Mean Square Error (MSE) is similar to Signal-to-Noise Ratio (SNR) except that it accounts for distortion and interference in addition to noise power.
Distortion may come from several sources such as bad Ethernet cables (poor shield, damaged, or low quality), path degradations such as multipath, or Fresnel zone encroachment.
Interference can come from other transmitters on the tower, as well as from sources inside an indoor shelter. High power transmitters inside a shelter can cause interference when near the PoE device or when located very close to the cabling.
There are maximum acceptable MSE values for each modulation which are useful in determining the quality of the link. The MSE value reported is only relevant to one tx-rx path, so the MSE of each tx-rx path must be evaluated to verify the link is operating as expected. The lower the number the better, so a -35dB is better than a -30dB.
Other possible causes for unacceptable MSE
XPIC parameters are incorrect
Insufficient isolation between polarisations on an XPIC link
Insufficient performance to support high QAM modulation
Inbalance between paths on an XPIC dual polarity link
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70GHz to 80GHz bands (E-band) are used for Point to Point (P2P) Microwave (Millimeter Wave, MMW) Radio Links
Sources of Data and Graphics
All contents (C) OFCOM and taken from:
OfW48 UK Frequency Allocations for Fixed (Point-to-Point) Wireless Services and Scanning Telemetry This document shows the current bands managed by Ofcom that are available for fixed terrestrial (point to point) links and scanning telemetry in the UK.
The Radio Equipment and Telecommunications Terminal Equipment Directive
99/5/EC (R&TTED) has been implemented in ‘The Radio Equipment and Telecommunications Terminal Equipment Regulations 2000, Statutory
Instrument (SI) 730. In accordance with Articles 4.1 and 7.2 of the R&TTED
• IR2000: The UK Interface Requirement 2000 contains the requirements for the licensing and use of fixed (point-to-point) wireless services in the UK.
• IR2037: The UK Interface Requirement 2037 applies for scanning telemetry services.
• IR2078: The UK Interface Requirement 2078 applies for the 60 GHz band
Notes specific to the frequency charts
The first column describes each available frequency band, represented by a diagram (not to scale). The frequency band limits are listed below the diagram; frequencies below 10 GHz are represented in MHz, while those above 10 GHz are in GHz. The width of each guard band is shown above the diagram, and is always specified in MHz.
The channel arrangements in some bands are staggered, so that the width and position of the guard band vary for different channel spacings. In these cases, a table underneath gives details of the guard bands for different spacings (with all frequencies in MHz).
The first column also includes the title of the relevant international recommendations for each band, produced by the European Conference of Postal and Telecommunications (CEPT) or the International Telecommunication Union (ITU). CEPT recommendations are available at https://www.cept.org/ecc/ and ITU Recommendations at https://www.itu.int.
The final column contains the channel spacing for duplex operation in each frequency band except for bands above 60 GHz. Details of standard systems assigned in the UK are shown in the relevant technical frequency assignment criteria.
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1024QAM Microwave Links for High Capacity Wireless Transmission
High Capacity Microwave Links from leading vendors use 1024QAM modulation to achieve high capacity, spectral density and efficiency without sacrificing reliability. This technology sets a new benchmark for microwave transmission capacity for operators including 4G / LTE Backhaul links for mobile operators as well as last-mile links, backbone and other applications.
High Capacity Links require High Order QAM modulation
Leading long-haul microwave equipment vendors are now using dependable long-distance transmissions using 1024 QAM. Relative to the industry-standard 256 QAM, this represents a 25% increase in capacity (and up to double the capacity of legacy SDH links), with all other factors the same. Compared to older 4QAM modulation the increase to 1024QAM is five-fold. Operators of long-haul microwave links will certainly enjoy the boost to their capacity with 1024 QAM, especially when these upgrades are relatively painless and generally require only a minor and quick swap of equipment.
Adaptive Coding and Modulation (ACM)
Leading microwave equipment vendors are able to keep their long-haul transmission links operational even in transient fade and noisy conditions. The enabling technology is ACM: Adaptive Coding and Modulation. Microwave links with ACM technology automatically sense the quality of the transmission link and can automatically decrease the modulation technique in case of degraded signal quality due to interference or other microwave propagation problems such as weather. So, if a microwave transmission is operating at maximum capacity using 1024QAM and suddenly encounters interference or high rainfall, a system such as the CableFree microwave system automatically steps down the modulation to lower levels until the transmission network, although at lower capacity now, maintains the ultra high level of link reliability and availability. As the temporary weather effects disappear, the microwave system automatically re-applies more efficient higher-order modulation techniques to regain full capacity.
Overcoming Tradeoffs due to High Order QAM Modulation
With increasing modulation the receiver sensitivity is greatly reduced, and generally transmit power has to be reduced due to linearity constraints in the transmitter. For fixed modulation speeds the result is either increase of antenna size or reduced distances, which may prevent an operator upgrading to higher capacity. The use of ACM allows use of 1024QAM whilst avoiding sacrifice of distance or antenna sizes, by graceful step-down of modulation to lower rates during rare periods of high rainfall.
Use along with other bandwidth-enhancing technologies such as XPIC
1024QAM modulation is fully compatible with other methods to increase capacity such as XPIC (Cross Polar Interference Cancellation). An advanced microwave modem featuring 1024QAM and XPIC can greatly increase capacity. XPIC alone offers double the capacity compared to a single polarised non-XPIC solution.
1024QAM Microwave Summary
These latest advancements in advanced microwave modulation offer network operators an easy and inexpensive upgrade path to higher capacities to meet demand. Advanced modulation technology of 1024QAM is fully shipping and available today and offers a very cost-effective way to boost capacity in long-haul microwave applications.
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Rain 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.
Two models are generally used for Rain modelling: Crane and ITU. The ITU model is generally preferred by microwave planners. A global map of Rain distribution according to the ITU model is shown below:
Used in conjunction with appropriate planning tools, this data can be used to predict the expected Operational Availability (in %) of a microwave link. Useful Operational Availability figures typically vary from 99.9% (“three nines”) to 99.999% (“five nines”), and are a function of the overall link budget including frequency band, antenna sizes, modulation, receiver sensitivity and other factors.
Another useful Rain Fade map is shown here, showing the 0.01% annual rainfall exceedance rate:
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Modulation is a data transmission technique that transmits a message signal inside another higher frequency carrier by altering the carrier to look more like the message. Quadrature Amplitude Modulation (QAM) is a form of modulation that uses two carriers—offset in phase by 90 degrees—and varying symbol rates (i.e., transmitted bits per symbol) to increase throughput. The table in this blog post (Figure 1) describes the various common modulation levels, associated bits/symbol and incremental capacity improvement above the next lower modulation step.
2. Must all operators who use microwave backhaul use higher-order QAMs?
Higher-order QAMs are not necessarily a must-have for all network operators. However, higher-order modulations do provide one method of obtaining higher data throughput and are a useful tool for meeting LTE backhaul capacity requirements.
3. What is the main advantage of using higher-order QAMs with microwave radios?
The main advantage is increased capacity, or higher throughput. However, capacity improvement diminishes with every higher modulation step (i.e., moving from 1024QAM to 2048QAM the improvement is only about 10 percent!), so the real capability of higher-order modulations alone to address the objective of increasing capacity is very limited. Other techniques will be needed.
4. What are the tradeoffs of higher-order QAMs on RF performance?
First, with each step increase in QAM the RF performance of the microwave radio is degraded as per the Carrier-to-Interference (C/I) ratio. For example, going from 1024QAM to 2048QAM will produce an increase of 5 dB in C/I (Figure 2). This results in the microwave link having much higher sensitivity to interference, making it more difficult to coordinate links and reducing link density. Along with this increase in phase noise there will be an increase in design complexity cost.
Also, by increasing from 1024QAM to 2048QAM, system gain will decrease from above 80 dB to just above 75 dB (Figure 2). With much lower system gain microwave links will have to be shorter and larger antennas will have to be employed—increasing total cost of ownership and introducing additional link design and path planning problems.
All of the above are the results of linear functions: they degrade in a one-to-one relationship with the move to higher-order QAMs. Meanwhile, the capacity increases derived from higher-order QAMs are the function of a flattening curve: Each step increase in QAM results in a reduced percentage increase in capacity compared to prior increases in QAM. The added capacity benefits are diminished when considering the added costs of higher C/I and lower system gain.
5. Do you need to use Adaptive Coding and Modulation (ACM) while using higher-order QAMs?
ACM should be implemented while employing high-order QAMs to offset lower system gain. However, while ACM does help mitigate the effects of more difficult propagation when using higher-order modulations, it cannot help offset increased C/I.
6. What gives CableFree a “heads-up” here when other big name companies seem to be supporting the technology?
CableFree realizes higher-order modulations are not a panacea—a cure-all. While every minor technology improvement in throughput can help, a focus on technologies that grow capacity in hundreds of percentage points vs. tens of percentage points is most critical now. CableFree believes that these hundreds-of-percentage-points-of-improvement-in-capacity solutions will be the most important moving forward. It is in these technologies that CableFree has a “heads-up.” Such techniques include deploying more spectrum—particularly in the form of multichannel RF bonding (N+0) solutions—to achieve a minimum of 200 percent capacity increase. This technique is subject to frequency availability, but with flexible N+0 implementations (such as being able to use frequency channels in different bands and different channel sizes) many congestion issues can be avoided.
Second, intelligently dimensioning the backhaul network based on proven rules, best practices and L2/L3 quality of service (QoS) capabilities is another technique to provide potentially very large gains in backhaul capacity. Higher-order modulations can be one tool to achieve required capacity increases in the backhaul network. However, their inherent drawbacks should be well understood, while the most attention should be paid to other techniques that deliver more meaningful and quantifiable benefits.
7. Will operators need to “retrofit” microwave radios to be capable of higher-order QAM operation in their existing microwave infrastructure? Or will completely new hardware be required?
This depends on the age and model of the existing radios. Older microwave systems will likely need to be “retrofitted” to support 512QAM and higher modulations. Recently installed microwave systems should be able to support these technologies without new hardware.
8. How will QAM evolve in the future? Is the introduction of higher-order QAMs an indefinite process, with no end in sight?
The introduction of higher-order QAMs is not an endless process. As per Figure 1 above in this blog post, the law of diminishing returns applies: Throughput percentage improvement declines as modulation rates increase. The cost and complexity of implementing higher-order QAMs probably is not worth the capacity increase benefits derived—not past 1024QAM, in any event.