WiMAX - Engineering Seminar Paper

ABSTRACT

New and increasingly advanced data services are driving up wireless traffic, which is being further boosted by growth in voice applications in advanced market segments as the migration from fixed to mobile voice continues. This is already putting pressure on some networks and may be leading to difficulties in maintaining acceptable levels of service to subscribers.

For the past few decades the lower band width applications are growing but the growth of broad band data applications is slow. Hence we require technology which helps in the growth of the broad band data applications. WiMAX is such a technology which helps in point-to-multipoint broadband wireless access with out the need of direct line of sight connectivity with base station.

This paper explains about the WiMAX technology, its additional features in physical layer and MAC layer and the benefits of each feature.

This paper focuses on the major technical comparisons (like QOS and coverage) between WiMAX and other technologies. It also explains about the ability of the WiMAX to provide efficient service in multipath environment.

   

Introduction:

For the past couple decades, low-bandwidth applications such as downloading ring tones and SMS are experiencing sharp growth, but the growth of broadband data applications such as email and downloading/ uploading files with a laptop computer or PDA has been slow. The demand for broadband access continues to escalate worldwide and lower-bandwidth wire line methods have failed to satisfy the need for higher bandwidth integrated data and voice services. WiMAX is radio technology that promises two-way Internet access at several megabits per second with ranges of several miles. It is believed that the technology can challenge DSL (Digital Subscriber Line) and cable broadband services because it offers similar speeds but is less expensive to set up. The intention for WiMAX is to provide fixed, nomadic, portable and, eventually, Mobile wireless broadband connectivity without the need for Direct line-of-sight with a base station.

What is wimax?

WiMAX is an acronym that stands for “Worldwide Interoperability for Microwave Access”. IEEE 802.16 is working group number 16 of IEEE 802, specializing in point-to-multipoint broadband wireless access. It also is known as WiMAX. There are at least four 802.16 standards: 802.16, 802.16a, 802.16-2004 (802.16), and 802.16e.

WiMAX does not conflict with WiFi but actually complements it.  WiMAX is a wireless metropolitan area network (MAN) technology that will connect IEEE 802.11 (WiFi) hotspots to the Internet and provide a wireless extension to cable and DSL for last km broadband access. IEEE 802.16 provides up to 50 km of linear   service   area range and allows user’s connectivity without a direct line of sight to a base station. The technology also provides shared data rates up to 70 Mbit/s.

The portable version of WiMAX, IEEE 802.16 utilizes Orthogonal Frequency Division Multiplexing Access (OFDM/OFDMA) where the spectrum is divided into many sub-carriers. Each sub-carrier then uses QPSK or QAM for modulation. WiMAX standard relies mainly on spectrum in the 2 to 11 GHz range. The WiMAX specification improves upon many of the limitations of the WiFi standard by providing increased bandwidth and stronger encryption

For years, the wildly successful 802.11 x or WiFi wireless LAN technology has been used in BWA applications. When the WLAN technology was examined closely, it was evident that the overall design and feature set available was not well suited for outdoor Broadband wireless access (BWA) applications. WiMAX is suited for both indoor and outdoor BWA; hence it solves the major problem.

In reviewing the standard, the technical details and features that differentiate WiMAX certified equipment from WiFi or other technologies can best be illustrated by focusing on the two layers addressed in the standard, the physical (PHY) and the media access control (MAC) layer design.

WIMAX PHY Layer

 The first version of the 802.16 standard released addressed Line-of-Sight (LOS) environments at high frequency bands operating in the 10-66 GHz range, whereas the recently adopted amendment, the 802.16a standard, is designed for systems operating in bands between 2 GHz and 11 GHz. The significant difference between these two frequency bands lies in the ability to support Non-Line -of-Sight (NLOS) operation in the lower frequencies, something that is not possible in higher bands. Consequently, the 802.16a amendment to the standard opened up the opportunity for major changes to the PHY layer specifications specifically to address the needs of the 2-11 GHz bands. This is achieved through the introduction of three new PHY-layer specifications (a new Single Carrier PHY, a 256 point FFT OFDM PHY, and a 2048 point FFT OFDMA PHY);

  Some of the other PHY layer features of 802.16a that are instrumental in giving this technology the power to deliver robust performance in a broad range of channel environments are; flexible channel widths, adaptive burst profiles, forward error correction with concatenated Reed-Solomon and convolutional encoding, optional AAS (advanced antenna systems) to improve range/capacity, DFS (dynamic frequency selection)-which helps in minimizing interference, and STC (space-time coding) to enhance performance in fading environments through spatial diversity. Table 1 gives a high level overview of some of the PHY layer features of the IEEE 802.16a standard.

 b) IEEE 802.16a MAC Layer

The 802.16a standard uses a slotted TDMA protocol scheduled by the base station to allocate capacity to subscribers in a point-to-multipoint network topology. By tarting with a TDMA approach with intelligent scheduling, WiMAX systems will be able to deliver not only high speed data with SLAs, but latency sensitive services such as voice and video or database access are also supported. The standard delivers QoS beyond mere prioritization, a technique that is very limited in effectiveness as traffic load and the number of subscriber’s increases. The MAC layer in WiMAX certified systems has also been designed to address the harsh physical layer environment where interference, fast fading and other phenomena are prevalent in outdoor operation.

WiMAX Scalability:

At the PHY layer the standard supports flexible RF channel bandwidths and reuse of these channels (frequency reuse) as a way to increase cell capacity as the network grows. The standard also specifies support for automatic transmit power control and channel quality measurements as additional PHY layer tools to support cell planning/deployment and efficient spectrum use. Operators can re-allocate spectrum through sectorization and cell splitting as the number of subscribers grows.

In the MAC layer, the CSMA/CA foundation of 802.11, basically a wireless Ethernet protocol, scales about as well as does Ethernet. That is to say - poorly. Just as in an Ethernet LAN, more users results in a geometric reduction of throughput, so does the CSMA/CA MAC for WLANs. In contrast the MAC layer in the 802.16 standard has been designed to scale from one up to 100's of users within one RF channel, a feat the 802.11 MAC was never designed for and is incapable of supporting.

a) Coverage:

The BWA standard is designed for optimal performance in all types of propagation environments, including LOS, near LOS and NLOS environments, and delivers reliable robust performance even in cases where extreme link pathologies have been introduced. The robust OFDM waveform supports high spectral efficiency over ranges from 2 to 40 kilometers with up to 70 Mbps in a single RF channel. Advanced topologies (mesh networks) and antenna techniques (beam-forming, STC, antenna diversity) can be employed to improve coverage even further. These advanced techniques can also be used to increase spectral efficiency, capacity, reuse, and average and peak throughput per RF channel. In addition, not all OFDM is the same. The OFDM designed for BWA has in it the ability to support longer range transmissions and the multi-path or reflections encountered. In contrast, WLANs and 802.11 systems have at their core either a basic CDMA approach or use OFDM with a much different design, and have as a requirement low power consumption limiting the range. OFDM in the WLAN was created with the vision of the systems covering tens and maybe a few hundreds of meters versus 802.16 which is designed for higher power and an OFDM approach that supports deployments in the tens of kilometers.

b) Quality of service:

The 802.16a MAC relies on a Grant/Request protocol for access to the medium and it supports differentiated service The protocol employs TDM data streams on the DL (downlink) and TDMA on the UL (uplink), with the hooks for a centralized scheduler to support delay-sensitive services like voice and video. By assuring collision-free data access to the channel, the 16a MAC improves total system throughput and bandwidth efficiency, in comparison with contention-based access techniques like the CSMA-CA protocol used in WLANs. The 16a MAC also assures bounded delay on the data. The TDM/TDMA access technique also ensures easier support for multicast and broadcast services. With a CSMA/CA approach at its core, WLANs in their current implementation will never be able to deliver the QoS of a BWA, 802.16 systems.

ROLE OF ‘OFDMA’ IN MULTIPATH ENIRONMENT:

Technologies using DSSS (802.11b, CDMA) and other wide band technologies are very susceptible to multipath fading, since the delay time can easily exceed the symbol duration, which causes the symbols to completely overlap (ISI). The use of several parallel sub-carriers for OFDMA enables much longer symbol duration, which makes the signal more robust to multipath time dispersion

a).Multipath: Frequency Selective Fading

                   This type of fading affects certain frequencies of a transmission and can result in deep fading at certain frequencies. One reason this occurs is because of the wide band nature of the signals. When a signal is reflected off a surface, different frequencies will reflect in different ways. In Figure below, both CDMA (left) and OFDMA (right) experience selective fading near the center of the band. With optimal channel coding and interleaving, these errors can be corrected. CDMA tries to overcome this by spreading the signal out and then equalizing the whole signal. OFDMA is therefore much more resilient to frequency selective fading when compared to CDMA.

OFDMA with Adaptive Modulation and Coding (AMC):

Both W-CDMA (HSDPA) and OFDM utilize Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM). It should be noted here that for WCDMA, AMC is only used on the downlink, since the uplink still relies on WCDMA which uses QPSK but not QAM. Modulation and coding rates can be changed to achieve higher throughput, but higher order modulation will require better Signal to Noise Ratio. Figure illustrates how higher order modulations like QAM 64 are used closer to the base station, while lower order modulations like QPSK are used to extend the range of the base station . Performance results conducted for one of the 3GPP Working Groups [2], show that while OFDM is able to achieve the maximum throughput of 9.6 Mbps (16QAM), WCDMA does not exceed 3 Mbps. From these results, it appears that even higher discrepancy may be found when utilizing higher modulation and code rates to yield even higher throughput for OFDM.

Adaptive Modulation and Coding (AMC) in a multipath environment may give OFDMA further advantages since the flexibility to change the modulation for specific sub-channels allows you to optimize at the frequency level. Another alternative would be to assign those sub channels to a different user who may have better channel conditions for that particular sub-channel. This could allow users to concentrate transmit power on specific sub-channels, resulting in improvements to the uplink budget and providing greater range. This technique is known as Space Division Multiple Access (SDMA).

With OFDMA, the client device could choose sub channels based on geographical locations with the potential of eliminating the impact of deep fades. CDMA-based technologies utilize the same frequency band regardless of where the user is.

ADVANCED RADIO TECHNIQUES:

a) Transmit and receive diversity schemes:

Transmit and Receive Diversity schemes are used to take advantage of multipath and reflected signals that occur in NLOS environments. By utilizing multiple antennas (transmit and/or receive), fading, interference and path loss can be reduced. The OFDMA transmit diversity option uses space time coding. For receive diversity, techniques such as maximum ratio combining (MRC) take advantage of two separate receive paths.

 b)  Smart Antenna Technology:

Adaptive antenna systems (AAS) are an optional part of the 802.16 standard. AAS equipped base stations can create beams that can be steered, focusing the transmit energy to achieve greater range as shown in the figure. When receiving, they can focus in the particular direction of the receiver. This helps eliminate unwanted interference from other locations.

802.16  is a group of broadband wireless communications standards for metropolitan area networks (MANs) developed by a working group of the Institute of Electrical and Electronics Engineers (IEEE). The original 802.16 standard, published in December 2001, specified fixed point-to-multipoint broadband wireless systems operating in the 10-66 GHz licensed spectrum. An amendment, 802.16a, approved in January 2003, specified non-line-of-sight extensions in the 2-11 GHz spectrum, delivering up to 70 Mbps at distances up to 31 miles. Officially called the WirelessMAN™ specification, 802.16 standards are expected to enable multimedia applications with wireless connection and, with a range of up to 30 miles, provide a viable last mile technology.

An earlier group of IEEE standards, the 802.11 specifications, provide a wireless alternative to Ethernet LANs (local area networks); 802.16 standards are expected to complement these by enabling a wireless alternative to expensive T1 links connecting offices to each other and the Internet. Although the first amendments to the standard are only for fixed wireless connections, a further amendment, 802.16e, is expected to enable connections for mobile devices.

A coalition of wireless industry companies, including Intel, Proxim and Nokia, banded together in April 2001 to form WiMAX, an 802.16 advocacy group. The organization's purpose is to actively promote and certify compatibility and interoperability of devices based on the 802.16 specification, and to develop such devices for the marketplace. According to the WiMAX Forum, the first products based on 802.16 technology are expected to hit the market in 2004.

Think about how you access the Internet today. There are basically three different options:

· Broadband access - In your home, you have either a DSL orcable modem. At the office, your company may be using a T1or a T3 line.

· WiFi access - In your home, you may have set up a WiFirouter that lets you surf the Web while you lounge with yourlaptop. On the road, you can find WiFi hot spots in restaurants, hotels, coffee shops and libraries.

· Dial-up access - If you are still using dial-up, chances are that either broadband access is not available, or you think that broadband access is too expensive.

The main problems with broadband access are that it is pretty expensive and it doesn't reach all areas. The main problem with WiFi access is that hot spots are very small, so coverage is sparse.

What if there were a new technology that solved all of these problems? This new technology would provide:

· The high speed of broadband service

· Wireless rather than wired access, so it would be a lot less expensive than cable or DSL and much easier to extend to suburban and rural areas

· Broad coverage like the cell phone network instead of small WiFi hotspots

This system is actually coming into being right now, and it is calledWiMAX. WiMAX is short for Worldwide Interoperability for Microwave Access, and it also goes by the IEEE name 802.16.

WiMAX has the potential to do to broadband Internet access what cell phones have done to phone access. In the same way that many people have given up their "land lines" in favor of cell phones, WiMAX could replace cable and DSL services, providing universal Internet access just about anywhere you go. WiMAX will also be as painless as WiFi -- turning your computer on will automatically connect you to the closest available WiMAX antenna.

WiMAX and the IEEE 802.16 Standard

The current WiMAX incarnation, Mobile WiMAX, is based upon IEEE Std 802.16e-2005, approved in December 2005. It is a supplement to the IEEE Std 802.16-2004, and so the actual standard is 802.16-2004 as amended by 802.16e-2005 — the specifications need to be read together to understand them.

IEEE 802.16e-2005 improves upon IEEE 802.16-2004 by:

· Adding support for mobility (soft and hard handover between base stations). This is seen as one of the most important aspects of 802.16e-2005, and is the very basis of 'Mobile WiMAX'.

· Scaling of the Fast Fourier transform (FFT) to the channel bandwidth in order to keep the carrier spacing constant across different channel bandwidths (typically 1.25 MHz, 5 MHz, 10 MHz or 20 MHz). Constant carrier spacing results in a higher spectrum efficiency in wide channels, and a cost reduction in narrow channels. Also known as Scalable OFDMA (SOFDMA). Other bands not multiples of 1.25 MHz are defined in the standard, but because the allowed FFT subcarrier numbers are only 128, 512, 1024 and 2048, other frequency bands will not have exactly the same carrier spacing, which might not be optimal for implementations.

· Advanced antenna diversity schemes, and hybrid automatic repeat-request (HARQ)

· Adaptive Antenna Systems (AAS) and MIMO technology

· Denser sub-channelization, thereby improving indoor penetration

· Introducing Turbo Coding and Low-Density Parity Check (LDPC)

· Introducing downlink sub-channelization, allowing administrators to trade coverage for capacity or vice versa

· Fast Fourier transform algorithm

· Adding an extra QoS class for VoIP applications.

SOFDMA (used in 802.16e-2005) and OFDM256 (802.16d) are not compatible thus equipment will have to be replaced if an operator is to move to the later standard (eg, Fixed WiMAX to Mobile WiMAX).

Physical layer

The original version of the standard on which WiMAX is based (IEEE 802.16) specified a physical layer operating in the 10 to 66 GHz range. 802.16a, updated in 2004 to 802.16-2004, added specifications for the 2 to 11 GHz range. 802.16-2004 was updated by 802.16e-2005 in 2005 and uses scalable orthogonal frequency-division multiple access (SOFDMA) as opposed to the fixed orthogonal frequency-division multiplexing (OFDM) version with 256 sub-carriers (of which 200 are used) in 802.16d. More advanced versions, including 802.16e, also bring multiple antenna support through MIMO (See WiMAX MIMO). This brings potential benefits in terms of coverage, self installation, power consumption, frequency re-use and bandwidth efficiency.

MAC (data link) layer

The WiMAX MAC uses a scheduling algorithm for which the subscriber station needs to compete only once for initial entry into the network. After network entry is allowed, the subscriber station is allocated an access slot by the base station. The time slot can enlarge and contract, but remains assigned to the subscriber station, which means that other subscribers cannot use it. In addition to being stable under overload and over-subscription, the scheduling algorithm can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control Quality of service (QoS) parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

Conclusion:

Thus WiMAX systems for portable/nomadic use will have better performance, interference rejection, multipath tolerance, high data quality of service support (data oriented MAC, symmetric link) and lower future equipment costs i.e., low chipset complexity, high spectral efficiencies. And hence WiMAX can complement existing and emerging 3G mobile and wireline networks, and play a significant role in helping service provides deliver converged service offerings