1 Overview of Wireless Communications. 1. HistoryofWirelessCommunications. 15 Cellular Systems and Infrastructure- Based Wireless Networks. PDF | Next-generation wireless (NextG) involves the concept that the next spectrum allocation, and utilization, in radio communications, networks, and. PDF | The ability to communicate with people on move has evolved remarkably specially in the domain of wireless technology. The mobile.
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of wireless communications and networks, and the associated technologies. in PDF (Adobe Acrobat) format, PowerPoint slides, and sign-up. Department of Electrical and Computer Engineering. Michigan State University. Introduction to Wireless. Communications and Networks. Tongtong Li. Dept. Wireless Networks Spring Outline of the course: Basic topics. Transmission Fundamentals o Analog and digital transmission o Channel capacity.
Soft capacity makes TDMA very flexible to unequal cell loading. Manual Ingles Cisco Aironet series punto de acceso. Using appropriate frequency planning and different transmission techniques including simulcast , a considerable part of the spectrum below 1 GHz could be freed up for alternative usage — a process that took place in U. This waiting time must be random; otherwise same frames will collide again and again. As these stations have a considerable influence on public opinion as well as lobbying power, frequency regulators are hesitant to enforce appropriate rule changes. The two channel group sizes depend on the stage of the splitting process.
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This technology has various functions and it is used commonly in the wireless communication market. Applications of wireless communication involve security systems, television remote control, Wi-Fi, Cell phones, wireless power transfer , computer interface devices and various wireless communication based projects. WiFi, WiMax, Bluetooth, Femtocell, 3G and 4G are some of the most important standards of Wireless technology The information which is given in this article will be helpful to the viewers.
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Hi Archana, Thanks for your Appreciation. Leave this field empty. Even though the devices are not mobile, the propagation channel they transmit over can change with time.
However, from one drop to the next, the environment can change radically. Laptops are typical examples: Low mobility: Cordless phones, as well as cellphones operated by walking human users are typical examples.
The effect of the low mobility is a channel that changes rather slowly, and — in a system with multiple BSs. Cellphones operated by people in moving cars are one typical example. These speeds pose unique challenges both for the design of the physical layer Doppler shift, see Chapter 5 and for the handover between cells. Energy Consumption Energy consumption is a critical aspect for wireless devices.
Most wireless devices use one-way or rechargeable batteries, as they should be free of any wires — both the ones used for communication and the ones providing the power supply. Rechargeable batteries: Standby times as well as operating times are one of the determining factors for customer satisfaction.
Energy consumption is determined on one hand by the distance over which the data have to be transmitted remember that a minimum SNR has to be maintained , and on the other hand, by the amount of data that are to be transmitted the SNR is proportional to the energy per bit.
One-way batteries: Furthermore, changing the battery is often not an option; rather, the sensor including the battery and the wireless transceiver is often discarded after the battery has run out.
Power mains: BSs and other fixed devices can be connected to the power mains. Therefore, energy efficiency is not a major concern for them. It is thus desirable, if possible, to shift as much functionality and thus energy consumption from the MS to the BS.
Use of Spectrum Spectrum can be assigned on an exclusive basis, or on a shared basis. That determines to a large degree the multiple access scheme and the interference resistance that the system has to provide: Spectrum dedicated to service and operator: A prime point in case is Prepared By A. Due to this arrangement, the operator has control over the spectrum and can plan the use of different parts of this spectrum in different geographical regions, in order to minimize interference.
Spectrum allowing multiple operators: Rather, users can set up qualified equipment without a license. Such an approach does not require or allow interference planning. The ISM band at 2.
Also for this case, each user has to adhere to strict emission limits, in order not to interfere too much with other systems and users. However, coordination between users in order to minimize interference becomes almost impossible — different systems cannot exchange coordination messages with each other, and often even have problems determining the exact characteristics bandwidth, duty cycle of the interferers.
Direction of Transmission Simplex systems send the information only in one direction — e. However, only one direction is allowed at any time. Walkie-talkies, which require the user to push a button in order to talk, are a typical example.
Note that one user must signify e. However, even in this case, full duplex capability is maintained. Service Quality The requirements for service quality also differ vastly for different wireless services.
The first main indicator for service quality is speech quality for speech services and file transfer speed for data services. It represents the average of a large number of subjective human judgments on a scale from 1 to 5 about the quality of received speech. An even more important factor is the availability of a service. Noise and Interference Limited Systems. Noise-Limited Systems Wireless systems are required to provide a certain minimum transmission quality.
Consider now a situation where only a single BS transmits, and a Mobile Station MS receives; thus, the performance of the system is determined only by the strength of the useful signal and the noise.
As the MS moves further away from the BS, the received signal power decreases, and at a certain distance, the SNR does not achieve the required threshold for reliable communications.
Therefore, the range of the system is noise limited; equivalently, we can call it signal power limited. Depending on the interpretation, it is too much noise or too little signal power that leads to bad link quality. As a first approximation, it is usually assumed that the environmental temperature is isotropically K. It is common to write Eq. Man-made noise: We can distinguish two types of man-made noise: Many electrical appliances as well as radio transmitters TXs designed for other frequency bands have spurious emissions over a large bandwidth that includes the frequency range in which wireless communications systems operate.
For urban outdoor environments, car ignitions and other impulse sources are especially significant sources of noise. In contrast to thermal noise, the noise created by impulse sources decreases with Frequency At MHz, it can be 20 dB stronger than thermal noise; at MHz, it is typically 10 dB stronger.
Furthermore, for communications operating in licensed bands, such spurious emissions are the only source of man-made noise. It lies in the nature of the license for which the license holder usually has paid that no other intentional emitters are allowed to operate in this band.
In contrast to thermal noise, man-made noise is not necessarily Gaussian distributed. However, as a matter of convenience, most system-planning tools, as well as theoretical designs, assume Gaussianity anyway. Several wireless communications systems operate in unlicensed bands. In these bands, everybody is allowed to operate emit electromagnetic radiation as long as certain restrictions with respect to transmit power, etc.
The most important of these bands is the 2. The amount of interference in these bands can be considerable. Receiver noise: The amplifiers and mixers in the RX are noisy, and thus increase the total noise power. As the amplifiers have gain, noise added in the later stages does not have as much of an impact as noise added in the first stage of the RX. The properties of the medium are well defined and time-invariant. The range over which communications can be The range that can be covered is limited both by performed without repeater stations is mostly the limited by attenuation by the medium and thus transmission medium attenuation, fading, and noise ; for optical fibers, the distortion of signal distortion and by the requirements of transmitted pulses can also limit the speed of spectral efficiency cell size.
Increasing the transmission capacity can be Increasing the transmit capacity must be achieved achieved by more sophisticated transceiver concepts and by using a different frequency on an existing smaller cell sizes in cellular systems , as the cable, amount of available spectrum is limited.
Interference and crosstalk from other users Interference and crosstalk from other users are either do not happen or the properties of the inherent in the principle of cellular interference are stationary.
Due to the mobility of the users, they also are time-variant. The delay in the transmission process is also The delay of the transmission depends partly on constant, the determined by the length of the cable and the distance between base station and Mobile Station group MS , and is thus time-variant. This means that SNR. Increasing the transmit power usually does a not lead to a significant reduction in BER. Due to the well-behaved transmission medium, Due to the difficult medium, transmission quality the is quality of wired transmission is generally high.
Jamming and interception of dedicated links Jamming a wireless link is straightforward, unless with special measures are taken. Interception of the on- wired transmission is almost impossible air signal is possible.
Encryption is therefore without necessary to prevent unauthorized use of the consent Prepared byBy theA. Devasena network operator Asso. Page 15 Establishing a link is location based. The connection is not associated with a connected to the outlet. Power is either provided through the MSs use rechargeable or one-way batteries. In neither case is energy consumption a major concern for the designer of the device. Interference-Limited Systems Consider now the case that the interference is so strong that it completely dominates the performance, so that the noise can be neglected.
Let a BS cover an area cell that is approximately described by a circle with radius R and center at the location of the BS. As a first approximation, we treat the interference as Gaussian.
This allows us to treat the interference as equivalent noise, and the minimum SIR, SIRmin, takes on the same values as SNRmin in the noise-limited case One difference between interference and noise lies in the fact that interference suffers from fading, while the noise power is typically constant averaged over a short time interval. This results in an overestimation of the true fading margin. Therefore, if we use that value in system planning, we are on the safe side. This can lead to different phases of MPCs, which lead to interference in narrowband systems.
In a system with large bandwidth, and thus good resolution in the time domain,3 the major consequence is signal dispersion: Assuming that no special measures4 are taken, this ISI leads to errors that cannot be eliminated by simply increasing the transmit power, and are therefore often called irreducible errors.
ISI is essentially determined by the ratio between symbol duration and the duration of the impulse response of the channel. This implies that ISI is not only more important for higher data rates but also for multiple access methods that lead to an increase in transmitted peak data rate e. Finally, it is also noteworthy that ISI can even play a role when the uration of the impulse response is shorter but not much shorter than bit duration. For wireless communications, the transmission medium is the radio channel between transmitter TX and receiver RX.
The signal can get from the TX to the RX via a number of different propagation paths. The number of these possible propagation paths are very large. As shown in figure 1 , each of the paths has a distinct amplitude, delay runtime of the signal , direction of departure from the TX, and direction of arrival; most importantly, the components have different phase shifts with respect to each other. In the following, we discuss some implications of the multipath propagation for system design.
The interference between them can be constructive or destructive, depending on the phases of the MPCs, Figure. For this reason, the interference, and thus the amplitude of the total signal, changes with time if either TX, RX, or IOs is moving.
This effect — namely, the changing of the total signal amplitude due to interference of the different MPCs — is called small-scale fading. In other words, even a small movement can result in a large change in signal amplitude. A similar effect is known to all owners of car radios — moving the car by less than 1m e. For cellphones, it can often be sufficient to move one step in order to improve signal quality.
As an additional effect, the amplitudes of each separate MPC change with time or with location. Obstacles can lead to a shadowing of one or several MPCs. Imagine, e. This is due to the fact that the MS is now in the radio shadow of the high-rise building, and any wave going through or around that building is greatly attenuated — an effect called shadowing. The MS has to move over large distances from a few meters up to several hundreds of meters to move from the light to the dark zone.
For this reason, shadowing gives rise to large-scale fading. Spectrum Limitations The spectrum available for wireless communications services is limited, and regulated by international agreements. For this reason, the spectrum has to be used in a highly efficient manner. Two approaches are used: In the following, we first review the frequency ranges assigned to different communications services. We then discuss the basic principle of frequency reuse for both regulated and unregulated access.
In its tri-annual conferences World Radio Conferences , it establishes worldwide guidelines for the usage of spectrum in different regions and countries. Further regulations are issued by the frequency regulators of individual countries, including the Federal Communications Commission FCC in the U. While the exact frequency assignments differ, similar services tend to use the same frequency ranges all over the world.
It is mostly systems that need good coverage, but show low user density. Also some emergency communications systems trunking radio make use of this band. The current secondgeneration cellular systems operate in this band, as do most of the third-generation systems. Many cordless systems also operate in this band.
Also, the frequency range between 5. Also car-to-car communications are working in this band. The amount of spectrum assigned to the different services does not always follow technical necessities, but rather historical developments. For example, for many years, the amount of precious low-frequency spectrum assigned to TV stations was much higher than would be justified by technical requirements.
Using appropriate frequency planning and different transmission techniques including simulcast , a considerable part of the spectrum below 1 GHz could be freed up for alternative usage — a process that took place in U. Broadcast stations tend to fight such a development, as it would require modifications in their transmitters. As these stations have a considerable influence on public opinion as well as lobbying power, frequency regulators are hesitant to enforce appropriate rule changes.
It is also noticeable that the financial terms on which spectrum is assigned to different services differ vastly — from country to country, from service to service, and even depending on the time at which the spectrum is assigned. Obviously, spectrum is assigned to public safety services police, fire department, military without monetary compensation. Even television stations usually get the spectrum assigned for free.
In the s, spectrum for cellular telephony was often assigned for a rather small fee, in order to encourage the development of this then-new service. Unregulated services, like WLANs, are assigned spectrum without fees.
Frequency Reuse in Regulated Spectrum Since spectrum is limited, the same spectrum has to be used for different wireless connections in different locations. To simplify the discussion, let us consider in the following a cellular system where different connections different users are distinguished by the frequency channel band around a certain carrier frequency that they employ.
If an area is served by a single BS, then the available spectrum can be divided into N frequency channels that can serve N users simultaneously. If more than N users are to be served, multiple BSs are required, and frequency channels have to be reused in different locations. For this purpose, we divide the area a region, a country, or a whole continent into a number of cells; we also divide the available frequency channels into several groups.
The channel groups are now assigned to the cells. The important thing is that channel groups can be used in multiple cells. The only requirement is that cells that use the same frequency group do not interfere with each other significantly. The large distance between the two cities makes sure that a signal from the MS in Stockholm does not reach the BS in Rome, and can therefore not cause any interference at all.
But in order to achieve high efficiency, frequencies must actually be reused much more often — typically, several times within each city. Consequently, intercell interference also known as co-channel interference becomes a dominant factor that limits transmission quality. Spectral efficiency describes the effectiveness of reuse — i. Since the area covered by a network provider, as well as the bandwidth that it can use, are fixed, increasing the spectral efficiency is the only way to increase the number of customers that can be served, and thus revenue.
Methods for increasing this spectral efficiency are thus at the center of wireless communications research. Since a network operator buys a license for a spectrum, it can use that spectrum according to its own planning — i. The network operator is allowed to use as much transmit power as it desires; it can also dictate limits on the emission power of the MSs of different users.
The operator can also be sure that the only interference in the network is created by its own network and users. Frequency Reuse in Unregulated Spectrum In contrast to regulated spectrum, several services use frequency bands that are available to the general public. For example, some WLANs operate in the 2. Anybody is allowed to transmit in these bands, as long as they i limit the emission power to a prescribed value, ii follow certain rules for the signal shape and bandwidth, and iii use the band according to the rather broadly defined purposes stipulated by the frequency regulators.
As a consequence, a WLAN receiver can be faced with a large amount of interference. This interference can either stem from other WLAN transmitters or from microwave ovens, cordless phones, and other devices that operate in the ISM band. For this reason, a WLAN link must have the capability to deal with interference. That can be achieved by selecting a frequency band within the ISM band at which there is little interference, by using spread spectrum techniques , or some other appropriate technique.
There are also cases where the Prepared By A. In that case, receivers might still have to deal with strong interference, but the structure of this interference is known. This allows the use of special interference mitigation techniques like dynamic frequency assignment, Dynamic frequency assignment can be seen as a special case of cognitive radio where a transmitter senses which part of the spectrum is currently unused in the location of interest, and dynamically moves the transmit frequency accordingly.
Limited Energy Truly wireless communications requires not only that the information is sent over the air not via cables but also that the MS is powered by one-way or rechargeable batteries. The requirement for small energy consumption results in several technical imperatives: Such amplifiers — specifically, class-C or class-F amplifiers — are highly nonlinear.
For example, constant envelope signals are preferred. This restriction has important consequences for the algorithms that can be used for interference suppression, combating of ISI, etc. The RX especially at the BS needs to have high sensitivity. This in turn would mean that — for identical talktime — the battery would have to be times as large — i.
But the high requirements on RX sensitivity have important consequences for the construction of the RX low-noise amplifiers, sophisticated signal processing to fully exploit the received signal as well as for network planning. In other words, transmit power should be adapted to the channel state, which in turn depends on the distance between TX and RX power control. If the MS is close to the BS, and thus the channel has only a small attenuation, transmit power should be kept low.
Several of the mentioned requirements are contradictory. For example, the requirement to build an RX with high sensitivity and thus, sophisticated signal Prepared By A. Engineering tradeoffs are thus called for. FDMA is a channel access method used in multiple-access protocols as a channelization protocol In FDMA, the entire frequency band is divided into fixed number of frequencies horizontally. One frequency is assigned to one individual user. If the user is not using the channel for sending or receiving the data means, then the channel will remain idle for that time.
This idle time causes wasting of resources. Simultaneously and continuously transmission of data is needed in order to avoid idle time. Guard bands between adjacent channels are needed. FDMA provides many users can be used different frequency channel but at same time.
Synchronization is not needed in FDMA.
This FDMA is usually implemented in narrow band systems. FDMA is commonly used for voice and data transmission. For first generation systems we generally prefer FDMA, because the signal used in first generation systems are analog signals. Advantages of FDMA: Disadvantages of FDMA: Allotted bandwidth of FDMA is In TDMA the entire frequency band is assigned vertically in the form of fixed number of slots to individual users for specific time.
The user has to utilize the assigned timeslot, by transmitting the data with high data rate, as soon as the user transmits the data, in the subsequent slots that user remains silent for next N-1 SLOTS. TDMA provides many users to access the same frequency channel but different time slots. In TDMA systems, each user is allowed to access the channel on time slot basis. The time slots are arranged into frames.
Messages generated by terminals are segmented into data units for fitting in TDMA time slots. Each time slot in a frame usually consists of user data bits as well as extra bits for synchronization, adaptation, control, guard time, etc.
As soon as the data transmission over, the transmitter is idle until it can transmit another packet of data to the destination. Frequencies are only reused in cells sufficiently distant in order to minimize interference. The capacity of the cellular radio system depends on radio capacity or cell capacity. The system capacity or cell capacity increased because of the averaging effect of the interference.
Therefore guard times between adjacent time slots are needed. TDMA systems transmit data in a buffer-and-burst method, thus the transmission for any user is non-continuous. High synchronization overhead is required in TDMA systems because of burst transmissions.
Due to incorrect synchronization, there can be interference between the adjacent time slots. Time slot structure is there in TDMA. This structure supports mobile assisted handoff MAHO.
Frequency hopping feature is present in TDMA system. This feature increases the capacity of TDMA systems with the help of frequency diversity and interface averaging technique. Soft capacity makes TDMA very flexible to unequal cell loading.
In TDMA slow frequency hopping is used to provide interferer diversity. With the help of DCA it is possible for us to eliminate the frequency planning easily. In DCA all channels are available in a common pool and according to the usage they are allotted dynamically to the users.
When the user wants to utilize the spectrum means with the help of DCA the idle channel is allotted immediately to the user.
If the number of channels in a spectrum is known means, then the FCA easily establish the frequency reuse pattern. Two types of FCAs are there. Two types of interference are there. They are Co- channel interference and adjacent channel interference.
Co-channel interference takes place within the channel itself. Adjacent channel interference takes place with the neighboring channels. We know that collection of cells is called as Cluster, collection of clusters is called as channels, and collection of channels is called as spectrum.
In TDMA systems, in order to avoid co channel interference, different frequency bands are utilized. All users share the same bandwidth with different sequences. Communication between the transmitter and receiver is identified by the sequence. The transmitter and receiver know the sequence in advance.
CDMA is based on spread spectrum technology. Spread spectrum is a transmission technology wherein data occupy a larger bandwidth than necessary. Bandwidth spreading is accomplished before transmission using a code that is independent of the transmitted data. The same code is used to demodulate the data at receiving end.
CDMA is originally designed for military to avoid jamming. Random signature sequences can be implemented in CDMA. With the help of direct sequence spread spectrum, it is possible for the user in the CDMA system to occupy the entire spectrum. With the help of this sequence, we can able to receive multiple packets simultaneously without losing any packets due to collision. The capability were the multiple packets can be received by the physical layer is known as multi packet reception MPR.
In this DSSS, each code symbol is spread by a unit energy chip waveform. The spreading sequence is independent from code symbol to code symbol and across users. A system is said to be wideband when the bit rate may be high so that multipath signals from one bit arrive over duration longer than that of one bit.
Near- far effect is present in CDMA systems. That is, the users near the receiver receive higher powers when compared to the users far away from the receiver. The users in the CDMA system can interfere with all other users. Because of this interference, performance degradation occurs in CDMA systems. The capacity of CDMA system is interference limited. CDMA system does not have a hard capacity limit.
In CDMA, the interference makes harder to increase system capacity . The capacity enhancement of CDMA can be done with the help of spatial diversity in space-time processing.
To do this, we can assume linear processing in both temporal and spatial time domains. Two dimensional matrix filters are implemented in these domains. For designing a CDMA systems care must be taken in the power control area. Because this power control area allows mobile users to utilize the radio resources or spectrum resources equally and efficiently.
Random Access Methods for Mobile data service The random access methods used in mobile data networks can be divided into two groups. The first group consists of ALOHA- based access methods for which the mobile terminals transmit their contention packet without any coordination between them.
The second group is the carrier sense based random access techniques for which the terminal senses the availability of the channel before it transmits its packets. ALOHA was the first protocol used in radio networks. This protocol derives its name from the ALOHA system, a communication network developed by Norman Abramson and his colleagues at the university of Hawaii and first put into operation in A shared communication system like ALOHA requires a method of handling collisions that occur when two or more systems attempt to transmit on the channel at the same time.
If another node transmits at the same time, a collision occurs, and the frames that were transmitted are lost. However, a node can listen to broadcast on the medium, even its own, and determine whether the frames were transmitted.
When two or more stations transmit simultaneously, there is collision and the frames are destroyed. In this protocol, whenever any station transmits a frame, it expects the acknowledgement from the receiver. If acknowledgement is not received within specified time, the station assumes that the frame has been destroyed.
If the frame is destroyed because of collision, the station waits for a random amount of time and sends it again. This waiting time must be random; otherwise same frames will collide again and again. Whenever two frames try to occupy the channel at the same time, there will be a collision and both will be damaged.
If first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted.
And it Prepared By A. The terminals transmit their packets as they become ready for transmission, and if there is a collision, they simply retransmit. Since the nodes send their frames without sensing the medium, there is a high probability for collisions to occur. In traditional SA, the user continues transmission in subsequent slots until a collision occurs.
The SA can be applicable for both voice and data transmission. In SA, each user passes the data in the form of packets to be transmitted to the nearby base station, then that base station passes those packets to the destination in a relay manner. When a new packet is generated, it approaches the channel at the beginning of the next slot. This phenomenon is known as immediate first transmission IFT. The result of the transmission of packets is broadcast with the help of another reliable acknowledgement channel.
Retransmission of the packets is scheduled after a random back off delay. This retransmission happens when the initial transmission of packets fails to reach the destination. Three types of retransmission policies are adopted. In slotted ALOHA, the time of the shared channel is divided into discrete intervals called slots and each slot interval corresponds to the time period of one frame.
The stations can send a frame only at the beginning of the slot and only one frame is sent in each slot. If any station is not able to place the frame onto the channel at the beginning of the slot i.
There is a possibility of collision if two stations try to send at the beginning of the same slot. Here chances of collision are reduced to one-half. Carrier sense means, a transmitter uses feedback from a receiver to determine whether another transmission is in progress before trying to send.