OFDMA System Analysis and Design (Mobile Communications)
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The new wavelet showed better performance than other traditional orthogonal wavelets. First, the OSTBC is extended to assess the performance of the scheme over extended receiver diversity order. Again, with the extended diversity conditions, the OSTBC is implemented for a multicarrier system over a doubly-selective fading channel. It was found that, during high mobile speed, it is better to transmit OFDM signals using lower operating frequencies.
The information theory for the 2-transmit antenna OSTBC does not support higher order implementation of multi-antenna systems, which is required for the future generation wireless communications systems. The performances of traditional QO-STBC methods are diminished by some off-diagonal interference terms such that the resulting system does not attain full diversity. Some methods for eliminating the interference terms have earlier been discussed. Type Thesis. Qualification name PhD. Collections Theses.
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MIMO-OFDM - Wikipedia
The extra guard bands necessarily add to the total bandwidth requirement. So is there a way to address these disadvantages and, at the same time, retain the advantage of transmitting multiple narrowband subcarriers? The answer is yes—in the form of OFDM. OFDM overcomes the problem of the large bandwidth requirement imposed by guard bands. The subcarriers generated this way do not need additional guard bands and can be placed closer together in the frequency domain.
The subcarriers are also orthogonal to each other over a set duration i. In addition, DFT and its inverse can be efficiently computed, eliminating the need for separate RF components for separate subcarriers. This high-rate stream of data symbols consists of blocks of complex data symbols, and each block contains K complex data symbols. In doing so, the serial-to-parallel converter assigns successive data symbols at its input to K separate substreams at its outputs.
So at any given time at the output of the serial-to-parallel converter, there is a set of K data symbols in parallel. Successive OFDM symbols 4. Note that an OFDM symbol is different from a data symbol, which encodes one or more user bits and is the input to the serial-to-parallel converter. In the transmitted OFDM signal, the subcarriers are separated such that they physically overlap in frequency, but the first zero crossings of one subcarrier fall on the peaks of the two adjacent subcarriers.
In fact, all zero crossings of a subcarrier fall on the peaks of all adjacent subcarriers. Chapter 4 discusses in more detail the OFDM operation that produces the overlapping subcarriers.
In other words, only one user transmits in a block an OFDM symbol. Note that the first zero crossings of subcarrier 2 fall on the peaks of adjacent subcarriers 1 and 3. As an example, Figure 1. The subcarriers are arranged in such a way that the zero crossings of one subcarrier coincide with the peaks of all other subcarriers. The spectrum is shown over the duration of one OFDM symbol. After an OFDM receiver demodulates the subcarriers to baseband, it ultimately recovers the data symbol at the peak i.
Because the peak of the corresponding subcarrier is where the zeros of all other subcarriers are, subcarriers do not interfere with one another over the period of one OFDM symbol. Thus in OFDM, subcarriers do overlap in frequency, but as far as the recovery of data symbols is concerned, they do not interfere with one another.
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This way, more than one user can access the air interface at the same time. OFDM assigns all subcarriers to a single user at the same time, and only one user can transmit at a time. If multiple users want to transmit using OFDM, then those users have to take their turns in time. So there are a total of JL complex data symbols in J groups. In doing so, the serial-to-parallel converter assigns successive data symbols at its input to JL separate low-rate substreams at its outputs.
So at any given time at the output of the serial-to-parallel converter, there is a set of JL data symbols in parallel. The subcarrier mapper maps JL data symbols to their respective subcarriers which are assigned to different users. Specifically, the subcarrier mapper assigns J groups of data symbols to J users in frequency. In effect, the subcarrier mapper reorders the parallel data symbols according to the particular subcarriers assigned to each user. The K transformed symbols in K substreams then pass through the parallel-to-serial converter that puts the K transformed symbols in series.
In particular, the spectrum of the transmitted signal is shown over J groups of data symbols, or one OFDM symbol, only. The figure shows that data symbols belonging to a user are carried by contiguous subcarriers. Note that, in this case, data symbols belonging to a user are carried by contiguous subcarriers.
In a distributed subcarriers arrangement, subcarriers are assigned pseudorandomly to users.
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In a contiguous subcarriers arrangement, subcarriers are assigned to users in continuous sets this is the scheme shown in Figure 1. Multiuser diversity occurs because different users at different locations would likely experience different channel responses, thus the system can 1. They show how a transmitter can send a single high-rate stream using multiple, narrowband orthogonal subcarriers.
However, there are practical issues, especially in a mobile environment, that can degrade the performance of such systems. This guard time is referred to as the cyclic prefix and is discussed in more detail in Chapter 4. Looking at Figure 1. If the center frequency of a subcarrier is shifted just a little bit, then that subcarrier is no longer orthogonal to its neighboring subcarriers, and intercarrier interference ICI will result.
As we know, users continue to demand broadband mobile applications that require a higher and higher bit rate Rb. If Rb increases, W necessarily has to increase as well to maintain the same Gp. If Rb increases but W is fixed e. In addition, the number of code channels that a traditional DSSS system can support depends on Gp [3, 5], so Gp relates to the capacity of the system also.
OFDM is a technique that belongs to a larger class of modulation schemes called multicarrier modulation. In multicarrier modulation, one high-rate stream is divided into many low-rate substreams, which are then transmitted over parallel frequency subcarriers. Initial works on multicarrier modulation were done in the s in the United States, and in the s it was demonstrated that DFT can be used to implement an FDM system . However, the simplified transmitter structures shown in this chapter do not take into account some practical implementation issues.
To help understand these W Freq Rb Figure 1. Chapters 4—6 cover the physical layer layer 1.
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Chapter 4 goes through those aspects of OFDM that are conducive to transmitting and receiving high-rate streams over a wireless channel, especially one that is time dispersive. After going through the general remedy that OFDM offers, the book proceeds to Chapter 5, which covers the specific format of the physical layer as implemented in IEEE Specifically, Chapter 5 addresses the physical layer from the perspective of channel structure i. Broadband wireless networks cannot only rely on the increase in raw bit rate provided by the enhanced physical layer.
To be sure, any increase in throughput as perceived by a higher-layer application also depends on how effectively the medium access control layer layer 2 utilizes the physical layer. Thus, any increase in throughput relies on enhancements made in layer 2. Chapters 7—9 use IEEE Specifically, Chapter 7 discusses layer 2 with respect to the data traffic it transports, whereas Chapters 8 and 9 discuss layer 2 from the perspective of its control and supervisory functions.
OFDMA System Analysis and Design
Chapters 10—12 discuss two special topics that are particularly relevant to operating modern and future broadband wireless networks: quality of service QoS and security. Chapter 10 presents QoS. The use of QoS has enabled different networks both wireless and wireline to deliver heterogeneous traffic based on the needs of individual users. Given that broadband wireless networks are inherently capacity-constrained, the application of QoS is an important topic to address.
www.uskudarevdenevenakliyat.org/wp-includes/9/1574.php In addition, a broadband wireless network carries with it unique security issues. Chapter 11 first reviews the fundamentals of network security, then Chapter 12 shows how important security features work in an actual network IEEE The book concludes with Chapters 13 and 14, which cover topics in the design of an OFDMA-based broadband wireless system.
Since a broadband wireless system effectively trades off coverage against capacity, the design aspects are presented in a coverage part Chapter 13 and a capacity part Chapter Ahmadi, S. Yang, S. Carlson, B. Weinstein, S.