Technical White Paper. WiMAX Modelling in Atoll 2.7.0

Transcription

1 February 2008 Technical White Paper WiMAX Modelling in Atoll WiMAX, OFDM, and SOFDMA Modelling in Atoll This white paper describes how WiMAX (IEEE d and IEEE e) is modelled in the Atoll WiMAX BWA module. Forsk 7 rue des Briquetiers Blagnac France

2 Table of Contents 1 Introduction WiMAX WiMAX Forum Atoll WiMAX BWA Module WiMAX Modelling in Atoll OFDM and SOFDMA: Concepts and System Parameters WiMAX Features Frequency Bands and Channel Bandwidths Quality of Service and Scheduling Adaptive Modulation and Coding Mobility Management WiMAX Base Stations Propagation Models for WiMAX Atoll WiMAX Traffic Model Bearers: Adaptive Modulation and Coding Services Terminals User Profiles Traffic Data Subscriber Database Raster, Vector, and Live Traffic Maps WiMAX Monte Carlo Simulations WiMAX Network Analysis Features in Atoll Preliminary Analysis Network Analysis Under Traffic Conditions Point Analysis Tool Advanced WiMAX Features Smart Antenna Systems Modelling in Monte Carlo Simulations Modelling in Coverage Predictions Multiple Input Multiple Output Systems Frequency Planning Fractional Frequency Reuse (Segmentation) Neighbour Planning WiMAX Network Planning Process in Atoll References Glossary of Terms Forsk 2008 WiMAX Modelling in Atoll 2

3 1 Introduction This white paper describes WiMAX and how its various features are modelled in Atoll. The document starts with introductions to WiMAX and to the Atoll WiMAX BWA module, and, in the following sections, explains how the Atoll WiMAX BWA module implements the different WiMAX features and performs calculations. 1.1 WiMAX WiMAX, or Worldwide Interoperability For Microwave Access, is the name given by the WiMAX Forum to a set of IEEE standards. The WiMAX MAC and PHY layers are described in the IEEE d and the IEEE e standards, which use OFDM and SOFDMA techniques respectively. The d standard is the complete specification for fixed broadband wireless access networks using OFDM-TDMA in downlink and OFDMA in uplink, and the e specifications describe mobile broadband wireless access networks which use SOFDMA, and support handovers and user terminal speeds of up to 100 km/hr. Several names are used to refer to WiMAX, such as IEEE , WirelessMAN, WirelessHUMAN, etc. Similar European (ETSI HiperMAN) and Korean (WiBro) standards also exist. The goal of all these standards is the same: provide broadband wireless access (BWA) for fixed and mobile users. As a highly flexible broadband wireless standard, WiMAX is able to provide a large variety of services, such as TV broadcast, cellular telephony using VoIP, wireless DSL or xdsl high speed internet, etc. It is also possible to do backhaul using the WiMAX air interface. 1.2 WiMAX Forum The WiMAX Forum is a worldwide consortium of companies that are interested in WiMAX and join efforts in developing the technology. The forum has more than a dozen board member companies, more than 100 principal members, and around 400 regular members including Forsk. The mission of the forum is to promote deployment of BWA network by using a global standard and certifying interoperability of products and technologies. It has principally supported the IEEE standards and proposes standard profiles for manufacturers and operators who are interested in WiMAX, thus guaranteeing interoperability. The forum also supports development and specification teams. The following figure summarizes important information about the standards approved and released by the IEEE working group. Figure 1 Interoperable subset of the IEEE standards 1.3 Atoll WiMAX BWA Module The Atoll WiMAX BWA module enables you to design IEEE d and IEEE e broadband wireless access networks. You can use Atoll to accurately predict the network s coverage and behaviour in simple (coverage-limited) as well as in advanced (traffic-limited) planning scenarios. Atoll also includes support for advanced antenna diversity techniques such as AAS and MIMO. Forsk 2008 WiMAX Modelling in Atoll 3

4 Atoll s highly flexible traffic model let s you create, import, and easily manage mobile and fixed traffic data. You can work with fixed and mobile users in WiMAX environments. You can carry out calculations on fixed subscribers as well as base your calculations on combinations of fixed and mobile user scenarios. Calculations include interference prediction, resource allocation and scheduling, and throughput calculations. 2 WiMAX Modelling in Atoll 2.1 OFDM and SOFDMA: Concepts and System Parameters Orthogonal Frequency Division Multiplexing works by dividing the carrier bandwidth into a large number of orthogonal subcarriers. The subcarrier waveforms, generated using Fast Fourier Transform, are said to be orthogonal because the peak of each subcarrier is located at the nulls of its adjacent subcarriers as shown in Figure 2. Orthogonal subcarriers mean that there is nearly no inter-(sub)carrier interference (ICI) in an OFDM system. In case of an OFDM-based cellular network, this means that there is almost no intra-cell interference. This is an important benefit of OFDM over conventional FDM (Frequency Division Multiplexing) in which each pair of carriers require filters and guard bands between them to reduce the adjacent channel interference. The spectrum usage is also better in OFDM than in FDM. Figure 2 OFDM carrier and subcarriers Dividing a wideband carrier into a large number of narrowband subcarriers increases the duration of each data symbol, which in turn makes the system more robust against multipath and inter-symbol interference (ISI). ISI is almost completely eliminated by adding a cyclic prefix to each symbol. However, the addition of a cyclic prefix to each symbol reduces the throughput as a trade-off to eliminating ISI. Therefore, cyclic prefix has to be considered as a time-domain overhead when calculating throughput. Figure 3 Symbol duration for wideband carrier Forsk 2008 WiMAX Modelling in Atoll 4

5 Figure 4 Symbol duration for narrowband subcarriers Figure 5 OFDM symbol and cyclic prefix The carrier is composed of different types of subcarriers as shown in Figure 6. Guard subcarriers on the left and right are not used in order to avoid interfering the adjacent carriers, the centre subcarrier (DC subcarrier) is not used either, the remaining subcarriers (used subcarriers) comprise pilot and data subcarriers. In WiMAX d networks in Atoll, you can set these values for the network. Figure 6 Subcarriers in a carrier Subcarriers in the frequency domain and symbols durations in the time domain together form what is known as a frame, as shown in Figure 7. Figure 7 OFDM frame A WiMAX frame starts with a preamble over which all the base stations transmit their identification information, and mobiles use the preambles to recognize their serving cells and to synchronize their frames with those of the cells. Other important signalling messages follow the preamble. These messages include the frame control header (FCH), the downlink and uplink channel descriptors (DCD and UCD), and the downlink and uplink maps (DL-MAP and UL-MAP). All these signalling messages constitute additional time-domain overheads, i.e., these parts of the frame are not used for user data transfer. The downlink and uplink maps list the locations of the data regions (bursts) allocated to each mobile. Forsk 2008 WiMAX Modelling in Atoll 5

6 Figure 8 WiMAX frame Figure 8 shows a TDD frame. The transmit time guard (TTG) and the receive time guard (RTG) are only valid for TDD frames, with a downlink and an uplink subframe at the same carrier frequency. The division of the frame into downlink and uplink subframes is also an important parameter. For FDD systems, the TTG and RTG do not exist, and the lengths of the downlink and uplink subframes are the same as the frame itself. The smallest resource unit in the frequency domain that can be allocated to a user is a subchannel. A subchannel is a group of subcarriers. The frame structure (for both d and e) and the channel configuration for d (indicated with the red rectangle in Figure 9) can be set up in Atoll as shown below. Figure 9 Network parameters dialogue In e, the subcarriers used in each subchannel can be either physically adjacent or distributed over the channel depending on the subchannel allocation mode. In IEEE e, there are various subchannel allocation modes which can be used in different sections of the frame. Each section is called a permutation zone, as shown in Figure 10. Figure 10 Permutation zones Forsk 2008 WiMAX Modelling in Atoll 6

7 The numbers of subcarriers of different types (total, pilot, traffic) and the numbers of subchannels per channel vary for each subchannel allocation mode. Each frame can have as many as 8 permutation zones in downlink and 3 in uplink, with each permutation zone using different numbers of these parameters. This highly flexible aspect of the e standard is modelled in Atoll by frame configurations. Figure 11 Frame configurations and permutation zones Each frame configuration is a set of permutation zones with the OFDMA and calculation parameters of each permutation zone defined separately as shown in Figure 11. The permutation zones are defined by their subchannel allocation modes, the numbers of subcarriers and subchannels, and the antenna diversity technique that they support. Apart from these parameters, there are also some radio parameters which characterize the permutation zone and are used during calculations for allocating traffic to the zones, such as the minimum required quality, maximum coverage distance, and maximum vehicular speed supported by the zones. It is possible to modify any of the parameters of existing frame configurations, and to create as many frame configurations as needed. The resources are allocated to users in a WiMAX frame as bursts, shown in Figure 8. In Atoll, these bursts are automatically assigned to users during Monte Carlo simulations, and dynamically formed according to the traffic demands of the users. 2.2 WiMAX Features WiMAX is a very flexible technology. All of the RF and system parameters, such as carrier bandwidths, frequency spectrum, frequency reuse, frame structures, etc., can vary from one equipment manufacturer to the other, or from one network operator to the other. As shown in the sections below, this highly flexible nature of WiMAX is fully modelled in Atoll Frequency Bands and Channel Bandwidths The WiMAX Forum is the certification and regulation authority for all aspects of WiMAX. However, the ITU and the telecommunications regulatory authorities of each region and country also play their roles in their respective fields. For example, different frequency bands are, and will be in the future, available in different regions of the world for deploying WiMAX networks, as shown in Figure 12. The figure also shows the initial and future profiles to be used for WiMAX deployment. Forsk 2008 WiMAX Modelling in Atoll 7

8 Figure 12 Frequency spectrum allocation worldwide and frequency bands for initial and future WiMAX profiles Moreover, the WiMAX standards offer flexibility to the equipment manufacturers by allowing them to make equipment working with different channel bandwidths. Atoll offers an easy-to-use method for defining different frequency bands using different channel bandwidths. Figure 13 Frequency bands and channels in Atoll As you can see in Figure 13, different frequency bands supporting TDD and FDD can be created and used in the same document. Atoll fully considers the effects of co-existence of TDD and FDD networks Quality of Service and Scheduling WiMAX networks can offer a number of services using different quality of service classes. The WiMAX QoS classes include, in the order of resource allocation priority, the following: Unsolicited Grant Service (UGS) Extended Real-Time Polling Service (ErtPS) Real-Time Polling Service (rtps) Non-Real-Time Polling Service (nrtps) Best Effort Service (BE) The scheduler, or the radio resource management algorithm, is available in each base station, and performs resource allocation to users for each WiMAX frame in accordance with the QoS classes assigned to the service being accessed by each user. The Atoll WiMAX BWA module includes a number of scheduling and RRM algorithms such as Proportional Fair, QoS Class Biased, Max Aggregate Throughput, and Proportional Demand. Forsk 2008 WiMAX Modelling in Atoll 8

9 For more information on how the WiMAX QoS classes are modelled in Atoll, please refer to 3.2 Services. And, for more information on how the radio resource management is performed in Atoll, please refer to 3.6 WiMAX Monte Carlo Simulations Adaptive Modulation and Coding WiMAX systems use AMC for optimizing the usage of channel resources. Different modulation and coding schemes are assigned to users under different radio conditions. If a user s radio conditions allow him to access the network using a less robust MCS providing a high spectral efficiency, he will be assigned that MCS. Under bad radio conditions, the user will have to use a more robust MCS, which provides lower throughput. For more information on how adaptive modulation and coding is modelled in Atoll, please refer to 3.1 Bearers: Adaptive Modulation and Coding. Figure 14 Adaptive modulation and coding Mobility Management IEEE e networks support mobile users. Users can be handed over to neighbouring cells as they move from one cell s coverage area to the next one s. Neighbour management is an important feature available in Atoll. For more information on neighbour planning in Atoll, please refer to Neighbour Planning. Apart from handovers, different subchannel allocation modes are suited for different user speeds. For example, the obligatory PUSC zone usually covers the handover regions, and is well suited for users moving at higher speeds. AMC zones are suited for fixed or pedestrian users. This aspect is fully modelled through defining user speed limitations for each permutation zone in the frame configurations (see Figure 11.) Moreover, user speeds have significant influence on the channel characteristics between the base station and the user. In Atoll WiMAX, you can define different channel characteristics, i.e., channel models, by assigning different values for different user speeds to the related parameters WiMAX Base Stations Detailed modelling of WiMAX base stations is available in Atoll. A base station in Atoll is a site with one or more transmitters. You can create a network by placing base stations, single or in groups, based on station templates. This allows you to build your network quickly with consistent parameters. Atoll comes with default station templates, which you can modify or you can create new ones as required. It is also possible to import or paste existing data into your document to create base stations. Each site can have a number of transmitters. For each transmitter, you can define a number of transmission and reception parameters. Base stations can be simple or use advanced antenna diversity techniques such as smart antenna systems or multiple-input-multiple-output systems. All the RF parameters of the base station are modelled in cells associated to transmitters. Forsk 2008 WiMAX Modelling in Atoll 9

10 Figure 15 Base station parameters Propagation Models for WiMAX Many propagation models have been proposed for use in WiMAX. The propagation model adopted by the IEEE group is the Stanford University Interim model. It is an empirical model based on measurement data collected and proposed by Vinko Erceg and others [9]. The measurements were taken at 1900 MHz with the receiver at a height of 2 m in different cities of the USA. This propagation model is particularly suitable for suburban areas. A few correction factors were then introduced in the equation to extend the propagation model to other frequencies and other receiver heights. This model is available in the default propagation models library in Atoll. Figure 16 Erceg-Greenstein (SUI) propagation model Another propagation model available in Atoll, which is highly recommended for use with WiMAX, is the Standard Propagation Model. This model can be automatically calibrated using measurement data, and gives highly accurate results in all types of environments. Forsk 2008 WiMAX Modelling in Atoll 10

11 Figure 17 Measurements and Model Calibration Apart from the propagation models available in Atoll s library, it is also possible to work with 3 rd party propagation models, such as ray-tracing models like Volcano from Siradel, WaveSight from WaveCall, and WinProp from AWE Communications. 3 Atoll WiMAX Traffic Model The traffic model available in Atoll is based on the definition of bearers, services, and terminal equipment and speed. It is possible to create as many services and user equipment as required, and to create and import traffic data in many different formats. Atoll lets you create and import raster, vector, as well as live traffic data in the form of maps. The Atoll WiMAX BWA module has introduced the concept of fixed subscriber databases in order to model the FWA traffic that the IEEE d and e networks support. It is possible to study the behaviour of the network for fixed subscribers and traffic maps separately as well as together. 3.1 Bearers: Adaptive Modulation and Coding Bearers in Atoll define the modulation and coding schemes and their respective properties. Bearers support data transfer for all the different services that the network might offer. Bearers can be modified and created as required. Figure 18 WiMAX bearers The most important parameter of a bearer is its efficiency, which is the number of useful data bits that the bearer can transfer in one symbol of the WiMAX frame. Forsk 2008 WiMAX Modelling in Atoll 11

12 Figure 19 A symbol WiMAX reception equipment, available at the base stations and terminals, model the RF aspects of the bearers. You can modify and create different reception equipment as required. Reception equipment list the CINR requirements for selecting the bearers and various CQI characteristics. Figure 20 Reception equipment: WiMAX bearer characteristics Coverage predictions can be easily created to analyse the coverage of different modulation and coding schemes as shown in Figure 21. Figure 21 Coverage by WiMAX bearers and histogram of a throughput coverage prediction showing the effect of adaptive modulation and coding Forsk 2008 WiMAX Modelling in Atoll 12

13 3.2 Services Different services that a WiMAX network offers to its subscribers can be modelled, modified, and created as required. For each service, you can define the WiMAX QoS class and its throughput requirements. Figure 22 Service properties dialogue You can also model services that use different VoIP codecs (G.711, G.729, etc.) 3.3 Terminals User equipment, referred to as terminals, can be modified or created as required. Each terminal groups all the necessary radio parameters, such as the transmission power range for uplink power control, the noise figure, reception capabilities, the type of antenna diversity supported, etc. Moreover, some terminals may also be equipped with a directional antenna. This is most often the case with fixed terminals on subscriber rooftops, for example. These cases are fully supported in the subscriber database, Monte Carlo simulations, as well as in raster coverage predictions. Directional antennas have the benefit of providing a gain in the direction of the serving base station and attenuating considerably the interference from other cells. Figure 23 Terminal properties dialogue Forsk 2008 WiMAX Modelling in Atoll 13

14 3.4 User Profiles Figure 24 Downlink CINR coverage predictions for a frequency plan of N = 1 With an isotropic receiver (top left) With a receiver with a directional antenna (bottom right) User profiles model the behaviour of different types of users. For example, a business user would connect to the internet, have videoconference sessions, and use VoIP telephony, and a home user would connect to the internet to download media or files. These characteristics are modelled in Atoll using user profiles. Figure 25 User profile 3.5 Traffic Data Network traffic data can be input in Atoll in various forms. You can then choose which data to consider in calculations to study the network behaviour under traffic Subscriber Database Subscribers with fixed locations and specific CPEs can be modelled in Atoll using the subscriber database. You can add subscribers to the subscriber database using the mouse (clicking on the map), importing the data from external files, and by simply copying and pasting the information in Atoll. Each subscriber can have different service usage characteristics which are used as inputs to the simulations. Forsk 2008 WiMAX Modelling in Atoll 14

15 You can carry out calculations on the subscriber database directly without having to carry out simulations. Subscribers may have directional antennas which can be automatically pointed towards their serving base stations by Atoll. Figure 26 A subscribers list from a subscribers database Raster, Vector, and Live Traffic Maps The different types of traffic data sources are: The OMC (Operations and Maintenance Centre) Marketing statistics Population statistics 2G network traffic statistics Figure 27 Multi-layer traffic Atoll provides four types of traffic maps for WiMAX projects. These maps can be used for the different types of traffic data sources as follows: Live traffic data from the OMC: Traffic maps per transmitter and per service where traffic is spread over the cell's coverage area and each coverage area is assigned either the total throughput demand or the number of users. The OMC (Operations and Maintenance Centre) collects data from all the cells in a network. This includes, for example, the number of active users in each cell and the traffic characteristics related to different services. You can use this data to create traffic maps containing the number of active users in each cell or the data transfer characteristics of all the services in each cell. Marketing-based traffic data: Traffic vector maps based on user profiles where each vector (polygon or line) carries densities of user profiles. The marketing department can provide information which can be used to create traffic maps. This information describes the behaviour of different types of users. In other words, it describes which type of user accesses which Forsk 2008 WiMAX Modelling in Atoll 15

16 services and for how long. There may also be information about the type of terminal devices they use to access different services. Population-based traffic data: Traffic raster maps based on user densities where each pixel has an actual user density assigned. Population-based traffic data can be based on population statistics and user densities can be deduced from the density of inhabitants. In the traffic maps based on population statistics, you can enter the number of active or potential users per unit surface area, i.e., the density of users. 2G network statistics: Cumulated traffic maps. Atoll can cumulate the traffic of the traffic maps that you select and export it to a file. The information exported is the number of active users per km² for a particular service. This allows you to export your 2G network traffic and then import these maps as traffic density maps into your WiMAX document. These maps can then be used in traffic simulations like any other type of map. 3.6 WiMAX Monte Carlo Simulations WiMAX simulations are used to study the network behaviour under different traffic conditions. Traffic data is taken as input to generate user distributions on the map and then to carry out calculations based on this traffic scenario. The calculations performed during a Monte Carlo simulation include determining the serving cells for each mobile, allocating permutation zones to mobiles, performing power control and subchannelization, calculating CINR radio conditions, determining the best available bearers for mobiles, calculating channel throughputs at mobile locations, allocating resources to mobiles and calculating user throughputs. The accurate calculation of CINR includes conversing channel numbers into absolute frequency values for determining co- and adjacent channels, determining co- and adjacent channel overlaps for different carrier bandwidths, and full consideration of TDD and FDD coexistence in the same network as well. It also takes into account the effect of fractional frequency reuse by considering which segments are being used by different cells. In this case the interference depends on the preamble indexes defined for each cell, as well as on the secondary subchannel groups being used. The amount of resources available in each cell in the uplink and in the downlink are calculated taking into account the number of data subcarriers for each permutation zone, the number of symbols available, and by excluding the time and frequency domain overheads, such as the parts reserved for Preamble and the MAPs, and the guard subcarriers. The radio resource allocation algorithm takes into consideration the different QoS classes assigned to services being accessed by the users. For example, UGS-type services are first allocated the required resources before allocating resources to other services. Similarly, the best effort services are only allocated resources if there are remaining resources available after allocation to service of all the other QoS types. Different cells can use different schedulers. Apart from these calculations, comprehensive modelling of AAS and MIMO algorithms is also part of the simulation. Aggregate throughputs are also determined in the end for each cell. Figure 28 Displaying simulation results using tooltips Forsk 2008 WiMAX Modelling in Atoll 16

17 Coverage predictions can then be carried out based on the simulation results to display the network behaviour in the form of raster plots. For examples of coverage predictions based on results of a traffic analysis, see 4.2 Network Analysis Under Traffic Conditions. Monte Carlo simulation results are available in the map window and can be displayed based on different parameters. It is also possible to display detailed information about any mobile generated during the simulations in the form of a tooltip as shown Figure 29. Figure 29 Monte Carlo Simulation results display By activity status (top left) By throughput (top right) By service (centre) By uplink transmission power (bottom left) By number of used subchannels (bottom right) 4 WiMAX Network Analysis Features in Atoll There are various network analysis features available in Atoll for studying a WiMAX network in detail. The following sections describe these features briefly and show sample analysis results that can be easily obtained using these features. 4.1 Preliminary Analysis A preliminary analysis of a network can be done easily by carrying out coverage predictions that do not depend on traffic data. These coverage predictions represent the areas of preamble, traffic, and pilot coverage. Signal level analysis results are represented in the figures below. Forsk 2008 WiMAX Modelling in Atoll 17

18 Figure 30 Best server preamble coverage (top left) Coverage by preamble signal level (right) Coverage by number of serving cells per pixel (bottom) 4.2 Network Analysis Under Traffic Conditions You can study your WiMAX network under different traffic conditions by creating coverage predictions based on simulation results. The coverage predictions that depend on traffic conditions include all the coverage predictions that are based on the calculation of interference, i.e., CINR, bearer, and throughput coverage predictions. The calculations performed during coverage predictions include determining the serving cells for each pixel, allocating permutation zones to pixels, performing power control and subchannelization, calculating CINR radio conditions, determining the best available bearers and calculating the throughput depending on the available bearer. The accurate calculation of CINR includes conversion of channel numbers into absolute frequency values for determining co- and adjacent channels, determination of co- and adjacent channel overlaps for different carrier bandwidths, and full consideration of TDD and FDD coexistence in the same network as well. It also takes into account the effect of fractional frequency reuse by considering which segments are being used at different cells. In this case the interference depends on the preamble indexes defined for each cell, as well as the secondary subchannel groups being used. Apart from these calculations, comprehensive modelling of AAS and MIMO algorithms is also part of the calculation. Various coverage prediction examples are shown in Figure 31, Figure 32. Forsk 2008 WiMAX Modelling in Atoll 18

19 Figure 31 CINR coverage predictions: DL (top), UL (bottom) Figure 32 Throughput coverage predictions: DL (top), UL (bottom) Forsk 2008 WiMAX Modelling in Atoll 19

20 4.3 Point Analysis Tool The Point Analysis tool can be used for real-time prediction analysis. The tool window is dynamically linked to the map window. The displayed information is updated as the probe mobile is moved on the map. Figure 33 shows the Point Analysis tool. Figure 33 Point analysis tool 4.4 Advanced WiMAX Features Atoll combines comprehensive and accurate calculations with a highly flexible modelling approach in order to provide a powerful platform for WiMAX network planning. One of the many strengths of Atoll in terms of WiMAX is its capability of managing all sorts of combinations of the advanced features available in WiMAX. You can easily create and study a WiMAX network with simple base stations, base stations using smart antennas, and MIMO-capable base stations. Moreover, your network can also have traffic mixtures of simple, AAS-capable, and MIMO-capable user equipment. Atoll supports full as well as fractional frequency reuse cases. You can allocate channels as well as segments and preamble indexes easily to different cells. You can also manage the numbers of secondary subchannel groups used by the segmented PUSC zones. Atoll also uses the information from allocated preamble indexes during calculations. The following sections describe how smart antennas, and MIMO are modelled in Atoll, and how segmentation can be easily implemented in a WiMAX network in Atoll Smart Antenna Systems WiMAX supports adaptive antenna systems. TDD networks are more suitable for smart antennas compared to FDD because the uplink and downlink channel characteristics are similar, and information gathered from a mobile in the uplink can be directly used for downlink transmission by the base station. Smart antenna systems use digital signal processing with more than one antenna element in order to locate and track various types of signals to dynamically minimize interference and maximize wanted signal reception. Different types of smart antenna techniques exist, including beam-switching, beamsteering, beam-forming, etc. Adaptive antenna systems are capable of using adaptive algorithms to cancel out interfering signals. Atoll includes an advanced adaptive antenna systems model that performs beamforming in downlink and interference cancellation in the uplink using an MMSE (Minimum Mean Square Error) algorithm. The adaptive antenna system in Atoll represents a system that, in downlink, calculates and applies dynamic weighting to each antenna element in order to create beam patterns in real-time in the directions of wanted users. In uplink, the Minimum Mean Square Error algorithm models the effect of null steering towards interfering mobiles. The antenna patterns created for downlink transmission have a main beam pointed in the direction of the wanted signal. In the uplink, in addition to the main beam pointed in the direction of the wanted signal, there can also be one or more nulls in the direction of the interfering signals. If the adaptive Forsk 2008 WiMAX Modelling in Atoll 20

21 antenna system is using L antenna elements, it is possible to create L-1 nulls and, thereby, cancel L-1 interfering signals. In a mobile environment where the interference is not stationary but moving, the antenna patterns are adjusted so that the nulls remain in the direction of the moving interference. A system using adaptive antennas adjusts the weighting on each antenna element so as to achieve such a pattern. Atoll s MMSE smart antenna model supports linear adaptive array systems, such as shown in Figure 34. Figure 34 Linear adaptive array system (top) (a) Downlink beamforming (b) Uplink adaptive algorithm In Figure 34 θ is the angle of arrival for the wanted signal, φ is the angle at which we want to calculate the smart antenna gain, and d is the distance between two adjacent antenna elements Modelling in Monte Carlo Simulations During Monte Carlo simulations, the smart antenna model is used to calculate the downlink and uplink CINR accurately. In downlink, the smart antenna gain, for calculating the received carrier power (C), is calculated for the victim cell in the direction of the wanted user by determining the antenna element weights in that direction. Similarly, to calculate the interference (I) from any interfering cell, the smart antenna gain is calculated in the direction of the user being served by the interfering cell. This method is depicted in Figure 35. Figure 35 Downlink CINR calculation in simulations Forsk 2008 WiMAX Modelling in Atoll 21

22 In uplink, an inverse noise correlation matrix is calculated and interference cancellation is modelled using an MMSE adaptive algorithm. For each pair of victim and interfering users, the interference received and its direction is stored at the end of each simulation. The result is the angular distribution of the uplink noise rise which is calculated from the inverse noise correlation matrix obtained at the end of the simulation. The smart antenna simulation results include the geographical distribution of transmitted cell power (downlink) and noise rise (uplink) for each cell. These results are then used to carry out CINR-based coverage predictions for smart antenna base stations. The following figure shows some Monte Carlo simulation results with AAS. Figure 36 Monte Carlo simulation results with AAS Modelling in Coverage Predictions The simulation results shown in Figure 36 are used in coverage predictions. The behaviour of the network in different traffic and network scenarios, for example a mix of simple, AAS-capable, and MIMO-capable base stations and mobiles, can be easily studied in raster coverage plots. In downlink, the smart antenna gain, for calculating the received carrier power (C), is calculated for the victim cell in the direction of the wanted user by determining the antenna element weights in that direction. To calculate the interference (I), the simulation results for the angular distributions of downlink transmitted power are used in order to determine the power transmitted by an interfering cell in the direction of the wanted user, as shown in Figure 37. Figure 37 Downlink CINR calculation in coverage predictions In uplink, the simulation results of the geographical distribution of the uplink noise rise for each cell represent the values and the directions of noise rise (interference with respect to thermal noise) received by the cell itself. Uplink interference for each cell is therefore easily calculated by taking out the thermal noise from the noise rise in any direction. The CINR is also readily calculated from the calculated carrier power (C), interference (I), and thermal noise (N). The following figures show comparisons of downlink and uplink CINR coverage predictions with and without smart antennas. The improvement in CINR brought about by the smart antennas is quite clear, i.e., there is a significant improvement in the coverage of higher CINR values. Forsk 2008 WiMAX Modelling in Atoll 22

23 Figure 38 Downlink CINR without (top) and with (bottom) smart antennas Figure 39 Uplink CINR without (top) and with (bottom) smart antennas Multiple Input Multiple Output Systems Various Multiple Input Multiple Output (MIMO) techniques are available in WiMAX and modelled in Atoll. STTD (Space-Time Transmit Diversity) uses more than one transmission antenna to send the same data. The signals are combined by the receiver before extracting the data. As the receiver Forsk 2008 WiMAX Modelling in Atoll 23

24 gets more than one copy of the useful signal, STTD improves the CINR. STTD is often used for the regions that have bad CINR conditions. It is also referred to as STC (space-time coding) and Matrix A MIMO. During calculations, a MIMO-capable user connected to a cell that supports STTD, benefits from the STTD CINR gain defined for the numbers of transmission and reception antennas, and the clutter class where it is located. SM (Spatial Multiplexing) uses more than one transmission antenna to send different data streams on each antenna. The receiver can also have more than one antenna. SM using M transmission and N reception antennas can theoretically increase the throughput over the transmitter-receiver link by M or N times, whichever is smaller. SM improves the throughput (channel capacity) for given CINR and is used for the regions where CINR is sufficient. It is also referred to as Matrix B MIMO. During calculations, a MIMO-capable user connected to a cell that supports SM benefits from the SM gain in its throughput depending on its CINR, number of transmission and reception antennas, mobility, and its clutter class. The channel capacity gains are based on the increase in channel capacity, with respect to the SISO (1x1) capacity provided by SM for each MIMO configuration (2x2, 4x2, etc.). SM requires a rich multipath environment, without which the gains are reduced. In the worst case, there is no gain. This dependence of SM on the type of clutter where the user is located is also fully modelled. Uplink collaborative SM increases the system capacity in the uplink by receiving separate simultaneous data streams from different users at each antenna of the base station. Figure 40 Spatial multiplexing gains AMS (Adaptive MIMO Switch) is a technique for switching from spatial multiplexing to spacetime transmit diversity as the CINR conditions get worse than a given CINR value. Figure 41 Adaptive MIMO switching AMS can be used in cells to provide spatial multiplexing gains to users under good CINR conditions and space-time transmit diversity gains to users in bad CINR conditions. AMS provides the optimum solution using both MIMO techniques to their best. Forsk 2008 WiMAX Modelling in Atoll 24

25 Figure 42 SISO, MIMO (SM), and MIMO (AMS) throughput coverages The following figures depict the effect of spatial multiplexing gain. The histograms show the increase in throughput when using 4x2 MIMO compared to the SISO case. Figure 43 Effect of spatial multiplexing (Matrix B MIMO) Forsk 2008 WiMAX Modelling in Atoll 25

26 4.4.3 Frequency Planning WiMAX networks can be single or multi-carrier networks. In single-carrier, or single-frequency, networks, the frequency plan would be an N=1 reuse, with all the cells using the same carrier. In multicarrier networks, however, different frequency plans can be exist, such as N=3 reuse for example. WiMAX e provides another level of frequency planning using fractional frequency reuse, or segmentation. The principle of fractional frequency reuse is to divide the carrier bandwidth into segments that can be allocated to different cells that are using the same carrier. This type of frequency plan is also referred to as a pseudo-n=3 reuse. The next section describes how segmentation is modelled in Atoll. Figure 44 CINR coverage predictions with N = 1 (top) and N = 3 (bottom) reuse patterns Fractional Frequency Reuse (Segmentation) Permutation zones in a WiMAX frame can be segmented in order to use a fraction of the channel bandwidth. The advantage of this feature is that each permutation zone of a cell can cover different regions. For example, an FUSC zone (not segmented) can cover an inner zone using the entire channel bandwidth, supporting users that are close to the base station. The power spectral density of the transmitted power in the FUSC zone will be low, and the transmissions from this zone will interfere other cells less. A segmented PUSC zone in the base station, which uses a fraction of the channel bandwidth, would have a higher power spectral density and would cover the inner coverage of the FUSC zone as well as an outer coverage area. A segmented PUSC zone would interfere other cells only over the fraction of the channel bandwidth used. Atoll supports all the different possible scenarios, i.e., non-segmented coverage, segmented coverage, and a mix of segmented PUSC zone and non-segmented FUSC zone. The following figures show sample coverage predictions with and without segmentation. In case of segmentation, the first PUSC DL permutation zone is segmented and each cell of the same site uses 1 primary and 1 secondary subchannel group, i.e., 1/3 rd channel bandwidth in each segment. The Preamble Indexes have been so allocated to each cell that the cells of the same site use mutually exclusive sets of subcarriers. In other words, each cell of the same site uses a different segment, but the same external permutation seed (ID_Cell). Forsk 2008 WiMAX Modelling in Atoll 26

27 Figure 45 CINR coverage predictions Non-segmented FUSC zone (top) (Frequency reuse: N = 1) Segmented PUSC zone (centre) (Frequency reuse: fractional-n = 3) Segmented PUSC + non-segmented FUSC zone (bottom) (Frequency reuse: pseudo-n = 3) Neighbour Planning Neighbour relations are important in WiMAX e networks that support handovers. Neighbour plans can be easily created, edited, and interactively displayed in Atoll. Clicking a transmitter displays all the neighbour links on the map window. Any type of links (outwards, inwards, or symmetrical) can be created and edited, or deleted using the mouse. In addition, you can also import existing neighbour plans easily into documents. Figure 46 Graphically creating neighbour relations Forsk 2008 WiMAX Modelling in Atoll 27

28 5 WiMAX Network Planning Process in Atoll The network planning process for an RF planning engineer working with Atoll is summarized in Figure 47. The planning process starts by creating the network, importing geographic data (DTM, clutter maps, etc.), and setting up all the network parameters and elements, such as base stations and other equipment. The initial positioning of the base stations can be studied by carrying out basic preamble coverage predictions that predict the cell coverage areas. Once the positioning of the base stations has been validated according to the coverage requirements, you can proceed to studying the behaviour of the network under different traffic load conditions. Figure 47 WiMAX network planning process in Atoll To study the network under different load conditions, you can either define the network loads yourself, from statistics collected in the OMC, or use Atoll s powerful Monte Carlo simulation engine to create realistic network traffic scenarios and let Atoll calculate the simulated network loads for you. Atoll s calculation engine performs detailed and accurate calculations for all types of scenarios. In case of AAS-capable base stations, beamforming and MMSE interference cancellation is performed during Monte Carlo simulations and coverage predictions. Similarly, simulations as well as coverage predictions support all types of MIMO with an easy-to-use yet comprehensive modelling approach based on space-time transmit diversity (Matrix A) CINR gains and spatial multiplexing (Matrix B) throughput gains. Any MIMO configuration (2x2, 4x2, 4x4, etc.) can be set up and used in Atoll. The dependency of MIMO gains on different types of environments is also fully modelled. A number of tools are available for studying the network in detail. Among others, these tools include detailed simulation results, easy-to-generate and exportable reports on coverage predictions, and profile and point analysis tools. Coverage prediction reports can be based on different geographic data such as population maps. Apart from studying the behaviour of the network under different traffic conditions, you can also study the effects of different frequency plans (full or fractional), and carry out simulations and coverage predictions to study the network in various frequency planning scenarios. Moreover, other tools are also available for studying and verifying your WiMAX network. You can use the frequency search tool to verify the frequency allocation of your network. Using this tool you can search for channels, segments, and preamble indexes. You can also carry out measurement campaigns and import test mobile data into Atoll for comparison with predicted results, and other verifications. Forsk 2008 WiMAX Modelling in Atoll 28

29 6 References [1] Atoll User Manual 2.7.0, February [2] Atoll Technical Reference Guide 2.7.0, February [3] IEEE : Standard for local and metropolitan area networks Part 16 (IEEE e-2005) [4] Nuaymi, Lutfi, WiMAX Technology for Broadband Wireless Access, John Wiley & Sons, [5] Andrew, J. G., Ghosh, A., Muhamed, R., Fundamentals of WiMAX, Prentice Hall, [6] Liu, H., Li, G., OFDM-Based Broadband Wireless Networks, John Wiley & Sons, [7] The WiMAX Forum website [8] The IEEE Working Group website [9] Erceg, V., et al., An Empirically Based Path Loss Model for Wireless Channels in Suburban Environments, IEEE Journal on Selected Areas in Communications, Glossary of Terms AAS AMC AMS BE BER BLER bps BPSK CINR CNR CP CPE CQI ErtPS FBSS FCH FDD / TDD FEC FER FFR FFT FUSC ICI IEEE IP ISI MAC / MAP MCS MDHO MIMO MMSE nrtps OFDM OFDMA PUSC QAM QoS QPSK RLC RRM RTG / TTG rtps SISO SM SOFDMA STTD SUI TUSC UGS VoIP WiMAX adaptive antenna systems adaptive modulation and coding adaptive MIMO switch best effort bit error rate block error rate bits per second binary phase shift keying carrier-power-to-interference-plus-noise ratio carrier-power-to-noise ratio cyclic prefix customer premises equipment channel quality indicator extended real-time packet service fast base station switching frame control header frequency division duplexing / time division duplexing forward error correction frame error rate fractional frequency reuse (segmentation) fast Fourier transform full usage of subcarriers inter-carrier interference Institute of Electrical and Electronics Engineers Internet protocol inter-symbol interference media access control / media access protocol modulation and coding scheme macro-diversity handover multiple input/multiple output minimum mean square error non real-time polling service orthogonal frequency division multiplexing orthogonal frequency division multiple access partial usage of subcarriers quadrature amplitude modulation quality of service quadrature phase shift keying radio link control radio resource management receive time guard / transmit time guard real-time polling service single input/single output spatial multiplexing scalable OFDMA space/time transmit diversity Stanford University Interim tile usage of subcarriers unsolicited grant services voice over Internet protocol worldwide interoperability for microwave access Forsk 2008 WiMAX Modelling in Atoll 29