Telecommunication baseband system

Chapter 1

1 Introduction

1.1 Project Overview

As 4G is emerging and the world is moving toward communication with more data rate and more mobility. Till now UMTS and WLAN are fulfilling demands of people to some extent but the demand is increasing day by day. The technology evolving now to fulfill this demand is WiMAX (World Wide Interoperability for Microwave Access). Using orthogonal frequency division multiplexing OFDM and multiple input multiple output MIMO, WiMAX combines the high data rate of WLAN and the Range of UMTS to serve users with greater speed at greater mobility.

WiMAX is an emerging technology which can be used in a variety of ways. Firstly it can be used in a fixed environment and on the other hand it can be used in a moving or mobile environment. For fixed environment, WiMAX has the standard IEEE 802.16d and for mobile, IEEE 802.16e (OFDMA).

In this project, we have designed a baseband processor i.e. working of base station for WiMAX. We designed a transmitter, a channel and a receiver in MATLAB SIMULINK. The design was first simulated through simple AWGN channel. After that, the design of the 2*2 Rayleigh fading channel was then introduced for which OSTBC was used. The simulator is thoroughly checked and analyzed for different modulation schemes and a comparison is made between them. It is also expected that we will (INSHALLAH) implement the Rayleigh channel in DSP platform to check the hardware results. The specifications of WiMAX and its implementation in SIMULINK are explained in chapter 2 and 3 of this report.

1.2 Project Objective

A question may be arising in your mind that why we have selected such kind of project? While using WiMAX to achieve higher data rates at lower power and lower cost, they systems to be designed become more complex and more difficult to test. Using MIMO can increase the capacity but the complexity, on the other hand, also increases. Because of these increased requirements, a flexible test bench is needed to assess system performance in real operating conditions. Once done, it can be used or can be enhanced for a variety of applications in actual implementation of WiMAX. In a sense, there are a lot of objectives that can be concluded for this project but following are the main:

  • To make a test bench for WiMAX baseband system
  • To include Rayleigh fading channel into WiMAX design
  • To analyse and compare SIMULINK results using different modulation schemes
  • To implement the 2*2 Rayleigh channel in DSP platform (optional)

Chapter 2

2 Background

2.1 Introduction

Telecommunication system is called broadband if a wide band of frequencies is available to transmit information. Telecommunication without any physical medium between transmitter and receiver is called Wireless communication. Wireless and broadband both have a great market share in telecommunication industry. Nowadays DSL (Digital Subscriber Line) and Cable Modem Technologies are used as broadband access. As land line telephony has been shifted to cellular telephony, the broadband is also going toward wireless. The two main types of broadband wireless are fixed and mobile broadband wireless. Fixed wireless broadband can be thought of as a competitive to DSL or cable modem. Mobile broadband, offers the additional functionality of portability, nomadicity, and mobility.

WiMAX (worldwide interoperability for microwave access) technology, the subject of our interest, is designed to accommodate both fixed and mobile broadband applications.

2.2 Background on IEEE 802.16 and WiMAX

Worldwide Interoperability for Microwave Access, WiMAX, is a telecommunications technology that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access. The technology is based on the IEEE 802.16 standard (also called Broadband Wireless Access). The name "WiMAX" was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard. In a typical cell radius deployment of three to ten kilometres, WiMAX Forum Certified systems can be expected to deliver capacity of up to 40 Mbps per channel, for fixed and portable access applications. This is enough bandwidth to simultaneously support hundreds of businesses with T-1 speed connectivity and thousands of residences with DSL speed connectivity. Mobile network deployments are expected to provide up to 15 Mbps of capacity within a typical cell radius deployment of up to three kilometres.

Initially the development of a LOS based point-to-multipoint wireless broadband system for operation in the 10GHz-66GHz millimeter wave band was developed under the standard based on a single-carrier physical (PHY) layer with a burst time division multiplexed (TDM) MAC layer.

The IEEE 802.16 group subsequently produced 802.16a, an amendment to the standard, to include NLOS applications in the 2GHz-11GHz band, using an orthogonal frequency division multiplexing (OFDM)-based physical layer. Additions to the MAC layer, such as support for orthogonal frequency division multiple access (OFDMA), were also included. Further revisions resulted in a new standard in 2004, called IEEE 802.16-2004, which replaced all prior versions and formed the basis for first WiMAX solution. These early WiMAX solution based on IEEE 802.16-2004 targeted fixed applications and we will refer to these as fixed WIMAX [1]. In December 2005, the IEEE group completed and approved IEEE 802.16e-2005, an amendment to the IEEE 802.16-2004 standard that added mobility support. The IEEE 802.16e-2005 forms the basis for the WiMAX solution for nomadic and mobile applications and is often referred to as mobile WiMAX [2].

The basic characteristics of various IEEE 802.16 standards are summarized in Table 2.1. The WiMAX Forum defined limited number of system profiles and certification profiles. A system profile defines the subset of mandatory and optional physical- and MAC-layer features selected by the WiMAX Forum from the IEEE 802.16-2004 or IEEE 802.6e-2005 standard. Currently, WiMAX Forum has two different system profiles: one based on IEEE 802.16-2004, OFDM PHY, called the fixed system profile; the other one based on IEEE 802.16e-2005 scalable OFDMA PHY, called the mobility system profile. A certification profile is defined as a particular instantiation of a system profile where the operating frequency, channel bandwidth, and duplexing mode are also specified. WiMAX equipment is certified for interoperability against a particular certification profile.

2.3 Physical Layer of WiMAX

Physical layer of WiMAX is based on orthogonal frequency division multiplexing. high-speed data, video, and multimedia communication can be possible using OFDM. OFDM is used by a variety of commercial broadband systems, including DSL, WiFi, Digital video broadcast-Handheld (DVB-H), besides WiMAX. for high data rate transmission in a NLOS or multipath environment OFDM is an elegant and efficient scheme.

2.3.1 OFDM Basics

A multicarrier modulation scheme, which includes OFDM, is based on the idea of dividing of a given high-bit-rate data stream into several parallel lower bit rate streams and modulating each stream on separate carriers. Intersymbol interference (ISI) is minimized using multi carrier modulation schemes by making the symbol time large enough. In high-data-rate systems in which the symbol duration is small (inversely proportional to the data-rate), splitting of the data stream into many parallel streams increases the symbol duration of each stream such that the delay spread is only a small fraction of the symbol duration.

OFDM is spectrally efficient version of multicarrier modulation, in which the sub-carriers are selected such that they are all orthogonal to one another over the symbol duration thereby avoiding the need to have non overlapping subcarrier channel to eliminate interccarrier interference. By selecting the first subcarrier to have a frequency such that it has an integer number of cycle in a symbol period, and setting the spacing between adjacent subcarriers (subcarrier bandwidth) to be Bsc=B/L, where B is the nominal bandwidth (equal to data rate), and L is the number of subcarriers ensures that all tones are orthogonal to one another over the symbol period. It can be shown that the OFDM signal is equivalent to the inverse discrete Fourier transform (IDFT) of the data sequence block taken L at a time. This makes it extremely easy to implement OFDM transmitters and receivers in discrete time using IFFT (inverse fast Fourier) and FFT, respectively.

In order to completely eliminate ISI, guard intervals are used between OFDM symbols. By making the guard interval larger than the expected multipath delay spread, ISI can be completely eliminated. Adding a guard interval, however, implies power wastage and a discrete in bandwidth efficiency. The amount of power wasted depends on how large a fraction of the OFDM symbol duration the guard time is. Therefore, the larger the symbol period (for a given data-rate, this means more subcarriers) the smaller the loss of power and bandwidth efficiency.

The size of the FFT in an OFDM design should be chosen carefully as a balance between protection against multipath, Doppler shift, and design cost/complexity. For a given bandwidth, selecting a large FFT size would reduce the subcarrier spacing and increase the symbol time. This makes it easier to protect against multipath delay spread. A reduced sub carrier spacing, however, also makes the system more vulnerable to inter carrier interface owing to Doppler spread in mobile applications. The competing influences of delay and Doppler spread in an OFDM design require careful balancing.

2.3.2 OFDM Parameters in WiMAX

The fixed and mobile versions of WiMAX have slightly different implementation of OFDM physical layer. Fixed WiMAX, which is based on IEEE802.16-2004, uses a 256 FFT-Based OFDM physical layer. Mobile WiMAX, which is based on the IEEE802.16e-2005 standard, uses a scalable OFDMA-based physical layer. In the case of mobile WiMAX, the FFT sizes can vary from 128 bits to 2,048 bits.

Fixed WiMAX OFDM-PHY: For this version the FFT size is fixed at 256, which 192 subcarriers used for carrying data, 8 used as pilot subcarriers for channel estimation and synchronization purposes, and the rest use as guard band subcarriers. Since the FFT size is fixed, the subcarrier spacing varies with channels bandwidth. When larger bandwidths are used, the subcarriers spacing increases and the symbol time decreases. Decreasing symbol time implies that a larger fraction needs to be allocated as guard time to overcome delay spread as table shows, WiMAX allows a wide range of guard times that allow system designers to make appropriate trade-offs between spectral efficiency and delay spread robustness for maximum delay spread robustness, a 25% guard time can be used, which can accommodate delay spreads up to 16s when operating in a 3.5 MHz channel and up to 8s when operating in 7MHz channel in relatively benign multipath channels, the guard time overhead may be reduced to as little as 3%.

MOBILE WiMAX OFDMA-PHY: In Mobile WiMAX, the FFT size is scalable from 128 to 2,048. Here, when the available bandwidth increases, the FFT size is also increased such that the subcarrier spacing is always 10.94 kHz. This keeps the OFDM symbol duration, which is the basic resource unit, fixed and therefore makes scaling have minimal impact on higher layers. A scalable design also keeps the cost low. The subcarrier spacing of 10.94 kHz was chosen as good balance between satisfying the delay spread and Doppler spread requirements for operating in mixed fixed and mobile environment. This subscriber spacing can support delay spread values up to 20 s and vehicular mobility up to 125kmph when operating in 3.5GHz. A subcarrier spacing of 10.94 kHz implies that 128, 512, 1,024, and 2,048 FFT are used when the channel bandwidth is 1.25MHz, 5MHz, 10MHz, and 20MHz, respectively. It should, however, be noted that mobile WiMAX may also include additional bandwidth profiles.

2.3.3 Subchannelization: OFDMA

The available subcarriers may be divided into several groups of subcarriers called subchannels. Fixed WiMAX based on OFDM-PHY allows a limited form of subchannelization in the uplink only. The standard defines 16 subchannels, where 1, 2, 4, 8, or all sets can assigned to a subscriber station (SS) in the uplink. Uplink subchannelization in fixed WiMAX allows subscriber stations to transmit using only a fraction (as low as 1/16) of the bandwidth allocated to it by the BS, which provides link budget improvements that can be used to enhance range performance and/or improve battery life of subscriber stations. A 1/16 subchannelization factor provides a 12 db link budget enhancement.

Mobile WiMAX based on OFDMA-PHY, however, allows subchannelization in both the uplink and the downlink, and here, subchannels form the minimum frequency resource-unit allocated by the BS. Therefore, different subchannels may be allocated to different users as a multiple-access mechanism. This type of multi-access scheme is called orthogonal frequency division multiple access (OFDMA), which gives the mobile WiMAX PHY its name.

Subchannels may be constituted using either contiguous subcarriers or subcarriers pseudo randomly distributed across the frequency spectrum. Subchannels formed using distributed subcarriers provide more frequency diversity, which is particularly useful for mobile applications. WiMAX defines several subchannelization schemes based on distributed carriers for both the uplink and the downlink. One, called partial usage of subcarriers (PUSC), is mandatory for all mobile WiMAX implementations. The initial WiMAX profiles define 15 and 17 subchannels for the downlink and the uplink, respectively, for PUSC operation in 5MHz bandwidth. For 10MHz operation, it is 30 and 35 channels, respectively.

The subchannelization scheme based on contiguous subcarriers in WiMAX is called band adaptive modulation and coding (AMC). Although frequency diversity is lost, band AMC allows system designers to exploit multi-user diversity, allocating subchannels to users based on their frequency response. Multi-user diversity can provide significant gains in overall system capacity, if the system strives to provide each user with a subchannel that maximizes its received SINR. In general, contiguous subchannels are more suited for fixed and low-mobility applications.

2.3.4 Slot and Frame structure

The WiMAX PHY layer is also responsible for slot allocation and framing over the air. The minimum time-frequency resource that can be allocated by a WiMAX system to a given link is called a slot. Each slot consists of one subchannel over one, two, or three OFDM symbols, depending on the particular subchannelization scheme used. A contiguous series of slots assigned to a given user is called that users data region; scheduling algorithms could allocate data regions to different users, based on demand, QoS requirements, and channel conditions.

Figure 2.1 shows an OFDMA and OFDM frame when operating in TDD mode. The frame is divided into two subframes: a downlink frame followed by an uplink frame after a small guard interval. The downlink-to-uplink-subframe ratio may be varied from 3:1 to 1:1 to support different traffic profiles. WiMAX also supports frequency division duplexing, in which case the frame structure is the same except that both downlink and uplink are transmitted simultaneously over different carriers. Some of the current fixed WiMAX systems use FDD. Most WiMAX deployments, however, are likely to be in TDD mode because of its advantages. TDD allows for a more flexible sharing of bandwidth between uplink and downlink, does not require paired spectrum, has a reciprocal channel that can be exploited for spatial processing, and has a simpler transceiver design. The downside of TDD is the need for synchronization across multiple base stations to ensure interference-free coexistence. Paired band regulations, however, may force some operators to deploy WiMAX in FDD mode.

As shown in figure 2.1, the downlink subframe begins with a downlink preamble that is used for physical-layer procedures, such as time and frequency synchronization and initial channel estimation. The downlink preamble is followed by a frame control header (FCH), which provides frame configuration information, such as the MAP message length, the modulation and coding scheme, and the usable subcarriers. Multiple users are allocated data regions within the frame, and these allocations are specified in the uplink and downlink MAP messages (DL-MAP and UL-MAP) that are broadcast following the FCH in the downlink subframe. MAP messages use the burst profile for each user, which defines the modulation and coding scheme used in that link. Since MAP contains critical information that leads to reach all users, it is often sent over a very reliable link, such as BPSK with rate coding and repetition coding. Although the MAP messages are an elegant way for the base station to inform the various users for its allocations and burst profiles on a per-frame basis, it could form a significant over head, particularly when there are a large number of users with small packets (e.g.: VoIP) for which allocation needs to be specified. To mitigate the overhead concern, mobile WiMAX system can optionally use multiple sub-MAP messages where the dedicated control messages to different users are transmitted at higher-rates, based on their individual SINR conditions. The broadcast MAP messages may also optionally be compressed for additional efficiency.

WiMAX is quite in terms of how multiple users and packets and multiplexed in a single frame. A single downlink frame may contain multiple burst of varying size and type carrying data for several users. The frame size is also variable on a frame-by-frame basis from 2 ms to 20 ms, and each burst can contain multiple concatenated fixed-size or variable-size packets or fragments of packets received from the higher layers. At least initially, however, all WiMAX equipment will support only 5 ms frames.

The uplink subframe is made up of several uplink bursts from different users. A portion of the uplink subframe is set aside for contention-based access that is used for a variety of purposes. This subframe is used mainly as a ranging channel to perform closed-loop frequency, time and power adjustments during network entry as well as periodically afterward, the ranging channel may also be used by SS or MS to make uplink bandwidth requests. In addition, best-effort data may be sent on this contention-based channel, particularly when the amount of data to send is too small to justify requesting a dedicated channel. Besides the ranging channel and the traffic bursts, the uplink subframe has a channel-quality indicator channel (CQICH) for the SS to feedback channel-quality information that can be used by the BS scheduler and an acknowledgment (ACK) channel for the SS to feedback downlink acknowledgments.

To handle the time variations, WiMAX optionally supports repeating preambles more frequently. In the uplink, short preambles, called midambles, may be used after 8, 16, or 32 symbols; in the downlink, a short preamble can be inserted at the beginning of each burst. It is estimated that having a midamble every 10 symbols allows mobility up to 150 kmph.

2.3.5 Adaptive Modulation and Coding in WiMAX

WiMAX supports a variety of modulation and coding schemes allows for the scheme to change on a burst-by-burst basis per link, depending on channel conditions. Using the channel-quality feedback indicator, the mobile can provide the BS with feedback on the downlink channel quality. For the uplink, the BS can estimate the channel quality, based on the received signal quality. The base station scheduler can take into account the channel quality of each users uplink and downlink and assign a modulation and coding scheme that maximizes the throughput for the overall system capacity, as it allows real-time trade-offs between throughput and robustness on each link.

Table 2.3 lists the various modulation and coding schemes supported by the WiMAX. In the downlink, QPSK, 16 QAM, and 64 QAM are mandatory for both fixed and mobile WIMAX; 64 QAM is optional in the uplink. FEC coding using convolutional codes is mandatory. Convolutional codes are combined with an outer REED-Solomon code in the downlink for OFDM-PHY. The standard optionally supports turbo codes and low-density parity check (LDPC) codes at a variety of code rates as well. A total of 52 combinations of modulation and coding schemes are defined in WiMAX as burst profiles.

2.3.6 Physical Layer Data Rates

Because the physical layer of WiMAX is quite flexible, data rate performance varies based on the operating parameters. Parameters that have a significant impact on the physical-layer data rate are channel bandwidth and the modulation and coding scheme used. Other parameters, such as number of channels, OFDM guard time, and over sampling rate, also have an impact.

Table 2.4 lists the Physical layer data rate at various channel bandwidths, as well as modulation and coding schemes. The rates shown are aggregate physical layer data rate that is shared among all users in the sector of the TDD case, assuming a 3:1 downlink-to-uplink bandwidth ratio. The calculations here assumes a frame size of 5 ms, a 12.5% OFDM guard interval overhead, and a PUSC subcarrier permutation scheme. It is also assumed that all usable OFDM data symbols are available for user traffic except one symbol used for downlink frame overhead. The numbers shown here do not assume spatial multiplexing using multiple antennas at the transmitter and the receiver, the use of which can further increase the peak rates in fixed multipath channels.

2.4 WiMAX Network Architecture

The IEEE 802.16e-2005 standard provides the air interface for the WiMAX but does not define the full end-to-end WiMAX network. The WiMAX Forums Network Working Group, is responsible for developing the end-to-end network requirements, architecture, and protocols for WiMAX, using IEEE 802.16e-2005 as the air interface.

2.4.1 Network Reference Model

The WiMAX NWG has developed a network reference model to serve as an architecture framework for WiMAX deployments and to ensure interoperability among various WiMAX equipment and operators. The network reference model envisions a unified network architecture for supporting fixed, nomadic, and mobile deployments and is based on an IP service model. Figure 2.3 shows a simplified illustration of an IP-based WiMAX network architecture. The overall network may be logically divided into three parts: (1) mobile stations used by the end user to access the network, (2) the access service network (ASN), which comprises one or more base stations and one or more ASN gateways that form the radio access network at the edge, and (3) the connectivity service network (CSN), which provides IP connectivity and all the IP core network functions.

Business entities:

The architecture framework is defined such that the multiple players can be part of the WiMAX service value chain. More specifically, the architecture allows for three separate business entities: (1) network access provider (NAP), which owns and operates the ASN; (2) network service provider (NAP), which provides IP connectivity and WiMAX services to subscribers using the ASN infrastructure provided by one or more NAPs; and (3) application service provider (ASP), which can provide value-added services such as multimedia applications using IMS (IP multimedia subsystem) and corporate VPN (virtual private networks) that run on top of IP. This separation between NAP, NSP, and ASP is designed to enable a richer ecosystem for WiMAX service business, leading to more competition and hence better services.

Functional entities:

The network reference model developed by the WiMAX Forum NWG defines a number of functional entities and interfaces between those entities. Figure 2.3 shows some of the more important functional entities.

Base stations (BS): The BS is responsible for providing the air interface to the MS. Additional functions that may be part of the BS are micromobility management functions, such as handoff triggering and tunnel establishment, radio resource management, QoS policy enforcement, traffic classification, DHCP (Dynamic Host Control Protocol) proxy, key management, session management, and multicast group management.

Access service network gateway (ASN-GW): The ASN gateway typically acts as a layer 2 traffic aggregation point within an ASN. Additional functions that may be part of the ASN gateway include intra-ASN location management and paging, radio resource management and admission control, caching of subscriber profiles and encryption keys, AAA client functionality, establishment and management of mobility tunnel with base stations, QoS and policy enforcement, foreign agent functionality for mobile IP, and routing to the selected CSN.

Connectivity service network (CSN): the CSN provides connectivity to the internet, ASP, other public networks, and corporate networks. The CSN is owned by the NSP and includes AAA servers that support authentication for the devices, users, and specific services. The CSN also provides per user policy management of QoS and security. The CSN is also responsible for IP address management, support for roaming between different NSPs, location management between ASNs, and mobility and roaming between ASNs. Further, CSN can also provide gateways and interworking with other networks, such as PSTN (public switched telephone network), 3GPP, and 3GPP2.

Decomposition and combination of functional entities:

The WiMAX architecture framework allows for the flexible decomposition and /or combination of functional entities when building the physical entities. For example, the ASN may be decomposed into base station transceiver (BST), base station controllers (BSC), and an ASN-GW analogous to the GSM model of BTS, BSC, and serving GPRS support node (SGSN). It is also possible to collapse the BS and ASN-GW into a single unit, which could be thought of as a WiMAX router. Such a design is often referred to as a distributed, or flat, architecture. By not mandating a single physical ASN or CSN topology, the reference architecture allows for vendors/operators differentiation.

Reference points:

In addition to functional entities, the reference architecture defines interfaces, called reference points, between function entities. The interfaces carry control and management protocols mostly IETF-developed network and transport-layer protocols in support of several functions, such as mobility, security, and QoS, in addition to bearer data. Figure 2.4 shows an example.

The WiMAX network reference model defines reference points between: (1) MS and the ASN, called R1, which in addition to the air interface includes protocols in the management plane, (2) MS and CSN, called R2, which provides authentication, service authorization, IP configuration, and mobility management, (3) ASN and CSN, called R3, to support policy enforcement and mobility management, (4) ASN and ASN, called R4, to support inter-ASN mobility, (5) CSN and CSN, called R5, to support roaming across multiple NSPs, (6) BS and ASN-GW called R6, which consists of intra-ASN bearer paths and IP tunnels for mobility events, and (7) BS to BS, called R7, to facilitate fast, seamless handover. The network reference model along with the reference points are shown in figure 2.5.

2.5 MIMO Theory and OSTBC

MIMO (multiple-input-multiple-output) is a technique in which multiple antennas are used at the transmitter and receiver to tackle fading channel impairments shown in figure 2.6. Orthogonal space-time block coding in combination with orthogonal frequency-division multiplexing converts frequency-selective (MIMO) channels into a set of equivalent scalar Gaussian channels. There has been considerable work on a variety of new codes and modulation signals, called Space-time codes, in order to approach the huge capacity of such MIMO channels. One scheme of particular interest is the Alamouti scheme for two transmits antennas. More general schemes referred to as orthogonal space-time block codes (OSTBC) with the same properties as the Alamouti scheme like, e.g., a remarkably simple maximum-likelihood decoding algorithm.

The performance of orthogonal space-time block codes with respect to mutual information was analyzed for the uncorrelated Rayleigh fading. OSTBC exploit multiple antennas at both the transmitter and receiver in order to obtain transmit and receive diversity and therefore increase the reliability of the system.

Orthogonal space-time block coding (OSTBC) has emerged as one of the most promising techniques to exploit spatial diversity and to combat fading in multiple-input multiple-output (MIMO) systems. The special structure of OSTBCs implies that, assuming that the MIMO channel is known at the receiver, the optimal maximum likelihood (ML) decoder is a simple linear receiver, which can be seen as a matched followed by a symbol-by-symbol detector.

Chapter 3

3 Design, explanation and Analysis

3.1 Introduction

Our design is the model of WiMAX baseband processor using SIMULINK in MATLAB. The model is a useful tool for performance evaluation of WiMAX under different data rates, coding schemes and channel conditions. Standards from IEEE and ETSI have been used to develop this model. The model is built with a standard input using convolutional encoding rate of 5/6 with QPSK modulation and transmitted with 256 carrier OFDM symbols.

As any other communication system WiMAX has three basic elements, a transmitter, a receiver, and a channel over which the information is sent. The main components of a WiMAX communication system are shown in Figure 3.1.

Besides these three components, we have also introduced MIMO systems. The use of multiple antennas at transmitter and receiver opens a new dimension in a reliable communication system. The main idea in the MIMO transmitter is STC (space time coding) in which signal processing in time is completed with signal processing in spatial dimension by using multiple spatially distributed antennas at both link ends. The feedback mechanism is optional that allows the transfer of information about the channel state back to the transmitter. This is done through adaptive modulation and coding (AMC) scheme.

3.2 Transmitter

Our model for Transmitter of WiMAX consists of the blocks shown in the figure 3.2. First of all the input data is randomized with PN sequence generator. Then block encoding is performed using Reed Solomon encoder followed by convolutional encoding, then data is interleaved to minimize burst errors probability and modulated into symbols. Each OFDM symbol is composed of 192 data subcarriers, 1 zero DC subcarrier, 8 pilot subcarriers, and 55 guard carriers. These symbols are assembled and then send to the OSTBC Encoder (Orthogonal Space Time Block Coding). Each block is explained in detail below.

3.2.1 Randomizer

Randomization is the first process carried out in the physical layer after the data packet is received from the higher layers. We have selected the input data from the standard [1]. Each burst in downlink and uplink is randomized. Randomizer operates on bit by bit bases. The purpose is to convert long sequences of 0s and 1s in a random sequence to improve the coding performance.

The main component of the data randomization is Pseudo Random Binary Sequence Generator. We defined the PN Generator as [1000001000000001] or 1+x6+x15. The XOR operation is done on input data and PN Sequence to randomize the data. 8 bits are added to the data just before it is presented to the Encoder stage as this is necessary for Reed Solomon Encoding.

3.2.2 Encoder

The encoding process consists of two encoding schemes, namely Reed Solomon Encoding and Convolutional Encoding. The Whole process of Encoder is depicted in figure 3.4

3.2.2.1 Block Encoding

The main component in block encoding is Reed Solomon Encoder as shown in figure 3.5. Reed Solomon Codes are mostly used in applications where errors occur in burst. RS Encoding is performed on the symbols instead of bits, thats why a bit to integer converter is used in start and an integer to bit converter is used in end. It is a coding scheme which works by first constructing a polynomial from the data symbols to be transmitted, and then sending an over-sampled version of the polynomial.

A Reed Solomon code is specified as RS ( n, k, t ) where

n: number of bytes after coding

k: number of bytes before coding

t: number of data bytes that can be corrected

The encoding process of RS encoder is based on Galois Field Computation to do the calculations of the redundant bits. A Galois field is widely used to represent data in error control coding. Galois field is denoted by GF (2m). WiMAX uses a fixed RS Encoding Scheme Based on GF (28) which is denoted as RS (n=255, k=239, t=8).

Eight tail bit were added in the randomization stage, now Reed Solomon stage requires 2 polynomials for its operation called Code Generator Polynomial g(x) and Field Generator Polynomial p(x). The code generator polynomial is used for generating the Galois field array whereas the field generator polynomial is used to calculate the redundant information bits which are appended to the start of output data. These polynomials are defined by standard [1] as shown below.

g(x)=(x + ?0)(x + ?1)(x + ?2)(x + ?3)(x + ?2T-1)

p(x)=x8 + x4 + x3 + x2 + 1

More information about Reed Solomon Codes is given in [4] and [5].

The Extra zeros which were padded before RS encoder, are now discarded in the selector block, this process is called shortening. The process of shortening once has done the number of symbols that can be corrected, t change, and also the number of symbols going in and out of the RS encoder.

3.2.2.2 Convolutional Encoder and Puncturing

Convolutional encoding is performed on the data bits instead of bytes. It has a rate of and a constraint length of 7. The generator polynomials used to derive its output code bits, denoted by X and Y, are specified in the following expressions:

G1= 171OCT for X (1111001 in binary)

G2= 133OCT for Y (1011011 in binary)

K bits are entered in convolutional encoder which generates n coded bits as a result. It has a shift register of length L segments, where L=7, is constraint length. Figure 3.6 shows how convolutional encoding is done. The outputs X and Y are then sent for the puncturing process.

In the puncturing process, data rate is changed from to 5/6 of, the original data before convolutional coding. In this process, bits from the output of convolutional encoder are deleted according to a vector defined as [1101100110] and known as puncturing vector. Bits from X and Y placed alternately are selected according to this vector, and then the bits against 1 are transmitted and the bits against zeros are discarded. Figure 3.7 shows the process of puncturing.

3.2.2.3 Interleaver

Interleaver can be considered as a randomizer but it is different from the randomizer in a sense that is does not change the state of the bits but it works on the position of the bits. It is done by spreading the coded symbols in time before transmission to avoid long error bursts. The incoming data into leaver is randomized in two permutations. First permutation ensures that adjacent bits are mapped onto nonadjacent subcarriers. The second permutation maps the adjacent coded bits onto less or more significant bits of constellation thus avoiding long runs of less reliable bits.

Interleaving is done on all encoded data bits with a block size corresponding to the umber of coded bits per OFDM symbols. The number of coded bits depends on the modulation technique used in the physical layer of WiMAX. WiMAX defines two permutations for interleaver.

3.2.3 Modulation Mapper

After the encoding process the encoded data is sent to the modulation block. Here the bits are mapped to a subcarrier amplitude and phase, which is represented by a complex in-phase and quadrature-phase (IQ) vector. The IQ plot for a modulation scheme shows the transmitted vector for all data word combinations. We have used 4-QAM with normalization constant Cm = 0.7071 and symbol alphabet As= (1+j, 1-j, -1+j, -1-j).

3.2.4 Pilot Symbols, DC subcarrier and Guard Bands

Pilot symbols are used to perform frequency offset compensation at the receiver. They can be used for channel estimation in fast time-varying channels. Pilot symbols allocate specific subcarriers in all OFDM data symbols. These pilots are obtained by a pseudo-random binary sequence (PRBS) generator that is based on a polynomial.

WiMAX specifications for the 256-point data FFT OFDM PHY layer define three types of subcarriers; data, pilot and null. The way these three are implemented in our simulation is shown in the figure 3.8.

192 of the total 256 subcarriers are used for data and 8 are pilot subcarriers. The rest of the potential subcarriers are nulled and set aside for guard bands and removal of the center frequency subcarrier.

3.2.5 Assembler

For constructing an OFDM symbol, a process to rearrange these carriers is needed. For this purpose, we use the assembling process. Assembling is done in our simulation through the matrix concatenation block shown in figure 3.8. As shown in the figure the DC subcarrier is inserted in the center, the pilot subcarriers are inserted according to the polynomial defined, and the guard subcarriers are inserted in the top and bottom.

3.2.6 Inverse Fast Fourier Transform

The IFFT is used to produce a time domain signal, as the symbols obtained after modulation can be considered the amplitudes of a certain range of sinusoids. This means that each of the discrete samples before applying the IFFT algorithm corresponds to an individual subcarrier. Besides ensuring the orthogonality of the OFDM subcarriers, the IFFT represents also a rapid way for modulating these subcarriers in parallel, and thus, the use of multiple modulators and demodulators, which spend a lot of time and resources to perform this operation, is avoided.

3.2.7 The Cyclic Prefix

The robustness of any OFDM transmission against multipath delay spread is achieved by having a long symbol period with the purpose of minimizing the inter-symbol interference. One way to perform the cited long symbol period is creating a cyclically extended guard interval where each OFDM symbol is preceded by a periodic extension of the signal itself. The guard interval, that is actually a copy of the last portion of the data symbol, is known as the cyclic prefix (CP). Copying the end of a symbol and appending it to the start results in a longer symbol time.

3.3 Channel

First we have simulated the design using an AWGN channel, and the design is working very well for that. After that we added a Rayleigh fading channel, which can only be made workable if we use space time coding (SPC).

3.3.1 Space Time Coding

Rayleigh channel of the form as shown is the figure 3.9 was designed and included in our model to represent the fading. To takle these impairments, we added a block of OSTBC encoder of 2*2 MIMO (Alamouti) in our SIMULINK model prior to the transmission. We set the value of maximum Doppler shift to 3Hz in the Rayleigh fading blocks.

3.4 Receiver

The receiver performs all the functions in reverse of the transmitter. Firstly the OSTBC combiner is used to combine all the scattered data and the best one is selected. After that the cyclic prefix is removed and the received signal is converted to the frequency domain using the FFT algorithm. The guard bands are removed and then, a disassembling is performed to obtain pilots, data, and any training sequence. Demapping is performed as a next step. After the data is demapped, it comes toward the final stage of the receiver which is decoder. The decoder includes the deinterleaving, Viterbi decoder and the Reed Solomon decoder.

Chapter 4

4 Background

4.1 WiMAX Model Test Results

There are several test cases and test vectors given in the WiMAX standard Document [6]. Following are the results of our simulation for a standard input which is also given;

Input Data from MAC:

45 29 C4 79 AD 0F 55 28 AD 87 B5 76 1A 9C 80 50 45 1B 9F D9 2A 88 95 EB AE B5 2E 03 4F 09 14 69 58 0A 5D

Randomized Data:

D4 BA A1 12 F2 74 96 30 27 D4 88 9C 96 E3 A9 52 B3 15 AB FD 92 53 07 32 C0 62 48 F0 19 22 E0 91 62 1A C1

RS Encoded Data:

49 31 40 BF D4 BA A1 12 F2 74 96 30 27 D4 88 9C 96 E3 A9 52 B3 15 AB FD 92 53 07 32 C0 62 48 F0 19 22 E0 91 62 1A C1 00

Convolutional Encoded Data:

3A 5E E7 AE 49 9E 6F 1C 6F C1 28 BC BD AB 57 CD BC CD E3 A7 92 CA 92 C2 4D BC 8D 78 32 FB BF DF 23 ED 8A 94 16 27 A5 65 CF 7D 16 7A 45 B8 09 CC

Interleaved Data:

77 FA 4F 17 4E 3E E6 70 E8 CD 3F 76 90 C4 2C DB F9 B7 FB 43 6C F1 9A BD ED 0A 1C D8 1B EC 9B 30 15 BA DA 31 F5 50 49 7D 56 ED B4 88 CC 72 FC 5C

Chapter 5

5.1 Conclusion

As the world now need higher data rate and more capacity, WiMAX can be a choice providing mobility and high data rates. Our design is a WiMAX simulator built in MATLAB SIMULINK to target 256-point FFT OFDM symbols. We have thoroughly studied, designed and analyzed different sub-blocks of our simulator. Throughout our report we described how transmitter of WiMAX is implemented. Transmitters different sub-blocks, coding schemes, modulation, and cyclic prefix length can be chosen by our own choice in this simulation. We have used AWGN channel first and our simulation design is working very good for that. In the end we added Rayleigh fading channel in our model, and for that purpose STC and MIMO (Alamouti scheme) are implemented. Due to 2 transmit antennas and 2 receive antennas it is easy to avoid fading channel impairments and get improved reliability and increased capacity. The increase in throughput is proportional to the number of antennas while implementing MIMO.

From the plots for different modulation schemes, we concluded that every modulation scheme has its own significance according to the channel conditions. If a user is near to the base station, he should be assigned 64-QAM because he needs less power and data rate. If a user is far away from the base station, he should be assigned 4-QAM as he needs more power and data rate.

Our model for WiMAX baseband simulator with Rayleigh fading and AWGN channel can be used for testing and calculating system performance. Our design can be used for different modulation schemes, just encoding and parameters need to be changed to get improved BER. The researchers can choose the best option for their requirement from this simulation.

5.2 Future Work

In future, our simulator can be used by different scholars and researchers to apply different schemes to fulfill their requirements. Our design can be expanded to include the components of MAC layer and upper layers resulting in a complete end to end WiMAX baseband system. Further adaptive modulation and coding (AMC) scheme can be used to get better performance according to different channel conditions. AMC adopts spectral efficiency to the variation in the channel SNR and maintain an acceptable BER. The feedback mechanism (AMC) performs channel estimation and accordingly selects a burst profile [1] for the particular user. Hence our WiMAX simulator design and this report can be guideline for the new students as well as the scholars who are new to WiMAX.

Bibliography

[1] IEEE. Standard 802.16-2004. Part16: Air interface for fixed broadband wireless access systems. October 2004.

[2] IEEE. Standard 802.16e-2005. Part16: Air interface for fixed and mobile broadband wireless access systems (Amendment for physical and medium access control layers for combined fixed and mobile operation in licensed band. December 2005.)

[3] Jeffrey G. Andrews, Arunabha Ghosh, and Rias Muhamed, Fundamentals of WiMAX: Understanding Broadband Wireless Networking, Prentice Hall PTR, Upper Saddle River, NJ, USA, 2007

[4] A. M. Michelson, and A. H. Levesque: Error Control Techniques For Digital Communications, Wiley-Interscience Publications, 1985.

[5] S. B. Wicker: Error Control Systems for Digital Communication and Storage, School of Electrical and Computer Engineering, Georgia Institute of Technology, Prentice Hall, 1995.

[6] ETSI TS 102 177 Version 1.3.1, February 2006, Broadband Radio Access Networks (BRAN); HiperMAN; Physical (PHY) Layer.

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