BACKHAUL DIMENSIONING INPUT

One result of the dimensioning of the access network is the required capacity by the backhaul network. A typical backhaul network consists of point-to-point or point-to-multipoint radio links or even fiber rings in recent networks. In most cases the information rate that is originated to and from the backhaul network should be calculated to evaluate an existing infrastructure or consider the cost to deploy a backhaul network along with WiMAX.

The total committed information rate of a WiMAX PoP is linear to the number of sectors and average sector throughput, which mainly depends on channel bandwidth and deployment scenario. Considering trisector cells, and average throughput of 9 Mbps (5 MHz channel bandwidth), the total TDD committed rate is 27 Mbps. To estimate the FDD equivalent, which is directed to and from the backhaul network, the total rate is multiplied by the DL/(DL + UL) ratio. Hence the traffic that should be reserved in the backhaul network for a PoP with 2/3 DL WiMAX traffic is 18 Mbps. For a dual layer cell or 10 MHz channel bandwidth the backhaul rate per PoP would be doubled, around 36 Mbps. It is acceptable to include a 5 percent margin as in practice it is common to observe small deviations in sector throughput. During the deployment and the operation the exact backhaul traffic can be monitored through the management system and design adjustments can be made accordingly.

In certain occasions the network designers are requested to propose a backhaul system along with the WiMAX network. Although this is not in the scope of this chapter, however there are some interesting observations that can be highlighted. Clearly the multipoint systems are more cost effective than point-to-point radio systems when the WiMAX PoP number is high, the PoP are quite close to each other and when their backhaul rate is such that more than 3–4 WiMAX PoP per multipoint sector can be served (around 10–12 per site). This indicates that the WiMAX site backhaul traffic should be from 15 to 25 Mbps, which mainly refers to trisector cells with 5 MHz channel. For higher bandwidth systems, such as dual layer cells or with 10 MHz bandwidth per site the point-to-point solution may offer the higher required capacity. Again in a dense urban environment where sites are usually deployed with separation of 0.3–0.7 km the use of fiber (if available) could be a better solution. Concluding, the best backhaul system strategy can be determined by evaluating the number of WiMAX PoP, their backhaul rate, and finally the PoP positioning. Note that although the WiMAX network dimensioning is done in such way to optimize the access network, this does not suggest that the backhaul network is also optimized and a separate study may be necessary.

JOINT DIMENSIONING

The number of PoP and sectors were estimated according to the requirements of a dimensioning project. The final step is to combine these results into the optimum BS configuration. Clearly the estimated PoP and sectors are the absolute minimum according the needs of coverage and capacity, respectively. At this stage joint consideration may suggest that more PoP or sectors may be necessary. There are three possible conditions:

• Balanced network: The number of PoP approaches 1/x of the number of sectors, which means that in each PoP roughly x sectors will be deployed. The number of sectors for blanket coverage should be 3 < x < 6, where x = 3 for Mobile WiMAX. This condition ensures both the integrity of the footprint and satisfies the capacity requirement.
• Coverage-limited network: The number of PoP is quite higher than 1/3 of sectors. The network is coverage limited and in this case the number of sectors should be increased until the previous condition is met. The fact that the original business plan leads to a coverage-limited network should be stated in the dimensioning study. CapEX is driven by coverage performance indicators, while the additional sectors will further increase the air-interface capacity. Operators may want to revise the size of the service area, or exploit the additional capacity.
• Capacity-limited network: The number of PoP is quite lower than 1/3 of sectors, which indicates either additional PoP or higher sectorization (sectors/PoP). Increasing the number of PoP will trigger additional CapEX and OpEX in terms of site acquisition and preparation. Therefore, if a higher sectorization scheme is possible, such as when the terminals are fixed-outdoor, fixed-indoor, or nomadic where handover is not necessary, it should be preferred as a cost-optimum solution. When the network needs to accommodate mobile terminals and provide handover capability, the sectors should be 3 < x < 4 and more PoP may be needed. An alternative approach would be to deploy a dual layer cell where 6 sectors of 120° are used, however each pair of sectors (i.e., 1 and 4) is assigned the same azimuth. For a dual layer cell at least 6 channels are necessary for the frequency reuse of 1.3.3.

The selected number of sectors per PoP defines the BS configuration in terms of frequency reuse/channel assignment, and antenna beamwidth/azimuth/tilt, while other air-interface parameters are not related to dimensioning. Capacity or coverage dimensioning should be revised based on the above-mentioned conditions, for coverage or capacity limited cases, respectively. A comparison between initial requirement and actual achievement should be included in the dimensioning study.

CAPACITY DIMENSIONING - 4G

Further to providing adequate radio coverage to customers, the next equally important objective is to ensure sufficient air-interface capacity (throughput) to offer a wide range of services. Given a business plan, the network capacity is driven by the potential subscriber number and the committed rates for data and VoIP services, as discussed in services dimensioning. The purpose of capacity dimensioning is to translate the capacity into number of sectors, which then have to be distributed for the estimated number of PoP. The missing parameter is average operating sector throughput, which finally determines how many subscribers can be served in a sector. Defining the average sector throughput is quite complex as it involves interpretation of a wide range of parameters such as

• Accurate equipment specifications: PHY modes and SINR thresholds, advanced antenna system impact, interference mitigation mechanisms, scheduling intelligence, and terminal types.
• Deployment considerations: Available spectrum, reuse factor and channel assignment method, coverage type(s), propagation environment, and site suitability.
• Designer capabilities: Very good knowledge of the IEEE 802.16e standard, formal training or academic expertise in planning methodologies and their impact on practical networks, past deployment experience.

The sector throughput is usually provided as recommendation by the system vendor, however it can vary a lot depending on the deployment scenario. During dimensioning a more accurate estimation can be made through two approaches: 

(1) By extensive RF planning simulations and statistical processing that would result in a PHY mode regions map and give the average throughput. If clutter data are available this approach may provide a better view of the network performance. 

(2) By previous experience and interpretation of the above factors based on system performance observation. The former approach is time consuming, however it is much more accurate and is usually followed during actual network design. The latter is more appropriate during dimensioning.

Typically the equipment specifications include a table that matches the available PHY modes with their SINR requirement and the resulting Ethernet throughput (for different channel bandwidths) as in Table 1. The SINR values are given in the standard. The provided Ethernet rates correspond to a 60/40 DL/UL asymmetry and hence the TDD rate is the sum of DL and UL rates. The upper throughput bound can be achieved for an interference-free sector where terminals are located only in the 64QAM, 3/4 region, achieving around 10 Mbps TDD for the 5 MHz bandwidth. During practical deployments, the terminals will be scattered across the whole cell footprint, hence operating in various modes. Furthermore, interference due to frequency reuse may further downgrade the PHY mode for a particular terminal, especially if the number of channels is limited. A default assumption is to consider, as average sector throughput, the one corresponding to 16QAM, 1/2. Thereafter if the deployment conditions are favorable, as in the case of fixed-outdoor terminals or when enough spectrum is available for relaxed reuse, higher throughput should be expected. The throughput can be also enhanced by means of MIMO techniques and this should also be taken into account. It should be noted that the values in Table 2 refer to full usage of subcarrier (FUSC) permutation scheme, where all subchannels are allocated to users, hence the whole channel is exploited. In case segmentation is considered, i.e., partial usage of subcarrier (PUSC), the users in a sector utilize a specific segment (1–6 subchannels), and therefore the throughput in this case is reduced accordingly. Based on the standard, there will be regions in the DL and UL subframes for both FUSC and PUSC and in this case an average throughput condition should be expected. In most products, during the sector configuration, an RF designer can select or exclude segmentation according to deployment conditions.

Table 1: Ethernet Rates per PHY Mode (IEEE 802.16e System) 

		5 MHz Bandwidth		10 MHz Bandwidth
PHY Mode (CTC)	SINR (dB)	DL (Mbps)	UL( Mbps)	DL (Mbps)	UL (Mbps)
QPSK 1/2	2.9	1.4	0.9	2.7	1.7
QPSK 3/4	6.3	2.2	1.3	4.3	2.7
16QAM 1/2	8.6	2.8	1.7	5.8	3.6
16QAM 3/4	12.7	4.3	2.6	8.6	5.4
64QAM 1/2	16.9	5.8	3.5	11.6	7.1
64QAM 3/4	18.0	6.5	3.9	12.9	8.2

The process of selecting frequency reuse and channel allocation in sectors is very important for both capacity and coverage. In mobile WiMAX this can be done in a flexible manner, although frequency planning cannot be avoided. This is due to the possibility of nonuniform network layout, in most cases, where frequency planning may improve performance. According to mobile WiMAX terminology the reuse is denoted as 1.x.y, where x denotes the cell sectors and y the available channels. There are two main schemes under consideration: global reuse 1.x.1, where a single channel is used everywhere and cell/cluster reuse 1.x.y, where y = nx, n = 1, 2, 3. The most appropriate scheme is adopted, based on the systems’s special capabilities to reject or tolerate interference (i.e., via BF). An indicative performance of the most common  schemes for nomadic/mobile terminals (most sensitive to interference) is presented in Figure 1. It can be observed that for the 1.3.1 scheme the SINR drops well below the 3 dB threshold for the lowest modulation scheme, QPSK, hence a significant part of the cell footprint, especially among adjacent sectors, has no coverage. In the case of the 1.3.3 scheme, the interference appears closer to the cell edges and hence the coverage blanks spots are much smaller. It should be noted that in the 1.3.3 scheme the higher order PHY mode schemes extend to a larger region, hence indicating an improved sector throughput. Furthermore, when employing PUSC instead of FUSC, the 1.3.1 scheme behaves essentially as 1.3.3, while 1.3.3 as 1.3.9. It is typical for a sector to operate in 1.3.1 FUSC mode for terminals with good link quality and short link distance and in PUSC mode, which is equivalent to 1.3.3 for terminals that would otherwise achieve low SINR (due to low signal strength or interference).

Figure 1: SINR map for (FUSC) and (PUSC) schemes.

Knowing the average sector throughput as described in previous paragraphs, capacity dimensioning can be completed as follows: Initially an analysis on the customers that can be accommodated in a sector is performed. This is done by analyzing the service plan and calculating the average data and VoIP CIR per service and customer. Then a graph that shows the throughput versus the subscribers is obtained, as in Figure 2. The average sector throughput is projected in the sector total graph indicating the corresponding subscriber number. Considering a 10 Mbps throughput, each sector can accommodate around 70 customers. Moreover a graph such as in Figure 2 is quite useful also during network operation as when the throughput takes a higher value, due to special circumstances, additional subscribers can be served. The second step is to divide the number of customers per area with the customers/sector to calculate the required number of sectors.

Figure 2: Estimated sector and services throughput versus subscribers.

DIMENSIONING PROCESS

The process of dimensioning involves a sequence of steps, which serve different requirements such as capacity or coverage estimations, to conclude to a final outcome as shown in Figure 1. The input consists of the business plan, the assets, and the key performance indicators (KPIs). The first action is then to verify that all necessary information is available and clarified, alternatively an interactive “questions and answers” session is necessary. During the input analysis, the designers utilize their theoretical and technological expertise to define the dimensioning strategy. In somecases this analysis identifies that potential business plan assumptions could affect the network design process, and a discussion is likely to be initiated with the customer. Such an approach is more appropriate after the award of a contract. The following step is the processing of service characteristics such as the Internet rates and Voice-over-Internet Protocol (VoIP) codecs. In most cases the services are provided in the business plan with their marketing description and it is necessary to identify the impact of each service on the WiMAX air-interface. In the next step, the coverage analysis is performed: the provided service areas are identified and the required number of points of presence (PoP) is estimated. A PoP refers to a WiMAX site, with at least three sectors, which is capable of providing a 360° footprint.

 

Figure 1: Network dimensioning process.

Further to the coverage analysis, the service areas should be also offered sufficient capacity as dictated by customer numbers and services profiles. Therefore the next step is to estimate the required number of sectors to achieve this capacity. The estimates of PoP and sectors are then used to determine the configuration of the BS in the final joint analysis step. It is common after the joint analysis for designers to evaluate the results and proceed with customizations to further improve the solution. In this case, coverage, capacity, and joint analysis may be revised several times before providing the final output. A description of the strategy, the final bill of material (BoM), and a discussion of the assumptions and methodology are mainly the outcome of a dimensioning study.

1 BUSINESS PLAN, ASSETS, AND KEY PERFORMANCE INDICATORS

A well-defined business plan can be the most significant driver behind a successful investment on WiMAX technology. From a cost point of view, a very ambitious plan in terms of subscribers/services will certainly impact CapEX and OpEX, whereas from a design point of view, the impact comes when the requirements are not extensively defined or they contradict the technology capabilities. A revision of the business plan during or after the network design may compromise the network performance or create additional design costs but most importantly it will result in implementation delays. A list with the most common information provided with a business plan is presented as follows:

1. Service area(s): defined with geocoded polygons, including the size in km², and the terrain profile details (i.e., urban, suburban, rural, average building height, etc.).
2. Coverage type: such as fixed-outdoor, on rooftop, or on outer walls, fixed-indoor, nomadic outdoor/indoor, mobile outdoor or any combination thereof.
3. Subscriber profile(s): such as residential, small business, corporate. Subscriber profiles may relate to a specific type of coverage and service.
4. Subscriber distribution: subscriber numbers per profile, per service area, and per deployment year, according to the scalability plan.
5. Service profile(s): such as VoIP, broadband Internet, VPN along with their distinct characteristics (i.e., VoIP codecs, peak information rates, contention factors, etc.). Service profiles may relate to specific subscriber profiles and coverage types.
6. Available spectrum: defined as paired, along with local regulations concerning the allowed channelization and duplex schemes.
7. Existing infrastructure: such as sites that can be reused, available backhauling equipment with Ethernet interface, and core network PoPs.
8. Cartographic data: such as high-resolution digital maps with buildings.
9. Key performance indicators: such as coverage objective in terms of percentage of the service area, differentiated per terminal type, where a stable QPSK link can be achieved.
10. Customer requirement: such as duplex scheme, number of sectors/BS, channel bandwidth, reuse scheme, type of sites, deployment strategy.

2 STRATEGY

During request for information (RFI)/RFP stages, a dimensioning exercise may be requested by a customer, mainly for two reasons: either to acquire know-how by differentiated proposals or to identify the more cost-efficient solution. In the first case, the requirements are usually relaxed so that the participant vendors/integrators can design with flexibility, while the provided information (i.e., business plan, assets, service areas) is hypothetical. The submitted studies will probably be presented in various formats and most certainly based on diverse assumptions. In such case a direct comparison among the studies is complicated, and usually a more defined exercise is the next step. In the second approach, the case study is well defined so that the design assumptions are either implied or directly mentioned. The results are now directly comparable, hence a clear ranking list can be obtained. From RF network designer point of view a different strategy should be followed: showing flexibility in the network design and perhaps providing several alternatives for the first approach, while a more strict, cost-optimum solution is more appropriate for the second approach.

DEPLOYMENT SCENARIOS

A major feature of WiMAX compared to other wireless access technologies is that it breaks the barrier of addressing a single customer profile. Global system for mobile communications (GSM)/universal mobile telecommunications system (UMTS) provide mainly voice and low speed internet to mobile subscribers, while local multipoint distribution service (LMDS)/wireless local loop (WLL) offer higher bandwidth services to fixed subscribers. WiMAX can offer broadband services to all fixed, nomadic, and eventually mobile subscribers, according to the aims of the latest IEEE 802.16e standard. This major advantage for WiMAX technology offers greater flexibility and scalability; however it presents more design challenges. A conceptual presentation of deployment scenarios, based on equipment, services, and potential customer profiles is presented in Figure 1. Each “sector” represents a WiMAX terminal profile:

• Fixed-outdoor units (including antenna, RF subsystem, modem), which can be installed on the rooftop or outer building walls for maximizing link performance. A cable connects the unit to an indoor interface terminal that provides Ethernet and VoIP ports.
• Fixed/portable indoor units (intergraded antenna, RF baseband and interface in a single box), which are installed indoors close to a window or the outer wall. The unit is portable within the indoor space, however it requires power supply.
• Nomadic/mobile units (PCMCIA cards, handheld devices), which are truly portable (mobile in future versions) and can be used in outdoor and indoor spaces.

 

Figure 1: Abstract of WiMAX deployment scenarios.

Each terminal profile is built with different performance capabilities and cost towards specific customer profiles. Fixed-outdoor terminals are capable of long range, robust links that can transfer high-bandwidth and delay sensitive services with low impact on network air-interface resources, hence they are more suitable for corporate, small-to-medium enterprises (SMEs), and small-offices-home-offices (SOHOs). The higher hardware and installation costs are balanced by higher revenues. Fixed-indoor terminals have considerably less cost and are self-installable, albeit with smaller link range. Such terminals address the mass market of residential access. Finally the nomadic and portable terminals require even greater network design margins and usually address individual customers at specific service areas (such as community/camp networks). Observing Section 1 it can be seen that business customers are likely to prefer a combination of fixed-outdoor and nomadic units, while residential and personal customers will probably select either a fixed-indoor or a nomadic unit. As WiMAX technology progresses, more system gain will be achieved in the air-interface thus resulting in higher cell ranges and increased percentage of nomadic terminals mainly at the expense of fixed-indoor units.

The continuous development of WiMAX technology from IEEE 802.16-2004 standard to the IEEE 802.16e amendment, has led to significant improvements in the air-interface. Recent advances include higher BS transmit power, advanced antenna systems (MIMO, beamforming (BF)), improved radio resource management through the OFDMA profile, improved coding techniques which reduce the signal-to-interference and noise ratio (SINR) thresholds, efficient uplink (UL) subchannelization, and flexible frequency reuse. The current amendment of WiMAX offers more than 15 dB increase in the system gain over previous versions which drastically extends the radio coverage, and can therefore reach indoor customers even when using portable/mobile terminals. As the WiMAX system gain increases due to the continuous enhancement of the air-interface, in the context of dimensioning, the network size for a specific deployment is reduced, and so is the up-front investment.

BENEFITS OF WiMAX

The WiMAX solution reflects the general trend in the communications industry toward unified packet-based voice and data networks. Fundamental benefits of this transition are reduced operation cost, improved network optimization, and better management of changes. The followings are some of the benefits of WiMAX.

Wireless. By using a WiMAX system, companies/residents no longer have to rip up buildings or streets or lay down expensive cables.

High bandwidth. WiMAX can provide shared data rates of up to 70 Mbps. This is enough bandwidth to support more than 60 businesses at once with T1-type connectivity. It can also support over a thousand homes at 1-Mbps DSL-level connectivity. Also, there will be a reduction in latency for all WiMAX communications.

Long range. The most significant benefit of WiMAX compared to existing wireless technologies is the range. WiMAX has a communication range of up to 40 km.

Multi-application. WiMAX uses the IP and is therefore capable of efficiently supporting all multimedia services from VoIP to high speed Internet and video transmission. It also supports a differentiated QoS enabling it to offer dynamic bandwidth allocation for different service types. WiMAX has the capacity to deliver services from households to small and medium enterprises, small office home office (SOHO), cybercafés, multimedia Tele-centers, schools and hospitals.

Flexible architecture. WiMAX supports several systems architectures, including point-to-point, point-to-multipoint, and ubiquitous coverage.

High security. The security of WiMAX is state of the art. WiMAX supports advanced encryption standard triple data encryption standard. WiMAX also has built-in VLAN support, which provides protection for data that is being transmitted by different users on the same BS. Both variants use privacy key management (PKM) for authentication between BS and SS station. WiMAX offers strong security measures to thwart a wide variety of security threats.

QoS. WiMAX can be dynamically optimized for a mix of traffic that is being carried.

Multilevel service. QoS is delivered generally based on the service-level agreement between the end user and the service provider.

Interoperability. WiMAX is based on international, vendor-neutral standard. This protects the early investment of an operator because it can select the equipments from different vendors.

Low cost and quick deployment. WiMAX requires little or no external plant construction compared with the deployment of wired solutions. BSs will cost under $20,000 but will still provide customers with T1-class connections.

Worldwide standardization. WiMAX is developed and supported by the WiMAX forum (more than 470 members). The WiMAX forum collaborates with different international standards organizations that are developing broadband wireless standards with the intent to provide interoperability among the standards. Some of the other broadband wireless standards include HiperMAN/HiperLAN (Europe) and WiBRO (South Korea). These standards are compatible with WiMAX at the physical layer. WiMAX will become a truly global technology-based standard for broadband and will guaranty interoperability, reliability, and evolving technology and will ensure equipment with very low cost.

VOIP AND IP | WIMAX APPLICATIONS

The WiMAX standard has been developed to address a wide range of applications. Based on its technical attributes and service classes, WiMAX is suited to supporting a large number of usage scenarios. Table 1 address a wide range of applications.

Table 1: Summary of WiMAX Applications 

Class Description	Real Time	Application Type	Bandwidth
Interactive gaming	Yes	Interactive gaming	50–85 Kbps
VoIP, video conferencing	Yes	VoIP	4–64 Kbps
		Videophone	32–384 Kbps
Streaming media	Yes	Music/speech	5–128 Kbps
		Video clips	20–384 Kbps
		Movies streaming	>2 Mbps
Information technology	No	Instant messaging	<250 byte messages
		Web browsing	>500 Kbps
		Email (with attachments)	>500 Kbps
Media content download (store and forward)	No	Bulk data, Movie download	>1 Mbps
		Peer to peer	>500 Kbps

Mobile WiMAX is an all-IP network. The use of OFDMA on the physical layer makes it capable of supporting IP applications. It is a wireless solution that not only offers competitive Internet access, but it can do the same for telephone service.

VoIP offers a wider range of voice services at reduced cost to subscribers and service providers alike. VoIP is expected to be one of the most popular WiMAX applications. Its value proposition is immediate to most users. Although WiMAX is not designed for switched cellular voice traffic as cellular technologies as are CDMA and WCDMA, it will provide full support for VoIP traffic because of QoS functionality and low latency. IPTV enables a WiMAX service provider to offer the same programming as cable or satellite TV service providers. IPTV, depending on compression algorithms, requires at least 1 Mbps of bandwidth between the WiMAX BS and the subscriber. In addition to IPTV programming, the service provider can also offer a variety of video on demand (VoD) services. IPTV over WiMAX also enables the service provider to offer local programming as well as revenue generating local advertising.