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authorHarald Welte <laforge@gnumonks.org>2019-09-10 02:39:47 +0200
committerHarald Welte <laforge@gnumonks.org>2019-09-10 02:39:47 +0200
commitc3f7f15b10d7e703910696c5bbfa116f0aee3e6a (patch)
tree8bc1aaca389f641a9913e9bb292e5768b447d69b /2019
parent8b75b3371ea6162561ca7ba2031a8c0ea90952b0 (diff)
initial checkin of slides for CCCB Datengarten
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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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personal git repositories of Harald Welte. Your mileage may vary