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+\documentclass[11pt]{beamer}
+\usetheme{default}
+%\setbeamertemplate{frametitle}{}
+\newenvironment{myline}
+    %{\usebeamerfont{frametitle}\usebeamercolor[fg]{frametitle}\vfill\centering}
+    {\usebeamerfont{frametitle}\vfill\centering}
+    {\par\vfill}
+
+\usepackage[pdf]{graphviz}
+\usetheme{Warsaw}
+\usecolortheme{whale}
+
+\title{Cellular Base Station Technology}
+%\subtitle{Subtitle}
+\author{Harald~Welte}
+\date[September 2019, CCCB]{September 2019, CCCB Datengarten}
+\institute{osmocom.org / sysmocom.de}
+
+
+\begin{document}
+
+\begin{frame}
+\titlepage
+\end{frame}
+
+
+\begin{frame}{Outline}
+	\tableofcontents[hideallsubsections]
+\end{frame}
+
+
+\begin{frame}{About the speaker}
+\begin{itemize}
+	\item Free Software + OSHW developer for more than 20 years
+	\item Used to work on the Linux kernel from 1999-2009
+	\item By coincidence among the first people enforcing the GNU GPL in court
+	\item Since 2009 developing FOSS in cellular communications (Osmocom)
+	\item Living and working in Berlin, Germany.
+\end{itemize}
+\end{frame}
+
+
+\section{Introduction}
+
+\begin{frame}{What is a Cellular Base station?}
+\begin{columns}
+	\column{0.38\linewidth}
+	\centering
+	\includegraphics[width=50mm]{gsm-tower.jpg}
+	\column{0.58\linewidth}
+	\begin{itemize}
+		\item transmits and receives signals from/to mobile phones
+		\item converts wireless signals to wired signals
+		\item sits between the {\em air interface} and {\em back-haul}
+		\item is the most visible part of cellular networks
+	\end{itemize}
+\end{columns}
+\end{frame}
+
+\begin{frame}{The 3GPP Specification point-of-view: 2G}
+\includegraphics[width=100mm]{GSM_structures.png}
+
+{\tiny Image credits: tsaitgaist via Wikipedia}
+\end{frame}
+
+
+\begin{frame}{The 3GPP Specification point-of-view: 3G}
+\includegraphics[width=100mm]{UMTS_structures.png}
+
+{\tiny Image credits: tsaitgaist via Wikipedia}
+\end{frame}
+
+\begin{frame}{The 3GPP Specification point-of-view}
+What do we learn from this?
+\begin{itemize}
+	\pause
+	\item The telecom world loves acronyms
+	\pause
+	\item Specifications deal with functional / logical network elements
+	\item Cellular network contains lots of elements
+	\item Today, we only want to look at real-world base stations
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Terminology across cellular generations}
+\begin{table}
+\begin{tabular}{c | c | c | c | c}
+	Generation & Name & Base Station & Back-haul & Next element \\
+\hline \hline
+	2G & GSM/GPRS & BTS & Abis & BSC \\
+	3G & UMTS & NodeB & Iub & RNC \\
+	4G & LTE & eNodeB & S1 & MME + SGW \\
+	5G & NR & gNodeB & N2 + N3 & AMF + UPF
+\end{tabular}
+\end{table}
+\end{frame}
+
+\begin{frame}{Site vs. Cell}
+\begin{description}
+	\item[Site] A single tower and associated equipment
+	\begin{itemize}
+		\item could in theory be omnidirectional
+		\item in reality almost always sectorized
+		\item classic setup is three-sector site (120 degree per sector)
+	\end{itemize}
+	\item[Cell] A logical cell in one cellular network generation
+	\begin{itemize}
+		\item typically illuminated by one (set of) antenna
+	\end{itemize}
+\end{description}
+
+\begin{itemize}
+	\item Result: Single site often has 9 cells
+	\item three sectors for each of 2G, 3G and 4G
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Components of a cellular base station}
+\begin{itemize}
+	\item Tower/Pole (civil engineering part)
+	\item Antenna
+	\item Coaxial Cable
+	\item Actual Base Station Electronics
+	\item Back-haul connection to the rest of the network
+	\item Power Supply / Environment (Fans, AC, UPS, ...)
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{Simplified Rx/Tx chain}
+\begin{itemize}
+	\item Simplified Receiver chain:
+\digraph[scale=0.30]{rxsimple}{
+	rankdir=LR;
+	Antenna -> Duplexer -> RF_Filter -> LNA -> Mixer -> BB_Filter -> ADC -> PHY -> L2_L3
+}
+	\item Simplified Transmitter chain:
+\digraph[scale=0.30]{txsimple}{
+	rankdir=RL;
+	L2_L3 -> PHY -> DAC -> BB_Filter -> Mixer -> PA -> RF_Filter -> Duplexer -> Antenna;
+}
+\end{itemize}
+	Reality is more complex in many cases (circulator, active predistortion, rx diversity, ...)
+\end{frame}
+
+
+\begin{frame}{Even more Simplified Rx/Tx chain}
+\begin{itemize}
+	\item Simplified Receiver chain:
+\digraph[scale=0.40]{rxsimple2}{
+	rankdir=LR;
+	Antenna -> Mixer [label=RF];
+	Mixer -> ADC [label="Analog Baseband"];
+	ADC -> PHY [label="Digital Baseband"];
+	PHY -> L2_L3 [label="Primitives"];
+}
+	\item Simplified Transmitter chain:
+\digraph[scale=0.40]{txsimple2}{
+	rankdir=RL;
+	L2_L3 -> PHY [label="Primitives"];
+	PHY -> DAC [label="Digital Baseband"];
+	DAC -> Mixer [label="Analog Baseband"];
+	Mixer -> Antenna [label="RF"];
+}
+\end{itemize}
+\end{frame}
+
+\section{Evolution of Cell Sites}
+
+\subsection{Classic Cell Sites}
+
+\begin{frame}{Classic Cell Site (year 2000)}
+\begin{columns}
+	\column{0.28\linewidth}
+	\centering
+	\includegraphics[width=36mm]{RBS2206.jpg}
+	\column{0.70\linewidth}
+	The traditional way of building cell sites:
+	\begin{itemize}
+		\item (multiple) large racks full of equipment
+		\item installed in [air conditioned] shelters
+		\item all active electronics on ground level
+		\item long lines of coaxial cable up the tower
+		\item only passive element (antenna) up tower
+		\item half of transmitted power lost in cable
+	\end{itemize}
+{\tiny Image: Timur V. Voronkov via Wikimedia Commons (CC-BY-SA)}
+\end{columns}
+\end{frame}
+
+\begin{frame}{Slightly less Classic Cell Site}
+\begin{columns}
+	\column{0.28\linewidth}
+	\centering
+	\includegraphics[width=36mm]{nokia_flexi.jpeg}
+	\column{0.70\linewidth}
+	The fist step of logical evolution:
+	\begin{itemize}
+		\item equipment becomes smaller (partial rack)
+		\item no strict need for large shelter anymore
+		\item all active electronics on ground level
+		\item long lines of coaxial cable up the tower
+		\item only passive element (antenna) up tower
+		\item half of transmitted power lost in cable
+	\end{itemize}
+	Equipment gets smaller, less power hungry and dissipates less heat
+{\tiny Image: Peter Schmidt @33dBm}
+\end{columns}
+\end{frame}
+
+\begin{frame}{Coaxial Cables...}
+Why don't we like long coaxial cables
+\begin{itemize}
+	\item good cabling is 1/2" to 1" in diameter and costs a lot
+	\item installation is more like plumbing than cabling
+	\item looses lots of energy over length of tower; compensated by
+	\begin{itemize}
+		\item downlink: more PA; waste of energy; causs more heat dissipation
+		\item uplink: tower-mounted amplifier (TMA)
+	\end{itemize}
+	\item higher frequencies have even more losses (and we went from 900 MHz to 1800 MHz to 2100 MHz to 2600 MHz)
+	\item more bands mean more coaxial cables in parallel
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{Towards Remote Radio Heads}
+So why not do he logical thing and ...
+\begin{itemize}
+	\pause
+	\item Generate the RF closer to the antenna?
+\end{itemize}
+Answer:
+\begin{itemize}
+	\item Requires much more compact radios
+	\item Requires passive cooling
+	\item Difficult installation (heavy)
+	\item Environmental protection (sun, rain, temperature cycles)
+	\item Hard to service / replace
+\end{itemize}
+\end{frame}
+
+\subsection{(Remote) Radio Heads}
+
+\begin{frame}{(Remote) Radio Heads}
+Solution: Instead of moving all equipment up the tower,
+\begin{itemize}
+	\item Move only the Analog parts of the chain up
+	\item Transport digital samples up/down the tower
+	\item Base Station split in two parts:
+	\begin{itemize}
+		\item Baseband processing ({\em digital unit})
+		\item Radio processing ({\em radio unit})
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Base Station split with Radio Heads}
+\begin{itemize}
+	\item Incredibly Simplified Receiver chain:
+\digraph[scale=0.30]{rxsimple2split}{
+	rankdir=LR;
+	Antenna -> Mixer [label=RF];
+	subgraph cluster_0 {
+		label="Radio Head";
+		Mixer -> ADC [label="Analog Baseband"];
+	}
+	ADC -> PHY [label="Digital Baseband Samples"];
+	subgraph cluster_1 {
+		label="Baseband Unit";
+		PHY -> L2_L3 [label="Primitives"];
+	}
+}
+	\item Incredibly Simplified Transmitter chain:
+\digraph[scale=0.30]{txsimple2split}{
+	rankdir=RL;
+	subgraph cluster_0 {
+		label="Baseband Unit";
+		L2_L3 -> PHY [label="Primitives"];
+	}
+	subgraph cluster_1 {
+		label="Radio Head";
+		PHY -> DAC [label="Digital Baseband Samples"];
+		DAC -> Mixer [label="Analog Baseband"];
+	}
+	Mixer -> Antenna [label="RF"];
+}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Cell Sites with (Remote) Radio Heads}
+\includegraphics[width=100mm]{antennas-and-rrus.jpg}
+\end{frame}
+
+\begin{frame}{Cell Sites with (Remote) Radio Heads}
+\includegraphics[width=100mm]{cellular-tower-2172041_1920.jpg}
+\end{frame}
+
+\begin{frame}{Cell Sites with (Remote) Radio Heads}
+\includegraphics[width=92mm]{lots-of-radioheads.jpeg}
+
+{\tiny Image: Peter Schmidt @33dBm}
+\end{frame}
+
+\begin{frame}{New term: front-haul}
+\begin{itemize}
+	\item {\em back-haul} is the connection between cell and core
+	\item {\em front-haul} is the newly-introduced term for the link between radio head and baseband unit
+	\item physical medium
+	\begin{itemize}
+		\item typically fiber-optic
+		\item copper only if radio next to baseband unit
+	\end{itemize}
+	\item physical layer
+	\begin{itemize}
+		\item OBSAI (Open Base Station Architecture Initiative)
+		\begin{itemize}
+			\item Started in 2002 by Hyundai, LG, Nokia, Samsung, ZTE
+			\item Mostly obsolete now
+		\end{itemize}
+		\item CPRI (Common Public Radio Interface)
+		\begin{itemize}
+			\item Ericsson, Huawei, NEC, Alcatel-Lucent
+			\item more adoption particularly in recent years
+		\end{itemize}
+		\item eCPRI showing up on the horizon
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{from fiber-based front-haul to C-RAN}
+As digital baseband samples are transmitted over fiber optics
+\begin{itemize}
+	\item can cover distances way above height of the tower
+	\item single-mode transceivers allow for dozens of kilometers
+	\item allows for cell sites without any shelter or rack
+	\item leads to some people proclaiming {\em cloud-RAN} or {\em centralized RAN}
+	\begin{itemize}
+		\item don't distribute baseband compute power in the field
+		\item bring all your baseband samples into the cloud
+		\item perform CPU-intensive baseband function in data center
+	\end{itemize}
+	\item bit rates are high. A single LTE 2x2 MIMO carrier at 20MHz needs 2Gbps CPRI bandwidth
+	\begin{itemize}
+		\item site with 3 sectors and multiple carriers exceeds 10Gbps
+	\end{itemize}
+	\item latency constraints are biggest limiting factor
+\end{itemize}
+\end{frame}
+
+%%\section{Antennas}
+
+\begin{frame}{Antennas}
+\begin{itemize}
+	\item You learned some antenna basics
+	\item You think about an omnidirectional dipole
+	\item Almost no cellular base station antenna is like that
+	\item Complexity of those antennas has grown significantly
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Vertical polarization vs. X-Pol}
+\begin{itemize}
+	\item Nominally, cellular signals are emitted in vertical polarization
+	\item Industry has moved to two radiators at +45 / -45 degrees polarization
+	\item This apparently gives polarization gain, as signals reflected (by buildings) don't arrive in
+		vertical polarization
+	\item Isolation between radiators typically 20..30dB, allowing use cases like
+	\begin{itemize}
+		\item operating two transmitters without combiner
+		\item operating Rx + Tx without duplexer
+		\item diversity reception within one antenna (polarization diversity)
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Single-Band vs. Multiple Bands}
+\begin{itemize}
+	\item So you rolled out a GSM network in 900 MHz
+	\begin{itemize}
+		\item then added more GSM on 1800 MHz
+		\item then added 3G on 2100 MHz, ...
+	\end{itemize}
+	\item Do you add one new set of three sector antennas per band?
+	\begin{itemize}
+		\item space and weight constraints on tower
+		\item they may affect each others' radiation pattersn
+	\end{itemize}
+	\item Industry responds with multi-band antennas
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Electrical Tilt}
+\begin{itemize}
+	\item For RF planning, you want to determine where your cell physically ends
+	\item Tilting antennas downwards means RF signals emitted eventually will hit the ground
+	\item Adjusting the network by climing up the tower and mechanically adjusting tilt is cumbersome
+	\item Industry responds with {\em Electrical Tilt}
+	\item Rods are controlled by motors leading to {\em Remote Electrical Tilt (RET)}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{MIMO}
+\begin{itemize}
+	\item MIMO means Multiple-In / Multiple-Out
+	\item uses spatial diversity to establish multiple signals between different antennas
+	\item 2x2 MIMO is standard with LTE today
+	\item 5G / New Radio specified for massive MIMO (32-64 antennas in base station!)
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Antennas with many ports}
+\includegraphics[width=85mm]{multiport-antenna.jpg}
+\end{frame}
+
+\begin{frame}{Where will it end?}
+\includegraphics[width=115mm]{kathrein_hepta.png}
+\end{frame}
+
+\subsection{Antenna Integrated Radio}
+
+\begin{frame}{Further integration}
+\begin{itemize}
+	\item the radio head has moved up the tower
+	\item coaxial cables are shorter than ever
+	\item ... but we have more and more of them
+	\item So what do we do?
+	\pause
+	\item Integrate radio head inside antenna!
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Antenna Integrated Radio}
+\begin{columns}
+	\column{0.28\linewidth}
+	\centering
+	\includegraphics[width=36mm]{RAS.jpg}
+	\column{0.70\linewidth}
+	\begin{itemize}
+		\item Systems like {\em Nokia RAS} / {\em Ericsson AIR}
+		\item Radio heads completely integrated with antenna
+		\item no coaxial cable at all
+		\item CPRI over fiber directly into the antenna
+		\item Everything Great? New problems
+		\begin{itemize}
+			\item enormous weight not suitable everywhere
+			\item complicated measurements (field technicians)
+		\end{itemize}
+	\end{itemize}
+\end{columns}
+\end{frame}
+
+
+\section{back-haul, hardware, software}
+
+\subsection{Evolution of cellular back-haul}
+
+\begin{frame}{Classic 2G back-haul}
+\begin{itemize}
+	\item 2G (GSM) was specified while ISDN was hot
+	\item back-haul of GSM BTS is done via E1/T1 (ISDN PRI)
+	\item E1 has 30 usable timeslots of 64kBps each
+	\begin{itemize}
+		\item use one for signaling (A-bis RSL + OML)
+		\item use one quarter (16kBps) sub-slot for each voice call
+	\end{itemize}
+	\item While GSM is still deployed today, 3GPP never specified any other transport
+	\item Every vendor came up with their own proprietary kludge on how to carry Abis over IP
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Classic 3G back-haul}
+\begin{itemize}
+	\item 3G (UMTS) was specified when ATM was the next hot thing
+	\item back-haul of eNodeB is done via ATM
+	\item in reality, often Inverse ATM Multiplex (ATM over 4xE1 ISDN)
+	\item 3GPP at least later adapted specs for IP based transport
+	\begin{itemize}
+		\item Every 20ms voice codec frame split over three different UDP packets. yay!
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{4G back-haul}
+\begin{itemize}
+	\item 4G is first 3GPP cellular technology transported over IP from day one
+	\item Therefore, no exotic physical layers
+	\item Ethernet in most cases
+	\item Problem: Where do we get clock from?
+	\begin{itemize}
+		\item ISDN/E1/ATM always provided clock reference
+		\item Ethernet doesn't provide clock reference
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{IP-based back-haul and base station clocking}
+\begin{itemize}
+	\item cellular base stations need super stable clock reference
+	\begin{itemize}
+		\item requirement of 30 ppb is almost 1000 times more accurate than crystal
+		\item even ovenized crystals (OCXOs) not long-term stable enough
+	\end{itemize}
+	\item in the post-ISDN/PDH/SDH days, pick your poison:
+	\begin{itemize}
+		\item go for a GPS-DO and create a single point of failure, or
+		\item use Synchronous Ethernet and loose the advantage of low-cost COTS Ethernet Switches, or
+		\item use IEEE PTP and hope your switches don't introduce too much jitter, or
+		\item let your base stations hammer your NTP server and pray
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\subsection{Base Station Electronics}
+
+\begin{frame}{Base Station Electronics: Baseband}
+\begin{itemize}
+	\item Typically some multi-core DSP
+	\begin{itemize}
+		\item e.g. TI Keystone2 (eight 64bit 1.2GHz DSPs)
+		\item built-in coprocessors (FFT, crypto, Turbo Decoder, Viterbi)
+		\item built-in CPRI/OBSAI Controller
+		\item four ARM Cortex A-15 for L2/L3 processing
+	\end{itemize}
+	\item Often also FPGAs + vendor-specific ASICs
+	\begin{itemize}
+		\item Ericsson big on ASICs
+		\item proprietary ASICs/SoCs with 10.5 billion transistors
+		\item that's comparable to Apple A12X / Huawei Kirin 990!
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Base Station Electronics: Radiohead}
+\begin{itemize}
+	\item Some RFIC (typically ADI)
+	\begin{itemize}
+		\item ADC + DAC
+		\item up/downconversion (mixer)
+		\item on-chip filters
+	\end{itemize}
+	\item Power Amplifier
+	\begin{itemize}
+		\item typically 2 stages of drivers + final PA
+	\end{itemize}
+	\item Circulator
+	\begin{itemize}
+		\item protect PA from power reflected back from antenna
+	\end{itemize}
+	\item Cavity Duplexer
+	\item [Digital] [Adaptive] Pre-distortion
+	\begin{itemize}
+		\item Ensure Linear PA even for high-PAPR signals
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+
+\subsection{Base Station Software}
+
+\begin{frame}{Base Station Software}
+\begin{itemize}
+	\item Don't expect too many familiar things here
+	\item decades of proprietary development by large corporations
+	\item Enea OSE (Operating System Embedded) popular with Ericsson + Nokia
+	\begin{itemize}
+		\item proprietary microkernel with custom-everything including filesystems
+	\end{itemize}
+	\item vxworks found in some equipment like Huawei radioheads
+	\item Linux found mostly only in small cells, inheriting software from femtocells
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Further Reading}
+\begin{itemize}
+	\item \url{http://cpri.info/}
+	\item FlexiWCDMA teardown: \url{https://www.youtube.com/watch?v=d5xT4p9FXIw}
+	\item Ericsson RBS600 teardown: \url{https://www.youtube.com/watch?v=qO127zY3voE}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Thanks}
+Thanks for your attention.
+
+	You have a General Public License to ask questions now :)
+\end{frame}
+
+\end{document}
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