ACM: Adaptive Coding and Modulation

Automatic Coding and Modulation (ACM)

Microwave Adaptive Coding and Modulation (ACM)
Microwave Adaptive Coding and Modulation (ACM)

Adaptive Coding and Modulation or Link adaptation is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link (e.g. the pathloss, the interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.). In a digital Microwave Link ACM uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. The process of link adaptation is a dynamic one and the signal and protocol parameters change as the radio link conditions change.

The Goal of ACM

ACM with 1024 QAM Modulation
ACM with 1024 QAM Modulation

The goal of Adaptive Modulation and Coding  is to improve the operational efficiency of Microwave links by increasing network capacity over the existing infrastructure – while reducing sensitivity to environmental interferences.
Adaptive Modulation means dynamically varying the modulation in an errorless manner in order to maximize the throughput under momentary propagation conditions. In other words, a system can operate at its maximum throughput under clear sky conditions, and decrease it
gradually under rain fade.  For example a link can change from 1024QAM down to QPSK to keep “link alive” without losing connection.  Prior to the development of Automatic Coding and Modulation, microwave designers had to design for “worst case” conditions to avoid link outage The benefits of using ACM include:

  • Longer link lengths (distance)
  • Using smaller antennas (saves on mast space, also often required in residential areas)
  • Higher Availability (link reliability)

Importance to Operators of ACM

CableFree Microwave Link using 30cm antenna benefits from ACM giving longer reach and higher availability
CableFree Microwave Link using 30cm antenna benefits from ACM giving longer reach and higher availability

Adaptive Coding and Modulation increases the capacity of microwave links without sacrificing distance or availability, and without requiring larger antennas.  The penalty – reduced capacity during heavy fade/rainfall – is usually considered an acceptable trade-off compared to the benefits, especially for IP networks where a variable capacity is generally considered acceptable, compared to legacy PDH (NxE1/T1) and SDH connections which are fixed capacity applications.  Conversely, ACM allows operators to minimise costs by using smaller antennas, meet higher availability targets (e.g. 99.999% availability) and customer SLA (service level agreement) and also fit within aesthetic and planning constraints in dense urban areas and regions of natural beauty where large antennas may be prohibited by planners or building owners.

For Further Information on ACM and Microwave Links

For more information on Microwave Links with ACM please Contact Us

Microwave Antenna Alignment

Alignment of Microwave Antennas for Digital Microwave Transmission Systems

This article contains generic instructions for alignment of Microwave antennas.  Specific products may have different features, in which case please refer to the documentation provided for those products:

CableFree Microwave Antenna Alignment
CableFree Microwave Antenna Alignment

Antenna Alignment for Microwave Links

This guide explains how to achieve the optimal antenna alignment of microwave antennas when used with modern digital microwave products.  Before attempting to do the alignment it is highly recommended that you read this guide in detail.  For specific commands please consult the manual of the product being installed

Step 1:  Preparation:

Mount the antenna on the tower according to the antenna installation instructions:  Ensure that the adjustment bolts move smoothly and the range of motion is sufficient for the expected angle of up and down (elevation) tilt. Ensure that the mount itself is attached securely and all safety precautions have been taken.

CableFree Microwave Antenna Alignment using DVM
CableFree Microwave Antenna Alignment using DVM

Step 2: Coarse Alignment:

Visually align the antenna with the far end.  The most common ways to do this are :

1)      If the visibility is good and the sun is in the correct position, have someone at the far end location reflect the sun with a mirror so the location is obvious.

2)      If visibility is poor, use GPS coordinates and a GPS compass to aim the antenna coarsely.

CableFree Microwave Antenna Alignment avoiding Sidelobes
CableFree Microwave Antenna Alignment avoiding Sidelobes

Step 3: Fine Alignment.

Before conducting fine alignment, the ODUs at both ends of the link must be attached properly to the antenna via the direct mount or remote mount (using Waveguide) and the far end ODU must be powered on and transmitting.  The ODU lightning surge suppressors and grounding provisions should be put in place as well before alignment. The local ODU must be powered on, but need not be transmitting.

Ensure that:

1)      Frequency of the far end transmitter matches the frequency of the local receiver.

2)      The TX output power is not set above the level of the license.

3)      ATPC is turned OFF on the far end.

4)      Alignment mode is ON for SP ODUs – Display on ODU and IDU will update at 5 times per second.

FINE ALIGNMENT PROCEDURE

1)      Adjust the azimuth over a 30 degree sweep by turning the adjustment bolt in increments of 1/10th turn to avoid missing the main lobe. When the highest signal has been found for azimuth, repeat for the elevation adjustment.

2)      Turn the local transmitter on to allow alignment at the far end.

3)      Move to the far end of the link and repeat step 1.

4)      Lock down the antenna so no further movement can occur.

5)      Install the antenna side struts supplied with the antenna.

6)      Verify the RSSI remains the same and is within 2-4 dB of the expected levels.

7)      Check the ODU connector seals.

8)      Turn alignment mode OFF

9)      The alignment is complete.

FDD and TDD Explained

The difference between FDD and TDD in Microwave Transmission

Microwave ODU with Antenna using FDD (Frequency Division Duplex)
Microwave ODU with Antenna using FDD (Frequency Division Duplex)

Microwave links typically use Frequency-division duplexing (FDD) which is a method for establishing a full-duplex communications link that uses two different radio frequencies for transmitter and receiver operation. The transmit direction and receive direction frequencies are separated by a defined frequency offset.

Advantages of FDD

In the microwave realm, the primary advantages of this approach are:

  • The full data capacity is always available in each direction because the send and receive functions are separated;
  • It offers very low latency since transmit and receive functions operate simultaneously and continuously;
  • It can be used in licensed and license-exempt bands;
  • Most licensed bands worldwide are based on FDD; and
  • Due to regulatory restrictions, FDD radios used in licensed bands are coordinated and protected from interference, though not immune to it.
Microwave FDD (Frequency Division Duplexing)
Microwave FDD (Frequency Division Duplexing)

Disadvantages to FDD

The primary disadvantages of the FDD approach to microwave communication are:

  • Complex to install. Any given path requires the availability of a pair of frequencies; if either frequency in the pair is unavailable, then it may not be possible to deploy the system in that band;
  • Radios require pre-configured channel pairs, making sparing complex;
  • Any traffic allocation other than a 50:50 split between transmit and receive yields inefficient use of one of the two paired frequencies, lowering spectral efficiency; and
  • Collocation of multiple radios is difficult.

TDD compared with FDD

Time-division duplexing (TDD) is a method for emulating full-duplex communication over a half-duplex communication link. The transmitter and receiver both use the same frequency but transmit and receive traffic is switched in time. The primary advantages of this approach as it applies to microwave communication are:

  • It is more spectrum friendly, allowing the use of only a single frequency for operation and dramatically increasing spectrum utilization, especially in license-exempt or narrow-bandwidth frequency bands ;
  • It allows for the variable allocation of throughput between the transmit and receive directions, making it well suited to applications with asymmetric traffic requirements, such as video surveillance, broadcast and Internet browsing;
  • Radios can be tuned for operation anywhere in a band and can be used at either end of the link. As a consequence, only a single spare is required to serve both ends of a link.

Disadvantages of TDD

The primary disadvantages of traditional TDD approaches to microwave communications are:

  • The switch from transmit to receive incurs a delay that causes traditional TDD systems to have greater inherent latency than FDD systems;
  • Traditional TDD approaches yield poor TDM performance due to latency;
  • For symmetric traffic (50:50), TDD is less spectrally efficient than FDD, due to the switching time between transmit and receive; and
  • Multiple co-located radios may interfere with one another unless they are synchronized.

 

Microwave Mobile Backhaul

Packet Microwave Radios for Mobile Backhaul

To deliver a compelling quality of experience for subscribers, you must respond quickly to growing traffic demands. Modern Packet Microwave Mobile Backhaul products help you maximize the network’s performance by enabling rapid deployment of scalable backhaul to cell sites.  Modern solutions include a portfolio of microwave products to address the backhaul needs of 2G, 3G, and LTE macro cells and 3G, LTE, and Wi-Fi® small cells. Radio spectrum is maximized using innovative techniques to maximize payload capacity to support the evolution to LTE and heterogeneous networks. Unique, common radio support for indoor and outdoor deployments enhances savings potential.

Packet Microwave Mobile Backhaul is a key component in a modern end-to-end mobile backhaul solution, which provides the flexibility, scale and operational simplicity to lower the total cost of ownership and simultaneously enhance the mobile service experience.

BENEFITS

CableFree Microwave for Mobile Backhaul
CableFree Microwave for Mobile Backhaul

Economic benefits

Rapidly support the optimal cell site location

  • Complete backhaul portfolio for macro cells and small cells
  • Support for all sites including both end and intermediate cell sites
  • Space and power efficiency
  • Full outdoor option to meet different microwave site space requirements

Achieve maximum spectral performance

  • Maximum bandwidth per band
  • Intelligent compression
  • Advanced quality of service levels supporting subscriber quality of experience

Scale the network cost effectively

  • Reliably bond radio channels to create larger microwave links
  • Any topology, any number of microwave link directions
  • Network awareness for both Carrier Ethernet and/or IP/MPLS networks

Be operationally efficient

  • Common radio for all cell sites
  • Evolutionary path from hybrid microwave to packet microwave at the touch of a button
  • Management beyond basic IP partner integration

Deployment, management, end-user benefits

Grow and retain subscribers by maximizing the mobile experience

  • Infrastructure support for increased subscriber bandwidth demands
  • Ability to react quickly to subscriber demand with optimally-located cell sites
  • Increased capacity that supports high bandwidth data applications

COMPONENTS

4G/LTE Mobile Backhaul
4G/LTE Mobile Backhaul

Packet Microwave Mobile Backhaul integrates a modern microwave portfolio with small cell optimized products to provide a complete backhaul offering for small cells and/or macro cells.

Read on in our following pages to find out more about technologies used in mobile backhaul applications

Microwave Link Technology

Introduction to Microwave

Example of a CableFree Microwave Link Installation
Example of a CableFree Microwave Link Installation

Microwave is a line-of-sight wireless communication technology that uses high frequency beams of radio waves to provide high speed wireless connections that can send and receive voice, video, and data information.

Microwave links are are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it.

Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

The higher part of the radio electromagnetic spectrum with frequencies are above 30 GHz and below 100 GHz, are called “millimeter waves” because their wavelengths are conveniently measured in millimeters, and their wavelengths range from 10 mm down to 3.0 mm. Radio waves in this band are usually strongly attenuated by the Earthly atmosphere and particles contained in it, especially during wet weather. Also, in wide band of frequencies around 60 GHz, the radio waves are strongly attenuated by molecular oxygen in the atmosphere. The electronic technologies needed in the millimeter wave band are also much more complex and harder to manufacture than those of the microwave band, hence cost of Millimeter Wave Radios are generally higher.

History of Microwave Communication

James Clerk Maxwell, using his famous “Maxwell’s equations,” predicted the existence of invisible electromagnetic waves, of which microwaves are a part, in 1865. In 1888, Heinrich Hertz became the first to demonstrate the existence of such waves by building an apparatus that produced and detected microwaves in the ultra high frequency region. Hertz recognized that the results of his experiment validated Maxwell’s prediction, but he did not see any practical applications for these invisible waves. Later work by others led to the invention of wireless communications, based on microwaves. Contributors to this work included Nikola Tesla, Guglielmo Marconi, Samuel Morse, Sir William Thomson (later Lord Kelvin), Oliver Heaviside, Lord Rayleigh, and Oliver Lodge.

Microwave Link over English Channel, 1931
Microwave Link over English Channel, 1931

In 1931 a US-French consortium demonstrated an experimental microwave relay link across the English Channel using 10 foot (3m) dishes, one of the earliest microwave communication systems. Telephony, telegraph and facsimile data was transmitted over the 1.7 GHz beams 40 miles between Dover, UK and Calais, France. However it could not compete with cheap undersea cable rates, and a planned commercial system was never built.
During the 1950s the AT&T Long Lines system of microwave relay links grew to carry the majority of US long distance telephone traffic, as well as intercontinental television network signals. The prototype was called TDX and was tested with a connection between New York City and Murray Hill, the location of Bell Laboratories in 1946. The TDX system was set up between New York and Boston in 1947.

Modern Commercial Microwave Links

CableFree Microwave Communication Tower
Microwave Communication Tower

A microwave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmit video, audio, or data between two locations, which can be from just a few feet or meters to several miles or kilometers apart. Examples of Commercial Microwave links from CableFree may be see here. Modern Microwave Links can carry up to 400Mbps in a 56MHz channel using 256QAM modulation and IP header compression techniques.  Operating Distances for microwave links are determined by antenna size (gain), frequency band, and link capacity.  The availability of clear Line of Sight is crucial for Microwave links for which the Earth’s curvature has to be allowed

CableFree FOR2 Microwave Link 400Mbps
CableFree FOR2 Microwave Link 400Mbps

Microwave links are commonly used by television broadcasters to transmit programmes across a country, for instance, or from an outside broadcast back to a studio. Mobile units can be camera mounted, allowing cameras the freedom to move around without trailing cables. These are often seen on the touchlines of sports fields on Steadicam systems.

Planning of microwave links

CableFree Microwave links have to be planned considering the following parameters:

  • Required distance (km/miles) and capacity (Mbps)
  • Desired Availability target (%) for the link
  • Availability of Clear Line of Sight (LOS) between end nodes
  • Towers or masts if required to achieve clear LOS
  • Allowed frequency bands specific to region/country
  • Environmental constraints, including rain fade
  • Cost of licenses for required frequency bands

Microwave Frequency Bands

Microwave Frequency Bands
Microwave Frequency Bands

Microwave signals are often divided into three categories:
ultra high frequency (UHF) (0.3-3 GHz);
super high frequency (SHF) (3-30 GHz); and
extremely high frequency (EHF) (30-300 GHz).
In addition, microwave frequency bands are designated by specific letters. The designations by the Radio Society of Great Britain are given below.
Microwave frequency bands
Designation Frequency range
L band 1 to 2 GHz
S band 2 to 4 GHz
C band 4 to 8 GHz
X band 8 to 12 GHz
Ku band 12 to 18 GHz
K band 18 to 26.5 GHz
Ka band 26.5 to 40 GHz
Q band 30 to 50 GHz
U band 40 to 60 GHz
V band 50 to 75 GHz
E band 60 to 90 GHz
W band 75 to 110 GHz
F band 90 to 140 GHz
D band 110 to 170 GHz

The term “P band” is sometimes used for ultra high frequencies below the L-band. For other definitions, see Letter Designations of Microwave Bands

Lower Microwave frequencies are used for longer links, and regions with higher rain fade.  Conversely, Higher frequencies are used for shorter links and regions with lower rain fade.

Rain Fade on Microwave Links

Microwave Link Rain FadeRain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow or ice, and losses which are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn. Rain fade is not limited to satellite uplinks or downlinks, it also can affect terrestrial point to point microwave links (those on the earth’s surface).

Possible ways to overcome the effects of rain fade are site diversity, uplink power control, variable rate encoding, receiving antennas larger (i.e. higher gain) than the required size for normal weather conditions, and hydrophobic coatings.

Diversity in Microwave Links

Example of a 1+0 Unprotected Microwave Link
Example of a 1+0 Unprotected Microwave Link

In terrestrial microwave links, a diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity plays an important role in combatting fading and co-channel interference and avoiding error bursts. It is based on the fact that individual channels experience different levels of fading and interference. Multiple versions of the same signal may be transmitted and/or received and combined in the receiver. Alternatively, a redundant forward error correction code may be added and different parts of the message transmitted over different channels. Diversity techniques may exploit the multipath propagation, resulting in a diversity gain, often measured indecibels.

The following classes of diversity schemes are typical in Terrestrial Microwave Links:

  • Unprotected:  Microwave links where there is no diversity or protection are classified as Unprotected and also as 1+0.  There is one set of equipment installed, and no diversity or backup
  • Hot Standby: Two sets of microwave equipment (ODUs, or active radios) are installed generally connected to the same antenna, tuned to the same frequency channel.  One is “powered down” or in standby mode, generally with the receiver active but transmitter muted.  If the active unit fails, it is powered down and the standby unit is activated.  Hot Standby is abbreviated as HSB, and is often used in 1+1 configurations (one active, one standby).
  • Frequency diversity: The signal is transmitted using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading. Microwave radio links often use several active radio channels plus one protection channel for automatic use by any faded channel. This is known as N+1 protection
  • Space diversity: The signal is transmitted over several different propagation paths. In the case of wired transmission, this can be achieved by transmitting via multiple wires. In the case of wireless transmission, it can be achieved by antenna diversity using multiple transmitter antennas (transmit diversity) and/or multiple receiving antennas (reception diversity).
  • Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization. A diversity combining technique is applied on the receiver side.

Diverse Path Resilient Failover

In terrestrial point to point microwave systems ranging from 11 GHz to 80 GHz, a parallel backup link can be installed alongside a rain fade prone higher bandwidth connection. In this arrangement, a primary link such as an 80GHz 1 Gbit/s full duplex microwave bridge may be calculated to have a 99.9% availability rate over the period of one year. The calculated 99.9% availability rate means that the link may be down for a cumulative total of ten or more hours per year as the peaks of rain storms pass over the area. A secondary lower bandwidth link such as a 5.8 GHz based 100 Mbit/s bridge may be installed parallel to the primary link, with routers on both ends controlling automatic failover to the 100 Mbit/s bridge when the primary 1 Gbit/s link is down due to rain fade. Using this arrangement, high frequency point to point links (23GHz+) may be installed to service locations many kilometers farther than could be served with a single link requiring 99.99% uptime over the course of one year.

Automatic Coding and Modulation (ACM)

Microwave Adaptive Coding and Modulation (ACM)
Microwave Adaptive Coding and Modulation (ACM)

Link adaptation, or Adaptive Coding and Modulation (ACM), is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link (e.g. the pathloss, the interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.). For example, EDGE uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. The process of link adaptation is a dynamic one and the signal and protocol parameters change as the radio link conditions change.

The goal of Adaptive Modulation is to improve the operational efficiency of Microwave links by increasing network capacity over the existing infrastructure – while reducing sensitivity to environmental interferences.
Adaptive Modulation means dynamically varying the modulation in an errorless manner in order to maximize the throughput under momentary propagation conditions. In other words, a system can operate at its maximum throughput under clear sky conditions, and decrease it
gradually under rain fade.  For example a link can change from 256QAM down to QPSK to keep “link alive” without losing connection.  Prior to the development of Automatic Coding and Modulation, microwave designers had to design for “worst case” conditions to avoid link outage The benefits of using ACM include:

  • Longer link lengths (distance)
  • Using smaller antennas (saves on mast space, also often required in residential areas)
  • Higher Availability (link reliability)

Automatic Transmit Power Control (ATPC)

CableFree Microwave links feature ATPC which automatically increases the transmit power during “Fade” conditions such as heavy rainfall.  ATPC can be used separately to ACM or together to maximise link uptime, stability and availability.  When the “fade” conditions (rainfall) are over, the ATPC system reduces the transmit power again.  This reduces the stress on the microwave power amplifiers, which reduces power consumption, heat generation and increases expected lifetime (MTBF)

Uses of microwave links

  • Backbone links and “Last Mile” Communication for cellular network operators
  • Backbone links for Internet Service Providers (ISPs) and Wireless ISPs (WISPs)
  • Corporate Networks for Building to Building and campus sites
  • Telecommunications, in linking remote and regional telephone exchanges to larger (main) exchanges without the need for copper/optical fibre lines.
  • Broadcast Television with HD-SDI and SMPTE standards

Enterprise

Because of the scalability and flexibility of Microwave technology, Microwave products can be deployed in many enterprise applications including building-to-building connectivity, disaster recovery, network redundancy and temporary connectivity for applications such as data, voice and data, video services, medical imaging, CAD and engineering services, and fixed-line carrier bypass.

Mobile Carrier Backhaul

CableFree Microwave Cellular Network
Microwave Backhaul in Cellular Networks

 

Microwave Links are a valuable tool in Mobile Carrier Backhaul: Microwave technology can be deployed to provide traditional PDH 16xE1/T1, STM-1 and STM-4, and Modern IP Gigabit Ethernet backhaul connectivity and Greenfield mobile networks.  Microwave is far quicker to install and lower Total Cost of Ownership for Cellular Network Operators compared to deploying or leasing fibre optic networks

Low Latency Networks

CableFree Low Latency versions of Microwave links uses Low Latency Microwave Link Technology, with absolutely minimal delay between packets being transmitted and received at the other end, except the Line of Sight propagation delay.  The Speed of Microwave propagation through the air is approximately 40% higher than through fibre optics, giving customers an immediate 40% reduction in latency compared to fibre optics.  In addition, fibre optic installations are almost never in a straight line, with realities of building layout, street ducts and requirement to use existing telecom infrastructure, the fibre run can be 100% longer than the direct Line of Sight path between two end points.  Hence CableFree Low Latency Microwave products are popular in Low Latency Applications such as High Frequency Trading and other uses.

For Further Information on Microwave

To find out more about Microwave Link Technology and how CableFree can assist with your wireless network, please Contact Us