BER PERFORMANCE SIMULATION IN LOS ENVIRONMENT

FOR FIXED BROADBAND WIRELESS ACCESS SYSTEM

Tang Min Keen , Tharek Abdul Rahman

Wireless Communication Centre, Faculty of Electrical Engineering,

Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

Keywords: BER performance, Channel modelling, Fixed Broadband Wireless Access

Abstract: This paper presents a straightforward bit error rate (BER) performance simulation methodology that can be

readily used for FBWA system with environment effects being taken into consideration. This work begins

with physical layer modelling of a current market available fixed broadband wireless access (FBWA)

system.Then, with the eight modelled line of sight (LOS) channels obtained from prediction and

measurement, BER performance of the system in the related environment is simulated. The FBWA system

is a high performance and high-speed wireless Ethernet bridge terminal, which operates in the Unlicensed

National Information Infrastructure (UNII) band of 5.8 GHz with orthogonal frequency division

multiplexing (OFDM) wireless transmission. Tests and verifications have been carried out in the simulation

tools in order to ensure the modelled system is conforming the standard and specifications of the actual

system. With the physical layer system template and the channel models that represent the real environment,

the BER computations are obtained

1 INTRODUCTION

Wireless local area networks (WLAN) supporting

broadband multimedia communication are being

developed and standardized around the world. IEEE

802.11a is one of the standards that provides an

internationally accepted standard defining

independent PHY and MAC layers at UNII

frequency band of 5 GHz. As mandated by Federal

Communications Commission (FCC), the IEEE

802.11a is applying to lower band (5.15-5.25 GHz)

for indoor applications, middle band (5.25-5.35GHz)

for indoor or outdoor applications and upper band

(5.725-5.825 GHz) for outdoor applications. Here,

the modelled FBWA system operates in the upper

band that is designed for outdoor point to point

application. It meets the standard requirements for

IEEE 802.11a.

The terminology, quality targets and

methodologies to be used in the planning of fixed

wireless systems are defined in F-series of the radio

communication sector of the International

Telecommunication Union (ITU-R)

recommendations. BER is one of the quality

parameters, which is used to define the performance

and range of radio systems. (Clark, 2000) Hence, to

know the performance range of the system used, the

system is modeled and simulated using a powerful

simulation software tool.

This paper is organized as follows: Firstly, a brief

description and results of transmitter tests that

include spectrum mask, error vector magnitude and

relative constellation error test are presented. This is

followed by receiver sensitivity level, adjacent and

alternate channel rejection test to confirm the

modelled receiver with related standard and

specifications. Then, a short summary about the

eight LOS channel models is explained in Section

IV. Finally, the results of physical layer software

simulation are given in the form of bit error rate

(BER) versus energy per bit to noise ratio (E

)

and discussed before this paper is concluded.

2 TRANSMITTER TEST

According to FCC regulations as stated in (IEEE

802.11a, 1999) section 17.3.9.1, the maximum

allowable output power is 40 mW (50 mW/MHz)

with up to 6dBi antenna gain. Though the maximum

power level is depending on the standard, the

166

Min Keen T. and Abdul Rahman T. (2004).

BER PERFORMANCE SIMULATION IN LOS ENVIRONMENT FOR FIXED BROADBAND WIRELESS ACCESS SYSTEM.

In Proceedings of the First International Conference on E-Business and Telecommunication Networks, pages 166-171

DOI: 10.5220/0001392701660171

 SciTePress

software of the system will determine the actual

power. The system has different actual transmit

power permitted for each channel according to the

modulation scheme. Yet, the defined value is lower

than the maximum allowable values by the relevant

standard. To achieve higher radiated power that is

allowed by the standard, a high gain directional

antenna will be used. In the software tool, the

maximum transmit power for the system is set.

In section 17.3.9.2, the transmit spectrum shall

have a 0 dBr (dB relative to the maximum spectral

density of the signal) bandwidth not exceeding 18

MHz, -20 dBr at 11 MHz frequency offset, -28 dBr

at 20 MHz frequency offset and –40dBr at 30 MHz

frequency offset and above. Here, the nine channels

that available in this system are measured. It is

found that all the transmit signal falls within the

allowable spectral mask shown in the Figure 1.

Figure 1: Transmit RF Spectrum

The error vector between the vector representing

the transmitted signal and the vector representing the

error-free modulated signal defines modulation

accuracy. The magnitude of the error vector is

called EVM. The purpose of this test is to verify that

the root mean square (rms) EVM measured on the

specific part of the burst meets the conformance

requirement. EVM and relative constellation error

measurements here are based on (IEEE 802.11a,

1999) section 17.3.9.6 and section 17.3.9.7. The test

for every sub carrier is performed over 20 frames

and the rms average is taken. Simulation results

show that all the EVM, averaged over sub-carriers,

OFDM frames and packets are less than 0.003%,

and the constellation are approximately –88 dB

which is much smaller than the specification

requirements.

3 RECEIVER TEST

The receiver performance requirements (IEEE

802.11a, 1999) are listed in Table 1. Firstly, the

packet error rate (PER) for rate-dependant input

levels in the table are tested less than 10% at a

physical sublayer service data units (PSDU) length

of 1000 bytes The minimum input levels are

measured at antenna connector with noise factor of

10 dB and 5 dB implementation margins. The

simulation result is displayed in Figure 2. It shows

that at PER

−

(10%), the received signal levels is

lower than the minimum sensitivity allowed.

Secondly, the adjacent channel rejection is tested

by setting the desired signal’s strength 3dB above

the rate-dependent sensitivity specified in Table 1

and raising the power of the interfering signal until

10% PER is caused for PSDU length of 1000 bytes.

The power difference between the interfering and

the desired channel is the corresponding adjacent

channel rejection. The interfering signal in the

adjacent channel is also a conformant OFDM signal,

unsynchronized with the signal in the channel under

test.

Thirdly, the same setting applies for the non-

adjacent channel rejection. The non-adjacent

channel rejection is also called alternate channel

rejection where the interfering signal is 40 MHz

from the channel under test. Figure 3 and 4 show the

power versus spectrum for data rate at 54 Mbps with

interference signal 10dB higher than the actual

signal for adjacent rejection channel and interference

signal 15dB higher than the actual signal for

alternate rejection channel rejection. The simulation

results for adjacent and alternate rejection tests are

shown in Figure 5 and 6. At PER equals to

−

(10%), all the adjacent and alternate channel

rejection for the eight data rate are higher than the

standard values and so the system fulfils the

requirements.

Table 1: Receiver performance requirements

Data

Rate

(Mbits/s)

Minimum

sensitivity

(dBm)

Adjacent

channel

rejection

(dB)

Alternate

channel

rejection

(dB)

6 -82 16 32

9 -81 15 31

12 -79 13 29

18 -77 11 27

24 -74 8 24

36 -70 4 20

48 -66 0 16

54 -65 -1 15

BER PERFORMANCE SIMULATION IN LOS ENVIRONMENT FOR FIXED BROADBAND WIRELESS ACCESS

SYSTEM

167

5.76 5.77 5.78 5.79 5.80 5.81 5.82 5.835.75 5.84

-120

-110

-100

-90

-80

-130

-70

Frequency (GHz)

ower

(dB

)

Power (dBm)

Frequency (GHz)

5.74 5.76 5.78 5.80 5.82 5.84 5.86 5.885.72 5.90

-120

-100

-80

-60

-140

-40

Frequency (GHz)

Power (dBm)

Frequency (GHz)

Figure 2: Receiver Sensitivity

Figure 3: Adjacent Channel Rejection

Figure 4: Alternate Channel Rejection

Figure 5: Adjacent Channel Rejection Test

Figure 6: Alternate Channel Rejection Test

4 CHANNEL MODELS

The channel models are obtained from a 3D Vertical

Plane Launch (VPL) ray tracing technique (Liang

and Bertoni. 1998) that incorporates site specific

environmental data in a newly constructed hostel in

Universiti Teknologi Malaysia (UTM) for 5.8 GHz

carrier frequency. Transmit site is at Wireless

Communication Centre (WCC). The receiver sites

consist of 8 LOS locations in a two u-shaped

hostel’s buildings, which are located around 50

meters lower than WCC. The terrain between WCC

and the hostel is a small oil palm plantation. Hence,

the site overlooked a terrain of light rolling hills with

moderate tree densities. From the highest floor of

WCC, we can see these buildings and the oil palm

plantation. The distances for these links are ranged

from 416 to 564 meter. Figure 7 and 8 show the

photo of WCC and the photo of the hostels that is

captured from WCC.

With building, terrain, and antenna databases, and

also transmitter and receiver locations databases, the

8 channel models are obtained from the propagation

prediction. However, this model excluded the

vegetation effects that appeared in the fresnel zone

clearance in the real site environment. Modification

is needed on predicted channel models to take

account of the obstruction loss. Path loss field

measurement has been conducted using the FBWA

system. To assure that propagation channel is

stationary in time, the measured data is averaged

over 30 instantaneously sampled values in 15

minutes. The deviation within free space loss and

measurement loss is used to consider the obstruction

loss of vegetation in fresnel zone. After adding

relatively the computed obstruction loss into each

component of the ray of the links, the complete sets

of output magnitudes are ready for BER

performance. The parameters of the eight channels

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are listed in the Table 2. A more detail explanation

of channel modeling can be found in (Tang and

Tharek, 2004)

Table 2: Channel Models

Figure 7: WCC

Figure 8: Hostel

5 SIMULATION RESULTS

The tested BFWA system is simulated under the 8

channel models. The block diagram for the BER

performance simulation is displayed in Figure 9.

Firstly, we have FBWA system signal source which

generates radio frequency (RF) OFDM signal, by

random data generation, scrambling, convolutional

coding, interleaving, mapping, inverse Fast Fourier

Transform, multiplexing, window function addition,

and idle insertion based on the IEEE 802.11a

Standard. The signal is then transmitted by an

antenna with location coordinates, height and gain,

going through the channel model with addition of

the noise according to the

/ N

. A receiver antenna

at a location with certain gain then captures the

signal. The FBWA receiver that owns full frequency

synchronization and reverse operations of FBWA

signal source receives the RF signal. The PSDU

from the receiver and the PSDU from the signal

source is synchronized by delaying the PSDU from

the signal source. Both the PSDU are compared to

obtain the BER performance. As the FBWA system

can only support until 36 Mbps, the simulations are

carried out at that data rate over Channel L1-L8.

Figure 9: Block Diagram for Simulation

To have an idea on how the BER performance is

effected by various parameters seperately, literature

study has been carried out. Numerical results from

(Yee and Linnartz, 1994) and (Witrisal et al., 1998)

revealed that the Rician K-factor has a significant

effect on the BER. The BER performance is getting

better as the value of K factor is higher at

/ N

higher than 5 dB.

On the other hand, results from (Doufexi et al.,

2002) indicated that system performance improves

as the RMS delay spread increases, until the excess

delay significatly exceeds the guard interval length.

This characteristic is due to OFDM exploits the

Channel

Num.

Distance

(meter)

Tap

Num.

Delay

(ns)

Average

Relative

Power

(dB)

RMS

Delay

Spread

(nsec)

Rician K

Factor

(dB)

L1 416.15 1 0 0 89.30 2.54

2 401.43 -12.59

L2 473.31 1 0 0 75.22 6.35

2 696.47 -19.19

3 26.27 -20.28

L3 478.06 1 0 0

L4 480.08 1 0 0

L5 491.00 1 0 0 8.93 6.11

2 29.03 -14.63

3 62.50 -18.27

L6 518.55 1 0 0 3.54 2.90

2 31.93 -19

L7 547.39 1 0 0 4.24 2.90

2 37.70 -18.86

L8 564.11 1 0 0 12.60 2.84

2 91.40 -17.04

BER PERFORMANCE SIMULATION IN LOS ENVIRONMENT FOR FIXED BROADBAND WIRELESS ACCESS

SYSTEM

169

increased frequency diversity that results from high

rms delay spread.

Besides, to improve the performance of the

system under certain channels, we may sacrifice the

speed of the data transmission. The performance of

the system increases as we decrease the data rate.

From Figure 10, we may observe that the BER

performance for the modeled system undergoing

channel L1 improving as data rate is lower.

The simulated BER performance results for

different channels are illustrated in Figure 11. We

found that this system performs better under

Channel L1 and L3, where the BER are near

−

when the

/ N

is in 12 and 14 dB range, comparing

with BER of other channels with

/ N

bigger than

16 dB. From our observation, L2 with longer

distance and bigger Rician K factor but a smaller

rms delay spread have worse performance than L1.

Here, the rms delay spread has much effect on the

performance. For channel L3 and L4 with single ray,

the longer distance link in L4 has worse

performance. The effect of rms delay spread can also

be seen from channel L2 and L5 with Rician K

factor 6 dB and channel L6 and L7 with Rician K

factor 2.9 dB. The bigger rms delay spread gives a

better performance. The longest distance link in L8

give worst performance although the rms delay

spread is bigger than channel L6 and L7.

The BER performances of the system over the

channels are found unpredictable without a

simulation and measurement. The performance

under these channels varies widely although all of

the channels are under LOS conditions. The

difference of the performances are not only due to

delay spread and Rician K-factor of each channel,

but also the distance, transmit power, and received

signal level of the links.

6 CONCLUSIONS

A high availability of a radio system is not only

depending on the design of the equipment, but also

the good location of radio antenna sites and a good

path planning. This paper highlights a good path

planning for a FBWA system. We model 8 LOS

channels with a physical propagation model and

enhance them with field measurement at the related

site. Then, the physical layer of FBWA system is

modeled and tested to conform its specifications and

standards. This is followed by the simulation on

BER performance of the system over the modeled

channels using a software simulator tool. BER

performance results have been presented and good

performance links are identified.

REFERENCES

Clark M. P., 2000, Wireless Access Network, John Wiley

& Sons. West Sussex P019 1UD, UK.

IEEE 802.11a, 1999, Part 11: Wireless LAN Medium

Access Control (MAC) and Physical Layer (PHY)

specifications: High-speed Physical Layer in the 5 GHz

Band.

Liang G. and Bertoni H.L. 1998. A new approach to 3-D

ray tracing for propagation prediction in cities. IEEE

Trans. Ant. Propagation, vol. 46, no. 6, pp.853-863.

Tang M.K. and Tharek A.R. 2004. Channel Modeling at

5.8 GHz for Fixed Wireless Access. In WOCN’04,

First IFIP International Conference on Wireless and

Optical Communications Network.

Tang M.K and Tharek A.R. 2004. Propagation Prediction

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Access. ICT’04,11th International Conference on

Telecommunication.

Yee. N and Linnartz J.P. 1994. Controlled Equalization of

Multi-Carrier CDMA in an Indoor Rician Fading

Channel. In IEEE 44

Vehicular Technology

Conference, vol. 3, pp.1665-1669.

Witrisal K., Yong H. K. and Prasad R. 1998. Frequency-

Domain Simulation and Analysis of the Frequency

Selective Radio Channel for the Performance Analysis

of OFDM. In Proc. 3, OFDM Fachgesprach,

Braumschweig, Germany.

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and IEEE 802.11a Wirelss LAN Standards.IEEE

Communications Magazine, May 2002, pp. 172-180.

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Figure 11: BER Performance For Different Channels

Eb/No

Figure 10: BER Performance For Channel L1

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