WiMAX Spectrum Overview and Possible Spectrum in China

WiMAX has developed system profiles covering the popular licence-exempted bands in 2.4 GHz and 5 GHz and other licensed bands in 2.3 GHz, 2.5 GHz and 3.5 GHz. At the moment, WiMAX will focus its conformance and interoperability test procedures on equipment that operates in 2.5 GHz and 3.5 GHz licensed bands and 5.8 GHz unlicensed band using 256 FFT OFDM modulation scheme. The flexible channel plan from 1.5 MHz to 20 MHz per channel will be adopted by WiMAX.

In China, Hong Kong is playing a pioneer role in WiMAX market. At present, radio equipment comply with the Exemption Order can be used in Hong Kong without a licence. WiMAX equipment can readily be operated in the license exempted 2.4 GHz and 5GHz bands provided that they meet the technical conditions given in the Exemption Order.

Some countries have already deployed wireless broadband services. In the U.S., the frequencies are in the 2.5- 2.7 GHz bands. Europe, Latin America and Asia including the Mainland have frequency allocations in the 3.5 GHz band. The actual frequency allocations are slightly different from each country, but most of them fall within the 3.4 – 3.6 GHz band.

There were discussions in the Radio Spectrum Advisory Committee (RSAC) in March 2004 on the frequency allocations for WiMAX systems in Hong Kong. In Mainland, the 3.4 –3.43 GHz / 3.5 –3.53 GHz band is opened for the implementation of BWA services. As the Mainland is a large market for BWA, Hong Kong will benefit if the spectrum allocation for BWA is harmonized with that adopted in the Mainland. OFTA is now considering the feasibility of opening more frequency bands to facilitate the future implementation of WiMAX application in Hong Kong.

The difference between Repeater and Relay Station..

The Relay Station nearly has all the functionality of a base station. In the following part, I will make some comparison between the Relay Station and the Repeater.

The advantages of using Repeater:

1. With the same coverage area, the investment of repeater is much lower.
2. The shape of coverage is more flexible. Normally a base station has circle coverage, while multiple repeaters can make multiple coverage shapes, such as "--", "L", "N" and "M".
3. In the beginning of forming the network, because the number of users is small and the benefit of investment is low, some repeaters can be used instead of base stations. When the users expand, they can be changed into base stations. And the replaced repeaters can be used in further places.
4. Because the building and transmission circuit are not needed, the network can be rapidly established.

The disadvantages of Repeater:

1. It cannot increase system capacity.
2. After the introduction of repeater, it will increase more than 3dB noise to base station. It makes the working environment of base station worse and decreases its coverage. So in a sector of one base station, the maximum of two repeaters can be used.
3. Repeater can only use frequency division, but not code division. One repeater usually amplifies the signal of multiple base stations and multiple sectors. If there are a lot of repeaters, it makes mess in base station phases and pilots. The improvement is hard and it increses a lot of unnecessary soft handovers.
4. The network management functionality and device detection is far from relay station. If there is something wrong with the repeater, it is not easily to be conscious.
5. Because of the limitation of separation requirement, the installation environment for a repeater is much more critical than a relay station. This makes its function not be sufficiently performed.
6. If there is interferencing source around the repeater, it will make severe influence on the original network. If the repeater antenna is high, it will effect larger area. CDMA is a co-frequency system, maybe all the base stations around it will be destroyed by congestion.

From some statistical results, in China, the installation ratio between repeaters and base stations is as high as or higher than 2:1. And because the people density of the country is quite high, the installation ratio should not be too large. If there is no fiber repeater, only for radio coupling outdoor repeater, this ratio should not be larger than 1. When it is the time for system plan, the repeaters work as transition devices for continuous development. The repeaters installed for long term use should be further decreased. In large or middle sized cities or high density areas, radio coupling outdoor repeaters should not be used.

On the other hand, because in CDMA system the frequency reuse is 1. So usage of the repeaters in CDMA system and the ones in GSM systems has difference. The use of repeaters has relation with the whole system performance. While in GSM sytem the use of repeaters only has relation with several corresponding channel performance. Therefore, proper plan of repeater network, strict project survey and implementation have much importance in improving CDMA network performance.

What is Repeater?

Nowadays the mobile communication is developing very repidly. No matter what kind of communication coverage, there may be weak signal and blind areas. While in some far away areas and blind areas with very few users, the cost of building analog or digital base station is very high and the basic installation is also quite complicated. Therefore it is necessary to provide a device with low cost, easy installation, but with the functionality of base station--Repeater. So mobile communication service providers start setting repeaters in radio blind areas such as inside the buildings, basement, etc., to satisfy the service requirements of users to the maximum extent.

In the beginning of formation of repeater market in China, radio device and FM broadcast repeaters were used. Since the later part of 90s', as the mobile phones were more and more popular, repeater market has been developed continuously. Repeater can enlarge the coverage of already built analog and digital base station and it is an efficient and unexpensive method to solve blind and remote area problems.

• Repeater definition

A repeater is an electronic device that receives a weak or low-level signal and retransmits it at a higher level or higher power with the same frequency, so that the signal can cover longer distances without degradation. To use repeater as "small capacity, large coverage" method is mainly because that with the condition of not increasing the number of base stations to ensure network cover and its cost is much lower than cell system with the same effect.

Compared with base station, it has the advantages of simpler architecture, lower cost an easier installation. It can be used in many blind and weak areas, such as shopping mall, hotel, airport, port, station, subway, tunnel, highway, island, etc., to improve communication quality.

• Repeater types:

· GSM mobile communication repeater
· CDMA mobile communication repeater
· GSM/CDMA fiber repeater

• Repeater applications:

-Urban area with high density: because of the large number of users and base stations, normally the large signal blind area problem does exit. Repeater is usually used to cover blind areas and the areas inside the building. Wireless repeaters are often used. With the increase of the number of buildings, the number of required repeaters increases. Then the situation of one base station with multiple repeaters appears. However, the introduction of repeater will definitely cause the interference to base station. And the interference will increase with the increase of the number of repeaters. Especially when large power repeater is introduced, the system interference is much more severe. Therefore low power repeater (less than 1W) should be applied.

-Edge of city: in the beginning of CDMA network establishment, because the small number of base stations, large power wireless or fiber repeaters are used. It is mainly to solve coverage problem. In the areas with fibers, it is better to use fiber repeater with 10W output power. When there is no fiber resource, wireless repeater with 5W/10W is used to extend coverage, which equals to a base station output.

-Suburban and village: it is also to solve coverage problem. In the areas with fiber, it is better to use large power fiber repeater (10W/20W) to enlarge coverage. In the areas where there is no fiber resource but can receive signal from base station, wireless repeater can be used to solve coverage problem. And frequency shift repeater can be utilized to increase coverage distance.

What is Relay Station (RS)?

Definition in Wikipedia:

Generally, an intermediate station that passes information between terminals or other relay stations.

Definition in IEEE802.16j Terminology:
Mobile Multihop Relay (MMR): The concept of relaying user data and possibly control information between an MMR base station and an IEEE Standard 802.16 compliant mobile station through one or more relay stations.

Informative Notes: Licensed spectrum is used for relay. The purpose of enabling relay is to enhance coverage, range, throughput, and capacity of an MMR–BS, and to enable very low power devices to participate in the network. The adjective “mobile” used here refers to the fact that both mobile subscriber stations and mobile relay stations are supported. It may be possible to establish multiple communication paths between an MMR-BS and a MS and communicate the same user data and/or control information through both paths to improve performance.

Why to develop IEEE802.16j?

Currently in the point-to-multipoint architecture, wireless communication easily has some hidden places. And the far away users' transmission speed decreases because of the drop of RSSI (Received Signal Strength Indication). IEEE 802.16 standard session started setting up Relay Task Group since the May of 2006 making the standard for supporting mobile user's multi-hop relay technology. The standard is designed to provide the signal relay between base station and mobile station to achieve the purposes of throughput enhancement and coverage extension. The IEEE802.16j is developed based on IEEE802.16e as its amendment.

• Scope

It specifies OFDMA physical layer and medium access control layer enhancements to IEEE Std 802.16 for licensed bands to enable the operation of relay stations. Subscriber station specifications are not changed.

• Purpose

The purpose of this amendment is to enhance coverage, throughput and system capacity of 802.16 networks by specifying 802.16 multihop relay capabilities and functionalities of interoperable relay stations and base stations.

• Reason

The multihop relay is a promising solution to expand coverage and to enhance throughput and system capacity for IEEE 802.16 system. It is expected that the complexity of relay stations will be considerably less than the complexity of legacy IEEE802.16 base station. The gains in coverage and throughput can be leveraged to reduce total deployment cost for a given system performance requirement and thereby improve the economic viability of IEEE802.16 systems. Relay functionality enables repid deployment and reduces the cost of system operation. These advantages will expand the market opportunity for broadband wireless access. This project aims to enable exploitation of such advantages by adding appropriate relay functionality to IEEE Std. 802.16 through the proposed amendment. Stakeholders include manufactures and operators of IEEE 802.16 networks.

WiMAX Forum and IEEE 802.16..

One of the main objectives of the WiMAX Forum is to create a single interoperable standard from the IEEE 802.16 and ETSI HiperMAN standards. This is achieved by the creation of System Profiles. Based upon what the WiMAX Forum sees in terms of service provider and vendor equipment plans, the WiMAX Forum has decided to focus first on profiles for the 256 OFDM PHY mode of the 802.16-2004 standard, which was ratified by the IEEE in June 2004. This physical layer (PHY) will be combined with a single media access controller (MAC), ensuring a uniform base for all WiMAX implementations.
Compliance with the 802.16 standard does not mean equipment is WiMAX Forum Certified™ or that it is interoperable with other vendors’ equipment. However, if a piece of equipment has earned the WiMAX Forum Certified™ designation, it is both compliant with the 802.16 standard and interoperable with other vendors’ equipment that is also WiMAX Forum Certified™.

IEEE 802.16a standardization focused on fixed broadband access. IEEE 802.16-2004 enhanced the standard by providing support for indoor CPE. The IEEE 802.16e standard is an extension to the approved IEEE 802.16-2004 standard. The purpose of 802.16e (also known as IEEE 802.16e-2005) is to add data mobility to the current standard, which is designed mainly for fixed operation.

IEEE approved the initial 802.16 standard for wireless MAN for the 10-66 GHz frequency range in December 2001. The 802.16a extension for sub-11 GHz was approved in January 2003. The 802.16-2004 standard was ratified by the IEEE in June 2004. The 802.16e-2005 standard was approved in December 2005.

The structure and process of WiMAX Forum certification is most like that of Wi-Fi Alliance, except that Wi-Fi Alliance only tests products for interoperability, whereas WiMAX Forum tests for both conformance to the technical standard and device interoperability. Testing conformance to the standard means that products just don’t work with each other out of luck, rather they are designed in a manner that allows them to implement the protocols in exactly the same way. This ensures that over time the products can be enhanced or new models can be issued with a higher likelihood of deployment in a common network.

What is WiMAX?

WiMAX (World Interoperability for Microwave Access), is a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL. WiMAX will provide fixed, nomadic, portable and mobile wireless broadband connectivity without the need for direct line-of-sight with a base station. In a typical cell radius deployment of 3 to 10 kilometers, WiMAX Forum Certified systems can be expected to deliver capacity of up to 40 Mbps per channel, for fixed and portable access applications. This is enough bandwidth to simultaneously support hundreds of businesses with T-1 speed connectivity and thousands of residences with DSL speed connectivity. Mobile network deployments are expected to provide up to 15 Mbps of capacity within a typical cell radius deployment of up to three kilometers. WiMAX technology already has been incorporated in notebook computers and PDAs, allowing for urban areas and cities to become “MetroZones” for portable outdoor broadband wireless access.

There are two main applications of WiMAX today:

Fixed WiMAX (IEEE802.16-2004) applications are point-to-multipoint enabling broadband access to homes and businesses, whereas mobile WiMAX (IEEE802.16e) offers the full mobility of cellular networks at true broadband speeds. Both fixed and mobile applications of WiMAX are engineered to help deliver ubiquitous, high-throughput broadband wireless services at a low cost.

Mobile WiMAX is based on OFDMA (Orthogonal Frequency Division Multiple Access) technology which has inherent advantages in throughput, latency, spectral efficiency, and advanced antenna support; ultimately enabling it to provide higher performance than today's wide area wireless technologies. Furthermore, many next generation 4G wireless technologies may evolve towards OFDMA and all IP-based networks as an ideal for delivering cost-effective wireless data services.