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Cambium College Webinar
Transmission Schemes and Frequency Bands
David Geitner
Systems Architecture Team
2
Cambium College – Topic Summary to date
Foundation3/21 Parts of a radio and PTP/PMP/WLAN
4/04 Radio wave propagation and antennas/reflectors
4/18 Interference and Mitigation techniques
5/02 Transmission Schemes and Frequency bands
5/16 WiFi Networks
Applications6/06 Real world applications – service providers
6/20 Enterprise and IoT applications
7/18 Streaming video and throughput uplink/downlink
8/01 MIMO and MU-MIMO
8/15 Guest Speaker: Network Operator
Optimization09/05 Planning a link and the network
09/19 IPv6 and configuration
10/03 Security and Encryption
10/17 Managing the Network
11/07 Maintaining the Network
3
RF Spectrum and Frequency Bands
What is RF Spectrum? Does it really matter?
Whenever we are communicating wirelessly in ANY manner, we need to use some portion of RF Spectrum.
Who determines this?
4
RF Spectrum and Frequency Bands
Radio SpectrumRadio spectrum is the part of the electromagnetic spectrum from 3 Hz to 3000 GHz (3 THz).
Electromagnetic waves in this frequency range, called radio waves, are extremely widely used in modern technology, particularly in telecommunication.
The Federal Communications Commission (FCC) is
the government agency that keeps track of who's
using which slivers of spectrum. This agency
grants companies licenses to use the spectrum… but
not all spectrum is Licensed! Unlicensed exits too!
6
RF Spectrum and Frequency Bands
Lower Frequencies (Less than 1Ghz) HIGH VALUE!
Travels farther distances with less energy
Obstacles (trees, buildings, things) have relatively small effect on attenuating LF RF energy
Relatively limited amount of Spectrum in this Range
Higher Frequencies (Greater than 1Ghz)
Takes more energy to travel far distances
Obstacles (trees, buildings, things) have greater effect on attenuating energy
More available Spectrum in this Range
Radio Spectrum – Key Valuation Points
7
RF Spectrum and Frequency Bands
Licensed Spectrum HIGH VALUE!
Application for usage required… can take time!
Usually requires a fee ($$) to use… can be substantial!
Provides protection for the user from Interference
Unlicensed Spectrum Can be used without Application or Fee… Immediate!
With new additional allocations, typically available for use
No Legal protection for the user from Interference
Radio Spectrum – Licensed vs Unlicensed
8
RF Spectrum and Frequency Bands
VHF (138-174 MHz) – The original standard for push-to-talk voice
– One of the first frequencies to be widely used is still the go-to for marine, air-to-ground and land mobile radio (LMR) radio users. Licensed spectrum.
UHF (406-470MHz) – A multipurpose performer in urban environments
– Another popular LMR frequency for push-to-talk voice, UHF offers better building penetration and tends to perform better in urban environments. UHF is also used for affordable SCADA applications. Licensed spectrum.
700 MHz – The new spectrum for public safety
– 700 MHz available only for public safety and related organizations. Licensed only for public safety.
800 MHz – Still popular for LMR trunking systems
– Public safety use is declining quickly in favor of 700MHz, but expect to see it continue for commercial applications like oil & gas plants. Licensed spectrum.
900 MHz – Sometimes overused license-free spectrum
– Widely used for everything from your cordless phone (if you still have one) to your rural wireless internet service provider. License-free spectrum.
9
RF Spectrum and Frequency Bands
2.4 GHz – The go-to for WiFi
– Another license-free and busy piece of spectrum. License-free
3.65 GHz – Working on the promise of WiMax
– Municipalities and wireless internet providers (WISPs) typically deploying point-to-multipoint with some comfort of more controlled spectrum. Lightly licensed
4.9 GHz – Broadband for public safety
– Set-aside protected for public safety and associate users. Ideal for point-to-point links for public safety applications like backhauling video surveillance and linking public safety towers and sites. Licensed
5.7/5.8 GHz – Broadband for the rest
– Excellent for both point-to-point and point-to-multipoint applications that need bandwidth and now also WiFi. It’s license-free which means spectrum cannot be protected, but good RF planning and design can deliver high-reliability bandwidth for a wide range of public and private sector uses. License-free
6–30 GHz – Microwave spectrum for high-bandwidth high-reliability requirements
– Used for high-performance point-to-point links when big bandwidth and/or high availability is critical. This is licensed spectrum which provides confidence that your deployment will have dedicated frequencies. Licensed
10
RF Transmission Schemes
• Simplex vs Duplex
• Time Division Duplex (TDD) vs Frequency Division Duplex (FDD)
• Single Carrier vs Multi-Carrier
• SISO vs MIMO
• Scheduled vs Random Access
• Synchronized vs Non-Synchronized
11
Simplex RF Communication Scheme
Simplex means Only One-Direction
Communication
Transmitter
Home
Wireless Examples
OTA Broadcast TV
AM/FM Radio
Public Address System
One-Way AMR
Receiver
12
Duplex RF Communication Scheme
Home
Transmitter
& Receiver
Half DuplexOnly One Direction at a Time
WiFi (Wireless LAN)
Two Way Communication
Full DuplexBoth Directions Simultaneously
Cellular Phones (LTE, 3G)
Transmitter
& Receiver
13
Time Division Duplex
Uses One Carrier Frequency; “Reverses” Many Times
• Typically “reverses” 200 times per second (5ms)
• The “guard time” separates the direction
TransmitterMediumReceiver
Transmitter Medium Receiver
Time
Time
14
Frequency Division Duplex
Uses Two Carrier Frequencies
• Forward/reverse or Downlink/Uplink
• Has “guard band” that separates the channels
TransmitterMediumReceiver
Transmitter Medium Receiver
15
Single Carrier vs Multi-Carrier OFDM
Single-carrier QPSK/QAM is proven technology. Time-division systems can
provide fast, dynamic capacity allocation, which is ideal for statistical multiplexing of
bursty sources
Orthogonal Frequency Division Multiplexing (OFDM) is a type of multicarrier
modulation. OFDM uses overlapped orthogonal signals to divide a frequency-selective
channel into a number of narrowband flat-fading channels.
Advantages Disadvantages
Proven and Simple Technology Susceptible to Interference & Multipath
Advantages Disadvantages
More Complex and Costly Can Mitigate Interference & Multipath
16
SISO vs MIMO
SISO – Single Input Single Output
MIMO – Multiple Input Multiple Output
These are techniques based on number of antennas
used at the transmitter and the receiver.
Difference between SISO and MIMOIn SISO system only one antenna is used at the Transmitter
and one antenna is used at the Receiver.
In MIMO case multiple antennas are used.
(Figure to the right depicts 2x2 MIMO case)
MIMO system achieves better Bit Error rate compared to
SISO counterpart at the same SNR. MIMO system also
delivers higher data rate due to transmission of multiple
data symbols simultaneously using multiple antennas, this
technique is called as Spatial Multiplexing (SM).
SISO
TX
SISO
RX
MIMO
TX
MIMO
RX
Antenna Polarization (V/H, +45/-45)
MIMO-A (Single Payload)
MIMO-B (Dual Payload)
MU-MIMO
XPIC
Related Concepts:
http://community.cambiumnetworks.com/t5/PMP-
FAQ/Throughput-vs-Range-An-Explanation/td-p/37199
17
Scheduled vs Random Access Transmission
Impacting Attributes:
• Latency of Data Transmissions
• Scalability of Radio technology
• Complexity of Configuration
• Flexibility of traffic services offered
• Ability to provide good QoS
• Standards based vs Proprietary
• Price delta
18
The Need for Synchronization:Interference Due to non-Synchronization
AP5
AP4 AP3
AP2
AP1AP6
SM
AP1
AP2
AP3
AP4
AP5
AP6
CMM
Each Access
Point is
transmitting at
a different time
AP: Access Point
Synchronization - Why
19
Synchronization: Common Sync Source Reduces System Interference
AP5
AP4 AP3
AP2
AP1AP6
SM
AP1
AP2
AP3
AP4
AP5
AP6
CMM
Each AP is
transmitting
at the SAME
time
AP: Access Point
Synchronization - How