diff --git a/a4.md b/a4.md
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+++ b/a4.md
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-# Channel access method for shared mediums
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+# Channel access method for shared mediums
+
+## Time Division (TDMA)
+assigns time-slots to each transmitter in which they are allowed to send.
+Dynamic slots can be used to adjust for variable bit-rate streams.
+
+## Frequency Division (FDMA)
+assigns parts of the total bandwidth into multiple channels.
+
+
+## Code Division (CDMA)
+Uses orthogonal codes assigned to transmitters, allowing them to send on the same time and frequency.
+
+Both TDMA and FDMA need guard intervals that remain relatively fixed with rising numbers of participants.
+CDMA in comparison is directly limited by the Shannon-Hartley channel capacity,
+meaning that an increased noise floor introduced by an increasing number of transmitters results in temporarily decreased call quality,
+whereas other channel division methods would usually result in permanently decreased quality.
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diff --git a/cdma.Rmd b/cdma.Rmd
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+---
+title: "CDMA"
+subtitle: "Software Defined Radio"
+output: pdf_document
+date: "`r Sys.Date()`"
+---
+# Assignment
+In the R environment for statistical computing, develop
+software support for generating a CDMA composite
+signal. Provide four parallel communication channels
+with corresponding characteristic code sequences of 5
+bits length. Define software support for accepting
+ASCII characters (via a terminal or reading from a data
+file), converting ASCII characters into binary data,
+coding with the respective code sequence of the given
+communication channel, and decoding and deciding
+on the receiving side.
+  
+```{r setup, include=FALSE}
+knitr::opts_chunk$set(echo = TRUE)
+```
+
+## Getting Input
+We first get input by reading a file `input.txt`, where each line (0-3) is one communication channel.
+Because UTF8 maintains compatibility with ASCII, the UTF8 conversion can be used assuming that the input is only ASCII.
+ASCII characters require 7 bits of information per character.
+```{r input}
+lines <- readLines('./input.txt')
+lines_bin <- unlist(lapply(lines, utf8ToInt))
+```
+
+```{r input-demo, echo=FALSE}
+print(lines)
+print(lines_bin)
+```
+
+## Orthogonal Code
+For an orthogonal code, we are looking for vectors $(a,b,...n)$ in  that satisfy pairwise orthogonality $a \perp b$.
+
+
diff --git a/input.txt b/input.txt
new file mode 100644
index 0000000..d68dd40
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+++ b/input.txt
@@ -0,0 +1,4 @@
+a
+b
+c
+d