Capacity scalability has beenidentified as one of the mostimportant technical challenges inmicrowave backhaul. Data trafficcontinues to double in size eachyear, driven by the increasingadoption of smartphones and thegrowth in data-hungry applicationssuch as HD video streaming. Inaddition, the deployment ofheterogeneous networks (HetNets)and small cells in LTE-A networksmeans that the data rates handledby each macrocell will continue toincrease. Consequently the wirelessindustry is facing a crisis in its abilityto keep up with the demand forbackhaul capacity, which will onlybecome worse when 5G isintroduced.

Connecting high-bandwidth datanetworking points by means ofoptical fibre is very expensive, andimpractical in many cases.Microwave links have been used for

many years, and already account formore than 50% of backhaul capacitywith the proportion growing year-on-year. Figure 1 shows the 2013

market for Ethernet radio links ofdifferent capacities and the forecastfor 2014 - 2018, extracted from areport by EJL Wireless Research3.

The radio capacity challengeA particular problem with the

forecast is that it is currently verydifficult to produce a 1 Gb/s radiowith current spectrum allocationsand radio technology.Channelization in the microwave

Simultaneous full-

duplex communication

can double spectral

efficiency

Achieving sufficient capacity tomeet the forecast backhaul

needs over the next few yearswill be a major challenge. This

white paper describes the tech-nique of Air Division Duplex-

ing (ADD), which makes use ofMIMO and spatial multiplex-ing techniques to achieve si-

multaneous transmission andreception of data on each car-

rier frequency, and has beenshown to achieve full duplex

data rates1 in excess of 1 Gb/sin a 28 MHz channel

allocation2 in the microwavebands.

CURABITUR FELISWhite Paper

Addressing the 4G and 5G backhaul capacitychallenge with Air Division Duplexing

Figure 1: Forecast IP radio sales by capacity 2014 - 2018 (Source EJLWireless Research)

bands (up to 42 GHz) is largely based on multiples eitherT1 or E1 fixed rates, with a base channelization of 3.5MHz. Generally the largest channel allocation is 28 MHz,with some rare occurrences of a 56 MHz or possibly 112MHz channel in some countries. The best data rates thatcan be achieved by currently available microwavesystems are typically 400 - 500 Mb/s in a 28 MHz channelallocation using both vertical and horizontal polarization inan XPIC configuration. Spectrum licenses can account forup to 40% of a backhaul operator's OPEX over a 10-yearperiod, and the leasing of tower space is also at apremium.

Until now, the only option available to operatorsdemanding a higher throughput has been to use themillimetre-wave frequency bands above 42 GHz,particularly the bands at 60 and 70/80 GHz wherechannel allocations are in multiples of 50 MHz for the 60GHz and 250 MHz up to 2 GHz for the 70/80 GHz. The

EJL Wireless Research report shows that radio sales inthe conventional microwave bands at 15 GHz, 18 GHz,23 GHz and 38 GHz have actually declined slightly since2010 because they have been unable to meet Gb/scapacity requirements. However, the millimetre-waveradios have disadvantages both in terms of distancebecause climatic and atmospheric attenuation is higher atthese frequencies and sometimes also in cost, since inmany countries the regulators charge more for licenses inrelation to the amount of spectrum used (millimetre-wavebands are license free in some countries, but not in mostdeveloped or developing countries).

The challenge therefore is that there is an escalatingneed for Carrier Ethernet data throughput, but in mostcases the telecom operators have limited radio frequencyspectrum available to them in order to meet their futurebackhaul capacity requirements.

Figure 2: Standard FDDmicrowave backhaul radiowith single polarization

Figure 3: FDD microwavebackhaul radio with Dual (orCross) polarization

Spectrum allocationTypically, traditional microwave frequency bands are

specified as two separate but linked frequency allocationsin the lower and upper halves of the band, knownrespectively as the 'Low Band' (LB) and 'High Band' (HB).A microwave link conventionally makes use of both thesefrequencies in order to achieve a link using FDD(frequency division duplexing), where each of the twofrequencies is used for transmission in one of thedirections, as shown in Figure 2. Increased capacity canbe achieved by using more complex modulation schemes,but this places higher demands on the linearity ofamplifiers, which adds to the cost, as well as requiringgreater single to noise ratios on the receiver side fordemodulation which reduces the overall system gain andrange while increasing antenna size and cost. Increasingthe order of modulation does not give a proportionalbenefit in capacity. 256 QAM is currently the most widelyused modulation, which allows typically up to 200 Mb/sthroughput to be achieved in 28 MHz channel bandwidthwithout data compression. When compression techniquesare also applied, the claims are to double the throughput,however this technique depends on the traffic type andthe sensitivity of the network to the additional latencyadded by the compression/decompression algorithms.Higher order modulations of 2048 QAM can achievesome 35% higher capacity than this but with performancetrade-offs in system gain, transmitter power, antenna sizeor range.

By using orthogonal polarization, known as Co-Channel Dual Polarization (CCDP) or Cross PolarInterference Cancellation (XPIC), a second transmissionpath can be added to the link in each direction, as shownin Figure 3. This can effectively double the capacity, andis a standard technique that is currently used by networkequipment vendors, operators and regulators.

MIMO techniquesAdditional spectral efficiency can be achieved by the

use of spatial multiplexing, known as MIMO (multipleinput multiple output), which combines multiple antennason each side of the link to increase the throughput. Theincrease in spectral efficiency is achieved because thetransmitted signal from each aperture is at the samefrequency.

However in order to resolve the MIMO transmissions,the radios need to be able to discriminate between themultiple signals, which requires there to be a minimumspacing between the antennas - this is proportional to linkdistance and inversely proportional to frequency. Thismeans that the overall dimensions of radio system tend tobe large (often up to 10m separation between radio unitson the same tower), which is unpopular with operators asmasts are already overcrowded. This implementationfurther doubles the operation costs as regulators mayalso charge a separate spectrum licence fee for eachaperture as they are not strictly co-located and similarlythe site owner may charge double the aperture rental feesfor the separate antennas.

Air Division DuplexingMIMOtech has taken the MIMO technique one step

further, with the development of an entirely newtechnology that uses spatial multiplexing to achievehigher spectral efficiency. Air Division Duplexing is apatented technique that also uses two antennas, but inthis case the separation between them is typically only100mm, allowing it to be considered as a quasi single-aperture antenna from the point of view of licensing, siterental cost and implementation.

Unlike the techniques described earlier, Air DivisionDuplexing exploits the possibility of using both the go andreturn frequency bands of a microwave link

Figure 4: Air DivisionDuplex microwavebackhaul radio

simultaneously from both sites. Rather than having atransmitted signal in the High Band in one direction and inthe Low Band in the opposite direction, both frequenciescan be used for transmission in both directionssimultaneously. A prerequisite for this is that theequipment must be optimized to cancel out self-interference.

Figure 4 illustrates the Air Division Duplexingtechnique. Because both frequencies and bothpolarizations are used simultaneously from bothdirections, this technology provides double the capacity ofmicrowave radios currently on the market.

The MIMOtech Janus AirDuplex™ range of ultra-highcapacity backhaul radios, shown in Figure 5, uses thistechnique to provide in excess of 1 Gb/s full duplex datarate1 in a 28 MHz channel allocation2 in the licensed andunlicensed bands up to 42 GHz, using 1024 QAMmodulation. This makes the radios suitable for small cell,microcell and macrocell mobile deployments for a rangeof technologies including LTE/LTE-A, providing a cost-effective alternative to fibre and millimetre-wave links.

ConclusionUsing the Air Division Duplexing approach doubles the

capacity and spectral efficiency when compared tocurrent solutions without the penalty of additional OPEXcosts going to alternative Line of Sight MIMOimplementations. Both CAPEX and OPEX are reduceddue to lower spectrum fees, lower site/tower rental feesand lower maintenance costs, while software definabilityoffers downstream savings in upgrade costs.

In addition, the Air Division Duplexing technologybrings reusability to frequency spectrum below 42 GHzthat up to now has been limited to backhaul applicationswith a maximum of several hundred Megabits per second.These frequency bands can be re-applied for Gigabitbackhaul applications.

The Janus AirDuplex™ product provides a highcapacity cost-effective alternative to fibre or millimetre-wave links for 4G and potentially 5G backhaul and linksfor enterprise and government applications.

Figure 5: The MIMOtech Janus Air-Duplex™ radio

MIMOtech Pty.64 White Rd. Retreat • Cape Town

7945 • South Africa+27 21 710 2463 •

sales@mimotechnology.comwww.mimotechnology.com

1Full duplex data rate is the available traffic data rate in each direction of the microwave link without payload compression being used. IntelligentHeader compression is applied.2The 28 MHz channel allocation is a Frequency Division Duplexing (FDD) channel allocation with go and return frequencies in both vertical andhorizontal polarizations. This is also known as a co-channel dual polarized (CCDP) allocation.3EJL Wireless Research, "Global Digital PTP Radio Market Analysis and Forecast, 2014-2018", 10th Edition April 2014