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| -rw-r--r-- | chapter07/content_ch07.tex | 122 | ||||
| -rw-r--r-- | common/acronym.tex | 3 |
2 files changed, 111 insertions, 14 deletions
diff --git a/chapter07/content_ch07.tex b/chapter07/content_ch07.tex index dcede28..919139d 100644 --- a/chapter07/content_ch07.tex +++ b/chapter07/content_ch07.tex @@ -1436,7 +1436,27 @@ Multiple access involves \index{multiplexing} \textbf{multiplexing}. Multiplexin \item Code \end{itemize} -\todo{Multiplexing scheme} +\begin{figure}[H] + \centering + \begin{tikzpicture} + \begin{scope}[shift={(-2,0)}] + \draw (0,0) -- (-1,-2) -- (-1,2) -- cycle; + \foreach \y in {1.5,1,0.5,0,-0.5,-1,-1.5}{ + \draw[-latex] (-2,\y) -- (-1,\y); + } + \node[anchor=east,align=right] at(-2.2,0) {Independent\\ signals}; + \end{scope} + \begin{scope}[shift={(2,0)}] + \draw (0,0) -- (1,-2) -- (1,2) -- cycle; + \foreach \y in {1.5,1,0.5,0,-0.5,-1,-1.5}{ + \draw[-latex] (1,\y) -- (2,\y); + } + \node[anchor=west,align=left] at(2.2,0) {Independent\\ signals}; + \end{scope} + \draw[very thick] (-2,0) -- node[midway,above,align=center]{Transmission\\ channel} (2,0); + \end{tikzpicture} + \caption[Principle of multiplexing]{Principle of multiplexing. Independent signals can be transmitted (quasi-) parallelly over the same transmission channel.} +\end{figure} Multiple access methods are implemented in both the \emph{physical layer} (\acs{OSI} layer 1) and the \emph{data link layer} (\acs{OSI} layer 2). \begin{itemize} @@ -1466,11 +1486,11 @@ A simple and \emph{non-spreading} method is \index{space-division multiple acces \subsection{Time-Division Multiple Access} -A multiple access method derived from \ac{THSS} is \index{time-division multiple access} \textbf{\acf{TDMA}}. +A \emph{multiple access} method derived from \ac{THSS} is \index{time-division multiple access} \textbf{\acf{TDMA}}. \begin{itemize} \item Each user obtains one of the $M$ time-slot for exclusive usage. - \item The \emph{spreading code} $C[m]$ is constant for each user and yields the time-slot number. - \item \ac{ISI} is an issue. Guard intervals must be inserted. + \item Usually, the time-slots are long enough so that a series of symbols can be transmitted for one user. In contrast to pure \ac{THSS}, where one symbol is transmitted as a chip, \ac{TDMA} transmits a series of symbols for user 1 and then hands over the transmission channel to the next user. + \item \ac{ISI} between the users is an issue. Guard intervals must be inserted. \end{itemize} \begin{remark} @@ -1507,7 +1527,7 @@ Drawbacks: \subsection{Frequency-Division Multiple Access} -A multiple access method derived from \ac{FHSS} is \index{frequency-division multiple access} \textbf{\acf{FDMA}}. +A \emph{multiple access} method derived from \ac{FHSS} is \index{frequency-division multiple access} \textbf{\acf{FDMA}}. \begin{itemize} \item Each user obtains one of the $M$ sub-bands for exclusive usage. \item The \emph{spreading code} $C[m]$ is constant for each user and yields the sub-bands number. @@ -1547,15 +1567,66 @@ Drawbacks: \item Guard band required \end{itemize} +\subsection{Hybrid TDMA and FDMA} + +\ac{TDMA} and \ac{FDMA} can be combined to a hybrid system. +\begin{itemize} + \item The allocation for both sub-bands and time-slots changes dynamically for each user. + \item The pattern is determined by the \ac{MAC} layer (\acs{OSI} layer 2). + \item Some users may lose allocations for the sake of enabling other users to use the transmission medium. +\end{itemize} + +\begin{figure}[H] + \centering + \begin{tikzpicture}[ + x={(0.5cm,0cm)}, + y={(0cm,0.5cm)}, + ] + \draw[-latex] (0,0) -- (9,0) node[below right,align=left]{Time $t$}; + \draw[-latex] (0,0) -- (0,9) node[above left,align=right]{Frequency $f$}; + + \foreach \t/\f/\n/\c in {0/0/0/red, 0/1/1/blue, 0/2/2/green, 0/3/3/yellow, + 1/0/1/blue, 1/1/0/red, 1/2/3/yellow, 1/3/4/olive, + 2/0/2/green, 2/1/4/olive, 2/2/0/red, 2/3/3/yellow, + 3/0/4/olive, 3/1/5/gray, 3/2/2/green, 3/3/1/blue}{ + \draw[fill=\c!50,draw=black] ({(\t*2)},{(\f*2)}) -- ({(\t*2)+1.5},{(\f*2)}) -- ({(\t*2)+1.5},{(\f*2)+1.5}) -- ({(\t*2)},{(\f*2)+1.5}) -- cycle; + \node[align=center] at({(\t*2)+0.75},{(\f*2)+0.75}) {U\n}; + } + \end{tikzpicture} + \caption{Sub-band and time-slot allocation in an \acs{TDMA}/\acs{FDMA} hybrid system} +\end{figure} + +\begin{example}{2G cell phone -- \ac{GSM}} + \todo{GSM example} +\end{example} + \subsection{Code-Division Multiple Access} -A multiple access method derived from \ac{DSSS} is \index{code-division multiple access} \textbf{\acf{CDMA}}. +All \emph{spread spectrum} technologies rely on \emph{spreading codes}. +\begin{itemize} + \item The \emph{spreading codes} are the parameter of the \emph{spread spectrum} technology which defines how the signal power is spread. + \item Using \index{orthogonal spreading codes} \textbf{orthogonal spreading codes}, multiple \emph{spread spectrum} signals can superimpose without interfering each other. + \item Different \emph{orthogonal spreading codes} can be assigned to different users for \emph{multiple access}. + \item The receiver is able to split the simultaneously transmitted signals of the different users using the \emph{orthogonal spreading codes}. + \item Due to the code orthogonality, the users cannot interfere. The other user's signals will be noise to the receiver which is suppressed. +\end{itemize} + +The \emph{multiple access} method using \emph{orthogonal spreading codes} is \index{code-division multiple access} \textbf{\acf{CDMA}}. + +It can be subdivided according to the underlying \emph{spread spectrum} technology. +\begin{itemize} + \item \textbf{\acf{DS-CDMA}} uses \emph{\acf{DSSS}} with \emph{orthogonal spreading codes}. + \item \textbf{\acf{FH-CDMA}} uses \emph{\acf{FHSS}} with \emph{orthogonal spreading codes}. + \item \textbf{\acf{TD-CDMA}} uses \emph{\acf{THSS}} with \emph{orthogonal spreading codes}. +\end{itemize} + +\subsubsection{Direct Sequence CDMA} + +A \emph{\ac{CDMA} scheme} derived from \ac{DSSS} is \index{direct sequence code-division multiple access} \textbf{\acf{DS-CDMA}}. \begin{itemize} \item \emph{Spreading codes} with a length of $L$ have $K$ combinations which are orthogonal. - \item Each user obtains one of the $K$ orthogonal codes for exclusive usage. + \item Each user obtains one of the $K$ \emph{orthogonal spreading codes} for exclusive usage. \item All users can transmit simultaneously using the whole bandwidth. - \item The receiver is able to split the simultaneously transmitted signals of the different users using the orthogonal codes. - \item Due to the code orthogonality, the users cannot interfere. \end{itemize} \begin{remark} @@ -1580,7 +1651,7 @@ A multiple access method derived from \ac{DSSS} is \index{code-division multiple \node[align=center] at(10,5,{(\n*2)+0.75}) {User \n}; } \end{tikzpicture} - \caption[Code allocation in a \acs{CDMA} system]{Code allocation in a \acs{CDMA} system. In a \acs{CDMA} system, each user obtains a spreading code which is orthogonal to all other user's codes. The whole bandwidth is used by all users simultaneously.} + \caption[Code allocation in a \acs{DS-CDMA} system]{Code allocation in a \acs{DS-CDMA} system. In a \acs{DS-CDMA} system, each user obtains a spreading code which is orthogonal to all other user's codes. The whole bandwidth is used by all users simultaneously.} \end{figure} Advantages: @@ -1596,6 +1667,30 @@ Drawbacks: \item Transmitters must be able to adjust their transmission power. Transmitters which are closer to the receiver must reduce their power. Otherwise, the receiver would be over-driven due to the limited dynamic range. It then cannot receive far transmitters whose signals are relatively weak. \end{itemize} +\subsubsection{Frequency-Hopping CDMA} + +A \emph{\ac{CDMA} scheme} derived from \ac{FHSS} is \index{frequency-hopping code-division multiple access} \textbf{\acf{FH-CDMA}}. +\begin{itemize} + \item In contrast to \ac{FDMA}, the frequency is not constant. + \item The frequency is changed as defined by the \emph{orthogonal spreading codes}. +\end{itemize} + +\todo{FH-CDMA plot} + +Benefits and drawbacks: See \ac{FDMA} + +\subsubsection{Time-Division CDMA} + +A \emph{\ac{CDMA} scheme} derived from \ac{THSS} is \index{time-division code-division multiple access} \textbf{\acf{TD-CDMA}}. +\begin{itemize} + \item In contrast to \ac{TDMA}, the time-slot is not constant. + \item The time-slot is changed as defined by the \emph{orthogonal spreading codes}. +\end{itemize} + +\todo{TD-CDMA plot} + +Benefits and drawbacks: See \ac{TDMA} + \subsection{Orthogonal Frequency-Division Multiple Access} The \index{orthogonal frequency-division multiple access} \textbf{\acf{OFDMA}} is an extension of the \ac{FDMA} using \emph{orthogonal sub-carrier}. The \ac{OFDMA} method is implemented by a \ac{OFDM} system. @@ -1604,13 +1699,12 @@ The \index{orthogonal frequency-division multiple access} \textbf{\acf{OFDMA}} i \item The carriers of the sub-bands are orthogonal. Guard bands are not required. The different users will not interfere. \end{itemize} -%\subsection{Hybrid Methods} - - \section{Orthogonal Codes} -%\section{Duplexing} +\todo{OVSF} + +%TODO \section{Duplexing} \nocite{ipatov2005} diff --git a/common/acronym.tex b/common/acronym.tex index 2e1c0e7..adeaf08 100644 --- a/common/acronym.tex +++ b/common/acronym.tex @@ -66,6 +66,7 @@ \acro{FDMA}{frequency-division multiple access} \acro{FEC}{forward error coding} \acro{FFT}{fast Fourier transform} + \acro{FH-CDMA}{frequency-hopping code-division multiple access} \acro{FHSS}{frequency-hopping spread spectrum} \acro{FIR}{finite impulse response} \acro{FPGA}{field-programmable gate array} @@ -75,6 +76,7 @@ \acro{GNSS}{global navigation satellite system} \acro{GPIO}{general-purpose input and output} \acro{GPS}{Global Positioning System} + \acro{GSM}{Global System for Mobile Communications} \acro{HDL}{hardware description language} \acro{HF}{high frequency} \acro{HPF}{high pass filter} @@ -123,6 +125,7 @@ \acro{OFDMA}{orthogonal frequency-division multiple access} \acro{OOK}{on-off keying} \acro{OSI}{Open Systems Interconntection} + \acro{OVSF}{orthogonal variable spreading factor} \acro{PAN}{personal area network} \acro{PC}{personal computer} \acro{PCM}{pulse-code modulation} |