread spectrum bandwidth without interfering with one another, spread spectrum systems become bandwidth efficient in a multiple user environment. Learning working make money