NETWORK DIMENSIONING AND DESIGN

Designing, deploying, and managing any wireless cellular system requires clear objectives to be identified from the outset. These includes definition of the footprint coverage, the estimated number of users, the traffic load distribution, the penetration and growth rate, and internetwork access and roaming. Mobile WiMAX, which will be deployed like 2G and 3G cellular networks, supports fractional frequency. Fractional frequency reuse takes advantage of the fact that mobile WiMAX user transmit on subchannels and does not occupy an entire channel such as in 3G. The objective of the radio network dimensioning and design activity is to estimate the number of sites required to provide coverage and capacity for the targeted service areas and subscriber forecast. This process is based on many assumption such as uniform distribution of subscribers, homogenous morphology, and ideal site location. The main inputs required for network dimensioning are site equipment-specific parameters, marketing-specific parameters, and licenses regulation and propagation models. Figure 1 shows the flow chart of activities performed in network design and planning, starting from data collection of marketing and design requirement input and achieving the business model to provide a nominal site plan using a network simulation software.

 

Figure 1: The cell planning process.

Mobile WiMAX is designed to complement existing 2G/3G access technologies with an “Always Best Connected” experience with voice and data connections. There is a large range of possible scenarios for the deployment of mobile WiMAX, but main four categories are

• Fixed and mobile operator with enhanced data for GSM evolution (EDGE)/3G who uses mobile WiMAX as a complementary extension for data services
• Mobile only operator with EDGE/3G who uses mobile WiMAX as a complementary extension for data services
• Fixed operator who uses mobile WiMAX to compete with 3G operators for data and voice services
• New entrant who uses mobile WiMAX to move into mobile market—threat to incumbent mobile operator.

WiMAX operates in a mixture of licensed and unlicensed bands. The unlicensed bands are typically the 2.4- and 5.8-GHz bands. Licensed spectrum provides operators control over the usage of the band, allowing them to build a high-quality network. The unlicensed band, on the other hand, allows independence to provide backhaul services for hotspots. Typical area licensed WiMAX spectrum allocations are

• Lower 700 MHz (US) with 2 × 6 MHz channels
• 2.5 GHz Multichannel Multipoint Distribution Service with 15.5 MHz in US and 72 MHz in Canada
• 3.5 GHz Wireless Local Loop with 2 × 2 MHz channel blocks
• 5.8 GHz UNI (license exempt) with 80 MHz allocation

WiMAX access networks are often deployed in point-to-multipoint cellular fashion where a single BS provides wireless coverage to a set of end users stations within the coverage area. The technology behind WiMAX has been optimized to provide both large coverage distances of up to 30 km under line-of-sight (LOS) situations and typical cell range of up to 8 km under NLOS. In an NLOS, a signal reaches the receiver through reflections, scattering, and diffractions. The signals arriving at the receiver consists of many components from direct and indirect paths with different delay spreads, attenuation, polarizations, and stability relative to the direct path. WiMAX technology solves or mitigates the problem resulting from NLOS conditions by using OFDMA, Subchannelization, directional antennas, transceiver diversity, adaptive modulation, error correction, and power control. The NLOS technology also reduces installation expenses by making the under-the-eaves customer premise equipment (CPE) installation a reality and easing the difficulty of locating adequate CPE mounting locations.

Both LOS and NLOS coverage conditions are governed by propagation characteristics of their environment, radio link budget, and path loss. In both the cases, relays help to extend the range of the BS footprint coverage allowing for a cost-efficient deployment and service.

TECHNOLOGIES EMPLOYED BY WiMAX

Mobile WiMAX operates in licensed frequency bands in the range of 2 to 6 MHz. The technologies employed by mobile WiMAX include the following:

• Scalable orthogonal frequency division multiple access (SOFDMA) on the physical layer
• MIMO
• IP
• Adaptive antenna systems (AAS)
• Adaptive modulation schemes (AMS)
• Advanced encryption standard (AES) encryption
  
  

PHYSICAL LAYER

Mobile WiMAX will initially operate in the 2.3, 2.5, 3.3, and 3.4–3.8 GHz spectrum bands using SOFDMA. OFDMA is perhaps the most important technology associated with WiMAX. SOFDMA is based on OFDMA which in turn is based on OFDM. OFDM is a form of frequency division multiplexing, but it has higher spectral efficiency and resistance to multipath fading and path loss compared to other multiplexing methods. It divides the allocated frequency spectrum into subcarriers which are at right angles to each other. This reduces the possibility of cross-channel interference thereby allowing the subcarriers to overlap. This reduces the amount of frequency spectrum required, hence the high spectral efficiency. The reduced data rate of each stream reduces the possibility of intersymbol interference because there is more time between the arrival of symbols from different paths. This feature of OFDM makes it resistant to multipath fading and ideal for nonline of sight (NLOS) applications. In OFDMA each frequency subcarrier is divided into subchannels which can be accessed by multiple users hence increasing the capacity of OFDM.

Scalable OFDMA is a form of OFDMA which allows variable channel bandwidth allocation from 1.25 to 20 MHz. SOFDMA has capabilities which make it ideal for the implementation of IP and hybrid automatic repeat request (HARQ). WiMAX also uses other features to enhance the performance of OFDMA. They include dynamic frequency shifting, MIMO, AAS, and software-defined radios. Dynamic frequency shifting monitors the signal and changes frequencies to avoid interference. Software-defined radios are controlled by changing software settings and this gives the equipment more flexibility when switching frequencies.

MIMO is a technology that has already found use in WiFi (IEEE 802.11n). MIMO multiplies the point-to-point spectral efficiency by using multiple antennas and RF chains at both the BS and the MS. MIMO achieves a multiplicative increase in throughput compared to single-input, single-output (SISO) architecture by carefully coding the transmitted signal across antennas, OFDM symbols, and frequency tones. These gains are achieved at no cost in bandwidth or transmit power.

AAS are spatial processing systems which combine antenna arrays with sophisticated signal processing. They reduce the effects of interference from multiple signal paths thereby also contributing to high capacity of the system and the use of mobile WiMAX in NLOS environments.

MAC SUBLAYER

The 802.16 MAC sublayer uses a scheduling algorithm for which the subscriber station only needs to compete for initial entry into the network. The scheduling algorithm also allows the BS to control QoS parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

WiMAX supports QoS differentiation for different types of applications. The 802.16 standard defines the following types of services:

• Unsolicited grant services (UGS): UGS is designed to support constant bit rate (CBR) services, such as T1/E1 emulation, and Voice-over-IP (VoIP) without silence suppression.
• Real-time polling services (rtPS): rtPS is designed to support real-time services that generate variable size data packets on a periodic basis, such as MPEG video or VoIP with silence suppression.
• Nonreal-time polling services (nrtPS): nrtPS is designed to support nonreal-time services that require variable size data grant burst types on a regular basis.
• Best effort (BE) services: BE services are typically provided by the Internet today for Web surfing.

CORE SERVICES NETWORK | WIMAX NETWORK ARCHITECTURE

The CSN is the transport, authentication, and switching part of the network. It represents the core network in WiMAX. It consists of the home agent (HA) and the AAA system and also contains the IP servers, gateways to other networks, i.e., public switched telephone network (PSTN), and 3G.

WiMAX has five main open interfaces which include reference points R1, R2, R3, R4, and R5 interface. The R1 interface interconnects the subscriber to the BS in the ASN and is the air interface defined on the physical layer and Medium Access Control (MAC) sublayer. The R2 is the logical interface between the mobile subscriber and the CSN. It is associated with authorization, IP host configuration management, services management, and mobility management. The R3 is the interface between the ASN and CSN and supports AAA, policy enforcement, and mobility management capabilities. The R4 is an interface between two ASNs. It is mainly concerned with coordinating mobility of MSs between different ASNs. The R5 is an interface between two CSNs and is concerned with internetworking between two CSNs. It is through this interface that activities such as roaming are carried out.

The unbundling of WiMAX divides the network based on functionality. The ASN falls under the network access provider (NAP). The NAP is a business entity that provides WiMAX network access to a network service provider (NSP). The NSP is a business entity that provides core network services to the WiMAX network and consists of the CSN. The Applications services fall under the applications service provider (ASP).

If network operator wants to reap the full benefits that WiMAX and its all-IP architecture can deliver, they need to carefully select the ASN and CSN solutions that best suit their requirements and provide all the functionality required while avoiding unnecessary complexity in their network.