62 

 

CHAPTER 3 : Diversity Analysis in WiFi and WiMAX Systems

This chapter imparts information on wireless LAN and MAN systems that have

perceived the theoretical potentials of diversity techniques. While Wi-Fi essentially a

WLAN(Wireless Local Area Network) has monopolized the domain of short range

wireless broadband systems, WiMAX (Worldwide Interoperability for Microwave

Access), IEEE 802.16a, on the other hand has made radical inroads into the wireless

communications market of WMAN (Wireless Metropolitan Area Network) tagged as the

‘Revolutionary broadband wireless technology of the current era’ as it opens up a realm

of schemes for multiple antenna techniques comprising of Diversity Techniques, Smart

Antenna Systems (SAS) and Multiple Input Multiple Output (MIMO) Systems.

To reinforce the existing system and further enhance the measure of performance

in multipath fading scenario, Wi-Fi and WiMAX systems are intertwined with space

diversity techniques, implemented using Alamouti coding and introduced with

interleaving/scrambling techniques that benefit from time diversity. Diversity technique

plays a pivotal role in enhancing the quality of the wireless system’s operations, by

providing optimum performance on par with least distorted path amongst various

multipaths at any given point of time and thereby substantiating the fading effect caused

in corruptive communication channels. The probability of experiencing a fade in the

composite channel is then proportional to the probability that all the component channels

simultaneously experience a fade, which is an unlikely event. Thereby diversity, a key

technique that exploits the inherent multipath nature of wireless channels has been

deployed in our work to mitigate transmission errors. Multiple transmit and multiple

63 

 

antennae have emerged as key enabling technologies for wireless transmission of

broadband data.

This analysis seeks to simulate the physical layer of Wi-Fi and WiMAX systems

with application of diversity schemes at the transmitter and receiver. 2X2 Alamouti

coding scheme has been implemented for exemplifying MIMO diversity and 2X1

Alamouti coding scheme for MISO diversity. Alamouti codes which have exhibited

remarkable results in flat fading channel and when coupled with OFDM structure have

been found to produce the same level of performance in multi path fading

environment[54,103,104]. Pertaining to Wi-Fi, IEEE 802.11b standard, the incorporation

of convolutional codes and DSSS has replaced the OFDM structure of WiMAX. The

study of BER of Wi-Fi and WiMAX systems, hosted with AWGN and Rayleigh

channels, have demonstrated the benefits of diversity reception. Studies have also been

undertaken to record the impact of modulation schemes like BPSK, Quadrature Phase

Shift Keying(QPSK) and 16-Quadrature Amplitude Modulation(QAM) on BER.

3.1. Combinational Diversity in Wi-Fi system

Wi-Fi, short for Wireless Fidelity is a wireless digital communication system that

operates in 2.4GHz ISM bandwidth. For LAN’s with coverage of only a few hundred

feet, the operating channel bandwidth is intended to be 25MHz. Any high-level protocol

can be used in a Wi-Fi network in a similar way as in an Ethernet network. The IEEE

802.11 standard is regarded as one of the earliest trailblazers in introducing bandwidths

of 1 to 2 Mbps. However, the original standard has undergone subsequent amendments to

optimize bandwidth that include the 802.11a, 802.11b and 802.11g standards, which are

64 

 

called 802.11 physical standards, to ensure enhanced security and compatibility of the

components[105].

3.1.1. Technological aspects of Wi-Fi system

The physical layer defines the radio wave modulation and signaling

characteristics for data transmission. The data link layer defines the interface between

the machine's bus and the physical layer, in particular an access method similar to the one

used in the Ethernet standard and rules for communication between the stations of the

network. The 802.11 standard actually has three physical layers as shown in Table 3.1,

which define alternate modes of transmission[105].

Table 3.1. OSI model for 802.11 standards

Not all diversity schemes have been proposed in the IEEE standards and amendments to

WLAN, but the proposed CTD scheme emphasize on MIMO techniques and can be

categorized to facilitate their implementation in base band and/or in the RF section of

transceivers. The diversity schemes in the base band signal level at the physical layer

may be implemented with respect to time, code and space with bit interleaving, DSSS

and space time block codes respectively.

Data Link Layer (MAC)

Physical Layer (PHY)

802.2

802.11

 DSSS FHSS Infrared

65 

 

Since the data to be transmitted in the wireless network is scattered in various

dimensions, this compensates the impact of long term and short term fading due to the

multipath channels. Thus, enabling the receiver to reconstruct intelligent information

from the corrupted copy received from the transmitted signal. Despite the numerous

benefits of wireless network such as support for mobility, dynamic topology, device

portability, etc., the wireless technology is highly vulnerable to data security threats and

multipath interference. While the security concerns are handled in the upper layers of the

OSI model, multipath propagation a fundamental characteristic of wireless

communication is addressed in the physical layer.

This work investigates the improvement in SNR for the MIMO design with time

diversity, code diversity and space diversity in the physical layer of basic WLAN

standard. These diversity schemes are exploited with scrambling and bit interleaving,

spread spectrum technique and Space Time Block codes. The performance of simple 1 X

1 transceiver is compared with 1X2 and 2X2 modes of WiFi architecture.

3.1.2. System Description

The schematic diagram of transmitter model of Wi-Fi System is shown in Fig.3.1.

To increase the reliability of transmission in a noise effective channel, it is necessary to

decrease the frequency of error events and this vital role is played by channel coding.

Fig.3.1. Transmitter model of the Wi-Fi system

Channel Encoder

Interleaver

DSSS Transmitter

Space Time Encoder

From Mod

66 

 

As per the standard, for the simulation of physical layer of Wi-Fi 802.11b,

convolutional codes are used as channel error control scheme and DSSS resulted with

code diversity. In addition to this, to encompass spatial and temporal diversity, STBC

encoder and interleaver are introduced to the existing standard. The schemes of CTD are

shown in italics in Table 3.2

Table 3.2. Technological aspects of WiFi 802.11b standard

Description Specification

Primary Application WLAN

Frequency Band 2.5 GHz ISM Band

Bandwidth 25 MHz

Modulation QPSK

FEC Convolutional Codes

Temporal Diversity Interleaver

Code Diversity DSSS

Space Diversity STBC coder

The convolutional codes are non-systematic binary codes and the encoder

operates on the incoming message bits in a sequentially manner[106]. The encoder of the

67 

 

binary convolutional code with rate 1/n is considered as a finite state machine. It consists

of an M-stage shift register with connections to n modulo 2 adders and a multiplexer that

serializes the output to the adders. An L-bit message sequence produces a coded output

in sequence of length n (L+M) bits. The code rate is given by

R= L / n (L+M) bits/symbol 3.1

Another important parameter to consider is the constraint length (K) of the convolutional

encoder i.e. the number of shifts over which the message bit can influence the encoder

output. In an encoder with ‘M stage’ shift registers the memory of the encoder that

equals ‘m’ message bits and requires K=M+ 1 shifts, for a message bit to enter the shift

register and come out finally. The encoded data is fed to the interleaver, which in turn

scrambles the data in a non-contiguous manner, so that no adjacent symbols come from

the same code word. This is done to ensure that even if noise corrupts one bit of data, the

other bits can be used to retrieve the original data, enhancing the performance of the

system by mitigating the burst errors, which may occur when data travels in a corrupted

multipath environment. The IEEE 802.11b data is encoded using DSSS technology which

is the most widely recognized form of Spread Spectrum technique

As per the standards of IEEE 802.11b and QPSK is used to encode 2 bits of

information in the same space, this modulation has an added advantage of increasing the

data rate and thereby causing reduction in the available process gain. For a given PN code

rate, the process gain is reduced as the bandwidth which sets the process gain is halved

due to the bi-fold increase in information transfer. As a consequence the systems in a

spectrally quiet environment benefit from the possible increase in data transfer rate. As

68 

 

the coverage range of Wi-Fi is limited to some hundreds of meters the interference is

comparatively low when compared to WMAN applications. So the Direct Sequence

Spread Spectrum technique using QPSK modulation in Wi-Fi system is found to have

acceptable levels of performance. This spectrum technique is used for a host of reasons

that include establishment of secure communications, prevention of spurious detection

and increasing resistance to natural interference and jamming.

With the use of Space Time Block Code, a certain amount of correlation between

these symbols transmitted over the channel is attained owning low receiver

complexities[107,108]. STBC coding and decoding is carried out as discussed in section

2.1.7. The receiver side of the Wi-Fi system consists of a De-Spreading block,

Demodulator, De- Interleaver and Viterbi Decoder that are used in decoding the

convolutional coded data as shown in Fig.3.2.

Fig.3.2. Receiver model of the Wi-Fi system

After the signal reception the signals are combined using Maximum Likelihood decision

rule to obtain the signal estimate. The received signal vector is:

rt= (rt1, rt

2,…………, rtnr) and rt= Htxt+ nt. 3.2

STBC

Decoder

DSSS

Receiver

De

Interleaver

Viterbi

Decoder

To Demodulator 

69 

 

The estimated signal is then fed to the de-spreading block. The transmitted and receiver

sequences need to be synchronized for the efficient working of de-spreading. When the

estimated signal correlates exactly with the PN signal generated at the receiver side the

output signal is maximized. The correlated signal is then filtered and the resultant de-

spread signal is sent to a QPSK demodulator.

Demodulation is followed by de-interleaving. The de-interleaved data is then imparted

to the Viterbi decoder. The function of Viterbi decoder is to estimate the path traversed

through the Trellis and correspondingly decode the data, after which the original message

is obtained[109].

3.1.3. Results of WiFi Diversity Analysis

BER is used as an index to compare the performance measures of various receiver

models.

Fig.3.3. Comparison of SISO, MISO and MIMO schemes

0 5 10 15 20 25 30 3510-16 

10-14 

10-12 

10-10 

10-8

10-6

10-4

10-2

100 

Eb/No (dB)

BER

MIMO,MISO&SISO Comparison in Wi-Fi

2x21x12x1

70 

 

From the Fig.3.3, it is observed that, for the maximum Eb/No at 30 dB, SISO

provides BER around 10-2 , MIMO gives BER around 10-9 . Hence 18 dB improvement

in Eb/No could be achieved. It is observed that there is a marked increase in performance

which can be attributed to the use of multiple antennae at both transmitter and receiver

side leads to better performance rather than using a single antenna at transmitter and

receiver side. This is due to the fact that, by using a single antenna scheme, the

cumulative density of SNR decreases due to fading effect.

3.2. Combinational Diversity in WiMAX system

To overcome the limitations that arise as a consequence of multipath fading,

implementation of simple and novel ideas in conjunction with key advantages of the

specific communication architecture can be adopted. The efficacy of inherent Non Line

Of Sight (NLOS) capableness of WiMAX standard enables long distance coverage

overriding the constraints of wireless systems and the limited foot-print of wired network.

Architecture of this kind, which devises the measures to overcome fading, has proven to

suit the technology for regional and rural areas.

3.2.1. Technological aspects of WiMAX system

WiMAX can provide broadband wireless access spanning 30 miles (50 km) for

fixed stations and 3 - 10 miles (5 - 15 km) for mobile stations. WiMAX operates on both

licensed and non-licensed frequencies, providing a regulated environment and viable

economic model for wireless carriers. Use of WiMAX for wireless networking runs

parallel to the more common Wi-Fi protocol. WiMAX, a second generation protocol,

71 

 

provisions for increased bandwidth use, interference avoidance and higher data

transmission rates over long distances. The Mobile WiMAX standard proposed by

WiMAX Forum[110] in August 2006 is the fulcrum for smart antenna technologies,

fractional frequency reuse and Multicast and Broadcast Service (MBS). Kim Ngan Trieu

Olmide has analyzed the transmission characteristics of two antenna systems in Mobile

WiMAX(802.16e) and discussed the robustness of OFDM to long delay spread and in

his dissertation report[111]. Though an adaptive modulation along with various channel

models was introduced for the physical layer of WiMAX in [112], the issue of signal

fading remained largely unaddressed.

This dissertation work depicts the method of introducing combinational diversity

techniques in the physical layer of IEEE 802.16. The performance of both SISO and

MIMO pattern is compared by simulating the standard architecture with and without

deployment of the diversity schemes. The MIMO systems with combining techniques

may be implemented in any of the signal domain representation as for divergence in time,

frequency and space with Rake receivers, OFDM and polarization concepts respectively.

In this effort frequency diversity at both the transmitter and receiver end is established.

Space time block and bit interleaving, mirrors the role of spatial and temporal scattering.

3.2.2. System Description

Reed Solomon Codes (RS codes) which are used for channel coding to simulate

the physical layer of WiMAX, are a linear, non-binary, block error correcting codes,

constructed and decoded through the use of finite field known as Galois Fields (GF).

72 

 

Comprising of two symbols, one is a redundant symbol for detecting error while the other

is for correcting code in a RS coding technique[109].

An amalgamation of interleaving and error control coding helps in overcoming

burst errors that result in performance enhancement. By interleaving, the bits in the data

packet are shuffled and arranged to form a new data, which enables to reconstruction of

the original data, by taking the advantage of temporal diversity. The Fig.3.4 shows the

WiMAX transmitter model adopted for the implementation.

Fig.3.4. Transmitter model of the WiMAX system

Analog Domain

Frequency Domain Time Domain

Channel encoder + Rate

matching

Ineter

leaver

Symbol

Mapper

I

F

F

T

I

F

F

T

D

/

A

 

D

/

A

Sub carrier

allocation + Pilot

Insertion

 

Space

Time

encoder

Sub carrier

allocation + Pilot

Insertion

Digital Domain

73 

 

Multi carrier systems such as WiMAX, with OFDM having long symbol period

and overlapped orthogonal subcarriers is an ideal candidate under heavy interference. The

narrow bandwidth of each carrier in the OFDM signal results in low symbol rate which

consequently reduces inter-symbol interference. RS coded data sequences are diverse in

time by interleaver followed by signal mapper where the necessary amplitude and phase

of the carrier is calculated based on modulation schemes used. Alamouti codes impart

spatial multiplexing and the required spectrum is divided into orthogonal carriers. OFDM

when reinforced with guard interval and cyclic prefix attains good resistance against

multipath fading. The resulting spectrum is then converted to time domain signal using

Inverse Fast Fourier Transform (IFFT). The factors which determine the value of Eb/No

can be enumerated as channel bandwidth, signal power, and the power spectral density of

the receiver noise.

Although the 802.16 standards are amply equipped with single carrier modulation

techniques, the vast majority of 802.16 compliant systems gravitate towards the OFDM

modes. Thus, OFDM based IEEE 802.16 WiMAX has evolved as the next generation

wireless connectivity provider for wideband communications. . With the clear intention

of obliterating multipath and its unwanted effects, pre-WiMAX systems are engaged to

obtain the desired effect. Achieving the LOS between the transmitter and receiver is

found to be conducive to the minimization or the annihilation of multipath.

74 

 

The technological aspects of WiMAX architecture is listed in Table 3.3. The

proposed combinational diversity schemes introduced in the existing IEEE standard[110]

are rendered in italics.

Table 3.3. Technological aspects of WiMAX(802.16) standard

Description Specification

Primary Application Broadband Wireless Access

Frequency Band Licensed, Unlicensed 2-11 GHz

Channel bandwidth Adjustable 1.25-25Mhz

Modulation BPSK, QPSK, 16 QAM

FEC Reed Solomon Code

Temporal Diversity Interleaving

Space Diversity STBC

3.2.3. R-S Performance

For a code to successfully combat the effects of noise, the noise duration needs to

be represented as a relatively small percentage of the codeword. For an increased and

more frequent occurrence of this phenomenon, the received noise should be averaged

over a long period of time. Thus, an increase in code block size dynamically impacts the

75 

 

efficiency of error correcting codes, making RS codes the most preferred choice for

desired long block lengths.

In order to gain an in-depth knowledge into encoding and decoding

principles of non-binary codes, such as R-S codes, it is necessary to venture into the area

of finite fields known as Galois Fields (GF). For any prime number ‘p’, there exists a

finite field containing ‘p’ elements denoted as GF (p). It is possible to extend GF (p) to a

field of ‘pm’ elements, called an extension field of GF (p) and is denoted as GF (pm),

where m is a nonzero positive integer. Note that GF (pm) contains a subset of elements

pertaining to GF (p). Symbols from the extension field GF (2m) are used in the

construction of Reed-Solomon (R-S) code. The binary field GF (2) is a subfield of the

extension field GF (2m), similar to the real number field which is a subfield of the

complex number field. Besides the numbers 0 and 1, there are additional unique elements

in the extension field that can be represented with a new symbol ‘α’. Each nonzero

element in GF (2m) can be represented as a power of α. An infinite set of elements ‘F’, is

formed by starting with the elements 0, 1 and α. Additional elements are generated by

progressively multiplying the last entry by α, which yields the following:

{ } { }...,,...,,,,0...,...,,,1,0 2102 JF αααααα == 3.3

To obtain the finite set of elements of GF (2m) from F, a constraint must be imposed on

F so that it comprises only 2m elements and is closed under multiplication. The condition

that closes the set of field elements under multiplication is characterized by the

irreducible polynomial shown below:

76 

 

01)12( =+−m

α 3.4

Using this polynomial constraint, any field element that has a power equal to or greater

than 2m - 1 can be reduced to an element with a power less than 2m – 1 as follows:

11)2()12( ++−− == nnmm

αααα 3.5

Thus, the finite sequence F* from the infinite sequence F is as follows:

{ },....,,,....,,,1,0* 212222 mmm

F ααααα −−= 3.6

Therefore, the equation defines that the elements of the finite field GF (2m) are as

follows:

{ }2210 ,....,,,0)2( −=mmGF ααα 3.7

A class of polynomials called primitive polynomials is of academic interest because such

functions define the finite fields GF (2m ) which in turn are needed to define R-S codes.

For increased efficiency, codes with higher code rates are the most preferred choice. RS

codes have efficiency of 80%. To correct a single error, RS codes employ a set of two

redundant symbols, since in this code does both error detection and correction, while one

symbol is used to detect the error and the other second symbol is used to correct the code.

3.2.4. OFDM in WiMAX

Orthogonal Frequency Division Multiplexing, a multicarrier modulation

technique has found widespread acceptance by a host of high data rate communication

systems and emerging wireless broadband systems. The systems riding high on the

77 

 

eminence of OFDM are Digital Subscriber Lines (DSL), Wireless LANs (802.11a/g/n),

Digital Video Broadcasting, WiMAX, Flash-OFDM and 4G/“Super 3G” cellular

systems.

OFDM works on the same principle as FDMA in that multiple user access is

achieved by fragmentation of the available bandwidth into multiple channels which are

then allocated to users. Interference between closely spaced carriers can be prevented by

aligning the carriers orthogonal to one another. This gives OFDM the added edge to

utilize the spectrum optimally as the channels are spaced closer together.

OFDM’s popularity for high-data rate applications stems primarily from its

efficient and flexible management of ISI in highly dispersive channels[113]. The

severity of ISI, potentially surges high when the channel delay spread becomes an

increasingly large multiple of Ts (Symbol Time). In a NLOS system such as WiMAX

that must transmit over moderate to long distances, the delay spread is generally large.

LOS between the Base Station (BS) and the Subscriber Station (SS) is expensive

and tough, owing to dense urban deployment scenario and the high incidence of SS being

stationed indoors. The answer to this predicament is WiMAX has been conceptualized to

operate in NLOS environments. WiMAX-OFDM based PHY level is especially suitable

for operating in a multipath environment. Modern multiple-antenna systems can be

designed to take advantage of multipath, rather than disregarding it as in the case of

traditional single antenna systems. When WiMAX is used in conjunction with multiple-

antenna systems, it is promising to accomplish an increased output and better error rate

performance, by imbibing the positives aspects of multipath effect. Multicarrier systems

78 

 

such as WiMAX with OFDM offer good functionality under heavy interference. On

account of its wide operating bandwidth, WiMAX faces strong frequency selective

fading. To contain the fading phenomena, the countermeasures need accurate and real-

time knowledge of the transfer function of the radio channel. This so called CSI (Channel

State Information) is a crucial factor concerning the true functionality of WiMAX. The

fact that the system needs accurate information of the channel state makes it vulnerable to

systems, that are capable of preventing a WiMAX device from getting this information.

3.2.5. OFDM and STBC Transceiver: System Architecture

OFDM’s splitting of the bandwidth into sub channels having frequencies lower

than the radio channel’s coherent bandwidth is regarded as a flat fading channel. The

data to be transmitted is spread onto the sub channels carriers empowering it to transmit

high data rates using long symbol times. Thus transmitting 1 Mbit/s on a 200 data

subcarrier would translate to a per subcarrier data rate of only 5kbit/s. During

transmission, a high–data- rate sequence of symbols is segregated into multiple parallel

low-data rate sequences which are utilized to modulate an orthogonal tone or a subcarrier.

The transmitted baseband signal, which is an ensemble of the signals in all the

subcarriers, can be represented as

3.8

where s[i] is the symbol carried on the ith subcarrier; Bc is the frequency separation

between two adjacent subcarriers, also referred to as the subcarrier bandwidth; ∆f is the

∑−

=

+∆− ≤≤=1

0

)(2 0,exp][)(L

i

tBcfj Ttistx π

79 

 

frequency of the first subcarrier; and T is the total useful symbol duration (without the

cyclic prefix). At the receiver, the symbol sent on a specific subcarrier is retrieved by

integrating the received signal with a complex conjugate of the tone signal over the entire

symbol duration T. The orthogonality between the subcarriers is preserved at the receiver,

when there is a perfect synchronization of time and frequency between the transmitter

and the receiver. When this synchronization is imperfect or distorted, the orthogonality

between the subcarriers is lost resulting in ICI. This timing mismatch occurs when there

is a misalignment of the clocks at both the transmitter & the receiver and also when there

is delay in the propagation of the channel.

The concept of independently modulating multiple orthogonal frequency, tones

with narrowband symbol streams that are equivalent to first constructing the entire

OFDM signal in the frequency domain and then using an inverse FFT to convert the

signal into the time domain. The FFT method is easier to implement, as it does not

require multiple oscillators to transmit and receive the OFDM signal. In the frequency

domain, each OFDM symbol is created by mapping the sequence of symbols on to the

subcarriers. To generate OFDM successfully, the relationship between all the carriers

must be carefully controlled to maintain the orthogonality of the carriers.. For this reason,

OFDM is generated by first choosing the spectrum required, input data, and modulation

scheme used. Each carrier thus produced is assigned data for transmission. To ensure the

carrier signals produced are orthogonal IFFT is used to perform the transformations

efficiently. The FFT transforms a cyclic time domain signal into its equivalent frequency

spectrum. This is done by finding the equivalent waveform, generated by a sum of

orthogonal sinusoidal components. The frequency spectrum of the time domain signal is

80 

 

represented by the amplitude and phase of the sinusoidal components. The IFFT performs

the reverse process, transforming a spectrum i.e. the amplitude and phase of each

component into a time domain signal and also converts a number of complex data points

of length which is a power of 2, into the time domain signal of the same number of

points. Each data point in frequency spectrum used for an FFT or IFFT is referred to as a

Bin and the parallel outputs are then simultaneously transmitted through different

antennae. However, only one antenna is used at the receiver side.This process at any

given instant will pick up a least faded signalthat occurs due to an independent fading of

the transmitted signal, resulting in an improved SNR.

Alamouti code can be generalized to an arbitrary number of antennae. If two

antennae are deployed at the transmitter side, then for a given symbol period, two

symbols can be simultaneously transmitted from them. Let the signal transmitted from

antenna one (A0) be denoted as S0 and that transmitted from antenna two (A1) be

denoted as S1. During the next symbol period let S1* be the signal transmitted from

antenna A0 and S0* be the signal transmitted from antenna A1, where * is the complex

conjugate operation. The two time slots are denoted as ‘t’and‘t + T’ .

For systems integrated with diversity techniques and for reception of the signal, it

is imperative that proper detectors and decoders be used at the receiving end. The

receiver block shown in Fig.3.5 consists of a demodulator, deinterleaver and a channel

decoder.

81 

 

Fig.3.5. Receiver model of the WiMAX system

On reception of the signals, they are combined using ‘Maximum likelihood

decision rule’ in order to obtain the estimated signal. Before de-interleaving, the signal is

first demodulated and then imparted to the channel decoder.

Fig.3.6. OFDM receiver

Onus of correcting a byte rests with the R-S decoder. The OFDM receiver

segment in the receiver block is shown in Fig.3.6 and its working principle is explained

in the previous section. It simply replaces the incorrect byte with the correct one

irrespective of the number of corrupted bits responsible for the error, giving an R-S code

a tremendous burst-noise advantage over binary codes. The original message is retrieved

once the channel decoding is completed.

Space Time Decoder

OFDM Receiver

De-Interleaver

Channel decoder

Received Signal from Combiner O/P

Baseband OFDM Signal

Serial Data Out

Parallel to

Serial

Serial to

Parallel

 

Guard Interval Removal

FFT

 

Modulation (QPSK, QAM, etc.) 

82 

 

3.2.6. Results of WiMAX Diversity Analysis

BER is used to determine the performance levels/measure of the receiver models.

Fig.3.7 shows the BER comparison graph of SISO, MISO and MIMO schemes. The BER

comparison graphs illustrate the performance of the three diversity techniques contrary to

multipath fading.

It has been found that performance is heightened with the use of multiple

antennae at both the transmitter and receiver side rather than with a single antenna. By

using a single antenna scheme, the cumulative density of SNR declines due to fading

effect. For an Eb/No ratio of 35 dB, while the error rate for SISO is around 10-2, it is

about 10-6 for MIMO scheme with CTD, implying the possibility of improvement in SNR

by 20dB.

Fig.3.7. Comparison of diversity techniques

Eb/No (dB) 

83 

 

Considering the configuration in which, diversity technique is deployed only at

the transmitter side and single antenna is used at the receiver side, it has been found that

there is no appreciable improvement in SNR when compared to the MIMO scheme which

employed multiple antennae at the receiver side.

It is observed from figure that for a maximum value of 35dB for Eb/No, SISO

provides BER in the range of 10-2 to 10-3 and MIMO offers BER of 10-7, inferring the

possibility of 15dB improvement in Eb/No.

Fig.3.8. Comparison of modulation schemes

In Fig.3.10, for the max Eb/No at 25 dB, BER for BPSK is around 10-1, QAM

around 10-2 and QPSK stays around 10-6. Thus, QPSK with its lowest BER is logically

the ideal scheme for WiMAX systems. In case the of urban environments, when most of

the receiver antennae receive weak faded signals due to independent fading and by

increasing the number of antennae at transmitter and receiver side, there is still a

Eb/No (dB) 

84 

 

probability of at least one antenna receiving the least faded signal which dramatically

reduces the bit error rate, when compared to MISO and SISO schemes.

3.3. GRC and USRP Implementation of OFDM system

It is clearly understood from the earlier study that the OFDM is a good contestant

in broadband wireless communication systems to trim down multipath fading through

frequency scattering. To reveal the real time wireless channel characteristics, OFDM

transceiver is simulated in GNU Radio Companion(GRC) as shown in Fig.3.9 and a

sample image is transmitted and received.

Fig.3.9. GRC implementation of OFDM Transceiver

85 

 

Fig.3.10. FFT plot of OFDM Transceiver

The FFT plot of OFDM transceiver simulated in GRC is shown in Fig.3.10. The

transmitted image in Fig.3.11 and received image in Fig.3.12 are compared to understand

the effect of poor channel quality. The degree of diversity can be increased by CTD to

perk up the quality of signal reception.

86 

 

Fig.3.11. Transmitted Image

Fig.3.12. Received Image

87 

 

In another attempt, OFDM transmitter and receiver is implemented in

USRP(Universal Software Radio Peripheral) with 890.12 MHz frequency band and a

text file is transmitted and received between nodes. USRP implementation of OFDM

transmitter and receiver with appropriate source and sink units are shown in Fig.3.13 and

Fig.3.14 respectively.

Fig.3.13. USRP implementation of OFDM Transmitter

Fig.3.14 USRP implementation of OFDM Receiver

88 

 

Multiple GSM(Global System for Mobile Communication) mobile phones were

in use while the transmission is initiated. This is to bring about the maximum interference

in the channel. The transmission is carried out with OFDM segment without channel

coding and is observed from the Fig.3.15 and Fig.3.16 that received file has got corrupted

in noisy channel. Since USRP provides real time implementation proposed CTD schemes

may also be tested.

Fig.3.15. Transmitted text file

Fig.3.16. Received text file

89 

 

3.4. Conclusion

It can be concluded stating that multipath fading has been substantially reduced in

wireless network, by using combinational diversity techniques. The BER comparison

graphs demonstrate that MIMO diversity technique offers least BER and exhibits

optimum performance for multipath scenario, there by facilitating Wi-Fi and WiMAX

technologies to best prevails over other broadband wireless technologies. The ultimate

goal of any wireless communication system is to provide services on par with wired

communication systems. Thus it is wise to integrate wireless network with MIMO

combinational diversity scheme to transform it into an efficient communication system.

From the analysis it is evident that it is possible to surmount the multipath errors

with deployment of diversity schemes in CDMA, Wi-Fi and WiMAX systems.

Furthermore, to overcome the bandwidth limitations of wireless communications,

schemes have to be introduced in the radio node which have cognition in spectrum. The

next chapter deals with diversity analysis in cognitive radio systems.