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OFDMA FOR WIRELESS COMMUNICATION
In recent years, Wireless Communication networks have become very popular. With the rapid development of our modern society and internet technologies, people have become to express higher demands to the Wireless Communication services.3G Communication performances are already insufficient to meet the needs of future high performance applications. The next generation (4G) wireless Communication technology is already a conceptual framework.
The next generation wireless Communication networks will support a variety of multimedia service with high speed wireless communication services. To support the high speed wireless services, the innovative OFDMA (Orthogonal Frequency Division Multiple Access).OFDMA resembles spread also to as multi-user OFDM. IT can also be described as a combination of spectrum, where user can achieve data rate by assigning different code spreading factor or different number of spreading codes to each other. OFDMA is used in the mobility mode of IEEE 802.16 wireless MAN air interface standard.
This paper presents an architectural overview of IEEE 802.16. A detailed analysis of the OFDMA sub-channels and how OFDMA works, frame structures, QOS Mechanism, generic MAC frame, And variations of OFDMA and how they differ. And along with current and future applications, and also major benefits of this technology.
802.16/WI-MAX, MAC, OFDMA, Applications.
Wireless Communication is used as a term for sending information from one place to another place. Without using cables. OFDMA as one of the most favourable new access technology due to its ability to deliver high data rates, and reduce the consumption of limited resources such as spectrum and transmit power [8, 9] .OFDMA can achieve higher spectral efficiency wireless MAN Air interface standard. In the IEEE 802.16 standard the scheduling is left unspecified. However it has significant impact on system performance and QOS. In this paper we deliberate this problem. In this, the BS centrally allocates the channels in different slots to different SS's for uplink and downlink. The balance of this paper is organized as follows. Section 2 architectural overview of IEEE 802.16,section 3 OFDMA sub-channels and how it works, section 4 frame structures, section 5 QOS mechanism, generic MAC layer, section 6 variations of OFDMA and how they differ, section 7 future applications and section 8 major benefits of this technology and finally, the paper ends with conclusion.
1.Architectural overview of IEEE 802.16
The IEEE 802.16 standards defining the PHY and MAC layers for the Wireless Metropolitan Area Network (MAN) and is frequently refer to as WiMAX. It is designed to offer multimedia services with capacities of up to 40Mbit/s per channel over the cell radius between 3 and 10kilometer. The 802.16 standards discover the various modes of system operation, including specifications for NLOS and environment, as well as for both license and license-exempt frequency bands. This is reflected in multiple specifications for the PHY layer.
The need for a truly worldwide wireless access standard crystallized in August 1998 during the meeting called by National wireless electronics systems Tested (N-WEST) of the U.S National Institute of standards and Technology. The initiative was then passed to IEEE 802, which created the 802.16 working group on broadband wireless access (BWA) operation since July 1999.The initial 802.16 standard defined PHY and MAC for licensed bands in the frequency range of 10-66 GHZ. The allowed NLOS operation results in a much more difficult radio condition (more multipath) and therefore methods to combat this adverse effect had to be developed these methods, also standardized in 802.16a, include OFDM and OFDMA, extending in this way the set of available PHY layers. Both OFDM and OFDMA PHY layers are extremely robust to multipath propagation and, in addition they expose other very useful features, such as bandwidth efficiency and flexibility for resource allocation in the frequency domain, Applying wireless MAN-OFDM, which poses less stringent requirements for implementation than wireless MAN-OFDMA air interface.
802.16 Network Architecture:
Ø Wireless MAN has a point-to-multipoint architecture.
Ø Base station (BS) connected to public networks.
Ø Base station serves subscriber stations
Ø BS and SS stationary
Ø SS serves a building(business or residence)
Ø Users inside building connect to SS with Ethernet or wireless LANs
Ø Multiple services with different QOS priority, simultaneously .
3.OFDMA sub-channels And How OFDMA works:
In an OFDMA system. The sub-carriers are normally grouped into logical sub-channel defines the granularity of the resource allocation in the frequency domain. In the time domain, the granularity is determined by the slot length within an OFDMA sub-channel, the same modulation and coding scheme (PHY mode) is used for each sub-carrier .
The design of these sub-channels has a crucial impact on the performance of an OFDMA system, the performance is determined by the diversity of the sub-carriers within a sub-channel and the diversity between the sub-cannels. Furthermore, the accuracy of the SNR estimation at the transmitter, which depends on the Doppler shift, is of significant relevance .
Orthogonal sub-carriers 
cyclic prefix in Time Domain 
Cyclic prefix in Frequency Domain [6
The quality of an OFDMA sub-channel from MAC point of view is described by an effective SNR level. This SNR level can be determined in different manners. In the following the effective SNR is the mean SNR of all sub-carriers of a sub-channel which is a common assumption, example sub-carriers with bad channel conditions can be compensate by sub-carriers with good conditions within an OFDMA sub-channel due to channel coding.
A subset of subcarriers is grouped together to form a sub-channel. A transmitter is assigned one or more sub-channels in DL direction (16 sub channels are supported in UL in OFDM PHY).sub-channels provide interference averaging benefits for aggressive reuse systems .
Ø Divides into Multiple sub-signals
Ø Orthogonal sub-carriers
Ø IFFT at Tx, FFT at Rx
Ø Time and frequency resources separate the multiple user signal
Ø Different groups of sub-carrier and OFDM symbols
Ø Allocation in frequency domain
Ø Formation of the sub-channel from sub-carriers
Ø Adjacent sub-carrier method (ASM) and diversity sub-carrier method (DSM)
Ø Time for ASM and DSM-Multiple of OFDM symbols .
Desired properties of frame structure for scalable OFDMA operation can be summarized as follows.
Ø Fixed tone spacing for unified frame structures.
Ø Symbol time trade off between guard time overhead for delay spread and ICI from Doppler spread.
Ø Short frame length for fast link control especially for TDD systems.
Ø Dynamic allocation of diversity channels for mobile users and frequency selective AMC channels for stationary users.
Ø Dedicated signalling channel for fast adaptive modulation coding and H-ARQ.
Ø Dedicated uplink time slot for initial random access to reduce interference to traffic channels.
For a TDD system, each frame starts with downlink, a BS to SS transmission. The downlink transmission begins with two preambles followed by a SICH symbol. In the uplink, transmission begins with control symbols. In order to allow BS to turn around, TTG and RTG shall be inserted between downlink (DL) and uplink (UL) in the middle of a frame and at the end of a frame, respectively. The number of downlink and uplink symbols can be changed with a granularity of six symbols.
A. Downlink Frame Structure:-
Preamble: Used for synchronization and equalization.
Ø Frame control sequence: Contains DL-MAP and UL-MAP messages along with downlink and uplink channel description (DCD, UCD).
Ø TDM portions carry data organized into bursts with different burst profiles and hence different levels of transmission robustness.
Ø DL-MAP message states the PS's at which different bursts begin in the downlink sub frame.
Ø UL-MAP message defines the usage of uplink and contains the grants addressed to SS's.
Ø DCD messages include following parameters :
Ø BS transmit power
Ø PHY type
FDD/TDD frame duration.
Downlink burst profile parameters like modulation type, FEC code type, last (shortened) codeword length .
B. Uplink Frame Structure:-
Ø SS's granted bandwidth in UL-MAP message in downlink sub frame.
Ø SS's transmit in the assigned allocation using burst profiles tagged with uplink interval usage code (UIUC) in UL-MAP message.
Ø Consists of contention based allocations for initial system access.
-Ranging requests (RNG-REQ)
-Contention resolved using the truncated exponential back off
Ø Transmissions have synchronization preamble.
Ø Ideally all data from a single SS is concatenated into a single PHY burst .
Quality of Service Mechanism:-
The data/control plane defines a means of quality of services (QOS) functionality based upon the concept of service flows. These service flows are unidirectional flows of data that are characterized by a set of quality of service parameters and are a means for the MAC to define transmission ordering and scheduling for packets within that service flow. However, the use of a named service class is optional and an explicit set of quality of service parameters can be defined for a service flow instead.
Although the IEEE 802.16 standard is already a very developed and sophisticated, the various protocol mechanisms, may be used to support QOS for both uplink and downlink through the SS and BS. The requirements for QOS include the following:
Ø A configuration and registration function for pre-configuring SS-based QOS service flows and traffic parameters.
Ø A signalling function for dynamically establishing QOS enabled service flows and traffic parameters.
Ø Utilization of MAC scheduling and QOS traffic parameters for uplink service flows.
Ø Utilization of QOS traffic parameters for downlink service flows.
Ø Grouping of service flow properties into named service classes, so upper-layer entities and external applications (at both the SS and BS) may request service flows with desired QOS parameters in globally consistent way .
The mechanism for providing QOS associate packets traversing the MAC interface into a service flow as identified by the transport CID.A service flow is a unidirectional flow of packets that is provided a particular QOS. The SS and BS provide this QOS according to this parameter set defined for the service flow. The importance of the QOS features defined in the standard is to define transmission ordering and scheduling on the air interface. A service flow is a MAC transport service that provides unidirectional transport of packets either to uplink packets transmitted by the SS or downlink packets transmitted by the BS. A service flow is characterized by a set of QOS parameters such as latency, jitter, and throughput assurances. SS and BS attributes include details of how the SS requests uplink bandwidth allocations and the expected behaviour of the BS uplink scheduler. It is useful to think of three types of service flows :
This type of service flow is known via provisioning by, for example, the network management system.
This type of service flow has resources reserved by the BS for its admitted QOS parasite, but these parameters are not active admitted service flows may have been provisioned or may have been signalled by some other mechanism.
This type of service flow has resource committed by the BS for its active QOS parasite; its active QOS parasite is non-null .
5.Medium access control layer:
The IEEE 802.16 MAC is a scheduling one where the subcarrier station (SS) that wants to connect to the network, has to compete once when it initially enters the network. A time slot allocation is made by the base station (BS) which can be enlarged and constricted. It remains to the subscriber station. It means that other stations are not supposed to use same resources. This scheduling algorithm has its advantages since it remains stable under overload and oversubscription, has more bandwidth efficiency and also it allows the base station to control QOS. The main aim of the MAC layer is to manage the resources of the air interface efficiently; Indeed, Access and bandwidth allocation algorithm must reserve hundreds of terminals per channel. Those terminals can eventually be shared by multiple end users. To support a large variety of services such as voice, data or internet connection, the 802.16 MAC must accommodate both continuous and burst traffic . The issues concerning the transport efficiency are also addressed at the interface between the MAC and the PHY layers the modulation and coding schemes are specified in a burst profile adjusted in function of each burst sent to each SS.
The MAC builds the downlink sub frame starting with a frame control section containing the DL-MAP and UL-MAP messages. These indicate PHY transitions on the downlink as well as bandwidth allocations and burst profiles on the uplink. The advanced technology of the 802.16 PHY requires equally advanced radio link control (RLC), particularly the capability of the PHY to transition from one well as the traditional RLC functions of power control and ranging .
The IEEE 802.16 MAC accommodates two classes of SS, differentiated by their ability to accept bandwidth grants simply for a connection or for the SS as a whole. Both classes of SS request bandwidth per connection to allow the BS uplink scheduling algorithm to properly consider QOS when allocating bandwidth In bandwidth grant per subscriber station(GPSS), base station grants bandwidth to the subscriber station(SS), and SS in turn may re-distribute bandwidth among its connections, maintaining QOS and service-level agreements . This method is suitable for many connections per terminal by off-loading base station work.
Subscriber station (SS) initialization has several steps:
Ø The SS scans for downlink channel and establishes synchronization with the BS. Then, it obtains transmit parameters.
Ø It performs ranging and negotiating basic capabilities then it is authorized by the BS and performs key exchange.
Ø It performs the registration and IP connectivity establishment. Then it's the turn of time day establishment and the transfer of operational parameters.
Ø It sets up the connections .
In this, we introduce the different techniques. Those are used in the physical layer: Orthogonal Frequency Division Multiple Access (OFDMA) those techniques have been developed for the last few years to deliver broad band services that can be compared to those of wired services in terms of data rates. The main issue apply for the PHY layer is to allocate the resources efficiently by assigning a set of subcarriers and by determining the number of bits to be transmitted for each subcarrier in an OFDMA system .
OFDMA Physical layer:
OFDMA is referred as multiuser-OFDM as it uses OFDM as multiple access method especially for the wireless communications.
The OFDMA PHY layer is based on OFDM and it supports sub-channelization schemes are supported to this. The OFDMA PHY layer supports both FDD and TDD operations CC is the required coding scheme. By the specification and these code rates are same as the ones supported by the OFDM PHY layer .
3. Variations of OFDMA and how they differ:
Various flavours of OFDMA Concept of sub-channelization allow four variations :
a. Frequency Division Multiplexing (FDM)  :
Frequency Division Multiplexing (FDM) sub-carriers
b.Orthogonal Frequency Division Multiplexing 
subcarriers in Orthogonal Frequency Division Multiplexing (OFDM)
c.Difference between OFDM and OFDMA 
uplink subchannelization in OFDM and OFDMA
OFDM: - Only one SC transmits in one timeslot.
OFDMA: - Several SC's transmitted at the same time slot over several sub-channels.
Two types of variations described below:
a.Scalable OFDMA (SOOFDMA)
Ø Adds scalability
Ø Reduces system complexity of smaller channels and improves performance of wider channels .
Ø First commercially deployed OFDMA based mobile broadband wireless communications system.
Ø Capability to work around interfering signals .
Applications of OFDMA:
OFDMA offers significant advantages as a multiple access scheme for BWA system. Below we list their most evident examples .
Ø OFDMA is robust to multipath propagation since it derives from OFDM scheme.
Ø OFDMA is bandwidth-efficient.
Ø OFDMA is relatively easy to implement in the digital domain by the use of (I) FFT.
Ø OFDMA partitions the frequency resource in a number of independent sub-carriers. This providing various levels of QOS.
Ø Its resource granularity opens up plenty of possibilities for various allocations of frequency resource to multiple users. They can be used either to minimize the transmit power, maximize the data rate, extend the system's coverage, or make a system more robust to difficult radio conditions.
Ø OFDMA, thanks to its inherent parallelism in the frequency domain, allows for independent per-subcarrier processing this may include per-subcarrier (or group of subcarriers such as sub channel in IEEE 802.16) adaptation of modulation modes or power control .
3.Major benefits of this technology:
A challenge in today's wireless systems is an effect called “Multipath”. Multipath results from reflections between a transmit and receive where by the reflections arrive at the receiver at different times. The time span separating the reflection is referred to as delay spread. This type of interference tends to be problematic when the delay spread is on the order of the transmitted symbol time. Typical delay spread is microseconds in length, which are close to CDMA symbol time. OFDMA symbol times tend to be on the order of 100 (microsecond), making multipath less of a problem. Another benefit of OFDMA is the use of advanced multi-antenna signal processing technical.
The two most common techniques are called multipath input, multipath output processing and beam forming
“OFDMA for Wireless Communications” is very essential with in growing of communication technology especially when most of them have got different platforms. This is only the one way to create them all technologies consistent with each other and this going to start an evolution in “Wireless Communication” world.
 Slawomir Pietrzyk. 1st edition, “OFDMA FOR BROADBAND WIRELESS ACCESS”. April 30, 2006, pages 47-50,-221.
Carl Eklund, Roger B.Marks, Kenneth L.Stanwood, Presented by.Manasi Navare
802.16/WIMAX, EECS 228a, spring 2006, shyam parekh