WiMAX Ing. Alessandro Leonardi
Content List Introduction System Architecture IEEE 802.16 standard Comparison with other technologies Conclusions
Introduction
Why WiMAX? (1/2) Main problems with actual standards supporting communications in Metropolitan Area Networks: Broadband wired access (DSL, T1 line) is pretty expensive and it doesn t reach all areas WiFi hot spots are very small, so coverage is sparse
Why WiMAX? (2/2) What if there were a new technology which provides: - The high speed of broadband service? - Wireless rather than wired access? - Broad coverage like the cell-phone network? This system is actually coming into being right now and it is called WiMAX
What is WiMAX? (1/2) A technology based on an evolving standard for point-to-multipoint wireless networking BWA (Broadband Wireless Access) solution for Wireless Metropolitan Area Network offering fast broadband connections over long distances Comply with European BWA standard IEEE 802.16/ETSI HiperMAN ( European Telecommunications Standards Institute's High-performance radio metropolitan area network )
What is WiMAX? (2/2) WiMAX is short for Worldwide Interoperability for Microwave Access: the forum comprises of industry leaders who are committed to the open interoperability of all products used for broadband wireless access The commercialization of IEEE 802.16 standard The technique or technology behind the standards is often referred as WiMAX
System Architecture
System Architecture (1/2) A WiMAX system consists of two parts: A WiMAX tower: similar in concept to a cell-phone tower can provide coverage to a very large area A WiMAX receiver: the receiver and antenna could be a small box or PCMCIA card, or they could be built into a laptop the way WiFi access is today
System Architecture (2/2)
WiMAX deployment types A WiMAX tower station can connect directly to the Internet using a high-bandwidth, wired connection for example, a T3 line) A WiMAX tower can also connect to another WiMAX tower using a line-of-sight (LOS) microwave link (this connection is often referred to as a backhaul)
WiMAX wireless services (1/3) WiMAX actually can provide two form of wireless services: Non-line-of-sight (NLOS), WiFi sort of service Line-of-sight (LOS) point-to-point service
WiMAX wireless services (2/3) Non-line-of-sight (NLOS) service : In this mode WiMAX uses a lower frequency range 2 GHz to 11 GHz. Lower-wavelength transmissions are not so easily disrupted by physical obstructions, they are better able to diffract, or bend, around obstacles. By virtue of NLOS propagation, 802.16a standard specifies a protocol that will enable a wireless alternative for cable, DSL and T1 level services for last mile broadband access.
WiMAX wireless services (3/3) Line-of-sight (LOS) service : In this mode a fixed dish antenna points straight at the WiMAX Tower. The line-of-sight connection is stronger and more stable, so it is able to send a lot of data with fewer errors and allows WiMAX to achieve its maximum range. Higher frequencies are used, with ranges reaching a possible 66 GHz. At higher frequencies, there is less interference and lots more bandwidth.
Network integration
WiMAX-WiFi integration The WiMAX base station would send data to a WiMAX-enabled router, which would then send the data to different computers on your local network (eventually a WiFi indoor connection)
IEEE 802.16 Standard
The IEEE 802.16 standard IEEE 802.16 was completed on Oct, 2004 Point-to-Multipoint (PMP) broadband wireless access standard for systems in the frequency ranges 10 66 GHz and sub 11 GHz.
The IEEE 802.16 chronology 802.16, published in April 2002 A set of air interfaces on a common MAC protocol Addresses frequencies 10 to 66 GHz Single carrier (SC) and only LOS 802.16a, published in January 2003 A completed amendment that extends the physical layer to the 2 to 11GHz both licensed and license-exempt frequencies. SC, 256 point FFT OFDM and 2048 point FFT OFDMA LOS and NLOS 802.16-2004, published in July 2004 Revises and replaces 802.16, 802.16a, and 802.16REVd. This announcement marks a significant milestone in the development of future WiMax technology P802.16-2004/Cor1 published on 8.11.2005 802.16e, Draft 12 was approved 7.12.2005 Extends the 802.16a standard for portability (mobile clients).
The family of 802.16 standard
IEEE 802.16 Covers MAC layer and PHY layer PHY layer Transmission Convergence sublayer MAC layer
Physical Layer 802.16 In the design of the PHY specification for 10 66 GHz, line-of-sight propagation was deemed a practical necessity. Because of the point-to-multipoint architecture, the BS basically transmits a TDM signal, with individual subscriber stations allocated time slots serially. The PHY specification defined for 10 66 GHz uses burst singlecarrier modulation with adaptive burst profiling in which transmission parameters, including the modulation and coding schemes, may be adjusted individually to each subscriber station (SS) on a frame-by-frame basis. Both TDD and burst FDD variants are defined.
Overview of 802.16 modulation Single Carrier QAM, Gray coded - QPSK - 16QAM Mandatory for Downlink, Optional for Uplink - 64QAM Optional for both Downlink & Uplink
Physical Layer 802.16a
Medium Access Control The 802.16 medium access control (MAC) layer supports many different physical layer specifications, both licensed and unlicensed.
MAC Addressing Subscriber station (SS) has 48-bit IEEE MAC Address Base Station (BS) has 48-bit Base Station ID - Not a MAC address - 24-bit operator indicator 16-bit Connection ID (CID) - Used in MAC PDUs
Frame Structure and PDU Each MAC packet consists of the three components, a) A MAC header, which contains frame control information. b) A variable length frame body, which contains information specific to the frame type. c) A frame check sequence (FCS), which contains an IEEE 32-bit cyclic redundancy code (CRC).
Generic MAC Header
MAC PDUs Transmission (1/2) MAC PDUs are transmitted in PHY bursts A single PHY burst can contain multiple Concatenated MAC PDUs The PHY burst can contain multiple FEC blocks MAC PDUs may span FEC block boundaries The TC (Transmission Convergence) layer between the MAC and the PHY allows for capturing the start of the next MAC PDU in case of erroneous FEC blocks
MAC PDUs Transmission (2/2)
Transmission Convergence sublayer This layer performs the transformation of variable length MAC protocol data units (PDUs) into the fixed length FEC blocks (plus possibly a shortened block at the end) of each burst. The TC layer has a PDU sized to fit in the FEC block currently being filled. It starts with a pointer indicating where the next MAC PDU header starts within the FEC block. The TC PDU format allows resynchronization to the next MAC PDU in the event that the previous FEC block had irrecoverable errors.
Transmission Convergence sublayer PDU Format
Security Authentication and registration are part of the 802.16 MAC common part sub-layer. Authentication is based on the use of PKI technologybased X.509 digital certificates. Each Subscriber Station contains both a manufacturer-issued factoryinstalled X.509 digital certificate and the certificate of the manufacturer. Privacy Sublayer uses privacy protocol that is based on the privacy key exchange management protocol of the DOCSIS BPI+ specification. PKM protocol uses X.509 digital certificates with RSA public key encryption for SS authentication and authorization key exchange. Traffic encryption uses DEC.
802.16 MAC operational modes The 802.16 standard specifies two modes for sharing the wireless medium: Point-to-Multipoint (PMP): The nodes are organized into a cellular-like structure where a base station (BS) serves a set of subscriber stations (SSs) within the same antenna sector in a broadcast manner, with all SSs receiving the same transmission from the BS. Transmissions from SSs are directed to and coordinated by the BS. Mesh (optional): The nodes are organized ad-hoc and access coordination is distributed among them.
802.16 MAC DL/UL connections Uplink and downlink data transmission are frame-based and in PMP mode they occur in separate time frames : Downlink (DL): The protocol employs TDM data streams from BS to SS. The BS transmits a burst of MAC Payload Data Units (PDUs) in broadcast and all SSs listen to the data transmitted by the BS. An SS is only required to process PDUs that are addressed to itself or that are explicitly intended for all the SSs. Uplink (UL) : Any SS transmits a burst of MAC PDUs to the BS in a Time Division Multiple Access (TDMA) manner
802.16 MAC DL transmissions Two kinds of bursts: TDM and TDMA All bursts are identified by a DIUC Downlink Interval Usage Code TDMA bursts have resync preamble allows for more flexible scheduling Each terminal listens to all bursts at its operational IUC, or at a more robust one, except when told to transmit Each burst may contain data for several terminals SS must recognize the PDUs with known CIDs DL-MAP message signals downlink usage
802.16 MAC: Downlink Map Message DL-MAP message defines usage of downlink and contains carrier-specific data DL-MAP is first message in each frame Decoding very time-critical typically done in hardware Entries denote instants when the burst profile changes
802.16 MAC UL transmissions Invited transmissions Transmissions in contention slots Bandwidth requests Contention resolved using truncated exponential backoff Transmissions in initial ranging slots Ranging Requests (RNG-REQ) Contention resolved using truncated exponential backoff Bursts defined by UIUCs Transmissions allocated by the UL-MAP message All transmissions have synchronization preamble Ideally, all data from a single SS is concatenated into a single PHY burst
802.16 MAC: Uplink Map Message UL-MAP message defines usage of the uplink Contains the "grants" Grants addressed to the SS Time given in mini-slots Time expressed as arrival time at BS
802.16 MAC Duplex techniques Downlink and uplink frames are transmitted using one of following techniques: Frequency Division Duplex (FDD) Time Division Duplex (TDD)
802.16 MAC QoS support (1/2) The MAC protocol is connection-oriented. At the start of each frame the BS schedules the uplink an downlink grants in order to meet the negotiated QoS requirements. Each SS learns of its allocation within the current frame by decoding the UL-MAP message. On the other hand, the DL MAP message contains the timetable of the downlink grants in the forthcoming frame. Both maps are transmitted by the BS at the beginning of each frame.
802.16 MAC QoS support (2/2)
Request/Grant scheme Self Correcting No acknowledgement All errors are handled in the same way, i.e., periodical aggregate requests Bandwidth Requests are always per Connection Grants are either per Connection (GPC) or per Subscriber Station (GPSS) Grants (given as durations) are carried in the UL-MAP messages SS needs to convert the time to amount of data using information about the UIUC
Come from the Connection Several kinds of requests: Implicit requests (UGS) Bandwidth Requests No actual messages, negotiated at connection setup BW request messages Uses the special BW request header Requests up to 32 KB with a single message Incremental or aggregate, as indicated by MAC header Piggybacked request (for non-ugs services only) Presented in GM sub-header and always incremental Up to 32 KB per request for the CID Poll-Me bit (for UGS services only) Used by the SS to request a bandwidth poll for non-ugs services
Classes of Uplink Service Characteristic of the Service Flow Unsolicited Grant Services (UGS) for constant bit-rate (CBR) or CBR-like Service Flows (SFs) such as T1/E1 Real-time Polling Services (rtps) for real-time-vbr-like SFs such as MPEG video Non-real-time Polling Services (nrtps) for non-real-time SFs with better than best effort service such as bandwidth-intensive file transfer Best Effort (BE) for best-effort traffic
802.16a MAC Features
Full mobility challenge 802.16e The IEEE 802.16e standard will provide full mobility to WiMAX Portable devices
Comparison with other technologies
WiMAX vs. 3G Comparison of OFDMA and CDMA based technologies WiMax systems for portable/nomadic use will have better performance Interference rejection Spectral efficiency Multipath tolerance High Data QoS support Lower future equipment cost WCDMA has advantage for voice communication
WiMAX vs. WiFi (1/2) At the PHY layer WiMAX supports flexible RF channel bandwidths and channel reuse. The standard also specifies support for automatic transmit power control and channel quality measurements. At the MAC layer 802.16 standard has been designed from one up to 100 s of users within one RF channel, a feat that 802.11 MAC was never designed for and is incapable of supporting. In a 802.11-based network more users results in a geometric reduction of throughput. The BWA standard is designed for optimal performance in all types of propagation environments, including LOS, near LOS and NLOS. The robust OFDM waveform supports high spectral efficiency over ranges from 2 to 40 Km with up 70 Mbps in a single RF channel. In contrast, WLANs systems use CDMA or OFDM (with a much different design) to support only low-power, limited range transmissions.
WiMAX vs. WiFi (2/2) The 802.16 MAC relies on a Grant/Request protocol for access to the medium and it supports differentiated service levels. By assuring collision-free data access to the channel, the 802.16a MAC improves total system throughput and bandwidth efficiency, in comparison with contention-based access techniques like the CSMA-CA protocol used in WLANs. The 802.16a MAC also assures bounded delay on the data (CSMA-CA by contrast, offers no guarantees on delay). 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 system.
Conclusions
WiMAX Promises In practical terms, WiMAX would operate similar to WiFi but at higher speeds, over greater distances and for a greater number of users. WiMAX could potentially erase the suburban and rural blackout areas that currently have no broadband Internet access because phone and cable companies have not yet run the necessary wires to those remote locations. WiMAX doesn t just pose a threat to providers of DSL and cable-modem service. The WiMAX protocol is designed to accommodate several different methods of data transmission, one of which is Voice Over Internet Protocol (VoIP). If WiMAX-compatible computers become very common, the use of VoIP could increase dramatically.