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
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
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
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5G Mobile networks, Microwave Backhaul and future trends in Mobile Networks
With 5G mobile communication becoming available around 2020, the industry has already started to develop a fairly clear view of the main challenges, opportunities and key technology components it involves. 5G will extend the performance and capabilities of wireless access networks in many dimensions, for example enhancing mobile broadband services to provide data rates beyond 10 Gbps with latencies of 1 ms.
Microwave is a key element of current backhaul networks and will continue to evolve as part of the future 5G ecosystem. An option in 5G is to use the same radio access technology for both the access and the backhaul links, with dynamic sharing of the spectrum resources. This can provide a complement to microwave backhaul especially in very dense deployments with a larger number of small radio nodes.
Today, microwave transmission dominates mobile backhaul, where it connects some 60 percent of all macro base stations. Even as the total number of connections grows, microwave’s share of the market will remain fairly constant. By 2019, it will still account for around 50 percent of all base stations (macro and outdoor small cells (see Figure 3). It will play a key role in last mile access and a complementary role the aggregation part of the network. At the same time, fibre transmission will continue to increase its share of the mobile backhaul market, and by 2019 will connect around 40 percent of all sites. Fibre will be widely used in the aggregation/metro parts of the networks and increasingly for last-mile access. There will also be geographical differences, with densely populated urban areas having higher fibre penetration than less populated suburban and rural areas, where microwave will prevail for both short-haul and long-haul links.
Spectrum efficiency (that is, getting more bits per Hz) can be achieved through techniques like higher-order modulation and adaptive modulation, the superior system gain of a well-designed solution, and Multiple Input, Multiple Output (MIMO).
The maximum number of symbols per second transmitted on a microwave carrier is limited by the channel bandwidth. Quadrature Amplitude Modulation (QAM) increases the potential capacity by coding bits on to each symbol. Moving from two bits per symbol (4 QAM) to 10 bits per symbol (1024 QAM) delivers a more than five-fold capacity increase.
Higher-order modulation levels have been made possible through advances in component technologies that have reduced equipment-generated noise and signal distortion. In the future there will be support for up to 4096 QAM (12 bits per symbol), but we are approaching the theoretical and practical limits. Higher-order modulation means increased sensitivity to noise and signal distortion. The receiver sensitivity is reduced by 3 dB for every increased step in modulation, while the related capacity gain gets smaller (in percentage terms). As an example, the capacity gain is 11 percent when moving from 512 QAM (9 bits per symbol) to 1024 QAM (10 bits per symbol).
Increasing modulation makes the radio more sensitive to propagation anomalies such as rain and multi-path fading. To maintain microwave hop length, the increased sensitivity can be compensated for by higher output power and larger antennas. Adaptive modulation is a very cost-effective solution to maximize throughput in all propagation conditions. In practice, adaptive modulation is a prerequisite for deployment with extreme high-order modulation.
Adaptive modulation enables an existing microwave hop to be upgraded from, for example, 114 Mbps to as much as 500 Mbps. The higher capacity comes with lower availability. For example, availability is reduced from 99.999 percent (5 minutes’ yearly outage) at 114 Mbps to 99.99 percent of the time (50 minutes’ yearly outage) at 238 Mbps. System gain Superior system gain is a key parameter for microwave. A 6 dB higher system gain can be used, for example, to increase two modulation steps with the same availability, which provides up to 30 percent more capacity. Alternatively it could be used to increase the hop length or decrease the antenna size, or a combination of all. Contributors to superior system gain include efficient error correction coding, low receiver noise levels, digital predistortion for higher output power operation, and power-efficient amplifiers, among others.
MIMO Multiple Input, Multiple Output (MIMO)
MIMO is a mature technology that is widely used to increase spectral efficiency in 3GPP and Wi-Fi radio access, where it offers a cost-effective way to boost capacity and throughput where available spectrum is limited. Historically, the spectrum situation for microwave applications has been more relaxed; new frequency bands have been made available and the technology has been continuously developed to meet the capacity requirements. However in many countries the remaining spectrum resources for microwave applications are starting to become depleted and additional technologies are needed to meet future requirements. For 5G Mobile Backhaul, MIMO at microwave frequencies is an emerging technology that offers an effective way to further increase spectrum efficiency and so the available transport capacity.
Unlike ‘conventional’ MIMO systems, which are based on reflections in the environment, for 5G Mobile Backhaul, channels are ‘engineered’ in point-to-point microwave MIMO systems for optimum performance. This is achieved by installing the antennas with a spatial separation that is hop distance-and frequency-dependent. In principle, throughput and capacity increase linearly with the number of antennas (at the expense of additional hardware cost, of course). An NxM MIMO system is constructed using N transmitters and M receivers. Theoretically there is no limit for the N and M values, but since the antennas must be spatially separated there is a practical limitation depending on tower height and surroundings. For this reason 2×2 antennas is the most feasible type of MIMO system. These antennas could either be single polarized (two carrier system) or dual polarized (four carrier system). MIMO will be a useful tool for scaling microwave capacity further, but is still at an early phase where, for example, its regulatory status still needs to be clarified in most countries, and its propagation and planning models still need to be established. The antenna separation can also be challenging especially for lower frequencies and longer hop lengths.
Another section of the microwave capacity toolbox for 5G Mobile Backhaul involves getting access to more spectrum. Here the millimeter-wave bands – the unlicensed 60 GHz bands and the licensed 70/80 GHz band – are growing in popularity as a way of getting access to new spectrum in many markets (see Microwave Frequency Options section for more information). These bands also offer much wider frequency channels, which facilitate deployment of cost-efficient, multi-gigabit systems which enable 5G Mobile Backhaul.
Throughput efficiency (that is, more payload data per bit), involves features like multi-layer header compression and radio link aggregation/bonding, which focus on the behaviour of packet streams.
Multi-layer header compression
Multi-layer header compression removes unnecessary information from the headers of the data frames and releases capacity for traffic purposes, as shown in Figure 7. On compression, each unique header is replaced with a unique identity on the transmitting side, a process which is reversed on the receiving side. Header compression provides relatively higher utilization gain for packets of smaller frame size, since their headers comprise a relatively larger part of the total frame size. This means the resulting extra capacity varies with the number of headers and frame size, but is typically a 5–10 percent gain with Ethernet, IPv4 and WCDMA, with an average frame size of 400–600 bytes, and a 15–20 percent gain with Ethernet, MPLS, IPv6 and LTE with the same average frame size.
These figures assume that the implemented compression can support the total number of unique headers that are transmitted. In addition, the header compression should be robust and very simple to use, for example offering self-learning, minimal configuration and comprehensive performance indicators.
Radio Link Aggregation (RLA, Bonding)
Radio link bonding in microwave is akin to carrier aggregation in LTE and is an important tool to support continued traffic growth, as a higher share of microwave hops are deployed with multiple carriers, as illustrated in Figure 8. Both techniques aggregate multiple radio carriers into one virtual one, so both enhancing the peak capacity as well as increasing the effective throughput through statistical multiplexing gain. Nearly 100 percent efficiency is achieved, since each data packet can use the total aggregated peak capacity with only a minor reduction for protocol overhead, independent of traffic patterns. Radio link bonding is tailored to provide superior performance for the particular microwave transport solution concerned. For example, it may support independent behaviour of each radio carrier using adaptive modulation, as well as graceful degradation in the event of failure of one or more carrier (N+0 protection).
Just like carrier aggregation, radio link bonding will continue to be developed to support higher capacities and more flexible carrier combinations, for example through support for aggregation of more carriers, carriers with different bandwidths and carriers in different frequency bands.
The next section of the capacity toolbox is network optimization. This involves densifying networks without the need for extra frequency channels through interference mitigation features like super high performance (SHP) antennas and automatic transmit power control (ATPC). SHP antennas effectively suppress interference through very low sidelobe radiation patterns, fulfilling ETSI class 4. ATPC enables the transmit power to be automatically reduced during favorable propagation conditions (that is, most of the time), effectively reducing the interference in the network. Using these features reduces the number of frequency channels needed in the network and could deliver up to 70 percent more total network capacity per channel. Interference due to misalignment or dense deployment is limiting backhaul build-out in many networks. Careful network planning, advanced antennas, signal processing and the use of ATPC features at a network level will reduce the impact from interference.
Looking to the future, 5G and Beyond
Over the coming years, microwave capacity tools for 5G Mobile Networks will be evolved and enhanced, and used in combination enabling capacities of 10 Gbps and beyond. Total cost of ownership will be optimized for common high-capacity configurations, such as multi-carrier solutions.
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.
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
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
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