Spectrum Utilization and Interface Analysis of WiMAX Broadband Wireless Access

fly

Spectrum Utilization and Interface Analysis of WiMAX Broadband Wireless Access [Copy link]

This article discusses the implementation of IEEE 802.16a/d/e WirelessMAN, where spectrum management organizations around the world have designated frequency bands for broadband deployment, both licensed and unlicensed. While discussing power issues and future spectrum allocations, this article also discusses the RF front-end to baseband (SoC) interface issues.

While Wi-Fi wireless local area networks (WLANs) are taking off quickly with the help of the IEEE 802.11 standard, the newly emerged MAN based on the IEEE 802.16 wireless metropolitan area network (MAN) standard is also about to start up quickly. In terms of spectrum, IEEE 802.16a/d/e WirelessMAN is also called WiMAX (Worldwide Interoperability for Microwave Access). While 802.16x sets the standardization and interoperability specifications, the WiMAX Forum, the global broadband wireless access (BWA) industry association provides quality control and certification to ensure successful standardized deployment. The primary mission of the WiMAX Forum is to unite a large number of global participants, including chip manufacturers, software developers, equipment manufacturers and service providers to support the IEEE WirelessMAN/ETSI HyperMAN standards and ensure global compatibility and interoperability, but also create a competitive field to reduce costs for service providers and users. IEEE 802.16 and WiMAX will advance BWA to accelerate the deployment of affordable global broadband networks.

Figure 1: Basic block diagram of a WiMAX subscriber station -
different RF front ends provide frequency band flexibility.

However, standardization does not mean global "consistency" and automatic interoperability of "all" deployed certified WiMAX devices. The standard defines and recommends key frameworks for the Media Access Control (MAC) layer, which packages or unpacks raw data according to standard protocols to provide data, voice and video, and the Physical Layer (PHY), which handles the air interface and modulation schemes based on user needs and the quality of the Radio Frequency (RF) connection. The IEEE 802.16 standard produces a variety of frameworks, but also allows providers to customize to meet specific or regional market needs, or to allow providers to differentiate themselves from competitors by adding value-added features.

Furthermore, there are variations in RF interfaces in different regions around the world. In this regard, spectrum managers, such as the Federal Communications Commission (FCC) in the United States, play a key role in determining the allocation of spectrum for different, sometimes competing services. Through these managers, governments can make certain spectrum available for a given service that is consistent or inconsistent with the rest of the world. This is also true for global deployments of WiMAX, and while there are indeed some very common RF issues, there is also a great deal of diversity in the allocation and management of spectrum.

However, it is not just the management issues that cause the RF band differences in the global deployment of WiMAX wireless MAN. Service carriers and wireless Internet service providers (WISPs) in a region also have the problem of band selection. The available and allocated spectrum includes different licensed and unlicensed (licensed-free) frequency bands. Service carriers can choose to use the licensed spectrum to provide services and/or choose to use unlicensed spectrum. Most WISPs choose to use unlicensed spectrum because it is free to use, which greatly reduces the cost to end users.

The spectrum diversity of WiMAX wireless metropolitan area network deployments has led to the need for base stations (BS) and subscriber stations (SS) with different RF. As shown in Figure 1, a typical WiMAX SS system includes a control processor, a MAC unit, a baseband processor (BBP), and an analog RF front end, which is responsible for putting 802.16x into a licensed or unlicensed frequency band. Equipment providers expect chip manufacturers to provide complete reference designs, bill of materials, components, software/firmware, and technical support so that they can quickly produce WiMAX equipment to meet these RF-differentiated market needs. This interface that serves a specific frequency band is the RF front end.

Focus on 802.16d - Beyond Line-of-Sight Point-to-Multipoint Spectrum for PMP Broadband Network Connections

WiMAX-certified BWA applications include cellular network backhaul, wired and wireless LAN backhaul, and wireless MAN, which brings BWA to homes or businesses as an alternative to DSL or cable access. However, the biggest explosive market growth will be when future versions of 802.16x solve the portability and mobility issues and bring BWA directly to end users. This "last mile" must be a point-to-multipoint structure that uses beyond-line-of-sight (NLOS) RF propagation. In this area, WiMAX networks will appear around the world in licensed and unlicensed frequency bands, replacing existing services before 802.16 in many cases.

Currently, people are focusing on the frequency bands between 2 GHz and 6 GHz. These are allocated bandwidths that are narrow relative to the available bandwidths between 10 GHz and 66 GHz. Microwave frequency bands below 10 GHz are called centimeter-wave bands, and those above 10 GHz are called millimeter-wave bands. Millimeter-wave bands have very wide channel bandwidths and offer large data capacities, so they are generally well suited for very high data rate line-of-sight backhaul applications (main ducts), while centimeter-wave bands are well suited for multipoint, beyond-line-of-sight, feeder, and last-mile distribution applications.

IEEE 802.16d supports fixed NLOS BWA to replace or supplement DSL and cable access in the last mile, which is the first wave of WiMAX deployment. Further, IEEE 802.16e will be approved in 2005, which will add mobile and portable features to support applications such as notebooks and PDAs in frequencies below 6GHz. In these deployments, both licensed and unlicensed spectrum will be utilized.

Licensed and Unlicensed Spectrum

Figure 2 shows the frequency bands available for BWA in the 2 GHz to 6 GHz frequency range. Note that these bands are labeled as licensed or unlicensed. Licensed bands are those owned by carriers who have paid for the use of these bands, while unlicensed bands are free for any experimental or enterprise applications. Wi-Fi based on IEEE 802.11a/b/g occupies unlicensed bands and has proven to be very stable despite the presence of competing technologies in these bands. Within each band, the channel spacing is relatively narrow, thus limiting the data rate relative to the channels in the higher frequency micron bands.

Many wireless ISPs seek to utilize unlicensed spectrum because it is free, saving not only the cost and time of deploying a local network, but also the cost to the user and providing an alternative to DSL and cable modem services. In the United States, unlicensed spectrum is also attractive because there is not much licensed spectrum available in the 2GHz-6GHz frequency range. On the other hand, major carriers that own licensed spectrum can market their services as "business-grade" because they are considered stable and reliable and enjoy the reputation of major brands.

Frequency band allocation

3.5GHz -- The 3.5GHz band is a licensed spectrum that can be used for BWA in many European and Asian countries, but not in the United States. This band is the most crowded and represents the largest global BWA market, covering 300MHz bandwidth from 3.3GHz to 3.6GHz. This band provides great flexibility for large-scale backhaul to wide area network (WAN) services. Major carriers use this licensed spectrum to provide competitive tariffs to users through economies of scale and low cost of WiMAX equipment.

5GHz U-NII & WRC Bands - The Unlicensed National Information Infrastructure (U-NII) bands have three major frequency bands: Low and Mid U-NII bands (5,150-5,350) (802.11a), WRC (new) (5,470-5,725), and Upper U-NII / ISM bands (5,725-5,850). Wi-Fi exists in the low and mid U-NII bands, which have proven to be suitable for BWA. The many overlapping 5GHz bands mark the growth of BWA around the world. The newly allocated World Radio Conference (WRC) 5,470-5,725MHz band has greatly increased the bandwidth of the unlicensed band. Most WiMAX operates in the upper U-NII 5,725-5,850MHz band because there is little competing traffic and interference in this band, which means that Wi-Fi and outdoor power are allowed in the 2 to 4 watt range, while the power in the low and mid U-NII bands is only 1W. Analysts and business people believe that WiMAX will have strong growth in unlicensed frequencies.

WCS - The two Wireless Communications Service (WCS) bands are two 15MHz frequency segments, 2,305MHz to 2,320MHz and 2,345MHz to 2,360MHz. The 25MHz frequency gap between the two is allocated for Digital Audio Radio Service (DARS), which creates potential interference issues caused by DARS terrestrial repeaters. Successful deployment in these bands will require very high frequency efficiency, such as Orthogonal Frequency Division Multiplexing (OFDM) used by Wi-Fi and WiMAX.

Figure 2: The 2 GHz to 6 GHz centimeter band can be used in BWA

2.4GHz ISM - The 2.4GHz Industrial, Scientific and Medical (ISM) band is unlicensed and provides approximately 80MHz of bandwidth for BWA deployment. Current Wi-Fi exists in this band and has proven to provide robust service for WLAN. Future WiMAX profiles that specify interoperable MAC and BBP requirements will bring both services together, providing complementary operation with wide-range mobility for users.

MMDS - The Multichannel Multipoint Distribution Service (MMDS) spectrum consists of 31 channels with 6MHz spacing in the range of 2,500MHz to 2,690MHz, and also includes the Instructional Television Fixed Service (ITFS). Due to its original purpose of educational television, this spectrum has not been fully utilized, and the US FCC has allocated this spectrum to BWA services. BWA providers gain access to this spectrum through FCC auctions and/or by leasing channels from ITFS. In the United States, Sprint and Nextel are the main spectrum owners here. Analysts expect that the BWA market will have a large growth in this frequency band in the next few years.

Due to the high growth and application potential, the WiMAX Forum focuses on its initial MMDS class establishment and certification work, the 3.5GHz licensed band and the unlicensed U-NII 5GHz band have less interference, can provide reasonable power levels and sufficient bandwidth. This helps to ensure the high growth rate of global WiMAX BWA services, because these bands represent the largest potential market and can achieve lower costs due to economies of scale.

Transmit and receive signal strength

The power level and power control of the transmitted and received signals are very important to the efficiency of any WiMAX system. The power level must be effectively managed to ensure stable communication and reduce potential interference. In addition, the power level is dynamically controlled for each user, depending on the user's specifications and distance to the BS.

As specified in the WiMAX standard, the receive power level is the same for the centimeter wave bands from 2GHz to 11GHz. The receiver must be able to accurately decode signals with a minimum channel power of -30dBm (1uW) and withstand strong signals of 0dBm (1mW) without damaging the front-end circuit. In addition, the Rx should be able to provide a minimum image suppression of 60dB. The WiMAX standard specifies that "the image suppression requirement must be included in all image conditions generated in the receiver RF and subsequent intermediate frequencies." Adherence to these requirements will ensure reliable operation under close and long-distance conditions.

Transmission requirements

Subscriber stations that do not utilize subchannels (single carrier) must provide power control with a 30 dB range. For SSs (OFDM) that utilize subchannels (such SSs would include all certified WiMAX SSs from 2 GHz to 11 GHz), the transmitter must have 50 dB of dynamic power control in steps of no less than 1-dB. Power control accuracy must be within +/-1.5 dB up to a 30 dB range, or +/-3 dB above 30 dB.

For the BS transmitter, the output power level must be controlled to not less than 10dB. The actual transmit power depends on the distance to the user, transmission characteristics, channel bandwidth and modulation scheme (BPSK, QPSK, 16QAM, 64QAM). BPSK is the modulation method with the lowest data efficiency and is used when the distance between the SS and the BS is very far, so a higher transmit power is required. 64QAM provides very high data efficiency (bits per symbol) and is used when the distance between the SS and the BS is relatively close, so a lower transmit power is required.

SoC and RF interface

Referring to Figure 1, the interface between the RF front end and the SoC involves handling the operation and internal management functions of the transmitter and receiver, as well as the interface control signals of the I/Q signals to the A/D and D/A data converters. The received data sent by the modulator circuit to the SoC should be differential "I" and "Q" signals. Attenuators can be used on the receiving end to handle calibration and gain control to ensure maximum bit utilization, conversion efficiency, and analog-to-digital converter (ADC) conversion efficiency.

Future spectrum of WiMAX

Currently, additional frequency bands are being considered for deployment of WiMAX and other similar broadband wireless access services around the world. In Japan, the 4.9GHz-5.0GHz frequency band will be used after 2007, and the 5.47GHz-5.725GHz frequency band is also being considered for future use. The former requires authorization when deploying the BS and will support 5MHz, 10MHz and 20MHz bandwidths, while the latter may not require authorization and may support 20MHz bandwidth.

The North American market is showing interest in deploying WiMAX in the 4.9GHz broadband public safety band. There is even interest in using the licensed 800MHz and unlicensed 915MHz ISM bands for deployment of WiMAX and similar services. The WiMAX standard will enable the long-awaited spectral efficiency and throughput required for user mobility, voice services, and high data rate applications. Due to its beyond-line-of-sight nature, it will allow for more user access, lower deployment costs, greater capacity, and penetration into the mass consumer market due to low CPE costs resulting from standardization and interoperability. Not to mention, it is a barrier-free path to broadband mobility that will be the foundation of "4G," providing true freedom of mobility.

Source: Fujitsu Microelectronics America