With frequency diversity, the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers. Time diversity techniques aim to spread the data out over time so that a noise burst affects fewer bits. Half of that is 5, km which is comparable to the east-to-west dimension of the continental U. While an antenna this size is impractical, the U. Defense Department has considered using large parts of Wisconsin and Michigan to make an antenna many kilometers in diameter.
Using Equation 5. From Table 5. The available received signal power is 20 — This approach is susceptible to sudden gain changes and is rather inefficient. A disadvantage of NRZ transmission is that it is difficult to determine where one bit ends and the next bit begins. For PM, the phase is proportional to the modulating signal.
For FM, the derivative of the phase is proportional to the modulating signal. First consider NRZ-L. For the remaining codes, one must first determine the average number of pulses per bit. For example, for Biphase-M, there is an average of 1.
These higher components cause the signal to change more rapidly over time. Hence, DM will suffer from a high level of slope overload noise. PCM, on the other hand, does not estimate changes in signals, but rather the absolute value of the signal, and is less affected than DM. The demodulator portion of a modem expects to receive a very specific type of waveform e. Thus, it would not function as the coder portion of a codec. The case against using a codec in place of a modem is less easily explained, but the following intuitive argument is offered.
If the decoder portion of a codec is used in place of the modulator portion of a modem, it must accept an arbitrary bit pattern, interpret groups of bits as a sample, and produce an analog output. Some very wide value swings are to be expected, resulting in a strange-looking waveform.
Given the effects of noise and attenuation, the digital output produced at the receiving end by the coder portion of the codec will probably contain many errors.
The actual step size, in volts, is: 0. Thus the actual maximum quantized voltage is: 0. The normalized step size is 2—8. The maximum error that can occur is one-half the step size. Only a recipient who knows the spreading code can recover the encoded information. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message. Each user uses a different spreading code. The receiver picks out one signal by matching the spreading code.
Cross-correlation, which is defined in Equation 7. Thus, to achieve the desired SNR, the signal must be spread so that 56 KHz is carried in very large bandwidths. Thus a far higher SNR is required without spread spectrum. Source: [SKLA01] 7. MFSK c. Same as for Problem 7. This is from the example 6. We need three more sets of 8 frequencies. The second set can start at kHz, with 8 frequencies separated by 50 kHz each.
The third set can start at kHz, and the fourth set at kHz. The first generator yields the sequence: 1, 6, 10, 8, 9, 2, 12, 7, 3, 5, 4, 11, 1,. The second generator yields the sequence: 1, 7, 10, 5, 9, 11, 12, 6, 3, 8, 4, 2, 1,. Because of the patterns evident in the second half of the latter sequence, most people would consider it to be less random than the first sequence. See [KNUT98], page 13 for a discussion. As discussed in the answer to Problem 10, this leads to results in which the right-hand digits are much less random than the left-hand digits.
Often, a and c are chosen to create a sequence of alternating even and odd integers. The simulation depends on counting the number of pairs of integers whose greatest common divisor is 1. With truly random integers, one-fourth of the pairs should consist of two even integers, which of course have a gcd greater than 1. This never occurs with sequences that alternate between even and odd integers.
For a further discussion, see Danilowicz, R. That is, it provides more information that can be used to detect errors. You could design a code in which all codewords are at least a distance of 3 from all other codewords, allowing all single-bit errors to be corrected.
Suppose that some but not all codewords in this code are at least a distance of 5 from all other codewords. Then for those particular codewords, but not the others, a double- bit error could be corrected. The source station receiving the NAK will retransmit the frame in error plus all succeeding frames transmitted in the interim. The modulo 2 scheme is easy to implement in circuitry. It also yields a remainder one bit smaller than binary arithmetic.
Each 1 bit will merge with a 1 bit exclusive-or to produce a 0; each 0 bit will merge with a 0 bit to produce a zero. The HDLC standard provides the following explanation. The addition of XK L X corresponds to a value of all ones. This addition protects against the obliteration of leading flags, which may be non-detectable if the initial remainder is zero.
The addition of L X to R X ensures that the received, error- free message will result in a unique, non-zero remainder at the receiver. The non-zero remainder protects against the potential non-detectability of the obliteration of trailing flags. The implementation is the same as that shown in Solution 3b, with the following strategy. At both transmitter and receiver, the initial content of the register is preset to all ones.
The final remainder, if there are no errors, will be For simplicity, we do not show the switches. The partial results from the long division show up in the shift register, as indicated by the shaded portions of the preceding table.
Compare to long division example in Section 8. Five additional steps are required to produce the result. For a codeword w to be decoded as another codeword w', the received sequence must be at least as close to w' as to w.
Therefore all errors involving t or fewer digits are correctable. Now suppose that the only error is in C8. Thus, the data word read from memory was The minimum value of n — k that satisfies this condition is The first column is filled after 21 bits are read in. Similarly, 21 bits must arrive before deinterleaving. This clears out the encoder, making it ready for use for the next transmission.
The sequence of states traversed is abdcbcbdcb. The output sequence is 10 11 10 01 01 01 11 10 01 00 8. Because only one frame can be sent at a time, and transmission must stop until an acknowledgment is received, there is little effect in increasing the size of the message if the frame size remains the same.
All that this would affect is connect and disconnect time. This would lower line efficiency, because the propagation time is unchanged but more acknowledgments would be needed. For a given message size, increasing the frame size decreases the number of frames. This is the reverse of b.
The first frame takes 10 msec to transmit; the last bit of the first frame arrives at B 20 msec after it was transmitted, and therefore 30 msec after the frame transmission began.
It will take an additional 20 msec for B's acknowledgment to return to A. Thus, A can transmit 3 frames in 50 msec. B can transmit one frame to C at a time. The larger the area of coverage, the more satellites must be involved in a single networked system.
General usage: commercial, military, amateur, experimental. In the case of a geostationary satellite, a single antenna is visible to about one-fourth of the earth's surface. Thus, satellite-to-satellite communication links can be designed with great precision.
An equatorial orbit is directly above the earth's equator. A polar orbit passes over both poles. Other orbits are referred to as inclined orbits. The traditional GEO satellite is in a circular orbit in an equatorial plane such that the satellite rotates about the earth at the same angular velocity that the earth spins on its axis. LEO satellites are satellites with much lower orbits, on the order of to 1, km high.
Finally, HEO satellites are characterized by an orbit that is an ellipse with one axis very substantially larger than the other. The height of the orbit can vary; it is the shape of the orbit that characterizes this type of satellite. Frequency reuse is more difficult because the antenna beam all other things being equal covers a much greater area from a GEO than from a LEO. On the other hand, tracking and handoff is not necessary for GEO satellites because they appear stationary relative to the earth.
LEO satellites, since they are so low travel very much faster, and cover less area than GEO so that tracking is more difficulty and passing off is frequent. HEOs require tracking and handoffs, as well. HEOs are primarily of use when coverage of areas near one of the poles is essential, such as the use of the Molniya satellites to cover the northern parts of the former Soviet Union.
LEOs are useful for point-to-point communication, and for extensive frequency reuse. Since LEOs have much less propagation delay they are useful for interactive data services. They also can cover polar regions. The received power will increase by a factor of 4 9.
The total available capacity is 60 Mbps. Frequency borrowing: In the simplest case, frequencies are taken from adjacent cells by congested cells. The frequencies can also be assigned to cells dynamically. Cell splitting: In practice, the distribution of traffic and topographic features is not uniform, and this presents opportunities of capacity increase. Cells in areas of high usage can be split into smaller cells. Cell sectoring: With cell sectoring, a cell is divided into a number of wedge-shaped sectors, each with its own set of channels, typically 3 or 6 sectors per cell.
Each sector is assigned a separate subset of the cell's channels, and directional antennas at the base station are used to focus on each sector. Microcells: As cells become smaller, antennas move from the tops of tall buildings or hills, to the tops of small buildings or the sides of large buildings, and finally to lamp posts, where they form microcells. Each decrease in cell size is accompanied by a reduction in the radiated power levels from the base stations and the mobile units.
Microcells are useful in city streets in congested areas, along highways, and inside large public buildings. In this case, the mobile unit is handed off to a neighboring cell based not on signal quality but on traffic capacity. Call dropping probability: the probability that, due to a handoff, a call is terminated.
Call completion probability: the probability that an admitted call is not dropped before it terminates. Probability of unsuccessful handoff: the probability that a handoff is executed while the reception conditions are inadequate. Handoff blocking probability: the probability that a handoff cannot be successfully completed. Handoff probability: the probability that a handoff occurs before call termination. Rate of handoff: the number of handoffs per unit time.
Interruption duration: the duration of time during a handoff in which a mobile is not connected to either base station. Handoff delay: the distance the mobile moves from the point at which the handoff should occur to the point at which it does occur.
This is because the power level is the sum from signals coming from a number of different paths and the phases of those paths are random, sometimes adding and sometimes subtracting. As the mobile unit moves, the contributions along various paths change.
Closed loop power control adjusts signal strength in the reverse mobile to BS channel based on some metric of performance in that reverse channel, such as received signal power level, received signal-to-noise ratio, or received bit error rate. Traffic intensity is a normalized version of mean rate of calls, and equals the average number of calls arriving during the average holding period.
Thus, traffic intensity is dimensionless. In particular, the first generation systems are designed to support voice channels using FM; digital traffic is supported only by the use of a modem that converts the digital data into analog form. Second generation systems provide digital traffic channels.
These readily support digital data; voice traffic is first encoded in digital form before transmitting. Of course, for second-generation systems, the user traffic data or digitized voice must be converted to an analog signal for transmission between the mobile unit and the base station.
Encryption: Because all of the user traffic, as well as control traffic, is digitized in second-generation systems, it is a relatively simple matter to encrypt all of the traffic to prevent eavesdropping. All second-generation systems provide this capability, whereas first generation systems send user traffic in the clear, providing no security. Error detection and correction: The digital traffic stream of second-generation systems also lends itself to the use of error detection and correction techniques.
The result can be very clear voice reception. Channel access: In first generation systems, each cell supports a number of channels. At any given time a channel is allocated to only one user. Second generation systems also provide multiple channels per cell, but each channel is dynamically shared by a number of users using time division multiple access TDMA or code division multiple access CDMA. Multipath resistance: The chipping codes used for CDMA not only exhibit low cross-correlation but also low autocorrelation.
Therefore, a version of the signal that is delayed by more than one chip interval does not interfere with the dominant signal as much as in other multipath environments.
Privacy: Because spread spectrum is obtained by the use of noise-like signals, where each user has a unique code, privacy is inherent. Thus the spreading sequences of the different users are not orthogonal and there is some level of cross-correlation. This is distinct from either TDMA or FDMA, in which for reasonable time or frequency guardbands, respectively, the received signals are orthogonal or nearly so.
Near-far problem: Signals closer to the receiver are received with less attenuation than signals farther away. Given the lack of complete orthogonality, the transmissions from the more remote mobile units may be more difficult to recover. Thus, power control techniques are very important in a CDMA system. Soft handoff: A smooth handoff from one cell to the next requires that the mobile acquire the new cell before it relinquishes the old.
Soft handoff: a mobile station is temporarily connected to more than one base station simultaneously. A mobile unit may start out assigned to a single cell.
If the unit enters a region in which the transmissions from two base stations are comparable within some threshold of each other , the mobile unit enters the soft handoff state in which it is connected to the two base stations.
The mobile unit remains in this state until one base station clearly predominates, at which time it is assigned exclusively to that cell. From part b, we know the number of channels that can be carried per cell for each system. The total number of channels available is just times that number, for a result of , , , , respectively. Source: [CARN99] Steps a and b are the same. The next step is placing the call over the ordinary public switched telephone network PSTN to the called subscriber.
Steps d, e, and f are the same except that only the mobile unit can be involved in a handoff. From there, steps c, d, e, and f are the same except that only the mobile unit can be involved in a handoff. For a given traffic level A and given capacity N , what is the probability of blocking P?
What traffic level can be supported with a given capacity to achieve a given probability of blocking? For a given traffic level, what capacity is needed to achieve a certain upper bound on the probability of blocking?
Source: [RAPP96] From Figure Thus, bit duration is: 0. The delay is the duration of 1 frame, which is 4. The amount of bandwidth allocated to voice channels BcNt must be no greater than the total bandwidth Bw. Multiply results of part a by 0. The table in the problem statement gives the value of A. Find N. Cell number 1 2 3 4 5 6 7 Channels 40 78 59 43 48 48 42 e.
Standardized cordless systems can operate in a number of environment. TDM is a multiplexing technique that allows multiple data sources to transmit over the same channel by taking turns. With full-duplex TDM, data are transmitted in both directions simultaneously. The P channel provides paging from the base station to the terminals.
In response to a page and at the time of handoff, a terminal uses the two-way M channel to exchange medium access control messages with the base station. Once a connection is established, the N channel provides a handshaking protocol. The C channel provides call management for active connections. Installation time: WLL systems typically can be installed rapidly. Selective installation: Radio units are installed only for those subscribers who want the service at a given time. Above 10 GHz, these attenuation effects are large.
As was pointed out in Chapter 5, Reflection occurs when an electromagnetic signal encounters a surface that is large relative to the wavelength of the signal; scattering occurs if the size of an obstacle is on the order of the wavelength of the signal or less; diffraction occurs when the wavefront encounters the edge of an obstacle that is large compared to the wavelength.
If the data stream is protected by a forward error-correcting code, this type of fading is easily handled. Hence MMDS can operate in considerably larger cells, thereby lowering base station equipment costs. It is designed to support data rates above 2 Mbps. The target market is small and medium size businesses. It is designed for data rates below 2 Mbps. The target market is residential and small business. So, the channel bandwidth becomes kHz with a bit rate of First, we need the attenuation from rain.
We use Equation From Table For the parameters a and b, we use Table At For a nomadic user, the user's Internet connection is terminated each time the user moves and a new connection is initiated when the user dials back in.
Registration: A mobile node uses an authenticated registration procedure to inform its home agent of its care-of address. Tunneling: Tunneling is used to forward IP datagrams from a home address to a care-of address. The co-located address is used if there is no foreign agent or all foreign agents on the foreign network are busy. Types of Small-Scale Fading. Rayleigh and Ricean Distributions. Statistical Models for Multipath Fading Channels.
Frequency Modulation vs. Amplitude Modulation. Angle Modulation. Digital Modulation: An Overview. Line Coding. Pulse Shaping Techniques. Geometric Representation of Modulation Signals.
Linear Modulation Techniques. Constant Envelope Modulation. Spread Spectrum Modulation Techniques. Modulation Performance in Fading and Multipath Channels. Fundamentals of Equalization. Training A Generic Adaptive Equalizer. Equalizers in a Communications Receiver. Survey of Equalization Techniques. Linear Equalizers. Nonlinear Equalization. Algorithms for Adaptive Equalization.
Fractionally Spaced Equalizers. Diversity Techniques. RAKE Receiver. Fundamentals of Channel Coding. Block Codes and Finite Fields. Convolutional Codes. Coding Gain. Trellis Coded Modulation. Turbo Codes. Characteristics of Speech Signals. Quantization Techniques. Frequency Domain Coding of Speech. Linear Predictive Coders. Choosing Speech Codecs for Mobile Communications.
The GSM Codec. Performance Evaluation of Speech Coders. Spread Spectrum Multiple Access. Packet Radio. Capacity of Cellular Systems. Introduction to Wireless Networks. Development of Wireless Networks. Fixed Network Transmission Hierarchy. Traffic Routing in Wireless Networks. Wireless Data Services. Signaling System No.
Protocols for Network Access. Network Databases. CT2 Standard for Cordless Telephones. US Wireless Cable Television. Summary of Standards Throughout the World. Rate Variance for Complex Voltage. Rate Variance for Power. Rate Variance for Envelope. The Gaussian Approximation. Pearson offers affordable and accessible purchase options to meet the needs of your students. Connect with us to learn more.
He is the editor or co-editor of four other books on the topic of wireless communications, based on his teaching and research activities at MPRG.
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