OVERVIEW OF IEEE 802.16J

In IEEE 802.16j low cost RSs are introduced to provide enhanced coverage and capacity. Using such stations, an operator could deploy a network with wide coverage at a lower cost than using only (more) expensive BSs to provide good coverage, and increasing significantly the system throughput. As network utilization increases, these RSs could be replaced by BSs as required. The mesh architecture defined in WiMAX is already used to increase the coverage and the throughput of the system. However, this mode is not compatible with the point-to-multipoint (PMP) mode with no support of the OFDMA PHY, fast route change for mobile station (MS), etc. Hence, the standards organization has recognized this as an important area of development, and today a task group is charged with drafting a new standard: the IEEE 802.16j mobile multihop relay design to address these issues. The first draft of the IEEE 802.16j standard has just finished in August 2007.

IEEE 802.16J SCOPE

The IEEE 802.16j is aiming to develop a relay mode based on IEEE 802.16e by introducing RSs depending on the usage model:

• Coverage extension
• Capacity enhancement

In other words, the relay technology is first expected to improve the coverage reliability in geographic areas that are severely shadowed from the BS or to extend the range of a BS. In both cases, the RS enhances coverage by transmitting from an advantageous location closer to a disadvantaged SS than the BS. Second, it is expected to improve the throughput for users at the edges of an 802.16 cell. It has been recognized in previous 802.16 contributions that subscribers at the edges of a cell may be required to communicate at reduced rates. This is because received signal strength is lower at the cell edge. Finally, it is expected to increase system capacity by deploying RSs in a manner that enables more aggressive frequency reuse. Figure 1 illustrates the different scenarios in which relay mode could be used. However, introducing such RSs considerably alters the architecture of the network and raises many issues and questions. It is still unclear what system design is appropriate and can be realized at a low cost while still providing good coverage with an enhancement of the throughput.

 
Figure 1: IEEE 802.16j example use cases.

The 802.16j task group’s scope is to specify OFDMA PHY and MAC enhancement to the IEEE 802.16 standards for licensed bands. These specifications aim to enable the operation of fixed, nomadic, and mobile RSs by keeping the backward compatibility with SS/MS. In other words, the standard will define a new RS entity and modify the BS to support Mobile Multihop Relay (MMR) links and aggregation of traffic from multiple sources. An MMR link represents a radio link between an MMR-BS and an RS or between a pair of RSs. Such link can support fixed, portable, and mobile RSs and multihop communications between a BS and RSs on the path. An access link is a radio link that originates or terminates at an SS/MS. Table 1 illustrates the main scope of the project.

Table 1: IEEE 802.16j Project Scope 

		Define New
No Change	Changes to BS	RS Entity	“802.16j Relay” Link Air Interface
To SS/MS To 802.16e OFDMA PMP link	Add support for MMR links Add support for aggregation of traffic from multiple RSs	Supports PMP links Supports MMR links Supports aggregation of traffic from multiple RSs	Support fixed, portable, and mobile RSs Based on OFDMA PHY MAC to support multi-hop communication Security and management

TECHNOECONOMICS OF DIMENSIONING

As already stated several times, the business plan and the dimensioning strategy are the main factors that affect the network size and overall investment. On top of the access network, a backhaul network should be implemented to connect the access with the core network, and there is also the core network infrastructure. The overall equipment depends on the number of PoP, almost in a linear manner. Further to the equipment costs, the deployment engineering costs should be also taken into consideration.

COST INCREASING FACTORS DURING DIMENSIONING

The dimensioning output may lead to an oversized/undersized network mainly for two reasons: either due to the business plan or due to a low quality study. In the first case it is the responsibility of the author of the business plan if it is not so realistic, while in the second case it is the responsibity of the designer if a study outcome is of low accuracy. The main parameters that impact the network size and hence the costs can be seen in Table 1. An ambitious business plan will most likely lead to a significant investment. However, in terms of network design and implementation, a huge network increases design complexity which results in longer time-to-market. A scalable deployment is preferable since a higher design quality can be obtained. In contrast, lack of scalability would further extend the transition period until the network implementation is finalized. The costs and complexity also depend on the number of PoP. If the distance between PoP is very small (i.e., less than 0.5 km) then the design complexity is increased significantly. For capacity-limited networks that require too many sectors it would be cost effective to use higher sectorization per BS or a dual sector layer if possible. Finally it should be mentioned that for areas under heavy construction the expected terrain changes should be considered since they may result in a long design and implementation period. Any condition that would result in longer engineering times (design, implementation) would also further increase costs. Furthermore, terrain changes may alter the coverage and hence a revision may be necessary after a period.

Table 1: Dimensioning Size and Cost Increasing Factors 

Factor	CapEX/OpEX	Complexity	Time-to-Market
Ambitious business plan	Very high	Very high	Very long
Unbalanced number of PoP	High	High	Long
Luck of scalability strategy	High	Very high	Long transition
Environmental changes	High	High	Long transition

SCALABILITY | STRATEGY INTERACTION

As mentioned previously, the most significant advantage of WiMAX is the flexibility and scalability of the air-interface. Therefore the concept “pay as you grow” is definitely applicable in WiMAX commercial networks. Scalability allows the reduction of initial investment and risks, and thereafter the network is expanded based on the market penetration and revenues. The majority of business plans provide the subscriber numbers and services per deployment year and in this context the dimensioning should be presented according to the scalability plan (network size/performance per year). The complexity in this case is not the increase of subscriber numbers or service area size, but when in parallel the coverage type is upgraded or new products (based on recently released standards) have to coexist with deployed equipment. The main types of scalability plan that have to be considered during dimensioning are presented as follows:

• Network expansion: Expansion may be in terms of subscriber numbers, service rates, or service areas or a combination and involves the deployment of new PoP or addition of sectors in existing PoP. A great challenge, in such scenario, is to optimize the positioning of PoP to achieve the best coverage and capacity outcome. It is more appropriate to determine the network size and PoP positions for the last deployment year, and then deploy only a subset of PoP that would satisfy the objectives of the initial phase.
• Coverage upgrade: It is common to allow the use of more demanding terminal profiles (i.e., nomadic/mobile) in the network after the first year so as to allow time to test the performance of the wireless network. In this case, not only the network is expanded from first to second year, but also the coverage should be upgraded too. The same approach “design for the future, deploy for present” as above should be applied, and the only difference is that a more dense network is probably required.
• Technology upgrade: The major change in terms of equipment is between the IEEE 801.16-2004 and 802.16e standards. The new products are based on software defined radio technology and therefore future standard releases will probably be implemented with minor changes. It is quite challenging to upgrade an existing fixed WiMAX network to coexist with mobile WiMAX, unless there is provision for additional spectrum. The major challenge is to replace subscriber equipment and restore access and it is likely that this transition phase will take a long time.