Standardization in 3GPP

Standardization in 3GPP
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Published Date:25-10-2017
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Standardization in IEEE 802.11, 802.16 10.1 IEEE Overview The IEEE- Standard Association’s involvement in electrical standards dates back to 1890, when the American Institute of Electrical Engineers (AIEE) proposed a recommendation for the practical unit of self-induction. As a pioneer in voluntary electrical and information technology standards activity, IEEE became a founding member of American National Standards Institute (ANSI) in 1918. In 1963, when the AIEE merged with the Institute of Radio Engineers (IRE) to form the IEEE, a formal standards body was established to support standards development. Envisioning the expanded role that standards were to play in the future and their impact on industry, IEEE formed its first Standards Board in 1973. As a standards body, the IEEE-Standard Association (IEEE-SA) has responded to changes in the marketplace and as a result, the IEEE-SA of today is quite different and innovative, but still committed to providing the most current, reliable standards knowledge. On average, the IEEE-SA publishes 80 new and revised standards annually and conducts over 245 standards projects ballots, in which a combination of approximately 13,000 individuals participate. Overview of IEEE Standards Association Organization is shown in Figure 10.1. The IEEE-SA is represented on the IEEE Board of Directors & IEEE Executive Committee. The IEEE-SA is assigned authority for the standardization activities of the IEEE by the IEEE Board of Directors. The IEEE-SA fulfills this assignment by activities such as, but not limited to:  Encouraging active development of needed standards. This involves, for example, promotion of open and innovative deliberations that result in broad consensus in accordance with due process procedures detailed in the bylaws and operations manuals of the subsidiary boards and committees of the IEEE-SA Board of Governors.  Building upon the strengths of the standards developing community by involving appropriate interests and outside organizations.382 Modulation and Coding Techniques in Wireless Communications Figure 10.1 IEEE Standards Association (SA) Organization Overview  Representing IEEE to external bodies on standards matters. This includes providing for cooperation with and IEEE participation in the activities of other organizations consistent with its scope and responsibilities.  Appointing IEEE members to participate in external bodies on standards matters based on nominations submitted by the IEEE Societies. The IEEE-SA also provides speakers to make presentations at meetings and conferences on subjects related to IEEE’s standards interests and to participate in panels on standards-related subjects. With the approval of the IEEE Board of Directors and as authorized, the IEEE-SA Board of Governors may establish groups:  To act for the IEEE in product testing or in certification of products or systems to comply with IEEE standards, or  To offer opinion in the name of the IEEE on the conformance of products or systems to the requirements of IEEE standards for their intended use and safe operation. Overview of IEEE Standard Association Standard Board Organization is shown in Figure 10.2. The IEEE-SA Standards Board (IEEE-SA SB) is established by the IEEE-SA Board of Governors. The IEEE-SA Standards Board is responsible on an Institute-wide basis for:  Encouraging and coordinating the development of IEEE standards.  Reviewing all proposed IEEE standards to determine whether the proposed standards conform to the requirements established by the IEEE-SA Standards Board and whether consensus has been achieved for approval of the proposed standards. – Consensus is established when, in the judgment of the IEEE-SA Standards Board, substantial agreement has been reached by directly and materially affected interest categories. – Substantial agreement means much more than a simple majority, but not necessarily unanimity.Standardization in IEEE 802.11, 802.16 383 Figure 10.2 IEEE Standard Association Standard Board (SA SB) Organization Overview – Consensus requires that all views and objections be considered, and that a concerted effort be made toward their resolution. Matters of standards policy, financial oversight, new directions in standardization, and other standards- related activities in fields of interest to the Institute are the responsibility of the IEEE-SA Board of Governors (BOG). Procedures Committee (ProCom): This committee is responsible for recommending to the IEEE-SA Standards Board improvements and changes in its bylaws, procedures, and manuals to promote efficient discharge of responsibilities by the IEEE-SA Standards Board and its committees. New Standards Committee (NesCom): This committee is responsible for ensuring that proposed standards projects are within the scope and purpose of the IEEE, that standards projects are assigned to the proper society or other organizational body, and that interested parties are appropriately represented in the development of IEEE standards. The committee examines Project Authorization Requests (PARs) and makes recommendations to the IEEE-SA Standards Board regarding their approval. Standards Review Committee (RevCom): This committee is responsible for reviewing proposals for the approval of new and revised standards and for the reaffirmation or withdrawal of existing standards to ensure that the proposals represent a consensus of the members of the official IEEE sponsor balloting group. The committee routinely examines submittals to ensure that all applicable requirements of the IEEE-SA Standards Board Operations Manual have been met and make recommendations to the IEEE- SA Standards Board regarding their approval. Audit Committee (AudCom): This committee provides oversight of the procedures used in the standards-development activities of IEEE Standards Sponsors and review of the procedures used by the Accredited Standards Committees for whom the IEEE serves as (co-)secretariat. This committee also oversees the submission of Sponsor annual reports. Patent Committee (PatCom): This committee provides oversight for the use of any patents and patent information in IEEE standards. This committee reviews any patent information submitted to the IEEE Standards Department to determine conformity with patent procedures and guidelines.384 Modulation and Coding Techniques in Wireless Communications Administrative Committee (AdCom): The Administrative Committee acts for the IEEE-SA Standards Board between meetings and makes recommendations to the IEEE-SA Standards Board for its disposition at regular meetings. Standards Coordinating Committees (SCCs): When the scope of a standards activity is too broad to be encompassed in a single IEEE Society, or a society may find itself in a position where it is unable to carry out the work needed to meet an identified need then the IEEE-SA Standards Board shall establish its own committees to perform the required functions. The IEEE is represented on the Standards Committees (ASCs) in which it has a substantial interest. This provides IEEE with an opportunity to have a direct influence on the development of an American National Standard generated by the ASC. IEEE 802 is a standard committee sponsored by the Computer Society. The Computer Society is one of the IEEE Societies and councils. All IEEE standards development is based on projects that have been approved by the IEEE-SA Standards Board, and each project shall be the responsibility of a Sponsor. Projects within IEEE 802, P802.xx, are sponsored by the Computer Society through LAN/MAN Standard Committee (LMSC). The Sponsor accepts responsibility for oversight of any of its assigned standards, including overseeing coordination, balloting, and making annual activity reports to the IEEE-SA Standards Board. Each Sponsor operates in accordance with a written set of policies and procedures (P & P). There are also operating procedures available for Sponsors developing a standard using the entity method of participation, and Sponsors utilize these procedures as the basis for entity standardization. These projects with P802.xx form a family of IEEE standards dealing with local area networks and metropolitan area networks. The services and protocols specified in IEEE 802 map to the lower two layers (Data Link and Physical) of the seven-layer OSI networking reference model. These standards deal with Ethernet, Token Ring, Bridging & Virtual bridged LANs, WLAN, Wireless Personal Area Net- work (WPAN): Bluetooth, Zigbee, in or around a body, and so on, Broadband Wireless Access Systems (WiMAX), Media independent handover and Wireless Regional Area Networks (WRAN).Workisorga- nized into a number of Working Groups (WGs) and Technical Advisory Groups (TAGs) operating under the oversight of a sponsor Executive Committee (EC). Current active working groups within IEEE 802 are:  802.1 Higher Layer LAN Protocols Working Group  802.3 Ethernet Working Group  802.11 Wireless LAN Working Group  802.15 Wireless Personal Area Network (WPAN) Working Group  802.16 Broadband Wireless Access Working Group  802.17 Resilient Packet Ring Working Group  802.18 Radio Regulatory TAG  802.19 Coexistence Working Group  802.20 Mobile Broadband Wireless Access (MBWA) Working Group  802.21 Media Independent Handover Services Working Group  802.22 Wireless Regional Area Networks 10.2 Standard Development Process Typically a Study Group (SG) is formed when a new area is first investigated for standardization. The SG can be within an existing WG or TAG, or it can be independent of the WGs. A new project within an existing group is typically developed by a Sub- Group or Task Group, while a new independent project can lead to the creation of a completely new WG. Each standard, recommended practice, or guide begins as a group of people with an interest in developing the standard. A Project Authorization Request (PAR) is normally submitted for approvalStandardization in IEEE 802.11, 802.16 385 within six months of the start of work. In order to avoid duplication, provide for effective management of overall efforts, and expedite approval of final documents, all requests for an initiation of a standards project, in the form of a PAR, shall be approved by the IEEE-SA SB. The IEEE-SA SB has assigned to NesCom the preliminary review of PARs and the responsibility for recommending final approval to the Board. Sponsors are required to submit a PAR at the earliest opportunity when a standards project is contemplated or work is started. PARs that have been submitted by Sponsors to the Secretary of the IEEE-SA Standards Board by the established deadline shall be submitted by the Secretary to the New Standards Committee (NesCom) for review. Unless specifically authorized by the IEEE-SA Standards Board, no proposed standard or revision shall be considered by RevCom without prior approval of the project by the IEEE-SA Standards Board. The lifetime of a PAR shall be four years. In LMSC, new projects require supporting material in the form of 5- Criteria to show that they meet the charter of LMSC. The draft PAR is voted on by the EC, and then it goes to the IEEE Standards Board New Standards Committee (NesCom) which recommends it for approval as an official IEEE Standards Project. Part of the PAR identifies if there will be liaisons with outside standards groups, for instance International Telecommunication Union (ITU) for some international standards. The liaisons help avoid conflicts or duplication of effort within an area. Technical proposals are presented and evaluated either in working group or task group meetings, and a draft standard is written and voted on by the WG. Typically there are six face-to-face meetings every year for working groups. Three of these meetings are called Plenary meetings. In Plenary meetings, all the working groups of IEEE 802 meet and three are called interim meetings. In interim meetings some working groups may meet and some may not. Further, for interim meeting working groups may decide to meet at different locations. When the TG/WG reaches enough consensus on the draft standard, a WG Letter Ballot is done to release it from the WG. It is next approved by the EC and then goes for Sponsor Letter Ballot. After the Sponsor Letter Ballot has passed and “No” votes are resolved, the draft Standard is sent to the IEEE Standards Board Standards Review Committee (RevCom). All IEEE standards shall be approved by the IEEE-SA Standards Board prior to publication. The IEEE-SA Standards Board has assigned to RevCom the review of standards submittals and the responsibility for recommending final approval to the IEEE-SA Standards Board. Proposed standards, together with the required documentation, that has been submitted by Sponsors to the Secretary of the IEEE-SA Standards Board by the established deadline shall be submitted by the Secretary to the Standards Review Committee (RevCom) for review. Approval by the IEEE-SA Standards Board indicates that the requirements of the IEEE-SA Standards Board Operations Manual and these bylaws have been satisfied and, specifically, that the final results of the ballot and statements submitted by other coordinating bodies who participated in the development of the standard indicate that consensus has been achieved and unresolved negative ballots have been considered together with reasons why the comments could not be resolved. Once it is recommended by RevCom and approved by the Standards Board, it can then be published as an IEEE standard. Some draft standards in LMSC are also sent to ISO at the time they go to Sponsor Letter Ballot. Parallel approval in ISO JTC1/SC6 (Joint Technical Committee 1, Subcommittee 6 - responsible for LANs) may lead to publication as an ISO standard. The process from start to finish can take several years for new standards, and less for revisions or addenda to existing standards. 10.3 IEEE 802.11 Working Group IEEE 802.11 is considered to be the home of WLAN standardization. Activities in 802 started in 1980. The number “802” was simply the next free number IEEE could assign. In 1987 activities in 802.4L (token bus) were started. 802.11 working group was established in 1990 as outgrowth from 802.4L. 802.11 published its first standard in 1997, standardizing three physical layers, infrared operating at 1 Mbps data rate, Direct sequence spread spectrum and Frequency hopping spread spectrum in 2.4 GHz band using 20 MHz bandwidth with 1 and 2 Mbps data rates.386 Modulation and Coding Techniques in Wireless Communications In 1999, 802.11a was standardized. This standard provided variable data rates from 6 Mbps to 54 Mbps in 5 GHz band using 20 MHz channels. It uses OFDM based air interface. Also in the same year, 802.11b was standardized. This standard uses same air interface (Direct sequence spread spectrum) as base 802.11 standard but increased the data rate to 11 Mbps in 2.4 GHz band. In practice 802.11b has a greater range than 802.11a because of lower frequency in which 802.11b operates but 802.11a offered higher data rates than 802.11b. 802.11c is a network interoperability standard that deals with bridge operation procedures in wire- less bridges or access points was published in 2001. It was incorporated in 802.1D in 2004. Also in 2001, 802.11d specified operation in additional regulatory domains. This includes addition of a country information element to beacons, probe requests, and probe responses. The country information ele- ments simplifies the creation of 802.11 wireless access points and client devices that meet the different regulations enforced in various parts of the world. In 2003, 802.11g introduced OFDM air interface in 2.4 GHz band and increased the data rate to 54 Mbps using 20 MHz channel bandwidth. This standard is backward compatible with 802.11b. Also in the same year 802.11h specified Spectrum and Transmit Power Management Extensions in 5 GHz band to solve problems like interference with satellites and radar. It was originally designed to address European regulations but is now applicable in many other countries. The standard provides Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) to the 802.11 MAC. In 2004, 802.11i specified security mechanisms for WLAN. It replaced the short authentication and privacy clause of the original standard with a detailed security clause. In the same year 802.11j standard was created for the Japanese market. It allows Wireless LAN operation in the 4.9 to 5 GHz band to conform to the Japanese rules for radio operation for indoor, outdoor and mobile applications. In 2005, 802.11e defined a set of Quality of Service enhancements for WLAN applications through modifications to the Medium Access Control (MAC) layer. This is of importance for delay-sensitive applications, such as Voice over Wireless IP and Streaming Multimedia. Task group TGma was authorized to incorporate many of the amendments made to base 802.11- 1999 standard. REVma or 802.11ma, as it was called, created a single document that merged eight amendments (802.11a, b, d, e, g, h, i, j) with the base standard 802.11-1999. Upon approval on 8 March 2007, 802.11REVma was renamed to the current base standard IEEE 802.11-2007. In 2008, 802.11k created a standard for a basic set of radio resource measurements. These mea- surements are required to provide services such as roaming, coexistence, and so on. It is necessary to provide these measurements and other information in order to manage these services. Also in 2008, 802.11r specified fast BSS Roaming/Transition within WLAN networks meeting the real-time handover requirements without compromising the security principles. Further, 802.11y created a standard which enables high powered Wi-Fi equipment to operate on a co-primary basis in the 3650 to 3700 MHz band in the United States, except when near a grandfathered satellite earth station. Recently in 2009, two new standards were created by 802.11. 802.11n added new features like multiple input multiple output, 40 MHz channel bandwidth and frame aggregation. Using these features 802.11n was able to increase the data rate capability to 600 Mbps (using four spatial stream in 40 MHz channel) from 54 Mbps provided by 802.11a and 802.11g. Also 802.11w created a standard to extended 802.11i to provide protection for selected management frames for example, Disassociation and Deauthentication frames and broadcast/multicast management frames. 10.4 IEEE 802.16 Working Group IEEE 802.16 is another WG of the IEEE 802 LAN/MAN Standards Committee. Officially the WG starts efforts since 1999 and the first official WG meeting was held in May 1999. The IEEE 802.16 WGStandardization in IEEE 802.11, 802.16 387 on Broadband Wireless Access Standards is responsible for developing standards and recommended practices to support the development and deployment of broadband Wireless Metropolitan Area Networks TM (WirelessMAN ). Although the 802.16 family of standards is officially called WirelessMAN in IEEE, it has been commercialized under the name of “WiMAX” (Worldwide Interoperability for Microwave Access) by the industry alliance called the WiMAX Forum. The mission of the Forum is to promote and certify compatibility and interoperability of broadband wireless products based on the IEEE 802.16 standards. Here we will focus on IEEE 802.16 WG only. The first issue of the standard specifics a set of PHY and MAC layer standards was the IEEE Standard TM 802.16-2001 1, completed in October 2001 and published on 8 April 2002, defined the WirelessMAN air interface specification for wireless metropolitan area networks (WMANs). The intention behind the first release of the standard was to define a technology for broadband wireless access (BWA) for fixed users, as an alternative to cabled access networks, such as digital subscriber line (DSL) links. For this reason, the original IEEE 802.16 defines a point-to-multi-point (PMP) network architecture where resources are shared with control from a central node called base station (BS) to a set of subscriber stations (SS). From its first release, the medium access control (MAC) layer was connection-oriented and supported quality of service (QoS). The standard, as approved in 2001, addresses frequencies from 10 to 66 GHz in line-of-sight (LOS) operations using single carrier transmission only. The standard employs QPSK, 16-QAM and 64-QAM as modulation schemes and the modulation scheme can be changed from frame to frame. Moreover, the standard was designed to evolve as a set of air interfaces based on a common MAC protocol, but with physical layer specifications dependent on the spectrum of use and the associated regulations. In 2003, a new version of the standard, IEEE 802.16a-2003, was published with support for non- LOS (N-LOS) operations in frequencies from 2 to 11 GHz, extending the geographical reach of the network. The subsequent milestone in standard development was IEEE 802.16-2004 2 which introduced support for two additional physical layers: orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA). In 2005, a new version of the standard was released to enable combined fixed and mobile operations in licensed bands. The aforementioned stan- dard, IEEE 802.16e-2005 3, was defined as an amendment to IEEE 802.16-2004 and added several features related to mobile operations and mobile stations (MSs), including power saving, idle mode, handover and an improved OFDMA physical layer. After the 2005 release, the standard development continued to define the management information base (MIB) for MAC and PHY (IEEE 802.16f) and the management plane and procedures (IEEE 802.16g), to improve the co-existence for license-exempt operation (IEEE 802.16h), to introduce relay capabilities (IEEE 802.16j), and to refine the MAC and PHY procedures for mobile operations (IEEE 802.16-2009). The latter is also known as the 2009 release, and brings the following major changes: half-duplex mobile terminal operations in OFDMA frequency division duplexing (FDD), load balancing, robust header compression (ROHC), enhanced mechanism for resource allocation (for example, persistent allocation), support for location-based services (LBSs) and multicast and broadcast services (MBSs) 10.11. The clean up for IEEE 802.16-2009 also involved incorporating the IEEE 802.16f and IEEE 802.16g amendments, and removing some stale features, such as the mesh mode. Targeting to meet the requirement of IMT-Advanced (IMT-A) air interface, by the end of 2006, IEEE 802.16 working group (WG) has set up a new task group IEEE 802.16m (TGm). The purpose of IEEE 802.16m TG is to propose an advanced air interface which will include enhancements and extension to IEEE 802.16-2004 and IEEE 802.16e-2005. In addition to this, the new radio interface will be contributed to ITU Radiocommunication Sector (ITU-R) as a radio technology proposal for IMT-A. The development of 802.16m is still ongoing, but the RIT proposal submission based on 802.16m was submitted to ITU in 2009.388 Modulation and Coding Techniques in Wireless Communications 10.5 IEEE 802.11 10.5.1 Overview and Scope Wireless Local Area Network (WLAN) technology based on the IEEE 802.11 standard has become very successful during the recent years. There is a huge deployed base of WLAN devices and the shipment rates have been constantly increasing. From a consumer point of view WLAN is becoming a basic connectivity feature even in low price category devices. The IEEE 802.11 standard specifies the physical layer (PHY) and the data link layer (layers 1 and 2, respectively) of the Open System Interconnection (OSI) model protocol stack. In this chapter the physical layer (layer 1) features of the most relevant parts of the standard are described. Furthermore, due to the wide scope of the baseline 802.11 specification and the numerous amendments, the focus is mainly on the most recent and commonly available variants of the technology. The original specification from year 1997 defines three different PHYs: frequency-hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS) and infrared (IR). None of the mentioned PHYs are widely used today. The original specification has since been extended with a large number of amendments over time. The following PHY related amendments of WLAN are discussed in this chapter:  802.11a: Orthogonal frequency division multiplexing (OFDM) PHY specification for the 5 GHz band  802.11b: High Rate direct sequence spread spectrum (HR/DSSS) PHY specification  802.11g: Extended Rate PHY specification  802.11n: High Throughput (HT) PHY specification Additionally some discussion is included about selected standardization activities that are currently ongoing in the 802.11. Specifically there is discussion about the very high throughput (VHT) task groups TGac and TGad. 10.5.2 Frequency Plan All the WLAN PHY variants of interest in the context of this discussion are either using the 2.4 GHz ISM (industrial, scientific and medical) or the 5 GHz U-NII (Unlicensed National Information Infrastructure) radio frequency bands. Both bands are unlicensed in most countries, and they are usually regulated by Table 10.1 List of available WLAN channels in Europe and North America Channel Frequency GHz Europe North America 1 2.412 X X 2 2.417 X X 3 2.422 X X 4 2.427 X X 5 2.432 X X 6 2.437 X X 7 2.442 XX 8 2.447 X X 9 2.452 X X 10 2.457 X X 11 2.462 X X 12 2.467 X 13 2.472 XStandardization in IEEE 802.11, 802.16 389 Table 10.2 Commonly available channels worldwide in 5 GHz band Band Channel Frequency GHz U-NII Lower 36 5.18 5.15–5.25 GHz 40 5.20 44 5.22 48 5.24 U-NII Middle 52 5.26 5.25–5.35 GHz 56 5.28 60 5.30 64 5.32 a national regulatory authority. For the most part regulation of these unlicensed bands is harmonized throughout the world. In the 2.4 GHz ISM band there are several designated channels that are spaced 5 MHz apart. WLAN signals require wider channel separation, and therefore adjacent channels overlap and cause interference to each other. In the following Table 10.1 there is a list of available 2.4 GHz WLAN channels and indication of commonly used non-overlapping channels in Europe and in North America. The regulation for the 5 GHz frequency band varies throughout the world. Generally there is a substantially larger amount of unlicensed band available than in the 2.4 GHz band. Typically the channel separation is 20 MHz, which corresponds better to the WLAN signal bandwidth. The list of available channels is country specific and from the user perspective also dependant on the supported channels by the WLAN equipment. In the following Table 10.2 there is a list of commonly available channels worldwide. 10.5.3 Reference Model The reference model provides an architectural overview of the system, where different parts of the system and the interaction model between the entities can be seen. The two major parts of an 802.11 system are the physical layer and the medium access control (MAC) sublayer. They correspond mostly to the OSI reference model layering. Also there is a distinction between the data plane and the management plane. The reference model is illustrated in Figure 10.3. Each protocol layer provides a service access point (SAP) towards upper layers in order to allow the upper layers to use its data plane services. In addition a SAP is defined towards a corresponding management entity for using the control plane services of each protocol layer. In the data plane the lowest layer specified by 802.11 is the physical medium dependent (PMD) sublayer. The PMD sublayer is located directly above the wireless physical medium and it provides services to upper layers for transmitting data frames over the medium using spread spectrum or carrier modulation techniques. Furthermore, the PMD sublayer provides carrier sense indication for detecting activity on the radio channel. This information can be used by the MAC sublayer in order to assess whether the channel is currently idle or busy. The upper part of the physical layer is the physical layer convergence procedure (PLCP) sublayer. It is using the service provided by the PMD sublayer. The main functionality of the PLCP sublayer is frame exchange between the PHY and MAC layers. Each of the different PHYs defined in the baseline 802.11 and in the amendments is unique in terms of PMD and PLCP details. The overall reference model and SAP definitions still apply for all of them.390 Modulation and Coding Techniques in Wireless Communications Data plane Management plane MAC sublayer MAC sublayer Station management entity management entity Data link layer PLCP sublayer PHY sublayer management entity Physical layer PMD sublayer Figure 10.3 802.11 system reference model 10.5.4 Architecture The most fundamental component of the WLAN architecture is a station (STA), which is connected to the wireless medium. A STA consists of at least a PHY and a MAC. When there are multiple STAs that communicate between each other, they form a basic service set (BSS) together. If there is no access point (AP) present, the configuration is said to be an independent BSS (IBSS). IBSS is also referred to as WLAN ad-hoc mode. In ad-hoc mode STAs communicate directly towards each other without a centralized forwarder device. When there is an AP present in addition to regular STAs, the configuration is commonly referred to as infrastructure BSS. Currently infrastructure mode is clearly the dominant a) b) STA 2 AP STA 1 STA 4 STA 5 STA 3 Figure 10.4 Illustration of a) ad-hoc mode and b) infrastructure modeStandardization in IEEE 802.11, 802.16 391 DS AP 1 AP 2 STA 1 STA 2 STA 3 STA 4 BSS 1 BSS 2 Figure 10.5 Example of an ESS operational mode in deployed WLAN networks. Ad-hoc mode is rarely used, although many devices have support for it also. Example physical network topologies for both operational modes are shown in Figure 10.4. Multiple BSSs may form an extended service set (ESS) by interconnecting with each other using a distribution system (DS). The DS can for example be based on IEEE 802.3 wired technology. The 802.11 MAC SAP is designed to be compliant with 802.3 MAC SAP. Also the MAC address formats and address spaces between the technologies are compatible. Therefore it is possible to bridge MAC frames directly between these networks. Figure 10.5 depicts an example ESS configuration. 10.5.5 802.11a The 802.11a amendment defines an OFDM PHY for the 5 GHz band. Originally the specification was released in 1999 and it was incorporated to the main 802.11 standard in the 2007 revision 8. It allows data rates up to 54 Mbit/s. Today the 802.11a WLAN variant is used mainly in corporate environments, because consumer class devices with 5 GHz capabilities have become available much later than 2.4 GHz devices. The 802.11a signal bandwidth is specified as 20 MHz. The OFDM uses 52 subcarriers with 0.3125 MHz spacing. At the target maximum bitrate the chosen number of subcarriers provides a practical compromise that allows a sufficient symbol guard interval while keeping the complexity and cost at a reasonable level. A guard interval is required between symbols to remove inter-symbol interference from the received signal. The required guard interval length depends on the delay spread of a typical usage environment for the technology. Forty-eight of the total 52 subcarriers are used for data and four are used as pilot subcarriers. Every data subcarrier is modulated using binary or quadrature phase shift keying (BPSK or QPSK) or using 16- or 64-quadrature amplitude modulation (16-QAM or 64-QAM). The OFDM symbol time is 4 µs, which corresponds to the symbol rate of 250 000 symbol/s. As forward error correction (FEC) mechanism 802.11a uses convolutional coding. The allowed coding rates are 1/2, 2/3, or 3/4. Coding is implemented across all subcarriers, which helps to even out the effects of narrowband interference or narrowband fading to the overall OFDM bit error ratio (BER). Decoding392 Modulation and Coding Techniques in Wireless Communications Table 10.3 802.11a modulation and coding schemes Modulation Coding rate Data rate Mbit/s BPSK 1/2 6 BPSK 3/4 9 QPSK 1/2 12 QPSK 3/4 18 16-QAM 1/2 24 16-QAM 3/4 36 64-QAM 2/3 48 64-QAM 3/4 54 by the commonly used Viterbi algorithm is recommended. Table 10.3 lists all possible modulation and coding schemes (MCS) for 802.11a and their corresponding data rates. The 802.11a PLCP protocol data unit (PPDU) consists of the PLCP preamble, the SIGNAL field and the data. The frame format is illustrated in Figure 10.6. The OFDM physical protocol unit starts with a preamble, which consists of 12 OFDM symbols. The main purpose of the preamble is to synchronize the transmitter and receiver to have common timing. The first part of the preamble is 10 short symbols each having 0.8 µs duration. They are transmitted without a guard interval. This is referred to as the short training sequence. It is used for signal detection, gain control, possible antenna selection and coarse synchronization. Following the short training sequence there is a long training sequence consisting of two 3.2 µs symbols preceded by a guard interval of 1.6 µs. It is used for channel estimation and fine frequency acquisition. After the preamble there is a SIGNAL field that is encoded within a single OFDM symbol using BPSK modulation and convolutional coding at 1/2 coding rate. SIGNAL includes the RATE and LENGTH fields. RATE defines the MCS used for the rest of the data unit. LENGTH is an integer value representing the number of octets in PLCP service data unit (PSDU). Additionally SIGNAL includes a parity bit for error detection and tail bits that are set to zero. Like all other 802.11a OFDM symbols the duration of the SIGNAL is 4 µs including a 0.8 µs guard interval. One OFDM symbol Variable length BPSK, r = 1/2 OFDM MCS according to RATE PLCP preamble SIGNAL DATA RATE Reserved LENGTH Parity Tail SERVICE PSDU Tail Pad bits PLCP header Figure 10.6 802.11a PPDU frame format 10.5.6 802.11b The 802.11b amendment extends the DSSS PHY of the original standard with increased throughput up to 11 Mbit/s. Like the 802.11a amendment, it was originally released in 1999 and it was incorporatedStandardization in IEEE 802.11, 802.16 393 2 bits DQPSK PLCP Scrambler Splitter modulator Code word selector 2 or 6 bits Figure 10.7 802.11b transmitter to the main 802.11 standard in the 2007 revision 8. It uses the unlicensed 2.4 GHz frequency band for communication. Since its release it has become one of the most common WLAN variants worldwide, and it is still supported by most WLAN capable devices today. The DSSS PHY in the original 802.11 specification defines data rates of 1 and 2 Mbit/s. The information signal is spread to wider bandwidth using static repeating chip sequences at a much higher chip rate than the information bit rate. 11-bit code words are transmitted 1 million times per second. Each code word carries one or two information bits depending on the data rate. Encoding more bits into a single 11-bit code word is no longer practical using regular phase shift keying techniques. Detecting finer phase shifts at the receiver especially in the presence of inter-symbol interference requires significantly more advanced and expensive technology. Therefore the higher data rates of 5.5 and 11 Mbit/s use an alternative encoding method. Complementary code keying (CCK) transmits chip stream in symbols of eight chips at the rate of 11 Mchip/s. Depending on the data rate either 4 or 8 bits are encoded to a single code word. Sophisticated transforms are used to derive the code words from the data. With both 5.5 and 11 Mbit/s data rates the two first bits of a transmitted data block are used for determining the phase shift of the modulated code word. The rest of the bits in the data block select the codeword to be modulated. The total size of a data block is 4 or 8 bits corresponding to 5.5 and 11 Mbit/s data rates respectively. A simplified transmitter structure is shown in Figure 10.7. All PPDU bits are scrambled before transmission by the DSSS PMD. The purpose of scrambling is to randomize long sequences of 0s or 1s within the data. The receiver can descramble the data after demodulation and de-spreading. The High Rate DSSS PHY defines also a PPDU format specific to the PHY. The frame consists of a PCLP preamble, PLCP header and PSDU. Two different preambles and headers are defined. The mandatory long preamble and header interoperate with the original 802.11 DSSS PHY. The short preamble and header are added in the 802.11b amendment as optional and are not interoperable with the legacy DSSS PHY. The frame format is illustrated in Figure 10.8. The first part of the preamble is the SYNC field, which is a string of scrambled 1s. The receiver uses it to detect the incoming signal and synchronize timing. Following SYNC comes the 16-bit start of frame delimiter (SFD). SFD marks the start of the PPDU frame. PLCP preamble PLCP header PSDU SYNC SFD SIGNAL SERVICE LENGTH CRC Figure 10.8 802.11b PPDU format394 Modulation and Coding Techniques in Wireless Communications Table 10.4 Long and short PPDU comparison Long PPDU Short PPDU PLCP preamble modulation 1 Mbit/s DBPSK 1 Mbit/s DBPSK PLCP preamble length 144 bits, 144 µs 72 bits, 72 µs PLCP header modulation 1 Mbit/s DBPSK 2 Mbit/s DQPSK PLCP header length 48 bits, 48 µs 48 bits, 24 µs PSDU bitrates 1, 2, 5.5 and 11 Mbit/s 2, 5.5 and 11 Mbit/s The PLCP header consists of the SIGNAL, SERVICE, LENGTH and CRC fields. The 8-bit SIGNAL field indicates the PHY data rate used for the PSDU. The integer value multiplied by 100 kbit/s equals the PSDU rate. The SERVICE field includes indication of the used modulation method and indication of the LENGTH field extension. The LENGTH field is a 16-bit integer value representing the duration of the PSDU in microseconds. CRC is used for error detection. If an error is detected, the receiving MAC can choose to reject the frame. The short PPDU format provides an enhanced throughput compared to the long PPDU format, but since the two formats are not compatible, the short PPDU format required all devices in the network to support 802.11b. The main differences are listed in Table 10.4. 10.5.7 802.11g The 802.11g amendment defines an extended rate PHY (ERP) for the DSSS PHY in the original specification and the high rate DSSS PHY in the 802.11b amendment. It supports data rates up to 54 Mbit/s in the 2.4 GHz unlicensed frequency range. The specification was originally released in 2003 and it was incorporated to the main 802.11 standard in the 2007 revision 8. The standard has been very widely adopted all over the world. An ERP system supports data rates specified in the DSSS and high rate DSSS PHY specifications, which makes it backwards compatible with the mentioned systems. Furthermore, the radio part imple- ments all mandatory modes of 802.11a using the 2.4 GHz frequency band and channelization plan. Table 10.5 includes the data rates and modulation schemes that are used in different ERP modes. An ERP system must support three different PPDU frame formats: the long and short DSSS frame formats specified in 802.11b and the OFDM frame format specified in 802.11a. Additionally there are Table 10.5 ERP modes Data rate Modulation scheme 1 Mbit/s DBPSK + DSSS 2 Mbit/s DQPSK + DSSS 5.5 Mbit/s DQPSK + DSSS/CCK 11 Mbit/s DQPSK + DSSS/CCK 6 Mbit/s OFDM 9 Mbit/s OFDM 12 Mbit/s OFDM 18 Mbit/s OFDM 24 Mbit/s OFDM 36 Mbit/s OFDM 48 Mbit/s OFDM 54 Mbit/s OFDMStandardization in IEEE 802.11, 802.16 395 optional frame formats specified in 802.11g. The receiver distinguishes the PHY mode using the PLCP preamble and demodulates the rest of the PPDU accordingly. The maximum throughput of 802.11g is comparable to 802.11a. However, if there are legacy DSSS mode devices present in the network, the aggregate throughput is decreased significantly. The reason is lower air time utilization efficiency DSSS mode due to the relatively long PLCP preamble and header duration. 10.5.8 802.11n 802.11n 10.9 is an amendment to the year 2007 revision of the original 802.11 specification. The target has been to significantly increase the network throughput of the preceding WLAN standards. The amendment specifies a new high throughput (HT) OFDM PHY in order to achieve the goal. The introduced enhancements in the PHY layer include multi-antenna MIMO techniques and 40 MHz channel bandwidth. In the MAC layer the new mechanisms include for example frame aggregation. The features of 802.11n are capable of supporting data rates up to 600 Mbit/s. The standard was ratified by the IEEE in October 2009. However, there have already been products available for several years before ratification that have been implemented using a draft version of the standard. The Wi-Fi Alliance has been granting certifications for interoperability based on the draft standards since 2007. The certified devices will remain compatible with the final version of the standard. An HT system may operate either in the 2.4 GHz or the 5 GHz frequency band. When operating in the 5 GHz range the HT system is backwards compatible with the 802.11a specification and similarly in the 2.4 range it is compliant with 802.11g. Support of both bands within a single device is optional. Similarly to the previous OFDM PHYs, 802.11n OFDM subcarriers are modulated using BPSK, QPSK, 16-QAM or 64-QAM. The subcarrier spacing is identical to the previous OFDM PHYs, but the maximum allowed number of data subcarriers is increased from 48 to 52 in a 20 MHz channel. The signal bandwidth may be either 20 MHz or 40 MHz. In practice the 40 MHz channel provides approximately double throughput compared to the 20 MHz channel. However, the availability of con- tiguous 40 MHz channels may be limited due to interference. The problem is prevalent especially in the 2.4 GHz range, which has a low number of non-overlapping channels and typically high amount of devices present. The support for 40 MHz channel bandwidth is optional for all devices. 802.11n supports different MIMO techniques such as spatial division multiplexing (SDM), receive diversity and beamforming. Support for two spatial streams in 20 MHz channel is mandatory for AP devices. All other MIMO features are optional. MIMO SDM in 802.11n is capable of supporting up to four spatial streams. Current implementations support typically at most two spatial streams. In practice the number is limited by implementation com- plexity and cost. SDM efficiency is also dependant on the spatial separation of the antennas. Achieving sufficient separation for significant performance increase may not be possible especially on small form factor devices. In the mandatory MCSs all streams use equal modulation (EQM). Optionally unequal modulation (UEQM) for individual streams is also supported. Other optional features that directly affect maximum achievable throughput by means of decreasing the physical protocol overhead are short guard interval (GI) and the greenfield frame format. The GI may be shortened from 800 ns to 400 ns with certain preconditions in environments that have sufficiently low delay spread. Typically the short GI is applied only after the highest MCS has been reached by rate adaptation. The greenfield frame format omits some unnecessary parts in the physical protocol headers in case there are no non-HT devices present in the network. There are a large number of combinations of the previously described parameters, which are supported by the standard. Table 10.6 shows the relationships of MCS parameters that are related to data rate. The table includes only a subset of all MCSs including only EQM modes for simplicity. MCS indexes 0-7 are mandatory for non-AP devices and MCS indexes 0-15 for AP devices.396 Modulation and Coding Techniques in Wireless Communications Table 10.6 Selected 802.11n MCSs Data rate Mbit/s 20 MHz channel 40 MHz channel MCS Coding Spatial 800 ns 400 ns 800 ns 400 ns Index Modulation rate streams GI GI GI GI 0 BPSK 1/2 1 6.5 7.2 13.5 15 1 QPSK 1/2 1 13.0 14.4 27 30 2 QPSK 3/4 1 19.5 21.7 40.5 45 3 16-QAM 1/2 1 26.0 28.9 54 60 4 16-QAM 3/4 1 39.0 43.3 81 90 5 64-QAM 2/3 1 52.0 57.8 108 120 6 64-QAM 3/4 1 58.5 65.0 121.5 135 7 64-QAM 5/6 1 65.0 72.2 135 150 8 BPSK 1/2 2 13.0 14.4 27 30 9 QPSK 1/2 2 26.0 28.9 54 60 10 QPSK 3/4 2 39.0 43.3 81 90 11 16-QAM 1/2 2 52.0 57.8 108 120 12 16-QAM 3/4 2 78.0 86.7 162 180 13 64-QAM 2/3 2 104.0 115.6 216 240 14 64-QAM 3/4 2 117.0 130.0 243 270 15 64-QAM 5/6 2 130.0 144.4 270 300 ... ... ... ... ... ... ... ... 31 64-QAM 5/6 4 260.0 288.9 540.0 600.0 ... ... ... ... ... ... ... ... In order to transmit multiple spatial streams the original data stream is first divided into multiple independent streams. Each spatial stream is then transmitted using a discrete transmit chain including analog and RF parts for all antennas. A corresponding number of antennas are required at the receiver in order to recover all the spatial streams. MIMO SDM can potentially increase data rate as a function of the number of spatial streams. However, the actual gain from spatial multiplexing is inversely proportional to the amount of spatial correlation between the streams. In addition to spatial separation, multipath signal propagation can reduce spatial correlation. In a system that has more receive antennas than the number of spatial streams, a technique known as receive diversity may be utilized to improve robustness. The signal from different receive antennas may be combined in a way that substantially increases SNR. The method is referred as maximal ratio combining (MRC). One optional feature of 802.11n is the usage of space-time block code (STBC). It is a coding technique that is used in order to improve reliability of data transfer by exploiting antenna diversity of a MIMO system. In STBC multiple encoded copies of the same data are transmitted from different antennas. The copies are combined at the receiver with an increased probability to correctly decode the original data. The STBC encoder divides the spatial streams further into space-time streams. If STBC is used in 802.11n, the number of space-time streams is always one or two larger than the number of spatial streams. The maximum number of space-time streams is four. Low density parity check (LDPC) code is an advanced coding method that has been introduced into 802.11n as an optional feature. Convolutional coding is still the only mandatory coding method. LDPC is a very efficient code and it can provide coding gain of a few decibels over convolutional code. Therefore it improves robustness of an 802.11n system, while maintaining the decoding complexity at a relatively low level.Standardization in IEEE 802.11, 802.16 397 Transmit beamforming (TxBF) is an optional capability that is provided in the 802.11n standard. In a MIMO system TxBF can be achieved by means of applying weights to the different transmit signals in such a way that reception is improved. It is possible to utilize TxBF for sending data and MRC for receiving data in a single multi-antenna device. An AP is a typical example of a device that benefits from such configuration. TxBF can provide relatively good performance improvements, but implies also increased hardware complexity. The TxBF method defined in 802.11n is based on knowledge of the propagation environment. To obtain the knowledge the channel needs to be sounded by transmitting a sounding packet between the two devices performing the TxBF. The transmit weighting matrix is calculated using the channel information. Commonly the singular value decomposition (SVD) method is used to solve the matrix. For simplicity, the details of SVD are not included in this discussion. If the communication channel is assumed to be reciprocal, implicit channel information feedback can be used. Implicit feedback means that channel sounding is done in the opposite direction as the TxBF. Explicit feedback on the other hand means that channel sounding and TxBF are done in the same direction. The explicit feedback method does not require reciprocal communication conditions, but there is overhead of sending the channel sounding results back to the transmitter. The TxBF SVD needs to be calculated and applied in digital baseband to each individual OFDM subcarrier before the RF and analog parts of the transmit chain. During the 802.11n development one requirement has been to provide some level of backwards compatibility with legacy 802.11a and 802.11g equipment. For this purpose the following three different PPDU frame formats have been defined.  Non-HT format  HT-mixed format  HT-greenfield format Support for the non-HT format and the HT-mixed format is mandatory, but the HT-greenfield format is optional. Packets of the non-HT format are legacy packets structured according to either 802.11a or 802.11g specification, depending on the used frequency band. In the HT-mixed format the PPDU contains both a legacy preamble and an HT preamble. Although 802.11a/g devices are only able to decode the legacy part of the PPDU, they will obtain information about the PPDU duration, which is utilized for the MAC protocol. The HT-mixed format therefore enables coexistence of 802.11n devices and 802.11a/g devices. The HT-greenfield format is intended for use in environments that contain exclusively 802.11n devices. Because it contains only the HT preamble, it provides the highest throughput of the different PPDU formats, but does not support optimal coexistence with 802.11a/g devices. 10.5.9 Future Developments Currently there are ongoing standardization activities in the 802.11 to further increase the throughput from 802.11n data rates. Originally the work has started in the very high throughput (VHT) study group even before the final version of 802.11n standard was ratified. Initially many different ideas were proposed to the group as the basis for the VHT work. Eventually two separate task groups emerged from the VHT study group. Each one of them is concentrating on different solutions. The 802.11ac task group (TGac) is developing a VHT WLAN standard for operation below 6 GHz frequency bands. The 802.11ad task group (TGad) is working on a 60 GHz VHT WLAN standard. The target aggregate throughput for multiple STAs in TGac is at least 1 Gbit/s measured at the MAC SAP interface. For single STA the target is 500 Mbit/s. There have been various PHY proposals for achieving the target. Some potential PHY techniques include using 80 MHz contiguous or even 160 MHz non-contiguous RF carrier, using higher order OFDM MCSs with 256-QAM and increasing the maximum number of spatial streams to eight in multi-user MIMO transmission.398 Modulation and Coding Techniques in Wireless Communications TGad is targeting for even higher data rates. For a single user the minimum target is 1 Gbit/s measured at the MAC SAP. There is a large amount of unlicensed spectrum available worldwide in the 60 GHz frequency band (typically between 57-66 GHz), which usually includes multiple 2 GHz RF carriers. The wide spectrum allows very high throughput, but the millimeter wave propagation has also some strict limitations. Path loss in 60 GHz is of a higher magnitude than in 2.4 or 5 GHz. To overcome the high path loss, directional antennas can be used both in transmitter and receiver side. Because the wavelength is relatively short, the antennas are also substantially smaller than with traditional WLAN devices. Consequently multi-element antenna arrays with a large number of antennas become a feasible solution. With directional antennas the multipath propagation is reduced, which makes MIMO SDM techniques less effective. Both single carrier and multi-carrier modulation methods are considered for 60 GHz. Physical layer coding is likely to build on 802.11n mechanisms. The options have different benefits and the selection will be based on requirements derived from usage models. 10.6 IEEE 802.16x In this section, we will focus on the standards with mobility support, in particular, IEEE 802.16e and IEEE 802.16m were selected. 10.6.1 Key PHY Features of the IEEE 802.16e In the following we give the key PHY features of mobile WiMAX technology (PHY and MAC are taken from IEEE 802.16e) and provide short descriptions. Scalable OFDMA OFDMA is the multiple access technique for mobile WiMAX. OFDMA is the Orthogonal Frequency Division Multiplexing (OFDM) based multiple access scheme and has become the de-facto single choice for modern broadband wireless technologies adopted in other competing technologies such as 3GPP’s Long Term Evolution (LTE) DL. OFDMA demonstrates superior performance in nonline-of-sight (N- LOS) multi-path channels with its relatively simple transceiver structures and allows efficient use of the available spectrum resources by time and frequency subchannelization.The simple transceiver structure of OFDMA also enables feasible implementation of advanced antenna techniques such as MIMO with reasonable complexity. Lastly, OFDMA employed in mobile WiMAX is scalable in the sense that by flexibly adjusting FFT sizes and channel bandwidths with fixed symbol duration and subcarrier spacing, it can address various spectrum needs in different regional regulations in a cost-competitive manner. TDD The mobile WiMAX Release 1 Profile has only TDD as the duplex mode even though the baseline IEEE Standards contains both TDD and Frequency Division Duplex (FDD). Even though future WiMAX Releases will have FDD mode as well, TDD is in many ways better positioned for mobile Internet services than FDD. First of all, Internet traffic is asymmetric typically with the amount of downlink traffic exceeding the amount of uplink traffic; thus, conventional FDD with the same downlink and uplink channel bandwidth does not provide the optimum use of resources. With TDD products, operators are capable of adjusting downlink and uplink ratios based on their service needs in the networks.Standardization in IEEE 802.11, 802.16 399 Advanced Antenna Techniques (MIMO and BF) Various advanced antenna techniques have been implemented in the mobile WiMAX Release 1 profile to enable higher cell and user throughputs and improve coverage. As a matter of fact, mobile WiMAX was the first commercially available cellular technology that actually realized the benefits of MIMO techniques promised by academia for years. With its downlink and uplink MIMO features, both operators and end-users enjoy up to twice the data rates of Single-Input Single-Output (SISO) rate, resulting in up to 37 Mbps for downlink and 10 Mbps for uplink sector throughput using just 10 MHz TDD channel bandwidth. Mobile WiMAX also enhances the cell coverage with its inherent beamforming (BF) techniques. Coupled with TDD operation, its powerful BF mechanism allows base stations to accurately form a channel matching beam to a terminal station so that uplink and downlink signals can reach reliably from and to terminals at the cell edge, thus effectively extending the cell range. Full Mobility Support Full mobility support is one of the main strength of the mobile WiMAX products. The baseline standard of mobile WiMAX was designed to support vehicles at highway speed with appropriate pilot design and Hybrid Automatic Repeat Request (HARQ), which helps to mitigate the effect of fast channel and interference fluctuation. The systems can detect the mobile speed and automatically switch between different types of resource blocks, called subchannels, to optimally support the mobile user. Furthermore, HARQ helps to overcome the error of link adaptation in fast fading channels and to improve overall performance with its combined gain and time diversity. Frequency Reuse One and Flexible Frequency Reuse From the operators’ perspective, securing greater frequency spectrum for their services is always costly. Naturally it is in their best interest if a technology allows decent performance in the highly interference- limited conditions with frequency reuse one. Mobile WiMAX technology was designed to meet this goal in a respectable way with its cell-specific subchannelization, low rate coding and power boosting and de-boosting features. It also enables real-time application of flexible frequency reuse where frequency reuse one is applied to terminals close to the cell centre whereas a fraction of frequency is used for terminals at the cell edge, thereby reducing heavy co-channel interference. The PHY transmission chains of OFDMA are illustrated in Figure 10.9. The blocks are the same with the small difference that OFDMA PHY includes a repetition block. The modulation is one of the four digital modulations described in the previous chapter: BPSK, QPSK, 16-QAM or 64-QAM. The modulated symbols are then transmitted on the OFDM orthogonal subcarriers. Physical PDU to be Randomization FEC encoder Interleaving transmitted TX signals Repetition Modulation IFFT Figure 10.9 OFDMA PHY Chain400 Modulation and Coding Techniques in Wireless Communications 10.6.2 IEEE 802.16m Targeted at IMT-Advanced (IMT-A), IEEE 802.16 working group (WG) had set up a new task group IEEE 802.16m by the end of 2006. The purpose of IEEE 802.16m TG is to propose an advanced air interface which will include enhancements and extension to IEEE 802.16-2004 1 and IEEE 802.16e- 2005 2. In addition to this, the new radio interface has been contributed to ITU-R as a radio technology proposal for IMT-A. The IEEE announced on 6th October 2009 that they had submitted a candidate radio interface technology for IMT-Advanced standardization in the Radio Communication Sector of the International Telecommunication Union (ITU-R). The proposal 4 documented that IEEE 802.16m met ITU-R’s challenging and stringent requirements in all four IMT-Advanced “environments”: Indoor, Microcellular, Urban, and High Speed. In this section we consider the layer 1 (L1) processing and procedures defined in 802.16m. In both the downlink and uplink direction, OFDMA is employed as a multiple access scheme. 802.16m supports both time division duplex (TDD) and frequency division duplex (FDD) modes, including half-FDD (H-FDD) 1 MS operation. Considering WirelessmMAN-OFDM system, the legacy WiMAX system which is built on top of 802.16e, it is very possible that TDD based system will be deployed first. Therefore, in this chapter we will focus on TDD features only. Frame Structure Basic Frame Structure The OFDM parameters defined in 802.16m are specified in Table 10.7. The 802.16m basic frame structure is illustrated in Figure 10.10. Each 20 ms superframe is divided into four equally-sized 5 ms radio frames. The first subframe of each superframe contains the super frame header (SFH). The SFH is divided into primary SFH (P-SFH) and secondary SFH (S-SFH).The P-SFH has a fixed size and it is transmitted in every superframe, whereas the S-SFH has a variable size. Advanced Preamble (A-Preamble) is transmitted at the beginning of every radio frame. When using the same OFDMA parameters as in Table 10.7 with the channel bandwidth of 5 MHz, 10 MHz, or 20 MHz, each 5 ms radio frame further consists of eight subframes for CP ratio G = 1/8 and 1/16. With the channel bandwidth of 8.75 and 7 MHz, each 5 ms radio frame further consists of seven and six subframes, respectively, for G = 1/8 and 1/16. In the case of G = 1/4, the number of subframes per frame is one less than that of other CP lengths for each bandwidth case. A subframe shall be assigned for either DL or UL transmission. There are four types of subframes: 1. Type-1 subframe consists of six OFDMA symbols, 2. Type-2 subframe consists of seven OFDMA symbols, 3. Type-3 subframe which consists of five OFDMA symbols, and 4. Type-4 subframe which consists of nine OFDMA symbols. This type shall be applied only to UL subframe for the 8.75MHz channel bandwidth when supporting the legacy 802.16e frames. The basic frame structure is applied to both FDD and TDD duplexing schemes, including H-FDD MS operation. The number of switching points in each radio frame in TDD systems is two, where a switching point is defined as a change of directionality, that is, from DL to UL or from UL to DL. In a TDD frame with DL to UL ratio of D:U, the 1st contiguous D subframes and the remaining U subframes are assigned for DL and UL, respectively, where D + U = 8 for 5, 10 and 20 MHz channel bandwidths, D + U = 7 for 8.75 MHz channel bandwidth, and D + U = 5 for 7 MHz channel bandwidth. The ratio of D:U shall be selected from one of the following values: 8:0, 6:2, 5:3, 4:4, or 3:5 for 5, 10 1 As in IEEE 802.16m Specification, the legacy WiMAX system is called WirelessMAN-OFDMA system.

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