Designing a MW Transmission NetworkA Basic TutorialSimon Baldwin

AgendaDigital SystemsDigital Transmission TechniquesSDHPDHArchitecturesMultiplex HierarchiesBenefits of eachPlanning ObjectivesAvailabilityError PerformanceComponents of the SystemDeployed EquipmentSystem Arrangements and CapacitiesPreplanningLocation of EquipmentPath ProfilesFresnel ZonesSignal PropagationFree Space PropagationFadingDiffraction and ReflectionDesigning LinksStages to the design

IntroductionModern digital MW radio systems make it possible to provide high capacity transmission links over distances of up to 80kmMW is an economic transmission option in terms of rapid deployment, network control and ownershipMW radio systems are easy to maintain and offer flexibility in reconfiguration and growth in transmission capacityIn telecoms networks the range of frequencies ranges from 2 38GHz. Capacities range from low (

Digital Systems

BasebandThe baseband for a digital system consists of a number of PCM (Pulse Code Modulation) traffic channels which are TDM (Time Division Multiplexed) togetherA 4kHz telephone traffic channel will be sampled at a frequency of 8kHz with each sample binary encoded into an 8 bit word which forms a 64kb/s data stream30 traffic channels + 2 control channels are interleaved together to form a standard 32 channel, 2mb/s data streamIn order to synchronise the bits, Line Coding is used which enables the bit clock information to accompany the data

Types of Network:PDHPlesiochronous Digital Hierarchy (meaning almost synchronous), PDH, is a transmission technology used in telecoms networks to transport large quantities of data over digital radio systems (MW and fibre)PDH allows transmission of data streams that are nominally running at the same rate, but allowing a variation on the speed of the nominal rateGlobally there are various versions of the hierarchies of bit rates on a PDH systemLevelUSJapanInternational056k56k64k11.544mb (T1/DS1)1.544mb (J1)2.048mb (E1)26.312mb (DS2)6.312mb (J2)8.448mb (E2)344.736mb (DS3)32.064mb (J3)34.368mb (E3)4139.264mb (DS4)97.728mb (J4)139.264mb (E4)In Europe the basic transfer rate is 2mb. For speech transmission this is broken down into:30 x 64k TDM timeslots for the conversation2 x 64k timeslots (TS0 for the synchronisation and TS16 for the signalling)Alternatively for data transmission the whole 2mb can be usedThe exact rate of the 2mb data stream is controlled by a clock in the equipment generating the data. This rate is allowed to vary slightly (+/- 50ppm) either side of the 2mb. This means that there are probably different 2mb data streams running in the network (this makes it plesiochronous)

PDH TechnologyIn order to move multiple 2mb data streams in the system, they are combined together (multiplexed) in groups of 4This is done by taking 1 bit from stream 1 followed by 1 bit from stream 2, then 1 from stream 3, then 1 from stream 4To nearly synchronise the 4 data streams allowing aggregation and combining, padding bits (known as justification bits) are inserted to the streamIn a synchronous network consecutive instances of slot 25 (for example) would be 125s apart. However once justification bits are added, the sequence is disrupted making it impossible to demultiplex slot 25 based upon synchronous timing.The only solution is to completely demultiplex the whole structure to determine whether these justification bits are present. The whole structure then needs to be remultiplexed ready for the next link. This prevents partial demultiplexing and allowing lower order rates to be easily extractedTo slow the process up even more the demultiplexing needs to be achieved in steps. Therefore the process of recovering a 64k channel from a 140mb stream is:140mb 34mb34mb 8mb8mb 2mb (from where the 64k channel can be recovered)

Types of Network:SDHSynchronous Digital Hierarchy, SDH (or SONET in the US) has been around since 1990. It was developed because the old system (PDH) was not able to economically provide higher order bandwidth (+140Mb)It is vendor independent and feature rich, allowing operators to develop new network applications and new topologiesSDH can carry PDHs hierarchy of bit rates except 8Mb (2, 34 &140Mb)The standard SDH transmission rate is 155Mb (STM-1). The maximum is around 10Gb (STM-64)SDH radio systems are compatible with SDH fibre systemsIn a mobile network SDH systems are typically deployed where high capacity, resilience, protection and restoration are importantIn the core networkTrunk transmissionEach SDH payload is transmitted in a container synchronous with the STM-1 frame. Selected payloads can be inserted or extracted from the STM-n without the need to fully hierarchically de-multiplex (as in PDH)All SDH equipment is SW controlled allowing centralised management of the network configuration

SDH TechnologySDH standards govern frame rates and formats, interface parameters, multiplexing methods, network operations, administration, maintenance and provisioning for high-speed transmission.SDH is more efficiently demultiplexed because there are pointers in the frame headers which indicate precisely where the sub-multiplex frames begin. Because of this a single slot can easily be picked out without having to dismantle the whole structureIn terms of the OSI model SDH is purely a Physical Layer technologySDH is designed to create an end-to-end connection between various networks. Communication between the networks is not a problem because SDH encodes signals from the source network into the SDH format and transmitting to the destination network. The SDH signal is decoded at the destination into the user formatSDH payloads consist of Virtual Containers (VCs) which are used to transport lower speed tributary signals (and the PDH data streams)VCs consist of:Payload:The information part of the message (PDH frames, ATM cells etc)Overhead:The signalling and protocol information (Path overhead, Payload information etc)

SDH ArchitecturesADMADMADMADMADMADMADMADMs are the main building blocks of the SDH networkThey allow lower order data rates to be dropped and addedto the SDH ring. E.g. drop add an STM-1 into an STM-4bit-stream without changing the STM-4 structure ADMADMADMADMSDH circuits counter rotating1 working1 standby

SDH BenefitsSoftware Control: Allows extensive use of intelligent network management software for high flexibility, fast and easy re-configuration and efficient network managementSurvivability: With SDH ring networks become more practicable and their use enables automatic reconfiguration and traffic rerouting when a link is downEfficient drop and insert: SDH allows simple and efficient cross-connect without full hierarchical multiplexing or demultiplexing. A single E1 tail can be dropped or inserted with ease even on Gbit linksStandardisation: SDH enables the interconnection of equipment from different suppliers through support of common digital and optical standards and interfacesRobustness and Resilience:Equipment Size and Operating Costs: Reduced by removing the need for banks of multiplexors and demultiplexorsBackward Compatible: PDH traffic is supportedFuture Proof: Will carry other technologies (ATM)

Multiplex Hierarchies9953.28mb/s2488.32mb/s622.08mb/s155.42mb/s139.26mb/s34.37mb/s2.048mb/s64kb/sSTM-64STM-16STM-4STM-1E4E3E1E0

Typical Cellular Backhaul NetworkSTM-4 RingADMADMADMSTM-16 RingADMBSCADMADMADMMSCPDHPDHPDHPDHPDHPDHPDHPDHPDHPDHPDH

Planning Objectives

Planning ObjectivesThe main aim of radio link planning is to achieve the required transmission objectives in the most economical wayTo do this several factors are taken into account:Ensuring that fading and disturbances between radio links remain within the performance objectives by performing interference calculationsReducing the effects of the terrain on transmission quality by means of adequate antenna heights and countering against ground reflectionDesigning proper fade margins in relation to allowed fading probabilities, including using diversity methodsThere are two basic performance objectives:Achieving required quality through various error performance parametersAchieving required availability of the linkA measure of the transmission quality is the quantity of errors that occur. These errors can arise through noise being introduced to the signalThermal noiseExternal interference which produces impulse noiseWhatever the noise source, there must be an objective which specifies the number of errors permitted over a period of time for it to be considered to be working properly

Error PerformanceDuring the available time the error performance is defined using the following concepts:Severely Errored Seconds (SES); BER >10-3, integration time = 1 second (this equates to 1 errored second every thousand)Degraded Minutes (DM); BER >10-6; integration time = 1 minute (equating to 1 errored minute in every million)Errored Seconds (ES); Intervals of 1 second containing at least 1 errorResidual Bit Error Ratio (RBER); The BER under non fading conditions measured over 15 minutes or moreThe design of radio hops in practice is usually based upon the SES if error performance is considered to be the limiting factor

AvailabilityAvailability objectives are set for a period of 1 yearA system is considered unavailable if one or both of the following conditions occurs for at least 10 consecutive seconds in at least 1 direction of transmission:The signal is interruptedThe BER is >10-3These 10 seconds are counted towards the unavailable timeThe unavailable time ends when, for both directions of transmission, both of the following conditions occur for at least 10 seconds:The signal is restoredThe BER is

Microwave Equipment Arrangements

Basic ArrangementA microwave terminal consisting of a transmitter and a receiver is located at each end of the linkThere may be a repeater in the middleThis arrangement is common when it is not necessary to provide duplicate protection against path outagesWhen the link is not considered critical1+01+0

Hot StandbyA microwave terminal consisting of a transmitter and a receiver is located at each end of the linkHSB configurations are used when protection is required on paths where propagation conditions are non critical to system performanceMost operators choose to protect all but the last link in a daisy chain (see the later example)2 transmitters are on and connected to the same frequencybut only 1 is connected to the antennaIf the operating transmitter fails, the second automatically connects to the antenna while the first is disconnected (within milliseconds)2 receivers are used and both are permanently connected to the antenna with both outputs being combined into 1 resultant signalHSB provides increased equipment reliability but does not increase propagation reliability1+11+1

Space DiversityFor improvements in propagation reliability, a space diversity arrangement can be usedIn this arrangement 1 transmitter and its associated antenna radiates on a transmit frequencyThis signal is received by 2 receivers which are tuned to the same frequency but connected to 2 separate antennas located at different positions on the towerThe receiver output signals can be combined to give a composite output, or switching can be done between the receivers, keeping the receiver with the best BER connected to the lineThe spacing between the receivers varies depending on the bandwidth of the signal but is seldom less than 5 metresSpace diversity provides a substantial increase in reliability, especially over highly reflective surfaces such as water or desert, however the increase in costs attributed to 2 receiving antennas, 2 receiving waveguide runs and stronger and taller towers makes it an expensive means of reliability1+1 configurations can be, but are seldom used1+01+0

Frequency DiversityFrequency diversity is a cost effective technique that provides equipment protection as well as protection from multi-path fadingThis method increases the total system reliability by providing both path and equipment duplication. There are two methods of frequency diversity:2 transmitters are on air simultaneously and both are modulated with the same baseband signal but are tuned to different frequencies. These two frequencies can be either within the same operating frequency band, or in two different operating bands. Both transmitters are connected to the same antenna which radiates the signals to the B end of the path. At the B end of the path there are two receivers, each one accepting the incoming signal to which it is tuned. These receivers, in turn, provides as an output the signal which modulated the transmitters. The two outputs are then combined using a combiner to provide one signal to the multiplexerDual polarisation antennas can be used to enable different frequencies and different polarisations to be transmitted and received across the same link1+11+11+1

PDH CapacitiesThe capacity of the link depends upon;The traffic that needs to be carriedThe capabilities of the vendor equipmentTransmission vendorRadio vendorAs a rule of thumb most vendors can multiplex 12 trxs onto a single 2mb linkTherefore a 4x4x4 configuration at the base station will require 2mbA 4x4x5 configuration will require 2 x 2mbMost operators choose to daisy chain base stations, therefore all the configurations in the daisy chained link need to be considered4x4x45x5x54x4x44x6x44x4x4Hub/ADM1x2mb2x2mb1x2mb2x2mb3x2mb1x2mb4x2mb6x2mbSite RequirementLink Requirement

PDH Capacity ConfigurationsThe basic PDH capacity configurations are:4 x 2mb (which means that in the link there are 4 x 2mb circuits available for use)8 x 2mb16 x 2mbThe initial configuration of a link to a cell site will usually be 4 x 2mb. If the number of trxs in use at the site totals no more than 12, only 1 of the 2mb will be used, leaving 3 spare. If more than 12 trxs but less than 24 are used, then 2 x 2mb will be used leaving 2 spareThe configuration is raised to 8 x 2mb when more than 4 E1s are required The configuration is raised to 16 x 2mb when more than 8 E1s are requiredAs a rule of thumb, configurations of 8 x 2mb and 16 x 2mb should always be in a 1+1 configuration because of the amount of traffic

Example PDH Capacity ConfigADMHubSDH Node4x4x44x4x44x4x44x4x44x4x44x4x44x4x44x4x44x4x44x4x44 x 2 Available1 x 2 Used1+0 Config4 x 2 Available2 x 2 Used1+1 Config4 x 2 Available1 x 2 Used1+0 Config4 x 2 Available1 x 2 Used1+0 Config4 x 2 Available2 x 2 Used1+1 Config4 x 2 Available1 x 2 Used1+0 Config4 x 2 Available4 x 2 Used1+1 Config4 x 2 Available3 x 2 Used1+1 Config4 x 2 Available2 x 2 Used1+1 Config16 x 2 Available10 x 2 Used1+1 ConfigEach cell site has 12 x TRXsEach cell site requires 1 x E1STM-n Ring

Deployed Equipment

Transmitters & ReceiversThe basic building blocks of the microwave system which make it possible to send and receive information at microwave frequenciesThe frequencies used depend primarily on those licenced by the licencing authorities and the distance of the hopThe table below is a rule of thumb guide and suggests the frequencies to be considered with different hop lengths

FrequencyMin DistMax Dist6GHz30km50km7GHz20km35km11GHz12km30km13GHz8km25km15GHz5km22km18GHz3km15km23GHz1km12km26GHz0.5km10km38GHz0km5km55GHz0km1.5km

AntennasParabolic or horn antennas are used to concentrate radiated energy into a narrow beam for MW transmission through free spaceThis results in the most efficient transmission or radiated power with a minimum of interferenceAntennas vary in sizes and are usually dependant on the distance of the hopTherefore a 600mm antenna used on a 23GHz hop is likely to provide similar performance to a 300mm antenna used on a 26GHz hopAntennas are often used in conjunction with Radomes which are protective coverings used to prevent snow, ice, water or debris accumulating on the antenna.Radomes can also reduce wind load across the towerOn the down side, using a Radome results in lower antenna gain across the link

Carrier MultiplexThe MW RF equipment has a wide bandwidth which is capable of carrying multiple channels of informationThe carrier multiplex terminal multiplexes groups of channels into a higher bit rate digital channel and demultiplexes them back into there individual channels at the other end of the hopTime Division Multiplexing (TDM) is the multiplexing technique used in microwave systems when combining 2 or more continuous channels over the linkTDM combines data streams by assigning each stream with a different time slot in a set.TDM repeatedly transmits a fixed sequence of time slots over a single transmission channelWithin the carrier system (E1, E3 etc) TDM combines PCM streams created for each conversation or data stream

RepeatersActive RepeatersThese are used at one or more intermediate points to regenerate the signal when:the distance between the transmitting and receiving equipment is too great to allow an acceptable receive levelWhen it is necessary to get around an obstacleWhen it is necessary to drop and insert channels at points in between the radio linkPassive RepeatersThese are used when there is an obstacle (mountain) within the LoS and the economics of installing an active repeater are prohibitive or there is no need to regenerate the signal. Two types exist:Reflective Passive Repeaters, which act as a mirror reflecting the signal to bypass the obstacleBack-to-Back Passive Repeaters, which receive the signal from the launch antenna and feeds it, via a waveguide, to another launch antenna (again bypassing the obstacle). This method involves a major loss of signal therefore it needs to be located as close as possible to the A end or B end of the radio link

Transmission FeedersFeeders exist to provide a means of coupling the transmitter and receiver to the antennaWaveguideA circular, elliptical or rectangular metal tube or pipe through which electromagnetic waves are propagated in microwave and RF communications. The wave passing through the medium is forced to follow the path determined by the physical structure of the guideCoaxial CableA type of wire that consists of a centre wire surrounded by insulation and then a grounded shield of braided wire. The shield minimizes electrical and radio frequency interference

TowersThe towers used in a MW system must be strong enough to support the necessary equipment to be installed on it and rigid enough to prevent deflection and rotation during windy conditions or ice loadingIn general two types of towers exist:Self supporting towers are either monopole or legged towersGuyed towers cost about a third of price of a supporting tower but are often restricted in use because of the land required for the installation of the guysThe height of the tower must be high enough to provide a LOS between transmitter and receiver sites, therefore it is determined by the terrain, the MW frequency, the propagation characteristics of the potential link and the distance

Power and AlarmsPrimary Power SourcesPrimary power sources are AC or DC as specified by the vendor. In some cases generators are used in areas where access to power is a problem but generally at each MW equipment location there is access to a utility power supplyStandby PowerIt is imperative that some type of standby power supply is available to maintain system operation in the event of power failure. Communication circuits are very important during times of emergency and in many cases it is a legal requirement of the operator to maintain communications at all timesAlarmsAlarms are necessary to report faults on a circuit to an attended office. This will expedite the maintenance of the system and reduce any circuit outage times.

Pre Planning

Microwave Pre-PlanningThe main objectives of the transmission network are to satisfy the capacity demands and provide a reliable serviceThis can be achieved if the following criteria are incorporated into the network design:All routes are sized to meet service demandsTraffic must be routed in the most economical mannerSurvivability must be built into the networkGenerally a mobile network grows rapidly within the first 3 years. To accommodate the expansion a transmission backbone originating from the switch locations must be builtThis backbone is the networks transmission infrastructure, generally high capacity SDH (STM-4)The hub sites for the PDH aggregation must also be capable of simplifying growthThe tower design (equipment shelter space, power & antenna type) should all be chosen with an eye towards expansionThe design packs should be produced using standard symbols and must be established and maintained as the network evolvesProtection systems need to be agreedA network management system should be in place

Location of Antenna SitesIn a mobile network the location of antenna sites will normally be decided upon by the RF designers and plannersIn order to get the signal from the nodes to these locations it might be the task of the Microwave planner to locate repeater sites in between already defined sitesIn general there are certain considerations to follow when positioning a site:Sites should be located on high points so that there will be a LOS path with some clearance over the terrain between the transmitting and receiving antennasThe sites should be easily accessible in all weathers to allow maintenance and upgrades to be completed economically and without major problemsPotential sites should be considered with respect to the availability and cost of the land and should take into account applicable zoning regulationsPotential sites should consider interference from other systems, proposed and existing, in the area

Path ProfilesIf the sites are acceptable under the antenna site considerations, it is now necessary to look at the profile of the terrain between the proposed sitesThis can be done using graph paper and topographic maps but nowadays all terrain data is loaded onto a system such as PLANET or Ellipse which is then used as the tool to plot the characteristics of the linkThe link characteristics such as earth curvature, obstacles and weather predictions are then used to determine how a signal will react along the link, for instance by how much will the signal propagateThis is the information that the designer and the design tool will use to determine equipment types and power requirements to ensure the link meets the specified performance objectives

Fresnel ZonesThe area around the visual LOS that radio waves spread out into after they leave the antenna is called the Fresnel ZoneThe first Fresnel Zone is bounded by points through which the distance between the transmitter and receiver = wavelength longer than the direct signalThe second Fresnel Zone is bounded by points through which the distance is 1 wavelength longer than the direct signalThe third is bounded by points through which the distance is 1 wavelengths longer than the direct signalThe first Fresnel Zone area must be clear or the signal strength will weaken.There are an infinite number of Fresnel Zones, the areas of all zones are equalThe energy received from each zone decreases with distance from the primary signal and in the first zone there remains about a quarter of the energy received from an unobstructed signal 1st Fresnel Zone

PolarisationThere are 3 general rules to follow when allocating either vertical or horizontal polarisation to a link:Vertical polarisation should be used as much as possible in areas subject to high rainfall. The attenuation produced by rain is polarisation dependent, a falling drop of rain is not spherical but flattened in the horizontal plane resulting in greater attenuation of horizontally polarised linksOn longer hop lengths it is wise to use vertical polarisation as greater distances increases the likelihood of localised rainfall somewhere along the linkHorizontal polarisation in a local area is done to minimise interference with other links (a good assumption is that other operators usually, by default, vertically polarise their hops)Horizontal polarisation is better suited to built up areas (areas with high rises, masts, flagpoles etc). In these areas there is a greater chance of getting vertically polarised reflection than horizontally polarised reflection, therefore when multipath fading is an issue, horizontal works better than vertical

Signal Propagation

Free Space PropagationThe benchmark by which we measure the loss in a transmission link is the loss that would be expected in free space that is the loss that would occur in a link which is free of all objects and phenomena that might absorb or reflect radio energy. Therefore the strength of the received signal depends only on the distance and frequency between the transmitter and the receiverFree SpaceThe likelihood, however, that the only losses across a link will be due just to free space is minimal, indeed only ever achieved in a vacuum. The reality is that atmospheric pressures cause the radio waves to bend as they interact with the molecules in the atmosphereAtmospheric Pressures

The Effects of the WeatherRadio energy is absorbed and scattered by the rain especially in a situation where the wavelength of the signal approaches the diameter of the raindropWhen the drops are large enough and sufficiently concentrated the absorption and scattering will noticably attenuate the signal of the link especially at frequencies above 10GHzThe amount of attenuation caused by the rain depends on the intensity of the storm, therefore it is rate of rainfall which is the determining factor and not total rainfall. This is especially true in Saudi Arabia where the rainfall is very low throughout the year, indeed rain is only anticipated in two or three months per year, therefore it is unwise to average out the rainfall across the year because the figure achieved would be significantly lower than the actual rate in the months when it rainedLocalised rainfall also needs to be considered especially along long hop lengths where it can be raining only along a proportion of the link.Propagation due to snow and fog becomes significant at frequencies above 30MHz and may cause additional path loss in the order of 1dB per km

Multipath FadingFading describes rapid changes in the amplitude of a radio signal over a short period of time or distance travelledIn a typical wireless environment the radio-waves emitted from the transmitter usually take different paths in getting to the receiver depending on the obstacles in the environmentAs a result the received signal is actually a sum of the various contributions, each of which differs in both amplitude and phaseIn many cases the signals combine in a destructive manner which severely degrades the signals strength so the receiver faces the difficult task of properly demodulating and decoding the signal into something that resembles the original

K FactorAs a radio wave moves forward it will travel in a straight line if air pressure, temperature and humidity allow all points on the front of the wave to travel at the same velocityHowever since these conditions are almost impossible to achieve, the upper portion of the wave front travels slightly faster than the lower portion causing the wave to be refracted towards the earth resulting in the radio horizon being slightly further away than the optical horizonThis effect is described using the K value, which gives the ratio of the effective earth radius to the real earth radius when the propagation path is a straight lineA factor of K = 1 would be appropriate where the actual earth curve and the effective earth curve are equal, however the normal K-value (median) is 4/3 or 1.33 which indicates that the effective earth has less curvature or is flatter than the true earthIt is possible for abnormal atmospheric conditions to refract the beam away from the earth, in which case the K factor would be less than 1Actual K-values will vary along a link due to temperature, air pressure and humidity differences, therefore they are averaged out along the hop length to reduce the variations

Diffraction and ReflectionDiffractionDiffraction may be considered as a modification of the radio waves as they are deflected or bent by the surface of the earth, hills and mountains or the edges of any opaque body. Diffraction of a radio wave concerns an engineer because obstructions which are not necessarily in the LOS may cause the beam to attenuateReflectionIf the terrain between the antennas reflects the radio waves efficiently, it is possible to receive strong reflected waves either in or out of phase with the direct wave, depending on the difference in lengths of the direct and reflected wave paths. Reflections are greatest when the point of reflection is over water, level moist earth, desert sand and other types of smooth terrain. It is desirable to adjust the tower heights or reroute the wave path so that the reflection point will be over rough terrain. Over rough terrain radio energy will either be absorbed or scattered, therefore the amount of reflected energy will only be a small percentage of the total energy

The Design

Stages to MW Link DesignAssuming that the site has already been placed by the RF designers and it has been loaded into PLANET and Ellipse, it is the job of the MW planner to connect that site to the backbone networkThe following steps are appropriate to the initial stages of the PCCW project where the responsibility of designing the link is within MSI and the responsibility of the LOS field surveys is that of an outside contractor:Using Ellipse, locate the new GSM site. This is now referred to as the A endDetermine at least 5 possible B end candidates that have connections to the network, or will be connected to the network as part of the new project, to generate a LOS matrix. The following information is required and should be printed as part of the candidate pack:Path profile to ensure no natural obstructionsLink distance to begin frequency planningThe candidate pack should be issued to the relevant field survey contractorThe candidate pack will be returned with an evaluation of each potential link. The design of the link can now beginDetermine which candidate is to be used as the B end of the linkChoose the correct frequency using the table described in the frequency slides

Stages to MW Link DesignUsing Ellipse, link the A end to the B end and input the other link characteristics:Equipment type and frequencyAntenna sizeAntenna heightFeeder typesHalf bandPolarisationChannel numberRain intensityUsing Ellipse generate a link budget report and ensure that the link achieves the performance objectives:Plot the final path profilePerform the frequency interference calculationsPrepare the FDED or Transmission Proposal including:DesignCWOMOPFrequency requestIssue the pack