SIMULATION RESULTS | Capacity Planning and Design

The average-based design models lead us to much smaller estimates of required capacity. Therefore, they run the risk of not being able to guarantee acceptable performance for many real-time applications such as voice or video where jitter must also be taken into account. We currently do not have design models that can take jitter into account so we need to evaluate whether the jitter remains acceptable in a system designed with an average delay method.

In this section, we present simulation results to study the delays encountered by the individual voice and video sources under various provisioning scenarios and compare them with the required delays for voice and video, respectively. We used ns-2 to conduct simulations. In this section, we only consider AF subclasses where multiple sources send packets to each subclass, and packets of each subclass are served in the order of their arrival while sharing bandwidth between the subclasses using PDD scheduling. The simulation model for AF class is shown in Figure 1. We simulate a voice source using a two state on–off model where it generates packets with a deterministic inter-arrival time of 15 ms in the on-state. On-periods are exponential with rate 2.5 and off-periods are also exponential with a rate 1.67. Each packet is of size 120 bytes. The video source is modeled using deterministic batch arrivals with batch inter-arrival time of 33 ms. The number of packets in a batch are geometrically distributed with an average of five packets. In each burst, the last packet has size distributed as uniform (0,1000) bytes. All other packets have 1000 bytes.

 
Figure 1: Simulation model for AF class.

Here also c_LB/r, c_OO/r, and c_P/r refer to overprovisioning required when, dimensioning for average delays using LB based model, using on–off-based model and Poisson-based model, respectively. Observe that the capacity computed using these models along with PDD-based scheduling for single, five, and ten voice and video sources. Now we use that capacity for the simulation and compare in Figures 2 through 7 the delays for single, five, and ten voice and video sources. We have plotted the observed mean delay and error bars corresponding to twice the sample standard deviation for voice and video sources. We also present a horizontal line showing the required average delay for each source.

 
Figure 2: Delay for single voice source.

 
Figure 3: Delay for single video source.

 
Figure 4: Delay for five voice source.

 
Figure 5: Delay for five video source.

 
Figure 5: Delay for ten voice source.

 
Figure 6: Delay for ten video source.

Note that for the Poisson-based capacity model with single sources, the actual mean delay is many times the target delay, both for voice and video. Moreover, some voice packets can have a delay as high as 400 ms and will be useless at the receiver. For video also, packets can have delays as much as 1 s. Such a capacity planning is not very useful and could lead to unsatisfied customers. When we multiplex five or ten voice and video sources, the average delays get closer to the target delays and for ten sources, they are even acceptable for both voice and video. However, there is still a large variance in the observed delays and voice packets could still have as high as 40 ms and video as high as 100 ms. Note that such high delays could be tolerable if they affect only a small number of packets.

Next, we consider on–off and LB-based design models. Observe that both the approaches provide acceptable delays, average as well as average along with two times standard deviation. The values are smaller than the required delays and hence a significant fraction of packets belonging to voice and video sources will encounter less than required delays. These models remain consistent for single, five, or ten sources and provide acceptable, performance to individual sources. Note that the LB-based model provides delays which are less than the target for both voice and video, although it requires lesser capacity than the on–off-based models. Observe that not only the delays are acceptable but also the variance is quite small.

Based on these results, it can be argued that LB-based model could be used to determine required capacity for a source requesting an average delay QoS. When allocating capacity for a small number of sources, it can achieve the multiplexing gain and provides minimal capacity to meet the required delays.

WIMAX NETWORK PLANNING PROCESS

WiMAX radio planning involves a number of steps ranging from tool setup to site survey. The process is similar to any wireless network. What differs between WiMAX and other technologies are the actual site configuration, KPIs, and the propagation environment as WiMAX may support mobile and fixed users where the latter may employ directional/rooftop antennas.

The final radio plan defines the site locations and their respective configuration. The configuration involves BTS height, number of sectors, assigned frequencies or major channel groups, types of antennas, azimuth and downtilt, equipment type, and RF power. The final plan will be tested against various KPI requirements mainly coverage criteria and capacity (or signal quality). Figure 1 can be used as a guide in developing a planning process. The planning process also largely depends on the planning tool used.

 
Figure 1: WiMAX radio planning process.

The planning process in Figure 1 includes measurements (i.e., drive test and verifications) after the site survey. This procedure is not mandatory for all sites if the site count is too large. Usually, site survey and the KPI analysis give an indication of which areas are expected to have poor RF quality and which sites are involved. This is usually done when the candidate site(s) are not located in ideal locations or if the site survey finds some discrepancies of the candidate(s).

The differences between WiMAX and 3G radio planning. WiMAX radio offers modest processing gain in a form of repetition coding and subchannelization. These features are only exploited when the signal quality demands more processing. To support high data rates, the radio plan must offer very good SINRs (signal to interference and noise ratio) even with very limited spectrum. For example, in the absence of subchannelization and repetition coding, the required SINR for the lower MCS (modulation and coding scheme) is around 5 dB and this needs to be achieved even with very tight frequency reuse factor of 1/3 or 1/4 in the presence of shadowing, where the reuse factor is the reciprocal of the number the cells using different frequencies and the sum of the frequencies presents the whole spectrum resource allocated to the planned system. Another consideration in the case of WiMAX planning is the high SINR requirements to support high data rates. Although a site is expected to support high data rates for CPEs closer to it, SINR values >30 dB are only possible in the absence of interference. This requires accurate modeling of the propagation and RF equipments. For example, in 3G, high data rates are possible even with C/(I + N) of < 10 dB as the processing gain enables the receiver to tolerate some amount of interference. In WiMAX, this is not the case as the processing gain is only provided through channel coding and limited coding repetition.

WiMAX—Architecture, Planning, and Business Model

The fixed/portable broadband wireless access is becoming a necessity for many residential and business subscribers worldwide. The demand is exploding as the pricing of broadband services is rapidly decreasing. The worldwide interoperability for microwave access (WiMAX) technology is an integral part of the portfolio by complementing 2G/3G mobile access, Digital Subscriber Line (DSL) broadband fixed access, and Wireless Fidelity (WiFi) hotspot access. An extended overview of WiMAX and its applications in higher generation wireless networks as a cost-effective solution to answering the challenges posed by the digital divide is presented. Technology behind WiMAX and its network architecture, design, and deployment are examined in addition to factors that impact WiMAX planning and performance. A WiMAX radio coverage simulation and analysis at different frequency bands for different demographic is presented. Furthermore, the WiMAX business models and a comparison with two enhanced third-generation (3G) technologies that are potential competitors to WiMAX are explored. 

Telecommunications has grown at a tremendous rate in the last ten to twenty years. Improved semiconductor and electronics manufacturing technology, and the growth of the Internet and mobile telecommunications have been some of the factors which have fueled this growth in telecommunications. The deployment of state-of-the-art telecommunications infrastructure and services has, however, been restricted to the developed world. The least developed countries have been left in the technological dark ages with few or none of the next-generation networks installed. Developed countries now boast high-speed connections with a large percentage of homes having access to the Internet and broadband services at an affordable fee. The underdeveloped countries are yet to enjoy such facilities. This is referred to as the digital divide. During the first World Summit on the Information Society (WSIS) held in Geneva in December 2003, the Digital Divide was defined as the unequal access to information and communication technologies (ICTs), where the least developed countries are separated from the developed countries because of a lack of technology particularly ICT.

The digital divide has persisted due to the relatively high cost of putting up modern telecommunications infrastructure. This is compounded by the fact that there are a number of different services available and each service requires its own technology and network. Therefore, existing technologies such as Wireless Fidelity (WiFi), Digital Subscriber Line (DSL), Global System for Mobile communications (GSM), Integrated Services Digital Network (ISDN), and the relatively new 3G technologies have not been able to provide a total solution to closing the digital divide. Figure 1 illustrates the main network types and the prevalent technologies associated with each, mapped against usage models and access modes.

 

Figure 1: Wireless network type and range. MAN—metropolitan area network (citywide, rural area), LAN—local area network (office, home, campus), and WAN—wide area network (countrywide, international).

WiMAX will boost today’s fragmented broadband wireless access market and mobile WiMAX promises to offer a solution to closing the existing digital divide. WiMAX can address the fixed wireless access and portable Internet market, complementing other broadband wireless technologies. Government initiatives to reduce the digital divide are making gains for broadband wireless countries such as Australia, South Korea, Taiwan, and the United States have programs in place today, and there has been a push by the European Commission for more flexible spectrum policies.

WiMAX access can be easily integrated within both fixed and mobile architectures, enabling operators to integrate it within a single converged core network, thereby providing new capabilities for a user-centric broadband world.

WiMAX addresses the following needs which may answer the question of closing the digital divide:

• Cost effective
• Offers high data rates
• Supports fixed, nomadic, and mobile applications thereby converging the fixed and mobile networks
• Easy to deploy and has flexible network architectures
• Supports interoperability with other networks
• Aimed at being the first truly a global wireless broadband network

WiMAX is a standard that is championed by the WiMAX forum which was formed in June 2001 to promote conformance to IEEE 802.16 standard. The WiMAX forum currently has more than 470 members comprising the majority of operators, component, and equipment companies in the communications ecosystem. The WiMAX forum promotes interoperability by working closely with IEEE and other standards groups such as the European Telecommunications Standards Institute (ETSI) which have their own versions of broadband wireless. Along these lines, the WiMAX forum works closely with service providers and regulators to ensure that WiMAX forum certified systems meet customer and government requirements.

The original WiMAX standard only catered for fixed and nomadic services. It was reviewed to address full mobility applications; hence, the mobile WiMAX standard, defined under the IEEE 802.16e specification was created. Mobile WiMAX supports full mobility, nomadic, and fixed systems to compete against DSL to cover isolated areas such as rural hot spots, private campus networks, and remote neighborhoods. Mobile WiMAX is more promising to be deployed as a cellular network that offers ubiquitous broadband services to mobile users to over large geographical areas. It can be deployed as a central office bypass to avoid using existing wired infrastructure for competitive local exchange carriers and wireless Internet service provider. Figure 2 shows the standard history for 802.16.

 

Figure 2: 802.16 standard evolution.