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    Intro30Nov204.doc.doc Intro30Nov204.doc.doc Document Transcript

    • 1 CHAPTER 1 INTRODUCTION 1.1 WIRELESS COMMUNICATION Wireless Communication has emerged as one of the largest sectors of the telecommunications industry due to mobility and flexibility of access. Cellular phones, cordless phones and paging services have experienced exponential growth over the last decade, and this growth continues unabated worldwide. Wireless communication has become a critical business tool and part of everyday life in most countries. The convergence of the telephone, cable and data networks into a unified network that supports multimedia and real- time applications like voice and video in addition to data and, the rapid growth in demand for Internet access suggest a promising future for wireless data services (Rappaport 1996). Wireless communication providing high-speed, high quality information exchange between portable devices located anywhere in the world is the vision for the future. People will operate a virtual office anywhere in the world using a small handheld device – with seamless telephone, modem, fax and computer communications. Wireless LANs will connect laptops and desktop computers anywhere within an office building or campus and will enable intelligent home appliances to interact with one another and with the Internet. Wireless video will be used to create remote classrooms, remote
    • 2 training facilities and remote hospitals anywhere in the world (Walrand and Varaiya 2000). The development of wireless systems to provide such services has been facing many technical challenges. Efficient utilization of the scarce radio spectrum, the unpredictable wireless channel characteristics, support for mobility, need for protocols to interface between wireless and wired networks are some of the issues to be considered when using the wireless medium for data communication. 1.2 EVOLUTION OF WIRELESS SYSTEMS First Generation Systems The first generation (1G) cellular systems used direct analog voice modulation and implemented a cellular architecture. Example of such a system is the advanced mobile phone service (AMPS) (Rappaport 1996, Lee 1995). The AMPS system uses the 850 – 950 MHz frequency band and a bandwidth of 30 kHz for each channel and frequency modulation is used for radio transmission. The limitations of AMPS systems include low system capacity and poor data communications with no room for spectrum growth. The 1G systems used different frequency bands in different countries. Second Generation Systems The second generation (2G) cellular systems used full digital communication over the radio channel with portable devices. Depending on the type of multiple access, 2G systems are classified into time division multiple access (TDMA) and code division multiple access (CDMA) systems. Important TDMA systems are the Global System for Mobile Communication (GSM) and
    • 3 Digital European Cordless Telephony (DECT) standards. The IS-95 standard is an example of a CDMA system. The initial transmission rate of GSM was 9.6 kbps and further research was focussed on increasing the bit rates supported by wireless systems and reducing the data rates needed for applications. General packet radio service (GPRS), also called GSM2+, employing packet switched resource allocation enables data rates up to 144 kbps and is termed as a 2.5G service. Enhanced data rate for GSM evolution (EDGE) was designed to be introduced in existing digital networks such as GSM. It is based on a higher level modulation and allows transmission at a data rate of up to 384 kbps. Third Generation Systems The main focus of 1G and 2G systems were to increase the system capacity in terms of established connections carrying constant bit-rate data streams. With the growing need for enhanced and rate demanding services, the need to support high and diverse data rates for heterogeneous applications like tele-commuting, home networking, video conferencing, fast wireless/ mobile internet access and multimedia, the idea of 3G systems which support such services gained momentum. Third generation (3G) systems provide higher data rates and support multimedia services. The 3G standards and specifications were guided by the global requirement developed by the International Telecommunications Union (ITU) which are referred to as the International Mobile Telecommunications-2000 (IMT-2000) standard. Wide-band CDMA (WCDMA) is an example of a 3G system which uses interference averaging techniques and supports data rates from 50-384 kbps in macro-cellular systems and upto 2 Mbps in micro-cellular systems (Prakash and Veeravalli 2002, Holma and Toskala 2002). Other 3G systems such as universal mobile
    • 4 terrestrial services (UMTS) and cdma2000 are envisioned to support rates of the order of 1 - 2 Mbps (Nanda et al 2000). The cdma2000 uses a CDMA air interface based on the existing IS-95 standard to provide wireline-quality voice service and high-speed data services, ranging from 144 kbps for mobile users to 2 Mbps for stationary users (Vijay K. Garg 2000). Bluetooth and HomeRF technologies provide such services at the 2.4 GHz band and support rates up to 1 Mbps (Amit Dhir 2001). Interference avoidance techniques provide better spectrum efficiency compared to interference averaging techniques (Pottie 1995). Wireless local area networks (WLAN) standards such as high performance LAN (HiPerLAN/2) and the IEEE 802.11 provide data rates up to 54 Mbps (ETSI 1999, IEEE 802.11 WG 1999). WLAN standards are primarily concerned with wireless connectivity in a short-range environment with localized mobility. The European telecommunications standards institute (ETSI) HiPerLAN/2 standard specifies a short range, high speed radio access system that can be used globally in the 5 GHz ISM band and can offer rates of about 50 Mbps (ETSI 1999). The IEEE 802.11 standard uses carrier sense multiple access with collision avoidance (CSMA/CA) and operates in the 2.4/5 GHz ISM band. The WLAN standards IEEE 802.11a and 802.11b (the latter also known as Wi-Fi) address localized transmission at the unlicensed bands of 5GHz and 2.4GHz and can achieve rates of 54 Mbps and 11 Mbps respectively (IEEE 802.11 WG 1999, 1999a). The HiperLan2 systems operate only in an infrastructure mode, i.e., with a centralized co-ordinator called the access point (AP). The IEEE 802.11 WLAN system could operate in an infrastructure mode (i.e., with the AP) or in an ad hoc mode (IEEE 802.11 WG 1999, 1999a). . The system performance can be improved by deploying WLANs in hotspots to provide high data rates over
    • 5 smaller areas and integrating them with 3G cellular services to provide larger coverage with low data rates (Shellhammer 2001). Another wireless technology, called Wi-Fi, provides high-speed internet access to suitably equipped computers within 50 metres or so of a small base-station. It is widely used in homes, offices and universities. But because of the short range of Wi-Fi technology, universal coverage is impractical. Although a dozen or so start-ups are working on ways to extend the range of Wi-Fi, it now takes hundreds of Wi-Fi base-stations to cover the same area as a single mobile-phone base- station. The IEEE 802.15 standard for wireless personal area networks (WPAN) focuses on short-range interconnectivity between different equipment (printers, PDAs, home appliances, etc.) at low cost and low power requirement. Such systems are also known as the ultra-wide-band (UWB) systems, which are expected to provide data rates up to 100 Mbps and a coverage of 10-15 meters. The IEEE 802.15.1 standard provides the MAC and PHY layer specifications that are identical to Bluetooth (Siep and Bisdikian 2001). The 802.15.2 WPAN specifies a coexistence model that quantitatively models the effects of the mutual interference of WPAN and WLAN devices on each other in the same location (Shellhammer 2001). In addition, the Task Group is to develop coexistence mechanisms that will facilitate coexistence of WPAN and WLAN devices in the same locale. The IEEE 802.15.3 standard supports bandwidths of 100 Mbps within 10 metres and 400 Mbps within 5 metre range (Allen and Gondalfo 2002). It supports a dynamic topology and high spatial capacity providing data services, high quality television and home cinema. The IEEE 802.15.4 standard specifies a low rate WPAN providing data rates of 20 – 250 kbps, e.g., sensor networks (Guttirez et.al 2003).
    • 6 The IEEE 802.16 wireless metropolitan area networks (WMAN) standard provides complementary services to a 3G cellular network with the objective to provide wireless services in different environments. The standard specifies fixed broadband wireless access (FBWA) at the 10 – 66 GHz band for buildings communicating through exterior antennas with central base stations (BSs) which are wired to the backbone network (IEEE P802.16a/D3–2001 (2002)). FBWA provides an alternative to cabled access networks such as fiber optic links, cable modems and digital subscriber line (DSL) links. Multi-channel multi-point distribution systems (MMDS) operate at the 2.5 GHz band and offer broadband packet services to residential users at rates of 10 Mbps. The 802.16 MAC provides a wide range of service types analogous to the classic asynchronous transfer mode (ATM) service categories as well as guaranteed frame rate (GFR) categories (Eklund et.al 2002). The IEEE 802.16a (IEEE P802.16a/D3–2001 (2002)) standard specifies extension of air interface support to lower frequencies in the 2-11 GHz band. Such services are oriented towards individual homes or small to medium sized enterprises. FBWA, that uses the 10 –66 GHz band, is a line of sight (LOS) system, where single carrier modulation and a burst design which allows both TDD and FDD, are employed. Design of the 2-11 GHz physical layer is driven by the need for non-line-of- sight (NLOS) operation. There has been a growing interest in fourth generation (4G) wireless systems that support global roaming across heterogeneous wireless and mobile networks (HWN). 4G/HWN wireless networks are also expected to provide IP interoperability for seamless mobile internet access and bit rates which are more than 50 Mbps (Varshney and Jain 2001). Research is being made on implementing fourth generation (4G) systems to provide higher data rates and
    • 7 higher mobility that will enhance multi-media services. The proposed IEEE 802.20 standard, formally known as the standard air interface for mobile broadband wireless access (MBWA) systems supporting vehicular mobility would support transmission speeds of up to 1Mbps in the 3-GHz spectrum band. The standard specifies a ubiquitous data wireless network that can support real-time traffic with extremely low latency at 20 milliseconds or less. The IEEE 802.20 interface seeks to boost real-time data transmission rates in wireless metropolitan area networks to speeds that rival DSL and cable connections (1 Mbps or more) based on cell ranges of up to 15 kilometres or more, and it plans to deliver those rates to mobile users even when they are travelling at speeds up to 250 kilometres per hour. This would make 802.20 an option for deployment in high-speed trains. 4G technologies combine Wi-Fi-like internet access with the blanket coverage, and fewer base-stations, of a mobile network. 4G systems are high-speed wireless networks covering a wide area, designed for carrying data, rather than voice or a mixture of the two. They can pipe data to and from mobile devices at “broadband” speed, typically 10-20 times faster than a dial-up modem connection. Such 4G wireless- broadband systems can be seen in two ways: as a rival to Wi-Fi that offers wider coverage, or as a wireless alternative to the cable and digital subscriber- line (DSL) technologies that now provide broadband access to homes and offices. The convergence of wireless and broadband, is called “nomadic broadband”; or “personal broadband”. 1.3 OVERVIEW OF CELLULAR AND WIRELESS SYSTEMS Cellular systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum and provide high quality
    • 8 of service. Two important features provided by cellular systems are (i) geographic reuse of the radio frequency channels and (ii) automatic handoff of a subscriber from one area to another (Cooper 1996). As mentioned in the previous subsection, cellular networks of the next generation are expected to provide data services along with voice. Obtaining high performance in cellular systems supporting the features (i) and (ii) mentioned above is challenging due to the time varying characteristics of the mobile radio propagation environment and the increasing demand for newer and better services over wireless systems from the user. Consequently, this is an important driver for research. Components The basic geographic unit of a cellular system is a cell. Each cell has a central control unit called base station (BS) (Rappaport 1996, Lee 1995, Stuber 1996, Zorzi and Chockalingam 2001). The coverage area of the cell depends on the transmitting power of the base station, the propagation characteristics, the environmental conditions and the geographical terrain. Base stations can be located anywhere in the cell, but are usually located at the centres of the cells. A cellular system with 61 cells is shown in Figure 1.1. The basic elements of a cellular system as shown in Figure 1.2 are: • Mobile station (MS) or user terminal, also called the mobile switching unit (MSU) which consists of a radio transceiver and a processor • Base station (BS) which co-ordinates communication between the users in the coverage area of the cell
    • 9 • Base Station Controller (BSC), which manages a set of base stations • Mobile Switching Center (MSC), also called a mobile telephone switching office, is responsible for connecting all mobiles to the public switched telephone network (PSTN). It controls a few tens of BSCs and provides additional functionalities such as call management and fault recovery. • Home location register (HLR), database which contains subscriber and location information of users registered in the area. 44 43 45 42 24 46 41 23 25 47 40 22 10 26 48 21 9 11 27 39 8 2 12 49 20 7 3 28 38 19 1 13 50 37 6 4 29 61 18 5 14 51 36 17 15 30 60 35 16 31 52 59 34 32 53 58 33 54 57 55 56 Figure 1.1 A cellular system with 61 cells
    • 10 • Visitor location register (VLR), contains information about roaming subscribers who are registered as subscribers in another area. • Authentication center (AUC) containing the Equipment identity register (EIR) used for security and authentication purposes. Communication between the base station and the mobiles is defined by a common standard air interface that specifies four different channels. The channel used for transmission from the base station to the mobiles is the forward or the downlink channel and the channel used for transmission from the mobiles to the base station is the reverse or the uplink channel. Forward and reverse channels are responsible for initiating calls and typically use 5% of the total available resources (Rappaport 1996). The combined channel with which the base station communicates with the users in its area is called the broadcast channel. Uplink Base station Visitor Location Home Controller Location Register Register MS Downlink Mobile Authentic- Switching ation Center Center MS PSTN Base station Controller Figure 1.2 Components of a cellular system
    • 11 1.4 MULTIPLE ACCESS SCHEMES Users in a cellular system employ several multiple access techniques to communicate with the base station. This enables the base station to distinguish between the users. Cellular systems employing frequency division multiple access (FDMA) and time division multiple access (TDMA ) are called channelized cellular systems, where “channel” refers to a time slot or a carrier frequency or a combination of both. In FDMA, users in a cell use different carrier frequencies to communicate with the base station. In TDMA, users access the base station on predefined slots in time. While 1G cellular systems use FDMA technique, 2G systems use a combination of FDMA and TDMA. In some 2G systems, all users transmit using the same frequency on all time slots. But the transmission bandwidth of the user is spread using a spreading sequence (or a spreading code), which is generated from a pseudo-noise (PN) sequence generator. Users are uniquely identified in the system by assigning different spreading codes for different users. Such a multiple access technique is called the code division multiple access (CDMA) (Viterbi 1995). The multiple access schemes are explained below. Multiple access schemes are employed to coordinate several users that need to access a common channel, so that the channel is shared and reused efficiently by them and user signals are distinguished at corresponding receivers. Three approaches can be identified in multiple access: connection- oriented (or fixed-assignment), connectionless (or random access) and demand- assignment methods. These methods are characterized by a tradeoff between coordination information overhead and risk of unsuccessful transmission.
    • 12 In connection-oriented multiple access, a separate connection is created for each user session and it is maintained for the entire duration of the session, regardless of whether or not the user transmits data. Connection- oriented access methods can be categorized as follows: • Frequency Division Multiple Access (FDMA). The spectrum is divided into orthogonal, non-overlapping frequency channels and each user is assigned to one frequency. A special case of FDMA is Orthogonal Frequency Division Multiple Access (OFDMA), which is the focus of this dissertation and is explained in detail in Section 1.9. • Time Division Multiple Access (TDMA). The spectrum is divided into orthogonal time slots and each user is assigned one (or more) slots. • Code Division Multiple Access (CDMA). All users transmit in the entire frequency spectrum at the same time, but each user is uniquely identified through its assigned signature sequence (code), with which it modulates the transmitted bits. Signature sequences can be deterministically computed or randomly generated. Furthermore, they can be orthogonal or non- orthogonal. CDMA falls within the category of spread-spectrum multiple access (SSMA) methods. Frequency-Hopped Multiple Access (FHMA), where carrier frequencies of users are varied in a random fashion is another SSMA method. • Space Division Multiple Access (SDMA). The separation of users is performed in space by directing the emitted energy towards
    • 13 each intended user through directional beams that are formed with an adaptive antenna array. Connectionless multiple access methods involve less coordination overhead than connection-oriented ones and are more suitable for networks with low traffic where the information stream to be transmitted is intermittent or bursty in nature. However, they are associated with higher risk of transmission failure due to potential simultaneous transmissions from other users. The ALOHA protocol, Carrier Sense Multiple Access (CSMA) scheme and their derivatives fall within this category. In ALOHA, a user transmits with certain probability whenever it has data to transmit, while in CSMA the user listens to the channel before transmitting and transmits when the channel is free. In CSMA with collision detection (CSMA/CD), a user can also detect collision while transmitting and can interrupt transmission if a collision occurs. In CSMA with collision avoidance (CSMA/CA), a user tries to avoid collision with other users before transmission by sensing the channel idle for a specified duration and waiting for a random amount of time before transmission. Demand-assignment techniques use random access methods in low traffic and fixed- assignment access in high traffic. Dynamic Assignment Multiple Access (DAMA) and Packet Reservation Multiple Access (PRMA) protocols belong to this class of methods. In DAMA, users can reserve traffic channels for packet transmission through contention-free assignment of request channels. When the number of users increases, users contend for the request channels. In PRMA, a user transmits with the ALOHA protocol, but if transmission occurs periodically, the user may also acquire contention-free transmission slots by reservation.
    • 14 1.5 ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING Orthogonal Frequency Division Multiplexing based multiple access (OFDMA) is one of the proposed multiple access techniques for wireless broadband access (Hanzo et.al 2001). The OFDM is used in the IEEE 802.11a and ETSI HiperLAN/2 standards for WLANs, as well as in the digital audio/video broadcasting (DAB/DVB) standards in Europe (Prasad and VanNee 2000). It has also been proposed for the IEEE 802.15.3 working groups for high speed wireless personal area networks (WPANs). Recently, OFDM systems have become popular for providing high data rate multimedia services over several multi-path channels (Hanzo et al 2001, Prasad and VanNee 2000). For data services, the offered peak bit rate is very important in determining the overall system performance, because of the highly bursty nature of internet traffic. The GPRS, EDGE and WCDMA systems support transmission rates up to 144 – 384 kb/s in macro-cellular environments. To achieve rates in the megabits-per-second range for all environments using the 5 MHz spectrum is challenging for both the physical layer as well as for the radio resource management design. Single carrier TDMA solutions are limited in supportable transmission bit rate by equalizer complexity. Low spreading gain or inter-code interference at high bit rates limits CDMA solutions. In OFDM, high rate data streams are split into a number of lower rate streams that are transmitted simultaneously over a number of sub-carriers. Hence, OFDM is a multi-carrier technique wherein data bits are encoded to multiple sub-carriers. Unlike single carrier systems, all the frequencies are sent
    • 15 simultaneously in time. Transmission of high data rate signals using conventional single carrier techniques suffer in terms of achievable bit rates even with interference suppression and space-time processing along with equalization. OFDM systems offer low complexity solutions. Since the symbol duration increases for the lower rate parallel sub-carriers, the relative amount of dispersion in time caused by multi-path delay spread is decreased. The use of OFDM with sufficiently long symbol periods of 100 – 200 µs for packet data transmission supports a high bit rate in time delay spread environments with performance that improves with increasing delay spread up to a point of extreme dispersion. Inter-symbol interference is eliminated almost completely by introducing a guard time in every OFDM symbol. Each OFDM symbol contains sub-carriers that are nonzero over a T-second interval. Hence, the spectrum of a single symbol is a convolution of a group of Dirac pulses located at the sub-carrier frequencies with the spectrum of a square pulse that is one for a T-second period and zero otherwise. The amplitude spectrum of the square pulse is equal to sinc(пfT), which has zeros for all frequencies f that are an integer multiple of 1/T. The effect is shown in Figure 1.3 which shows the overlapping spectra of individual sub-carriers. At the maximum of each sub-carrier spectrum, all other sub-carrier spectra are zero. As an OFDM sub-carrier essentially calculates the spectrum values at these points that correspond to the maxima of individual sub-carriers, it can modulate each sub-carrier free from other sub-carriers.
    • 16 f1 f2 f3 f4 f5 Figure 1.3 Frequencies with overlapping spectrum The frequencies used in OFDM system are orthogonal. Neighboring frequencies with overlapping spectrum can therefore be used as in Figure 1.3 where f1, f2 f3, f4 and f5 are orthogonal. This results in efficient usage of bandwidth. OFDM is therefore able to provide higher data rate for the same bandwidth. In the OFDM scheme, data is transmitted in parallel through several sub-carriers. OFDM offers flexibility in multiple access by suitably dividing the sub-carriers among several users. The block diagram of an OFDM system is shown in Figure 1.4. The data is modulated on the N sub-carriers by an inverse discrete fourier transform (IDFT) operation. After IDFT, the last G samples are copied and put as a preamble (cyclic prefix) to form an OFDM symbol. The addition of cyclic prefix mitigates the effect of inter-block interference. In addition to the advantages in suppressing inter-symbol interference (ISI), OFDM systems can offer flexibility in modulation and multiple access.
    • 17 Parallel to Serial Communication link Convertor Serial data stream .  . Add Serial toFigure Parallel Cyclic 1.4 OFDM System block diagram Convertor IDFT Prefix OFDM enables high speed transmission rates in very dispersive environments, and the system efficiency enhances with interference suppression which is supported by OFDM (Ojanpera and Prasad 1998). The sub-carrier spacing in OFDM results in much higher spectral efficiency than that of simple frequency division multiplexing. OFDM transmission increases the effective symbol duration and reduces the effective symbol transmission rate, since information is essentially transmitted over narrow band sub-carriers. Thus, it provides high immunity to ISI and delay spread. In addition, since the frequency selective broadband channel is divided into a set of frequency non- selective sub-carriers, the equalization procedure at the receiver simplifies to a scalar multiplication for each sub-carrier. Furthermore, OFDM provides additional flexibility in adapting transmission to varying link conditions, by allowing adaptation for each sub-symbol in a sub-carrier (Keller and Hanzo 2000). Since OFDM inherently provides frequency diversity over sub-channels, interleaving becomes possible in the frequency domain. OFDM when combined with diversity, interleaving and coding provides good link performance. On the upstream channel of a point-to-multi-point system, there are several multiple access techniques based on OFDM such as OFDM/TDMA and
    • 18 OFDMA. In OFDMA different carriers are assigned to different users. Carriers located in deep notches or strongly affected by interference are not assigned. OFDMA works on the same principle as OFDM with optimum bit loading on the downstream channel. Implementation of OFDMA requires knowledge of the channel by the base station. OFDMA is the most robust multiple access technique to narrow band interference. When the interference level exceeds a threshold, TDMA and CDMA offer a heavily degraded performance, whereas in OFDMA, only carriers affected by interference are rendered unusable for multiple access. If the channel is not known, OFDMA can be used by assigning a frequency hopping sequence rather than a particular carrier to a given user. In frequency hopped OFDMA (FH-OFDMA), it is not essential to have a random hopping sequence. A simple cyclic carrier assignment gives perfect performance averaging (Sari 2000). 1.6 DESIGN ISSUES IN A MOBILE WIRELESS ENVIRONMENT 1.6.1 Fading Radio signal propagation is affected by reflection, diffraction and scattering (Rappaport 1996, Jakes 1993) due to the ground terrain, the atmosphere and the objects in the path of propagation. Most cellular systems operate in areas where there is no direct line-of-sight path between the transmitter and the receiver. Hence signals propagating through the cellular radio channel are subject to severe impairments. The effect of propagation on the transmitted signal can be adequately described by three phenomena; multi- path fading, shadowing, and path loss. The performance of cellular systems is evaluated taking the influence of the above three phenomena into consideration.
    • 19 Propagation is influenced by reflection and scattering from the buildings and diffraction over and around them. Hence the radio waves travel along different paths of varying lengths creating a multi-path situation. At the receiver, these multi-path waves with randomly distributed amplitudes and phases add vectorially and give rise to a signal that fluctuates in time and space. Therefore, a receiver at one location may have a signal that is much different from the signal at another location, which is a short distance away because of the phase relationship between the incoming radio waves. This phenomenon of random fluctuations in the received signal is called fading. The short term fluctuation in the signal amplitude caused by the local multi-path is called small scale fading and is observed over distances of about half a wavelength. The long-term variation in the mean signal level is called large scale fading. Small scale fading can be further classified as flat or frequency selective fading, and slow or fast fading. A received signal is said to undergo flat fading, if the mobile radio channel has a constant gain and linear phase response over a bandwidth greater than the bandwidth of the transmitted signal. Under these conditions, the received signal has amplitude fluctuations due to the variations in the channel gain over time. However, the spectral characteristics of the transmitted signal remain intact at the receiver. If the mobile channel has a constant gain and linear phase response over a bandwidth lesser than that of the transmitted signal, the signal is said to undergo frequency selective fading. In this case, the received signal is distorted and dispersed, as it consists of multiple versions of the transmitted signal attenuated and delayed in time thus causing inter symbol interference (ISI). The Rayleigh distribution is a commonly accepted model for small-scale amplitude
    • 20 fluctuations in the absence of a direct line-of-sight (LOS) path (Sklar 1997). Large scale fading is a result of movement over distances large enough to cause gross variations in the overall path between the transmitter and the receiver. When there is relative motion between the transmitter and the receiver, Doppler spread is introduced in the received signal spectrum, causing frequency dispersion. If the Doppler spread is significant relative to the bandwidth of the transmitted signal, the received signal is said to undergo fast fading, which occurs for low data rates. If the Doppler spread of the channel is much less than the bandwidth of the baseband signal, the signal is said to undergo slow fading. 1.6.2 Shadowing The variation in the mean envelope (or the mean square envelope) of the radio wave over several tens of wavelengths is called shadowing. Shadowing differs from multi-path fading in two ways. (i) The duration of a shadow fade lasts for several seconds or minutes. Shadow fading is called as slow fading whereas multi-path fading is called fast fading. (ii) The attenuation due to shadowing is exponential in the width of the barrier that must be passed through. Experimental studies on macro-cellular environments were performed by Walker (1983) to investigate the statistics of shadowing. Similar studies were performed for macro cellular environments by Mockford and Turkmani (1988) and Mogensen et al (1991). In the above studies, it has been shown that the shadow loss typically follows a lognormal distribution with standard deviation in the range 5–12 dB. A commonly used value for the standard deviation of the lognormal shadowing is 8 dB.
    • 21 1.6.3 Path loss Path loss accounts for the signal attenuation due to the physical distance, d, between the transmitter and the receiver. In free space, the power is proportional to d-2 whereas in ground wave propagation there is an additional effect due to the vectorial combination of the direct (free-space) propagated signal and its reflection from the surface of the earth. In this case the path loss behaviour is modelled as d-η where η is called the path loss exponent. Path loss models accurate upto 1 dB for a distance of up to 20 km was obtained by Hata and Nagatsu (1980), and by Okumara et al (1968). The COST 231 study (1991) resulted in the COST 231 Hata model for urban propagation environments. Through various field measurement studies, the path loss exponent η, has been found to be between 2 and 6 in typical cellular environments. (Varshney and Jain 2001). A value of 4 is commonly used for η. 1.7 INTERFERENCE MODELLING Interference is a major factor which limits the performance of cellular and other wireless systems precisely because of the time varying wireless channel. The propagation models described in the previous sub-sections (1.6.1 – 1.6.3) are essential to model the interference at the base station or at the user in a cellular system. In the existing literature, several models have been developed to model the interference in cellular systems. To model the interference at a base station (or at a mobile), it is necessary to study the combined effects of path loss, lognormal shadowing, and fast fading. Since the three phenomena are independent, the interference from a mobile to a base station (or from a base station to a mobile) can be modelled to be lognormally
    • 22 distributed, conditioned on the path loss and multi-path fading. Since the interfering signals from different transmitters add up at the receiver, the total interference at a base station (or at a mobile) can be modelled to have a distribution of the sum of several lognormally distributed random variables. Hence it is important to study the distribution of the sum of several lognormally distributed random variables. The approximation that the sum of several lognormally distributed random variables can be approximated by another lognormally distributed random variable has been made in several studies. Fenton (1960) developed an approximation by taking into account the means and variances of several independent lognormally distributed random variables. Scleher (1977) applied the Gram-Charlier series to obtain another approximation. Schwartz and Yeh (1982) applied a higher order cumulant based approach to obtain the distribution of the sum of several lognormally distributed random variables. Beaulieu et al (1994) compared the above methods and pointed out that although the approaches by Scleher (1977) and that by Schwartz and Yeh (1982) are more accurate than the one by Fenton (1960), they are complex and cannot be applied to all cellular systems. In Schwartz and Yeh (1982), it has been mentioned that Fenton’s approach is the easiest to apply. 1.8 RESOURCE ALLOCATION The scarcity of resource to support emerging multimedia wireless applications requires efficient strategies for using the available bandwidth. The resources in a cellular system namely, the available bandwidth, the transmit/receive power, rate of transmission etc., should be managed efficiently
    • 23 to optimize the system performance. The system performance measures based on which the resources are to be allocated depend on the type of services the system offers and the requirements of the users in the system. Performance measures that meet the requirements of users are called as user centric performance measures, and the measures that characterize the network performance are called as the network centric performance measures. In cellular systems with voice-only traffic, the transmission rate on different carrier frequencies or time slots are equal. Depending on the availability, time slots or carrier frequencies are allocated to the different users in the system. One such measure of performance of a voice-only system is the blocking probability. It is defined as the probability that a call from any user gets blocked (the user is denied service by the system). Blocking occurs when all the channels are busy when a new call enters the system. Blocking probability is a network centric performance measure. Another performance measure which is user centric is the probability that the interference seen by a user exceeds a specified threshold, also called outage probability. In cellular and other wireless systems that support voice and data traffic, the data transmission rates on the different carrier frequencies/time slots are not essentially the same. Unlike voice-only traffic, where new calls may be blocked due to non-availability of resources, data calls can be buffered until the required resource becomes available. Voice calls cannot be delayed whereas data calls (in specific, non real-time data services) can be buffered adding delay to the data transfer. Once a voice call is initiated it is allowed to progress even if the quality deteriorates due to increased interference. A data call, if corrupted during the transmission due to poor channel quality/increased interference needs to be retransmitted, adding to the delay in data transfer. The performance
    • 24 measures in such cellular and other wireless systems are obtained by evaluating the mean delay for the data traffic and the average system throughput. The probability of the delay exceeding a limit or the variance of the delay is also a performance metric which is user centric. The average system throughput is the network centric performance measure. In a cellular system, the radio signal transmitted by a user is received not only by the base station of the cell to which the user belongs to, but also by all the other base stations in the system. Similarly, the transmitted signal from a base station will be received by users in cells other than the one which is controlled by the base station. The users in the system that transmit on the same channel as that of the user of interest are called the co-channel interferers to the user of interest. The signal received at a base station from the co-channel interferer to the user of interest is called co-channel interference. Users in the same cell or in different cells that use adjacent channels (channels spaced one time slot or carrier frequency apart) also interfere with each other due to the roll off characteristics of the filter at the receiver, resulting in adjacent channel interference. In channelized cellular systems no two users in the same cell can use the same channel, but the same channel can be used by cells that are geographically separated. This is called “channel reuse”. Channel reuse is achieved through (i) cell splitting and (ii) sectorization for interference management. Cell splitting means dividing a cell into smaller cells each with its own BS that has reduced antenna height and transmitter power. Sectorization involves directed transmission and reception using directional antennas at the base station. Both cell splitting and sectorization reduce the number of
    • 25 interferers and thus, support larger number of users. The probability that the interference level crosses a certain threshold is desired to be below a specified value for channels to be reused. Three cell reuse pattern in channelized cellular systems is shown in Figure 1.5, where cells 1,10,16,20,27,29 and 36 can reuse the same channel. The reuse distance is defined as the maximum distance within which two base stations cannot use the same set of frequency channels. The reuse distance varies according to the interference level that can be tolerated by the base stations, which depends on the tolerable bit error rate (BER). Lesser the reuse distance, higher is the co-channel interference experienced and higher is the bit error rate. In CDMA systems the same frequency can be used in all the cells at all times, which is known as “universal frequency reuse”. 21 20 22 37 8 23 36 19 9 24 18 2 10 35 7 25 3 17 1 11 34 6 4 26 16 5 12 33 15 13 27 32 14 28 31 29 30 Figure 1.5 Cellular system with 3-cell reuse Co-channel interference is handled using two techniques, namely interference averaging and interference avoidance. In addition, the effect of co-
    • 26 channel interference is limited using power control schemes in concurrence with the above techniques. In interference avoidance systems, the co-channel interference is reduced by assigning completely different sets of channels to adjacent cells. Frequency reuse is employed to improve the system capacity. FDMA and TDMA systems use interference avoidance. In systems employing interference averaging techniques, all the stations in the system can use the same frequency band. Spread spectrum techniques are used through which the co-channel interference effect is spread over a large bandwidth and its actual value in the band of interest becomes small. CDMA systems employ interference averaging For efficient utilization of the radio spectrum, a frequency reuse scheme that will provide increased capacity and minimal interference should be employed. Channelized cellular systems use the concept of interference avoidance. Channel allocation can be viewed as an integral part of multiple access that is performed at the MAC layer. Depending on the multiple access scheme, channels can be time slots, carrier frequencies or codes. If the set of users is given, an efficient channel allocation algorithm should try to minimize the number of channels needed to accommodate users and guarantee acceptable link quality for them. By minimizing the number of required channels at any time instant, the system can respond better to a potential sudden load increase or link quality deterioration. Hence, the likelihood of blocking a user is minimized. When the number of available channels is provided, the objective of channel allocation is to maximize system capacity, i.e., the number of accommodated users with acceptable link quality. If users have different rate requirements and need additional channels, the objective becomes to maximize the total achievable rate of users in the system. Channel assignment strategies
    • 27 can be classified as either fixed or dynamic. The choice of channel assignment strategy impacts the performance of the system (Tekinay and Jebbari 1991, Sung and Wong 1994). 1.8.1 Fixed Channel Allocation In channelized cellular systems with fixed channel allocation, a fixed number of nominal channels is allocated to each cell. Channel reuse is based on reuse distance which is obtained by calculating the maximum possible interference at the base station on the uplink (or at the mobile on the downlink). The total number of available channels is divided into subsets (not necessarily distinct or disjoint) and each subset is allocated to a cell. If two subsets have a common channel, then they are allocated to cells that are separated by more than the reuse distance. Each cell can admit calls only if any channel on the subset allocated to it is free. If all channels allocated to a cell are busy, a new call in that cell is blocked. Though fixed channel allocation is simple, it does not take into account the exact position of the users and the varying traffic conditions. Also, data traffic is bursty and the static nature of FCA results in inefficient allocation of resources thus making FCA unsuitable for wireless or cellular networks with data traffic. 1.8.2 Dynamic Channel Allocation Dynamic channel allocation (DCA) schemes are adaptive and there is no fixed relationship between channels and cells as in FCA. The available channels are kept in a central pool and assigned dynamically to cells as new calls arrive. After completion of the call, the corresponding channel is returned
    • 28 to the central pool. When more than one channel is available in the central pool to be assigned to a cell that requires a channel, the cost of each candidate channel is evaluated and the one with the minimum cost is selected subject to interference constraints. The selected cost function might depend on the future blocking probability in the vicinity of the cell, the frequency of usage of the candidate channel, the reuse distance, channel occupancy distribution under current traffic conditions, radio channel measurements of individual users and the average blocking probability of the system. In DCA, a channel is feasible, i.e., eligible for use in any cell, provided the interference constraints are satisfied. The assignment of a channel can be based on strategies like the best channel allocation, i.e., the feasible channel on which the measured interference is minimum or random channel allocation, i.e., any one of the feasible channels at random. DCA performs well in micro-cellular environments (Stuber 1996). DCA schemes could be centralized or distributed (Katzela and Nagshineh 1996). In centralized DCA, a channel from a central pool is assigned to a call in a cell for temporary use by a centralized controller. This scheme provides near optimum channel allocation, but at the expense of a high centralization overhead in terms of computational and communication complexity. Maximum packing algorithm (MPA) is an example of centralized DCA. In MPA, a call is blocked only if the call cannot be admitted after all possible global rearrangements of channel allocation in the system. The performance of MPA has been studied by Everitt and MacFayden (1983) Raymond (1991) has shown that the complexity of MPA can increase without bound as the number of cells in the network increases. The base station controller (BSC) in a GSM PLMN, that co-ordinates channel assignments across-the-network is an example of centralized controller.
    • 29 Distributed DCA is a cell based scheme that works purely with the local information in a cell. Instability may result under certain conditions with distributed DCA because this scheme does not take into account the fact that channel allocation in a cell may affect the signal quality in other cells. Distributed DCA requires no network planning. Using DCA, TDMA or FDMA systems can serve dynamic and non-uniform traffic demands without frequency planning (David Grace 1999). The flexibility of DCA results in better performance in terms of blocking probability as compared to FCA (Nanda and Goodman 1992). It is observed that the performance analysis of DCA is complex compared to the performance analysis of FCA. DCA is suitable for system with voice-only traffic as it requires resource allocation on demand and is connection oriented. Once allocated, the channel is held for the entire duration of the call. For data traffic, a variation of DCA called dynamic packet assignment (DPA) is more suitable. This is explained in detail in the following subsection. 1.8.3 Dynamic Packet Assignment The next generation cellular systems are expected to provide voice as well as data traffic services with high spectral efficiency and flexible data rate access. Applications such as web browsing using the wide-area cellular infrastructure require a peak downlink access rate of the order of 1-2 Mb/s. Very high efficiency is required for 1-2 Mb/s macro-cellular internet access. An advanced cellular internet service to achieve such high data rates was proposed by Cimini et al (1998). Measurement based DCA schemes can provide very high performance, particularly if channel selection is based on a combination of both mobile and base station measurements. DCA for which any carrier is
    • 30 Session Active Idle Active Idle DCA Channel held DPA Channel released Figure 1.6 DPA vs DCA allowed to be used in any time slot is a good approach to efficiently reuse spectrum for ACIS without requiring higher total bandwidth. DCA algorithm for packet access is called dynamic packet assignment (DPA). It is not possible to fully exploit the large potential capacity gain of DCA due to difficulties introduced by rapid channel reassignment and intensive receiver measurements required. DPA, based on the properties of an OFDM physical layer, reassigns transmission resources on a burst-by-burst basis using high-speed receiver measurements. OFDM has the ability to rapidly measure interference or path loss parameters on all candidate channels. DPA is relatively insensitive to errors in power control and provides good performance even without power control. Packetized data traffic consists of active bursts and idle periods. In DCA the allocated channel is held both during the active data bursts as well as during the
    • 31 idle periods, whereas in DPA, the channel is held during the active data bursts and released during the idle periods to be used by some other active burst as shown in Figure 1.6. Hence DPA is expected to perform better compared to DCA in terms of achieving high data rates. 1.9 OFDMA BASED WIRELESS SYSTEMS Orthogonal frequency division multiplexed access (OFDMA) is an efficient resource management technique that can support high data rates. In OFDMA based systems, carriers that are affected by interference are not assigned. The IEEE 802.16a WMAN is a system that deploys OFDMA. The IEEE 802.16a draft provides three air interface specifications, namely, the WirelessMAN – SC2 which uses a single carrier modulation format, the WirelessMAN-OFDM, which uses orthogonal frequency division multiplexing with a 256-point transform and TDMA access, the WirelessMAN-OFDMA, which uses orthogonal frequency division multiple access with a 2048-point transform. In the WirelessMAN-OFDMA system, multiple access is provided by addressing a subset of the multiple carriers to individual receivers. Performance analysis of resource management schemes in cellular systems with OFDMA like the 802.16a WMAN is of interest. 1.10PREVIOUS WORK Performance studies of cellular systems with distributed dynamic channel allocation (DCA) and the analysis for computing the blocking probability was performed by Cimini.et al (1994). Varghese (1999) presented a comparison of DCA schemes namely the clearest channel assignment
    • 32 algorithm, the nearest channel assignment algorithm and the random channel assignment algorithm and it was shown that the performances of the above three schemes were found to be similar. Anand et al (2003) proposed a two dimensional Markov chain to compute the blocking probability in cellular systems with DCA. The analysis in Anand et al (2003) took into account voice- only traffic. However, next generation cellular systems typically consist of voice as well as data traffic (Gummalla and Limb 2000). Several analytical and simulation models have been developed to study the performance of multiple access systems (Rubin and Tsai 1989, Rubin 1996, Khan and Peyravi 1998, Prakash and Veeravalli 2002). Rubin and Tsai (1989) obtained message delay distribution, using a discrete time priority queuing approach in a system with two classes of traffic. Rubin (1996) presented an analytical model for the delay in the forward signalling channels of a wireless cellular network. Khan and Peyravi (1998) compared the effects of five bursty distributions on buffer size and end to end delay in cellular data networks. Prakash and Veeravalli (2002) presented analytical techniques for cellular wireless packet data systems with incremental redundancy. The authors presented a time-scale separation approach to evaluate the mean delay and per- user throughput. The above models are quite complex and do not take into account the characteristics of the wireless channel. Chuang and Sollenberger (2000) have shown that fourth generation (4G) wireless systems can provide data transmission rates of 2-5 Mbps in macro-cellular environments, and upto 10 Mbps in micro-cellular environments by deploying dynamic channel assignment (DCA) based on interference avoidance, combined with orthogonal
    • 33 frequency division multiplexing (OFDM). Cimini et al (1998) proposed the concept of packetized DCA or dynamic packet assignment (DPA), which involves fast measurements on all sub-carriers in parallel. Chuang and Sollenberger (2000) proposed a system with DPA that makes fast channel measurements using the multi-carrier nature of OFDM. With the release of the broadband wireless access standard, the IEEE 802.16 WMAN has been the focus of research work.. Eklund et al (2002) have provided a technical overview of the WirelessMAN air interface for broadband wireless access, specifically the MAC and PHY features of the standard. Ramachandran et al (2002) have evaluated the performance of IEEE 802.16 for broadband wireless access. The authors have evaluated the delay and throughput performance of the various IEEE 802.16 PHY options under various channel conditions. 1.11CONTRIBUTION IN THIS THESIS It is of interest to develop an analytical means of determining the throughput and mean delay performance of OFDMA based cellular systems with DPA, taking into account the traffic conditions as well as the radio propagation characteristics. In this thesis, we present a performance analysis of DPA in OFDM based cellular systems with data traffic. We derive expressions for the mean data traffic delay and the average system throughput on the uplink (mobile-to-base station link), (Jayaparvathy and Srikanth 2003) and the downlink (base station-to-mobile link), (Jayaparvathy et.al 2004). Our analysis takes into account the interference conditions on the OFDM sub-carriers.
    • 34 In Chapter 2, we present a two dimensional CTMC to model the traffic and interference conditions in a cell on the uplink and at a user on the downlink. We solve the CTMC to obtain the mean delay and the average system throughput. We illustrate the accuracy of our model by validating our analysis with simulations. The performance of DCA is also provided for comparison. We show that there is an improvement in Erlang capacity that can be supported when DPA is used both in linear and two dimensional cellular systems on the uplink as well as on the downlink. 1.12OFDM BASED WIRELESS SYSTEMS In OFDM based wireless systems, devices use one wide frequency channel by breaking it up into several component sub-channels. Each sub- channel is used to transmit data. All the “slow” sub-channels are then multiplexed into one “fast” combined channel (Matthew Gast 2002). High system throughputs are achieved when OFDM is deployed, as it uses several sub-carriers in parallel and multiplexes data over the set of sub-carriers. The standards IEEE.802.11a and Hiperlan-2 for broadband wireless LAN systems specify OFDM as the modulation scheme because of the good performance of OFDM on frequency selective channels. The indoor propagation channel for wireless LAN applications is frequency selective due to multi-path fading. The link between the transmitter and the receiver is often a non-line of sight link. Also, OFDM exploits the frequency diversity by partitioning the bandwidth into narrow sub-bands, such that the fading in each is approximately flat.
    • 35 1.13IEEE 802.11 WIRELESS LOCAL AREA NETWORK (WLAN) The IEEE 802.11 standard for WLANs has become extremely popular due to high data rates and flexible wireless access (IEEE P802.11 1997, IEEE 802.11 Part II 1999). The IEEE 802.11 networks consist of four major physical components. The components of the IEEE 802.11 WLAN are shown in Figure 1.7. • Distribution system : Logical component of 802.11used to forward frames to their destinations, called as the backbone network • Access points : Perform the wireless – to – wired bridging function • Wireless medium : The standard uses the air interface to communicate between stations. • Stations : Computing devices with wireless network interfaces Access point Wireless medium Station Distribution system Figure 1.7 Components of a wireless LAN
    • 36 Basic service set (BSS) is the basic building block of an 802.11 network. BSS is a group of stations that communicate with each other. Stations in an independent Basic Service Set (IBSS) communicate directly with one another and must be within the direct communication range. IBSSs are composed of a small number of stations set up for a specific purpose and for a short period of time. Due to their short duration, small size and focussed purpose, IBSSs are called adhoc networks. Stations in an infrastructure BSS communicate via access points. The 802.11 standard defines two modes of operation; the distributed coordination function (DCF) and the point co- ordination function (PCF) (Matthew Gast 2002). The DCF is a mandatory protocol within the 802.11 specifications, while PCF is an optional protocol used for latency sensitive traffic like voice and video (Neil Reid and Seide 2003). The DCF is a distributed MAC protocol which is suitable for an ad hoc mode of operation, whereas, the PCF is a centralized MAC protocol. Special stations called point coordinators are used to ensure that the medium is provided without contention. Point coordinators reside in access points, so PCF is restricted to infrastructure networks. 1.13.1 The IEEE 802.11 DCF In the IEEE 802.11 DCF, the multiple access mechanism is carrier sense multiple access with collision avoidance (CSMA/CA). Carrier sensing is performed both at the physical layer (referred to as physical carrier sensing) and at the MAC layer (also known as virtual carrer sensing). A handshaking mechanism with request-to-send (RTS) and clear-to-send (CTS) signals is used to reserve the channel before data transmission. This is explained as follows.
    • 37 B A RTS Y DATA s D ACK CTS C X Figure 1.8 CSMA/CA in the IEEE 802.11 DCF Consider a network as shown in Figure 1.8, in which, transmissions from station S can be received by stations A, B, C, D and X. Similarly, transmission from station D can be received by stations S, X and Y. If S has data to be transmitted to D, station S senses the channel. If the channel is sensed idle for a time interval equal to a DCF inter-frame spacing (DIFS) (IEEE P802.11 1997), then it sends an RTS signal, which specifies the destination station, D, and the size of the data packet, which, in turn, specifies the time up to which the channel will be busy. The RTS is received not only by station D, but also by stations A, B, C and X. On receiving the RTS, stations A, B, C, and X set their network allocation vectors (NAVs) based on the packet size. Station D responds with a CTS, which is heard by station S and also heard by stations X and Y. The CTS piggy backs the packet size information. Station X ignores the CTS, while station Y sets its NAV based on the packet size information in the CTS. The NAV at each station gives the minimum amount of time the station needs to wait before it begins to sense the channel. Once the CTS is
    • 38 received by station S, data packets (DATA) are transmitted from station S to station D and station D responds with an acknowledgment (ACK). The hand shake mechanism using RTS, CTS, DATA and ACK is called as the four way hand shake, and is shown to overcome the hidden node problem (IEEE 802.11 1999). It is observed that the four way hand shake mechanism described above, could result in collision of data packets. Binary exponential backoff (BEB) is used to reduce collisions (IEEE P802.11 1999). The BEB procedure works as follows: When station S has data packets to be transmitted to station D, station S senses the channel. If the channel is sensed idle for a DIFS time interval, then station S generates a random integer, uniformly distributed in [0, CWmin]. A backoff counter is initialized to the random integer, b, and is decremented at discrete time slots at the rate of one per slot. The station S transmits RTS only when the backoff counter reaches zero. If the channel is sensed busy when the backoff counter reaches a value b’ such that 0<b’<b (i.e., if the channel is sensed busy when the backoff counter is being decremented but before it reaches zero), then the back off counter is frozen at b’, and the station continues to sense the channel. The backoff counter is then decremented (starting from b’) only if the channel is sensed idle for a DIFS period. Collision of packets occurs when two or more stations generate random integers such that the respective backoff counters reach zero at the same slot. If after transmission (i.e, after the backoff counter reaches zero) the station S does not receive the CTS within a time period specified by a short inter-frame spacing (SIFS) (IEEE P802.11 1997), then the RTS is assumed to have collided with the transmission from another station. Station S then senses
    • 39 the channel again, and if the channel is sensed idle for a time interval specified by the distributed inter-frame spacing (DIFS), then the backoff counter is initialized to a new random integer b2), which is uniformly distributed in [0, 2CWmin]. In general, the backoff counter is initialized to a random integer which is uniformly distributed in [0, CWk], where k ≥ 0 is called as the backoff stage or the retrial number. The value k=0 represents the first time transmission and value k ≥1 represents the kth retransmission. Also, CWk is given by CWk = min(2k(CWmin +1)-1, CWmax), where CWmin and CWmax are specified by the IEEE 802.11 standard (IEEE 802.11 Part II 1999). The above procedure (i.e., channel sensing and backoff) is continued till the CTS received successfully. If the CTS is not received successfully in seven transmission attempts of the RTS, then the packet is dropped from the system. Upon successful reception of the CTS, the packet is transmitted to station D, and if the ACK is not received by station S within an SIFS time interval, then the packet is assumed to be corrupted due to physical layer impairments. Corrupted packets are retransmitted following the four way hand-shake mechanism with BEB. If the ACK is not received successfully for seven transmission attempts of the packet, then the packet is dropped from the system. It is noted that the backoff counter is frozen when the channel is sensed busy for a time given by the NAV+DIFS for the first time transmission and NAV+EIFS for all retransmissions. It is also observed that the BEB only reduces the probability of collision, but does not eliminate collisions, i.e., the probability of collision in a system with BEB is also non-zero.
    • 40 1.13.2 The IEEE 802.11 PCF The centrally coordinated access mechanism of the IEEE 802.11 MAC, called the PCF, adopts a poll-and-response protocol to control the access to the shared wireless medium and eliminate contention among wireless stations. It makes use of the priority inter-frame space (PIFS) to seize and maintain control of the medium. The period during which the PCF is functional is called the contention-free period (CFP). In an IEEE 802.11 WLAN, a Contention-Free Period (CFP) and a Contention Period (CP) alternate over time periodically, where the centrally coordinated PCF is used during a CFP, and the contention based DCF is used during a CP. Once the PC has control of the medium, it may start transmitting downlink traffic to stations. Alternatively, the PC can also send contention-free poll (CF-Poll) frames to those stations that have requested contention-free services for their uplink traffic. During a CFP, a wireless station can only transmit after being polled by the PC. If a polled station has uplink traffic to send, it may transmit one frame for each CF-Poll received. Otherwise, it responds with a NULL frame, which is a data frame without any payload. Besides, in order to utilize the medium more efficiently during the CFP, it is possible to piggyback both the acknowledgment (CF-Ack) and the CF-Poll into data frames. During the CFP, the PC sends a frame to a wireless station and expects the reply frame, either a CF-Ack or a data frame or a NULL frame in response to a CF-Poll, within a short interframe space (SIFS) that is shorter than PIFS. Consider an example of uplink data frame transmission. The PC first sends a CF-Poll to the wireless station and waits for an uplink data frame. As shown in Figure 1.9, if a data frame is received correctly within an SIFS interval, the PC
    • 41 sends a CF-Ack + CF-Poll frame that allows the next uplink data frame transmission. If a data frame is received in error, the PC sends a CF-Poll requesting a re-transmission. However, if no reply frame is received within an SIFS interval possibly due to an erroneous reception of the preceding CF-Poll frame by the polled station, the PC reclaims the medium and sends its next CF- Poll after a PIFS interval from the end of the previous CF-Poll frame. The PC differentiates between an erroneous reception and lack of data from a polled station by using the NULL data frame. Downlink SIFS CFACK(D)+Cfpoll(1) CFACK(1)+Cfpoll(2) T Uplink Frame (1) Frame (2) SIFS SIFS Figure 1.9 Timing of successful frame transmissions in the PCF 1.14 PREVIOUS WORK Several simulation and analytical studies have been made to evaluate the IEEE 802.11 DCF performance (Muppala et al 1982 – German and Heindi 1999). A geometrically distributed backoff was considered by Crow (1996) and Ho and Chen (1996) and throughput performance studies were presented. Weinmiller et al (1997) studied the performance of the IEEE 802.11 WLAN as a function of the number of stations, and varying packet size, through extensive simulations. Chhaya and Gupta (1997) presented a renewal model to study the effect of hidden nodes in IEEE 802.11 with power control. Bianchi (2000) proposed an analytical model to obtain the saturation throughput of the IEEE 802.11 DCF. An embedded Markov chain based approach was presented,
    • 42 considering binary exponential backoff limited to two stages. Cali et al (2000) considered a geometrically distributed backoff and presented an upper bound for the MAC protocol capacity. The above mentioned references considered simplified backoff rule assumptions. Foh et al (2002) provided a queuing model to evaluate the DCF performance. The model took into account, the statistical characteristics of the protocol operations. Wu et al (2002) enhanced the analysis in (Bianchi 2000) to obtain the goodput of the system. The analytical model in the above studies did not take into account, the limit on the maximum number of packet retransmissions, and the effect of freezing of the back off counter due to channel capture by other stations. The advances in the theory of stochastic petri nets (SPN) (Peterson 1981) motivated SPN based analysis of the IEEE 802.11 DCF. German and Heindl (1999) formulated a detailed SPN model for performance evaluation of the IEEE 802.11 based WLAN. The authors derived two compact and analytically tractable models. Although the SPN model (German and Heindi 1999) captures most of the relevant system aspects, it does not exactly capture the freezing of the backoff counter at a station when some other station captures the channel. The limit on the maximum number of retransmissions has also not been taken into account. 1.15 CONTRIBUTION IN THIS THESIS Developing an analytical model capturing all the IEEE 802.11 DCF MAC protocol operations is challenging, because of the inherent synchroniza- tion required between the events. In this thesis, we propose a stochastic reward net (SRN) based approach to model the IEEE 802.11 DCF. Our model captures
    • 43 the freezing of the backoff counter in addition to capturing the other system as- pects as in (German and Heindi 1999). The SRN formulation not only provides the probability of retransmission of a packet due to collisions (and hence, the average system throughput) (Jayaparvathy et al 2003), but also provides the mean delay suffered by the first packet (i.e., the packet at the head of line (HOL)) at every station. To compute the mean delay of the subsequent packets at a station, we model each station as an M/G/1 queue, with the mean service time to be the mean delay suffered by the HOL packet (Jayaparvathy et al 2004). In Chapter 3 we present the SRN model for the IEEE 802.11 DCF. We obtain the numerical solution of the underlying CTMC of the SRN model. Using the Pollaczek –Khinchin formula for the M/G/1 queueing model, we evaluate the mean delay suffered by the packets in the system. We validate our analytical model by comparison with simulations. We show that for a given packet size, as channel bit rate is increased the mean delay reduces. 1.16 ORGANIZATION OF THE THESIS The rest of the thesis is organized as follows. In Chapter 2, we present the performance analysis of OFDMA based cellular systems with DPA. In Section 2.1, we describe the system model considered, the resource allocation strategy employed and the assumptions made in the analysis. Section 2.2 describes the two dimensional CTMC model and provides the analysis to derive the expressions for the mean delay and the average system throughput in the uplink as well as the downlink. Section 2.3 provides the numerical results and discussion. Section 2.4 provides the conclusions.
    • 44 In Chapter 3, we present the performance analysis of the IEEE 802.11 DCF using stochastic reward nets. In Section 3.1, we present the system model and the assumptions made in the analysis. In Section 3.2, we present the stochastic reward net model and provide the expressions to compute the mean delay and the average system throughput. Section 3.3 presents the numerical results and discussion. Section 3.4 provides the conclusions. In Chapter 4, we present the conclusions and the scope for future work from this thesis.