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| 79 | + | \def\SNR{\mbox{\small \sffamily SNR}} |
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| 173 | + | % Solution environment |
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| 175 | + | \newmdenv[linecolor=white,backgroundcolor=lightgray,frametitle=Solution:]{solution} |
| 176 | + | |
| 177 | + | |
| 178 | + | |
| 179 | + | \begin{document} |
| 180 | + | |
| 181 | + | \title{Problems: Coding and Capacity on Fading Channels\\ |
| 182 | + | ECE-GY 6023. Wireless Communications} |
| 183 | + | \author{Prof.\ Sundeep Rangan} |
| 184 | + | \date{} |
| 185 | + | |
| 186 | + | \maketitle |
| 187 | + | |
| 188 | + | |
| 189 | + | \begin{enumerate} |
| 190 | + | |
| 191 | + | \item \emph{Slow vs.\ fast fading:}. |
| 192 | + | For each scenario below state whether the variations would likely be |
| 193 | + | slow or fast fading relative to the coding block. |
| 194 | + | Use reasonable assumptions and explain your reasoning. There is no single correct answer. |
| 195 | + | \begin{enumerate}[label=(\alph*)] |
| 196 | + | \item A 5G NR base stations transmits over a channel with a \SI{100}{ns} delay spread, |
| 197 | + | to a UE moving at $v=$ \SI{30}{m/s} with a $180^\circ$ angular spread. |
| 198 | + | The carrier frequency is $f_c=$ \SI{28}{GHz}. |
| 199 | + | The transmission is over a \SI{100}{MHz} bandwidth in \SI{125}{\micro\second} slots. |
| 200 | + | \item A UAV is connected to a ground base station via point-to-point link with a line-of-sight. |
| 201 | + | So, there is no multipath fading. But the UAV rotates 360$^\circ$ about once a second. |
| 202 | + | The beamwidth of the UAV |
| 203 | + | antenna element is 60$^\circ$ and packets are transmitted once every \SI{1}{ms}. |
| 204 | + | \end{enumerate} |
| 205 | + | |
| 206 | + | \item \emph{Error rate on uncoded modulation}: |
| 207 | + | \begin{enumerate}[label=(\alph*)] |
| 208 | + | \item Use any reference to find the symbol error rate (SER) of 16-QAM as |
| 209 | + | a function of the SNR $\gamma_s = E_s/N_0$. Your expression will have a $Q$-function. |
| 210 | + | \item Find the SNR $\gamma_s$ requred for a SER of $(10)^{-3}$ assuming a constant channel. |
| 211 | + | You can use MATLAB to invert the $Q$-function. |
| 212 | + | \item Suppose that the channel is Rayleigh fading, so $\gamma_s$ is exponentially distributed. |
| 213 | + | Find the average SNR, $\Exp(\gamma_s)$ so that the average SER is $(10)^{-3}$. |
| 214 | + | \end{enumerate} |
| 215 | + | |
| 216 | + | \item \emph{Slow fading and outage probability:} An access point is installed in an office |
| 217 | + | area with four rooms. The path loss from the access point to each room and the percentage of |
| 218 | + | users in each room are as follows: |
| 219 | + | \begin{center} |
| 220 | + | \begin{tabular}{|c|c|c|} |
| 221 | + | \hline |
| 222 | + | % after \\: \hline or \cline{col1-col2} \cline{col3-col4} ... |
| 223 | + | Room & Path loss [dB] & Fraction users \\ \hline |
| 224 | + | 1 & 60 & 0.6 \\ \hline |
| 225 | + | 2 & 80 & 0.3 \\ \hline |
| 226 | + | 3 & 90 & 0.06 \\ \hline |
| 227 | + | 4 & 100 & 0.04 \\ \hline |
| 228 | + | \end{tabular} |
| 229 | + | \end{center} |
| 230 | + | The AP has a transmit power of \SI{15}{dBm} and bandwidth of \SI{18}{MHz}. |
| 231 | + | The thermal noise at the receivers, including noise figure is \SI{-165}{dBm/Hz}. |
| 232 | + | \begin{enumerate}[label=(\alph*)] |
| 233 | + | \item If there is no fading, what SNR can be guaranteed to at least 95\% of the users? |
| 234 | + | \item Now suppose that, at each location, there is Rayleigh fading that can be modeled as flat |
| 235 | + | over the transmissions. |
| 236 | + | Write an expression for the CDF of the SNR including variation in both location and fading. |
| 237 | + | \item What is the SNR that can be guaranteed to at least 95\% of the users if we need |
| 238 | + | to account for slow fading? You can use MATLAB to invert the expression in part (b). |
| 239 | + | \end{enumerate} |
| 240 | + | |
| 241 | + | \item \emph{Ergodic capacity:} A channel has two paths. One path would be received |
| 242 | + | at power, $P_1$ and delay $\tau_1$, and the second path would be received at power |
| 243 | + | $P_2$ and delay $\tau_2$ where $\tau_2 > \tau_1$. |
| 244 | + | Suppose you signal over a bandwidth $W \gg 1/(\tau_2 - \tau_1)$ and noise power spectral density |
| 245 | + | is $N_0$. |
| 246 | + | \begin{enumerate}[label=(\alph*)] |
| 247 | + | \item What is the average SNR over the band? |
| 248 | + | \item What is the ergodic capacity over the band? |
| 249 | + | \item Evaluate the expressions in (a) and (b) with $P_1/(W N_0)=$ \SI{8}{dB} |
| 250 | + | and $P_2/(W N_0)=$ \SI{5}{dB}. |
| 251 | + | \end{enumerate} |
| 252 | + | |
| 253 | + | |
| 254 | + | \item \emph{LLRs:} For each of the following channels, find the log likelihood ratio (LLR): |
| 255 | + | \[ |
| 256 | + | L(r) = \log \frac{ p(r|c=1) }{ p(r|c=0) } |
| 257 | + | \] |
| 258 | + | for the following channels: |
| 259 | + | \begin{enumerate}[label=(\alph*)] |
| 260 | + | \item Real-valued binary channel with fading: |
| 261 | + | \[ |
| 262 | + | r= Ax + w, \quad w \sim {\mathcal N}(0,N_0/2), \quad |
| 263 | + | x = \begin{cases} |
| 264 | + | \sqrt{E_x/2} & \mbox{if } c = 1, \\ |
| 265 | + | -\sqrt{E_x/2} & \mbox{if } c = 0. |
| 266 | + | \end{cases} |
| 267 | + | \] |
| 268 | + | The LLR $L$ should depend on $A$ and $N_0$. |
| 269 | + | |
| 270 | + | \item Binary symmetric channel: |
| 271 | + | \[ |
| 272 | + | r = c + w ~(\mbox{mod } 2), \quad w = |
| 273 | + | \begin{cases} |
| 274 | + | 1 & \mbox{with probability } p \\ |
| 275 | + | 0 & \mbox{with probability } 1-p |
| 276 | + | \end{cases} |
| 277 | + | \] |
| 278 | + | Thus, $r \in \{0,1\}$ where there is a bit error with probability $p$. |
| 279 | + | |
| 280 | + | \item Non-coherent channel: |
| 281 | + | \[ |
| 282 | + | r = \begin{cases} |
| 283 | + | h + n & \mbox{when } c = 1\\ |
| 284 | + | n & \mbox{when } c = 0, |
| 285 | + | \end{cases} |
| 286 | + | \quad h \sim {\mathcal CN}(0,E_s), ~n\sim {\mathcal CN}(0,N_0). |
| 287 | + | \] |
| 288 | + | |
| 289 | + | \end{enumerate} |
| 290 | + | |
| 291 | + | \item \emph{Bitwise likelihood}: Suppose that two bits $(c_0,c_1)$ are modulated to |
| 292 | + | a $4$-PAM constellation |
| 293 | + | (the real or imaginary component of a 16-QAM constellation): |
| 294 | + | \[ |
| 295 | + | r = x + n, \quad n \sim {\mathcal N}(0,N_0/2), |
| 296 | + | \] |
| 297 | + | where the transmitted symbol |
| 298 | + | \[ |
| 299 | + | x = \begin{cases} |
| 300 | + | -3A & \mbox{if } (c_0,c_1) = (00) \\ |
| 301 | + | -A & \mbox{if } (c_0,c_1) = (01) \\ |
| 302 | + | A & \mbox{if } (c_0,c_1) = (11) \\ |
| 303 | + | 3A & \mbox{if } (c_0,c_1) = (10) |
| 304 | + | \end{cases} |
| 305 | + | \] |
| 306 | + | Assume all the transmitted bits are equally likely. |
| 307 | + | \begin{enumerate}[label=(\alph*)] |
| 308 | + | \item Given a symbol energy, $E_s$, find $A$ such that $\Exp|x|^2 = E_s/2$. |
| 309 | + | \item Find the bitwise LLR for $c_0$: |
| 310 | + | \[ |
| 311 | + | L_0(r) = \log \frac{p(r|c_0=1)}{p(r|c_0=1)}. |
| 312 | + | \] |
| 313 | + | Use total probability |
| 314 | + | \[ |
| 315 | + | p(r|c_0) = \frac{1}{2}\left[ p(r|c_0,c_1=1) + p(r|c_0,c_1=0) \right]. |
| 316 | + | \] |
| 317 | + | Find the bitwise LLR for $c_1$ as well. |
| 318 | + | \end{enumerate} |
| 319 | + | |
| 320 | + | \item \emph{Row-column interleavers:} One simple way of doing interleaving is as follows. |
| 321 | + | The input is a sequence of bits of length $MN$ for some parameters $M$ and $N$. |
| 322 | + | We read the bits into an $M \times N$ array, one row at a time. Then, we read out |
| 323 | + | the bits one column at a time. If two bits are adjacent on the input what is the minimum separation on the output? |
| 324 | + | \end{enumerate} |
| 325 | + | |
| 326 | + | \end{document} |
| 327 | + | |
| 328 | + | |