OFDM matlab project full

Abstract

This thesis describes the design and implementation of a Matlab simulated IEEE 802.11g (WLAN) based Physical layer (PHY). As Wireless LANs are becoming a worldwide standard, it is important in studying how data communications happen in a WLAN system.

The result is an IEEE 802.11g (WLAN) based Physical layer simulation software (in Matlab), which mimics real-time data communication from one Station (STA) to another STA. The software simulates IEEE 802.11g based PHY system consisting of 3 major components: Transmitter, Communication Channel and Receiver.

The transmitter first codes and pre-processes the user data signal prior to transmission. The Transmitter Interoperations comprise of bit padding, scrambling, convolutional encoding, data interleaving, sub-carrier modulation mapping, pilot sub-carrier addition, and OFDM modulation.

Then, the transmitting signal goes through the communication channel. It is the wireless medium that is between the transmitter and receiver, and is modeled by the impulse response of a multipath fading channel and consists of Addictive White Gaussian Noise (AWGN).

Finally, the receiver decodes the received signal to suppress multipath and noise effects then reproduce the original user signal; by performing processing “complementary to those of the transmitter’s modulation processes”, The demodulation processes that the receiver perform are rake receiver for receiving multipath signals, OFDM demodulation (GI removal and FFT), de-mapping, de-interleaving, viterbi decoding, descrambling, and data recovery.

This software implementation allows studies on noise and multipath effects in the IEEE 802.11g (WLAN) based Physical layer, and understanding on how interferences can affect the quality of received signal.

1.1 Overview of Wireless LANs

A Wireless Local Area Network (WLAN) is a system of (usually mobile) nodes that access a common wireless channel within the same frequency band, for transferring data amongst each other, within a limited geographical area. Wireless LANs have quickly become a significant niche in the LAN market. As adjuncts to traditional wired LANs, they satisfy mobility, relocation, and ad hoc networking requirements and provide a way to cover locations that are difficult to wire [1]. Before advent of higher data rate modes of WLANs, few organizations used wireless LANs because they cost too much, low data rates, they posed occupational safety problems because of concerns about the health effects of electromagnetic radiation, and the spectrum used required a license. Today, however, these problems have largely diminished, and wireless LAN popularity is skyrocketing.

Wireless LAN products first appeared in the late 1980s, marketed as substitutes for traditional wired LANs. The motivation behind using a wireless LAN is to avoid the cost of installing LAN cabling and ease the task of relocating or otherwise modifying the network’s structure. For instance, building with large open areas such as “manufacturing plants, stock exchange trading floors, and warehouses” [1], make wired LANs awkward to install because of limited choices for cable placement. Also historical buildings often have insufficient twisted-pair cabling and prohibit drilling holes for new wiring. Finally, small offices often find it uneconomical to install and maintain wired LANs.

In most cases, an organization already has a wired LAN to support servers and some stationary workstations. For example, a manufacturing facility typically has an office area that is physically separate from the factory floor but must be linked to it for networking. Therefore, organizations will commonly link a wireless LAN into a wired LAN on the same premises. This kind of application, or LAN extension, can be achieved through one or more “Control Modules (CM)” in single or multiple cell configurations.

Fig. 1.1 shows the single-cell configuration, a simple wireless LAN strategy typical of many environments. It is so named because all the wireless end systems are within range of a single control module.

A backbone wired LAN, such as Ethernet, supports servers, workstations, and one or more bridges or routers to link with other networks. A control module (CM) acts as an interface to a wireless LAN. The module includes either bridge or router functionality to link the wireless LAN to the backbone and some sort of access control logic, such as a polling or token-passing scheme, to regulate access from the end systems. Some of the end systems are stand-alone devices, such as a workstation or a server. Hubs or other user modules (UMs) that control several stations off a wired LAN may also be part of the configuration.

Another common configuration is a multiple-cell wireless LAN, in which a wired LAN connects multiple control modules [1]. Each control module supports wireless end systems within its transmission range. An infrared LAN, for example, limits transmission to a single room, so each room in an office building would need one cell.

In a Nomadic Access configuration, the wireless LAN links a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop or notepad computer. Thus, for example, an employee returning from a trip can transfer data from a personal portable computer to an office server. Nomadic access is also useful in an extended environment such as a campus or a business operating from a cluster of buildings. In both cases, users can move around with their portable computers and access the servers on a wired LAN from various locations.

In an Ad hoc network configuration, a network is set up temporarily to meet some immediate need. It has no centralized server. Thus, in meetings, a group of employees, each with a laptop or palmtop computer, can link their computers in a network that lasts just as long as the meeting.

The IEEE 802.11 standard for wireless LANs is presently the dominant standard for Wireless LANs. It specifies the implementations of the Medium Access Control (MAC) and physical (PHY) layers.

1.2 The IEEE 802.11 Architecture

The smallest building block of a wireless LAN is a basic service set, which consists of stations that execute the same MAC protocol and compete for access to the same shared wireless medium [1, 2]. A basic service set may be isolated or, as shown in Fig. 1.2, connected to a backbone distribution system through an access point (AP), which functions as a bridge and is implemented as part of a station. A central coordination function housed in the access point controls the MAC protocol or the protocol may be fully distributed. The basic service set generally corresponds to a cell. The distribution system can be a switch, wired network, or wireless network. The portal integrates the IEEE 802.11 architecture with a traditional wired LAN. The portal logic is implemented in a device, such as a bridge or router, which is part of the wired LAN and attached to the distribution system.

These extensions to the Basic Service Set constitute an Extended Service Set as shown in Fig. 1.2, in which a distribution system connects two or more basic service sets. Typically, the distribution system is a wired backbone LAN, but it can be any communications network. The extended service set appears as a single logical LAN to the logical link control (LLC) level. The access point is the logic within a station that provides access to the distribution system by providing services in addition to acting as a station.

1.3 IEEE 802.11 Protocol Layers

Fig. 1.3 shows the standard’s layered protocol architecture [1, 2, 3, 4, 5]. The lowest (physical) layer defines the frequency band, data rate, and other details of the actual radio transmission. Above the physical layer is the medium access control (MAC) layer, which regulates access to the shared radio frequency band so that station transmissions do not interfere with one another [1]. The MAC layer has two sub layers. The lower one is the distributed coordination function, which uses an Ethernet-style contention algorithm that provides access to all traffic. Ordinary asynchronous traffic uses this coordination function. The upper MAC sub layer is the point coordination function, a centralized MAC algorithm that provides contention-free service by polling stations in turn. Higher priority traffic, traffic with greater timing requirements, uses this coordination function. Finally, the logical link control layer provides an interface to higher layers and performs basic link-layer functions such as error control.

1.4 IEEE 802.11 PHY Specifications

The IEEE issued the physical layer for 802.11 in four stages. The first part, issued in 1997, is called simply IEEE 802.11 [2]. As Fig. 1.3 shows, it includes the MAC layer and three physical layer specifications, all operating at data rates of 1 and 2 Mbps:

• Direct-sequence spread spectrum (DS-SS), operating in the 2.4-GHz ISM (Industrial, Scientific, and Medical) band;
• Frequency-hopping spread spectrum (FHSS), operating in the 2.4-GHz ISM band; and
• Infrared, operating at a wavelength between 850 and 950 nm.

Most of the early 802.11 networks used the FHSS scheme, which is simpler. Networks that used the DS-SS scheme were more effective, but all the original 802.11

products had data rates of at most 2 Mbps, which limited their usefulness.

In 1999, the IEEE issued the second and third physical layers, IEEE 802.11a and IEEE 802.11b, at roughly the same time. IEEE 802.11a operates in the 5-GHz band at data rates up to 54 Mbps. IEEE 802.11b operates in the 2.4-Ghz band at 5.5 and 11 Mbps. Because 802.11b is easier to implement, it has yielded products first.

IEEE 802.11b extends the IEEE 802.11 DS-SS scheme, providing data rates of 5.5 and 11 Mbps through the use of a more complex modulation technique Complementary Code Keying (CCK).

Although 802.11b is successful to some degree, the data rate is still too low for applications that need a truly high speed LAN. IEEE 802.11a targets this specific need. Unlike the other 802.11 standards, it specifies the 5-GHz band, and it replaces the spread-spectrum scheme with the faster orthogonal frequency-division multiplexing. OFDM, also called multi-carrier modulation, uses up to 52 carrier signals at different frequencies, sending some of the bits on each channel. Possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.

In 2003, the IEEE issued the fourth physical layer std. IEEE 802.11g. IEEE 802.11g PHY operates in 2.4 GHz band and the possible data rates are 1 and 2 Mbps (using DSSS), 5.5 and 11 (using DSSS/CCK), 6, 9, 12, 18, 24, 36, 48, and 54 Mbps (using OFDM).

1.5 Motivation for this Thesis

As there is a widespread use of wireless local area networks for data exchange, it is important to understand how such information is transmitted and received.

In July 2003 the new form of WLAN standard, IEEE 802.11g, came into existence. 802.11g is an extension to 802.11b, which is the basis of the majority of wireless LANs in existence today [6]. 802.11g broadens 802.11b's data rates to 54 Mbps within the 2.4 GHz band using OFDM (orthogonal frequency division multiplexing) technology. Because of backward compatibility, an 802.11b radio card will interface directly with an 802.11g access point (and vice versa) at 11 Mbps or lower depending on range.

Similar to 802.11b, 802.11g operates in the 2.4 GHz band, and the transmitted signal uses approximately 30MHz, which is one third of the band.

A big difference with 802.11a is that it operates in the 5 GHz frequency band with twelve separate non-overlapping channels [6]. As a result, one can have up to twelve access points set to different channels in the same area without them interfering with each other. This makes access point channel assignment much easier and significantly increases the throughput the wireless LAN can deliver within a given area. In addition, RF interference is much less likely because of the less-crowded 5 GHz band.

Similar to 802.11g, 802.11a delivers up to 54 Mbps, with extensions to even higher data rates possible by combining channels. Due to higher frequency, however, range (around 80 feet) is somewhat less than lower frequency systems (i.e., 802.11b and 802.11g).

A huge problem with 802.11a is that it is not directly compatible with 802.11b or 802.11g networks. In other words, a user equipped with an 802.11b or 802.11g radio card will not be able to interface directly to an 802.11a access point.

We have thus far seen the different WLANs standard such as IEEE 802.11b [3], IEEE 802.11a [4], and IEEE 802.11g [5]. So when this new standard find approval, the big question that arrived is that whether to go for the IEEE 802.11g from existing IEEE 802.11b and IEEE 802.11a or not. For this the performance analysis of different IEEE 802.11g operating modes in different channel conditions are required. Such as the performance in AWGN channels or performances in multipath fading channels should be available to decide whether to implement the system or not.

In this project, the performance of different operating modes of IEEE 802.11g PHY has been found though the simulation in MATLAB. The results for AWGN channel with and without multipath have been compared.

1.6 Organization of Thesis

The rest of the thesis is organized as follows:

Chapter 2 discusses the evolution of Wireless LANs and different Physical layer specifications for WLANs and gives the introduction of Complementary Code Keying and OFDM. Chapter 3 discusses the PHY of IEEE 802.11g in detail with End to End communication structure for ERP-OFDM mode. It also discusses the CCK encoding and decoding process. Chapter 4 gives the simulation details and setups. Chapter 5 gives the simulation results and discusses these results. Chapter 6 finally presents conclusions along with future work envisaged in this area.

Chapter 2: Literature Survey

2.1 Evolution of WLAN Standards

The first wireless Ethernet standard, IEEE 802.11, was adopted in 1997 [6]. This standard provided for three physical layer (PHY) specifications including infrared, 1-2 Mbps frequency hopping spread spectrum (FHSS) and 1-2 Mbps direct sequence spread spectrum (DSSS) in the 2.4 GHz ISM band. Because wired Ethernet LANs at the time were capable of speeds up to 10 Mbps and early products were quite pricey, the original 802.11 standard had limited success in the market.

Two years later, the original 802.11 standard evolved along two paths. The 802.11b specification increased data rates well beyond the critical 10 Mbps mark, maintained compatibility with the original 802.11 DSSS standard and incorporated a more efficient coding scheme known as complimentary code keying (CCK) [7] to attain a top-end data rate of 11 Mbps [8]. A second coding scheme, Packet Binary Convolutional Code (PBCC), was included as an option for higher performance in the form of range at the 5.5 and 11 Mbps rates, as it provided for a 3 decibel (dB) coding gain.

The second offshoot of 802.11 was designated as 802.11a [4]. It ventured into a different frequency band, the 5 GHz U-NII band, and was specified to achieve data rates up to 54 Mbps. Unlike 802.11b, which is a single carrier system, 802.11a utilized a multi-carrier modulation technique known as orthogonal frequency division multiplexing (OFDM). By utilizing the 5 GHz radio spectrum, 802.11a is not interoperable with either 802.11b, or the initial 802.11 WLAN standard.

In March 2000, the IEEE 802.11 Working Group formed a study group to explore the feasibility of establishing an extension to the 802.11b standard for higher data rates greater than 20 Mbps [6]. In July 2000, this study group became a full task group, Task Group G (TGg), with a mission to define the next standard for higher rates in the 2.4 GHz band. The new standard was which came into existence in July 2003 is named as IEEE 802.11g and operates into 2.4 GHz ISM band and the data rate is up to 54 Mbps.

For basic rate of 1 and 2 Mbps, IEEE 802.11g uses DSSS scheme, for 5.5 and 11 data rates IEEE 802.11g uses CCK encoding scheme, and for 6, 12 and 24 Mbps data rates IEEE 802.11g uses OFDM scheme. Theses all are mandatory modes for IEEE 802.11g PHY. Table 2.1 shows all the mandatory and optional modes for IEEE 802.11b, IEEE 802.11g, and IEEE 802.11a

2.2 Wireless Local Area Networks (WLANs) PHY

The 802.11 physical layer (PHY) is the interface between the MAC and the wireless media where frames are transmitted and received. The PHY provides three functions. First, the PHY provides an interface to exchange frames with the upper MAC layer for transmission and reception of data. Secondly, the PHY uses signal carrier and spread spectrum modulation to transmit data frames over the media. Thirdly, the PHY provides a carrier sense indication back to the MAC to verify activity on the media.

2.2.1 Original IEEE 802.11 PHY

The original IEEE 802.11 provides three different PHY definitions: Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), and Infrared [2]. Both Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) support 1 and 2 Mbps data rates.

2.2.1.1 Frequency Hopping Spread Spectrum (FHSS)

Frequency Hopping utilizes a set of narrow channels and "hops" through all of them in a predetermined sequence. For example, the 2.4 GHz frequency band is divided into 70 channels of 1 MHz each. Every 20 to 400 msec the system "hops" to a new channel following a predetermined cyclic pattern. There are 3 hopping sequence set with 26 hopping sequences per set. The minimum hope rate is 2.5 hops per second. The 802.11 Frequency Hopping Spread Spectrum (FHSS) PHY uses the 2.4 GHz radio frequency band, operating with at 1 or 2 Mbps data rate. The basic access rate of 1 Mbps uses a two level Gaussian Minimum Shift Keying (GMSK) while the enhanced access rate of 2 Mbps uses a 4 level GMSK.

2.2.1.2 Direct Sequence Spread Spectrum (DSSS)

The principle of Direct Sequence is to spread a signal on a larger frequency band by multiplexing it with a signature or code to minimize localized interference and background noise. To spread the signal, each bit is modulated by a code. In the receiver, the original signal is recovered by receiving the whole spread channel and demodulating with the same code used by the transmitter. The 802.11 Direct Sequence Spread Spectrum (DSSS) PHY also uses the 2.4 GHz radio frequency band. The basic access rate of 1 Mbps is encoded using Differential Binary Phase Shift Keying (DBPSK) while the enhanced 2 mbps rate is encoded using Differential Quadrature Phase Shift Keying (DQPSK).

2.2.1.3 Infrared

The Infrared PHY utilizes infrared light to transmit binary data either at 1 Mbps (basic access rate) or 2 Mbps (enhanced access rate) using a specific modulation technique for each. For 1 Mbps, the infrared PHY uses a 16-pulse position modulation (PPM). The concept of PPM is to vary the position of a pulse to represent different binary symbols. Infrared transmission at 2 Mbps utilizes a 4 PPM modulation technique. This specification was designed for indoor use only.

2.2.2 IEEE 802.11b PHY

PHY of IEEE 802.11b is known as High Rate, direct sequence spread spectrum (HR-DSSS). In addition to 1 and 2 Mbps data rate, IEEE 802.11b provides higher data rates of 5.5 and 11 Mbps. To provide the higher data rates, 8-chip complementary code keying (CCK) is employed as modulation scheme. The chipping rate is 11MHz, which is same as the DSSS system of IEEE Std 802.11, 1999 Edition, thus providing the same occupied channel bandwidth. The basic new capability described in this clause is called High Rate Direct Sequence Spread Spectrum (HR/DSSS) [3].

There is an optional mode replacing CCK modulation with packet binary convolutional coding (PBCC). This mode is denoted as HR/DSSS/PBCC.

Another optional mode is provided that allows data throughput at the higher rates (2, 5.5, and 11 Mbps) to be significantly increased by using a shorter PLCP preamble. This mode is called HR/DSSS/short or HR/DSSS/PBCC/short [3].

Four modulation formats and data rates are specified for the High Rate PHY. The basic access rate shall be based on 1 Mbps DBPSK modulation. The enhanced access rate shall be based on 2 Mbps DQPSK. The extended direct sequence specification defines two additional data rates. The High Rate access rates shall be based on the CCK modulation scheme for 5.5 Mbps and 11 Mbps. An optional PBCC mode is also provided for potentially enhanced performance.

2.2.2.1 Barker Sequence

The following 11-chip Barker sequence shall be used as the PN code sequence for the 1 and 2 Mbps modulation:

+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1

The leftmost chip shall be output first in time [10]. The first chip shall be aligned at the start of a transmitted symbol. The symbol duration shall be exactly 11 chips long [3]. The DBPSK encoder for the basic access rate is specified in Table 2.2. The DQPSK encoder is specified in Table 2.3. (In these tables, +jω shall be defined as counterclockwise rotation.)

3.3.2.1 Multipath Fading Channel

The communication channel is the medium which the transmitting radio signal goes through in order to reach the receiver. The channel can be modeled as a linear filter with a time varying channel impulse response [11].

A channel impulse response describes the amplitude and phase effects that the channel will impose on the transmitting radio signal, as it transmits through the medium. IEEE 802.11 communication channels are often modeled as a multipath fading channel, as it is the best modeling for a wireless communication channel.

The term ‘fading’ describes the small-scale variation of a mobile radio signal. As each transmitting signal is represented by a number of multipaths and each having different propagation delays, the channel impulse response is different for each multipath. Therefore, not only the channel response is time varying, the channel response is also functional dependent on the propagation delay. Hence, the channel impulse response should actually be summarized as h (t, τ), where t is the specific time instance, and τ is the multipath delay for a fixed value of t. As a result, the received signal in a multipath channel consists of a number of attenuated, time delayed, and phase shifted versions of the original signal, and the base-band impulse response of a multipath channel can be written as

(3.10)

where a_i(t,τ) and τ_i(t) are the amplitude and delay, respectively, of the i^th multipath component at time t. The phase term 2Пf_cτ_i(t) + Фi(t, τ) represents the phase shift due to free space propagation of the i^th multipath component, plus any additional phase shift which it encountered in the channel. And δ(τ- τ_i(t)) is the unit impulse function for the i^th multipath component with delay τ and at time instance t.

Fig. 3.8 illustrates an example of the channel response of a time varying discrete-time multipath fading channel.

3.3.2.2 Noise in Communication Channel

Within any communication channel, there is always noise present due to other surrounding radio signals. These noise can be white (colorless) or colored noise, and can interact differently with the transmitted user signal, for instance, the interaction can be additive, multiplicative or complex. In communication systems, a transmitting signal is very vulnerable to noise especially within the communication channel.

Noise is often classified as some unwanted signals or interference, which is present along with an information signal in a communication channel. And often, the level of noise present is incontrollable, as there are so many potential sources of noise in the channel. However, by determining the approximate power level of noise in a communication channel, the Bit-Error-Rate (BER) of a communication system can be greatly reduced, by adjusting the power level of the transmitting information signal.

In IEEE 802.11g WLAN, the channel noise analysis is often based on Additive White Gaussian Noise (AWGN).

3.3.2.3 White Noise

White noise is a type of noise that is often exists in communication channels. It is remarkably different from any other types of noise, due to the fact that its Power Spectral Density (PSD) is independent of the operating frequency. The word ‘White’ is used in the sense that white light contains all other visible light frequencies in the band of electromagnetic radiation. For any two different time samples of white noise, they will be uncorrelated, no matter how close they are to each other in time.

In addition, if the white noise is also Gaussian, then it’s also statistically independent, and exhibit total randomness. In IEEE 802.11g WLAN, communication channels are often modeled with Additive White Gaussian Noise (AWGN).

The adjective ‘Additive’ describes the interaction that happens between the noise interact with another signal during collision. When AWGN comes in contact with a user signal, the real and imaginary amplitude components of the two signals add up and form a new signal.

3.3.3 Receiver

The processing performed in the receiver terminal is complementary to those of the transmitter terminal modulation processes. The demodulation processes that the receiver must perform are rake receiver combining (I-Q demodulation and maximal ratio combining), OFDM demodulation (GI removal and FFT), de-mapping, de-interleaving, viterbi decoding, descrambling, and data recovery. A diagrammatic representation of the receiving processes is depicted

3.3.3.2 OFDM Demodulation

After rake receiver combining the guard band is removed from the symbols. The obtained symbols are then demodulated through FFT to get the mapped user data.

3.3.3.3 De-mapping

This is the complementary process of the mapping performed at the transmitter. During reception the receiver have prior knowledge of data rate and hence the modulation style. Depending upon the modulation type the corresponding normalization factor (K_MOD) that was multiplied at the transmitter is compensated. Now each symbol is decoded in 1, 2, 4, or 6 bits respectively for BPSK, QPSK, 16-QAM or 64-QAM.

3.3.3.4 De-interleaving

De-interleaving at the receiver acts to remove the effect of data interleaving applied to the data bit stream at the transmitter. The deinterleaver, which performs the inverse relation to that of the interleaver, is also defined by two permutations.

Here we denote by j the index of the original received bit before the first permutation; i be the index after the first and before the second permutation, and k be the index after the second permutation, just prior to delivering the coded bits to the convolutional (Viterbi) decoder.

4.2.7 OFDM

The time domain samples are produced by performing 64 point IFFT, cyclically extending them up to 80 points.

All the symbols are transmitted one by one after up converting to the desired carrier frequency.

4.3 Coding of Communication Channel

The implemented IEEE 802.11g communication channel is represented by a multipath fading channel and noise component. The channel imposes amplitude and phase attenuations to each of the multipath signal that passes through it, while the noise component causes further attenuations to each multipath signals both amplitude and phase.

4.3.1 Multipath Fading Channel

The communication channel is implemented as a multipath channel. It is represented by a number of randomly distributed objects, and each with an amplitude and phase gain. When a multipath signal reflects on one of these objects along its propagation, the multipath signal experiences amplitude and phase attenuations according the respective gains of the object, due to the interaction between the multipath signal and the object.

The objects are randomly generated and distributed in the channel. If we assume the channel as a Rayleigh fading channel then amplitude gains for different paths are Rayleigh distributed. In general, the amplitude gain α of an object will vary from 0 to 1, while the phase gain β will vary from 0 to 2П, but β is not equal to 0 or 2П. Fig. 4.1 shows a possible configuration of the multipath fading channel with 3 multipath signals.

6.1 Summary of Conclusions

In this thesis first we have studied the basics of wireless local area networks (WLANs).Then the architecture, and the protocol layers of WLAN std. IEEE 802.11 have been studied. This thesis also discusses different Wireless LAN standards and PHY specifications of different WLAN standards. The PHY of IEEE 802.11g has been discussed in detail including transmitter and receiver structure and communication channel.

We have discussed the different operational modes of IEEE 802.11g PHY, End to end communication structure for ERP OFDM modes, Transmitter processes as scrambling, convolutional encoding, data interleaving, modulation mapping, OFDM, the communication channel and noise for the WLANs. Receiver processes have also discussed in brief. CCK encoding and decoding processes have also been presented. Then the Matlab simulation process has been presented. Simulation processes from user data generation to generation of transmitted signal (at transmitter) and the receiver simulation which includes communication channel have discussed.

All the OFDM mandatory and optional modes and CCK modes have been simulated. The simulation results are in the form of BER v/s SNR. The results have been presented for all these modes for IEEE 802.11g Wireless Local Area Networks. Firstly the performance in terms of BER v/s SNR for AWGN channel is discussed for all modes, then the performances of all these modes have been compared with multipath fading channel results.

The different OFDM mandatory modes are compared with each other in presence of AWGN channel. The different OFDM optional modes have also been compared with each other in presence of AWGN channels. The performances in presence of multipath fading channels and in AWGN channel have been compared for all the modes.

Since the WLAN std. IEEE 802.11g is relatively new (July 2003), not much work is available to compare with the obtained results.

From the results it has been found that the performances of the OFDM modes are better than bare OFDM transmissions in presence of AWGN channel and this is due to the convolutional encoding which is employed before OFDM.

From the multipath results it has been found that CCK modulated systems are having poor performances in fading channel from OFDM systems about 20-30% in terms of SNR for same BER.

6.2 Future Directions

With this simulation one can include the MAC layer functionality and investigate the performance of WLAN in actual LAN scenario. More than one station can be taken for implementation.

The advanced channel modeling can be done and even the noise model can be enhanced by incorporating all other types of noises not just AWGN. Since AWGN is not the only type of noise present in a communication channel. Other type of noises such as colored nose can also be present.

References

1. W. Stallings, “IEEE 802.11: Moving closer to practical wireless LANs,” IEEE IT Professional, vol.3, no.3, pp.17-23, May 2001.

2. IEEE Std. 802.11,1999 edition, Information technology—Telecommunications and information exchange between systems— Local and metropolitan area networks— Specific requirements— Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.

3. IEEE Std. 802.11b-1999: Supplement to IEEE Standard for Information technology— Telecommunications and information exchange between systems— Local and metropolitan area networks— Specific requirements— Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band.

4. IEEE Std. 802.11a-1999: Supplement to IEEE Standard for Information technology— Telecommunications and information exchange between systems— Local and metropolitan area networks— Specific requirements— Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 5 GHz Band.

5. IEEE Std. 802.11g-2003: IEEE Standard for Information technology. Telecommunications and information exchange between systems. Local and metropolitan area networks. Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 4: Further Higher Data Rate Extension in the 2.4 GHz Band

6. William Carney, “IEEE 802.11g, New Draft Standard Clarifies Future of Wireless LAN,” Wireless Networking paper Texas Instruments, 2002.