ABSTRACT

Wireless communication is one of the most vibrant areas in the communication field today. The research thrust in the past decade has led to a much richer set of perspectives and tools on how to communicate over wireless channel. The transmission medium which is used normally cannot accept data to transmit in their natural form. Line codes convert digital data to digital signals, which is suitable for the transmission medium used. The performance of Return-to-zero (RZ), Non-return-to zero (NRZ) and Pseudoternary line codes for a particular wireless communication network (IEEE 802.11b) is analyzed and the results are presented in this paper. The IEEE 802.11b network is implemented with 11 bit barker spreading sequence and BPSK modulation. Throughput and Bit Error Rate (BER) performance of the different line coding techniques of wireless communication network have been compared using additive white Gaussian noise (AWGN) channel at various Eb/No. It is found that Pseudoternary line code is well suited for long distance communication since throughput and BER are better for this line code compared to RZ and NRZ coding sequences. Figure 1 shows the better BER performance of Pseudoternary code compared to other line coding techniques.

In these, binary information is transmitted through three-level rather than binary pulse codes for controlling the power distribution in the frequency spectrum, improving clock recovery, allowing error detection or for just increasing the binary data rate.

1. INTRODUCTION

1.0 IntroductionThe purpose of coding data is to efficiently transport it through a particular medium. Themedium may be circuit board traces, ribbon cable, twisted pair copper, copper coax, fiber opticor air. Each type of medium suffers from a group of impairments. These impairments mayinclude signal reflection, attenuation distortion, harmonic distortion, phase distortion,intermodulation distortion, dropout, echo, crosstalk, delay distortion manifesting itself as intersymbol interference (ISI), impulse noise, Gaussian noise and frequency shift, All theseimpairments in the medium affect the ability to transport data. In some cases, these factors cancause an excessive number of bit errors. For short transmission line lengths and low signalingrates, the simple linear lines codes may be employed. These codes may be unipolar or bipolarand may or may not have clocking information contained within the code. When the channel isbandwidth limited, more efficient codes are available. Such codes may utilize multi-level symbolsand alter the message data to allow the receiver to synchronize to it. The most sophisticatedcodes use block coding or convolutional coding to improve the performance of transmission.For a bandwidth limited channel, the maximum upper limit for reliable information transfer isgiven by the Hartley-Shannon Law. This law equates the channel bandwidth and the signal tonoise ratio to the maximum channel capacity and indicates the maximum number of symbols thatcan be transferred per second. This equation is given below:C = B * log2(1 + SNR) symbols/secondwhere: C is the channel capacityB is the channel bandwidthSNR is the signal to noise ratioThis equation implies that we can trade channel bandwidth for signal to noise ratio.

When the data is coded, the system can tolerate a lower signal to noise ratio for the same bitrate. This difference is called coding gain and is expressed in dB.

802.11 and 802.11x refers to a family of specifications developed by the IEEE for wireless LAN (WLAN) technology. The original IEEE 802.11 WLAN standard supports 1 Mbps and 2 Mbps data rates. IEEE 802.11b WLAN standard supports higher data rates of 5.5 Mbps and 11 Mbps while retaining compatibility with the original IEEE 802.11 standard. It supports three modulation schemes: DBPSK for 1 Mbps, DQPSK for 2 Mbps, and Complementary Code Keying (CCK) for both 5.5 Mbps and 11 Mbps.

2. PHYSICAL LAYER

The Physical layer is the interface between the MAC and wireless media, which transmits and receives data frames over a shared wireless media.Functions of Physical layer:

1. provides a frame exchange between the MAC and PHY under the control of the physical layer convergence procedure (PLCP) sublayer.

2. PHY uses signal carrier and spread spectrum modulation to transmit data frames over the media under the control of the physical medium dependent (PMD) sublayer.

3. PHY layer provides a carrier sense indication back to the MAC to verify activity on the media

The three physical layers originally defined in 802.11 included two spread-spectrum radio techniques (Frequency Hopping Spread Spectrum – FHSS and Direct Sequence Spread Spectrum – DSSS) and a diffuse infrared specification. The actual technique of spread spectrum transmission was developed by the military in an attempt to reduce jamming and eavesdropping.

2.1 Direct Sequence Spread Spectrum (DSSS)DSSS is a transmission technology used in WLAN transmissions where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. The chipping code is a redundant bit pattern for each bit that is transmitted, which increases the signal's resistance to interference. If one or more bits in the pattern are damaged during transmission, the original data can be recovered due to the redundancy of the transmission.

2.2 Barker Spreading

Barker coding is a modulation technique, that was used in the first specification of IEEE 802.11 and it provides 1 Mbps (2 Mbps) data rates while using BPSK (QPSK). An 11-bit Barker word is used as the spreading sequence and every station in an IEEE 802.11 network uses the same 11-bit sequence (+1,-1,+1,+1,-1,+1,+1,+1,-1,-1,-1). The binary adder effectively multiplies the length of the binary stream by the length of the sequence. This increases the signaling rate and makes the signal span a greater amount of frequency bandwidth. Fig. 1 shows the DSSS example with Barker spreading. 5.5Mbps and 11Mbps operation of 802.11b doesn't use the Barker sequence. Instead, 802.11b uses complementary code keying (CCK) to provide the spreading sequences at these higher data rates.

The ratio between the data and width of spreading code is called processing gain. The processing

Original signal

Spreading Code(Barker)

Spread Signal

1 0

gain for IEEE 802.11b when using Barker coding is 10.4dB . This is given in equation 1.

G=10 log ¿) = 10log(11) = 10.4dB (1)

3. LINE CODING

Line Coding is the process of converting digital data to digital signals. Line coding converts a sequence of bits to a digital signal. At the sender, digital data are encoded into a digital signal; at the receiver, the digital data are recreated by decoding the digital signal. Figure 2 shows the line coding that converts digital data to digital signal.

Line coding techniques can be broadly divided into three categories: Unipolar, Polar and Bipolar.Unipolar: In unipolar encoding technique, only two voltage levels are used. It uses only one polarity of voltage level as shown in Fig. 2.4.5. In this encoding approach, the bit rate same as data rate. Unfortunately, DC component present in the encoded signal and there is loss of synchronization for long sequences of 0’s and 1’s. It is simple but obsolete.

Fig.1. DSSS Example

Fig.2 Block diagram of Line coding

Figure 2.4.5 Unipolar encoding with two voltage levels

Polar: Polar encoding technique uses two voltage levels – one positive and the other one negative. Four different encoding schemes shown in Fig. 2.4.6 under this category discussed below:

Figure 2.4.6 Encoding Schemes under polar category

Non Return to zero (NRZ): The most common and easiest way to transmit digital signals is to use two different voltage levels for the two binary digits. Usually a negative voltage is used to represent one binary value and a positive voltage to represent the other. The data is encoded as the presence or absence of a signal transition at the beginning of the bit time. As shown in the figure below, in NRZ encoding, the signal level remains same throughout the bit-period. There are two encoding schemes in NRZ: NRZ-L and NRZ-I, as shown in Fig. 2.4.7.

The advantages of NRZ coding are:

Detecting a transition in presence of noise is more reliable than to compare a value to a threshold.

NRZ codes are easy to engineer and it makes efficient use of bandwidth.

The spectrum of the NRZ-L and NRZ-I signals are shown in Fig. 2.4.8. It may be noted that most of the energy is concentrated between 0 and half the bit rate. The main limitations are the presence of a dc component and the lack of synchronization capability. When there is long sequence of 0’s or 1’s, the receiving side will fail to regenerate the clock and synchronization between the transmitter and receiver clocks will fail. Return to Zero RZ: To ensure synchronization, there must be a signal transition in each bit as

shown in Fig. 2.4.9. Key characteristics of the RZ coding are:

Three levels Bit rate is double than that of data rate No dc component Good synchronization Main limitation is the increase in bandwidth

Figure 2.4.9 RZ encoding techniqueBiphase: To overcome the limitations of NRZ encoding, biphase encoding techniques can be adopted. Manchester and differential Manchester Coding are the two common Biphase techniques in use, as shown in Fig. 2.4.10. In Manchester coding the mid-bit transition serves as a clocking mechanism and also as data. In the standard Manchester coding there is a transition at the middle of each bit period. A binary 1 corresponds to a low-to-high transition and a binary 0 to a high-to-low transition in the middle. In Differential Manchester, inversion in the middle of each bit is used for synchronization. The encoding of a 0 is represented by the presence of a transition both at the beginning and at the middle and 1 is represented by a transition only in the middle of the bit period. Key characteristics are:

Two levels No DC component Good synchronization

• Higher bandwidth due to doubling of bit rate with respect to data rate The bandwidth required for biphase techniques are greater than that of NRZ techniques, but due to the predictable transition during each bit time, the receiver can synchronize properly on that transition. Biphase encoded signals have no DC components as shown in Fig. 2.4.11. A Manchester

code is now very popular and has been specified for the IEEE 802.3 standard for base band coaxial cables and twisted pair CSMA/CD bus LANs.

Figure 2.4.10 Manchester encoding schemes

4. BASIC DESCRIPTION OF THE SIMULATION

In this paper, IEEE 802.11b Physical Layer is simulated in Matlab for RZ, NRZ and Manchester line codes. The simulation is made for lower data rate (1 Mbps) using DBPSK modulation and the signal is spread with 11 chips long Barker Code.

This section shows the results and analysis for thedifferent transmission techniques of WLANsimulated under single path as well as multipathpropagating condition. The noise is assumed to beAdditive White Gaussian Noise (AWGN) [4]. The

effect of interference from another co-channelWLAN is also simulated. BER versus Signal toNoise Ratio (SNR) curves are used as the

performance metric.

0 2 4 6 8 10 12 14 16 18 200.033

0.0331

0.0332

0.0333

0.0334

0.0335

0.0336

0.0337

0.0338

0.0339

Data rate (Packets/Sec)

BE

R

Data rate versus BER for 802.11b

-6 -4 -2 0 2 4 6 8 10 122000

2500

3000

3500

4000

4500

Eb/no

thro

ughp

ut(K

ilo b

ytes

)

Eb/no versus Throughput for RZ, NRZ and Manchester line codes

manchester

nrzrz

0 50 100 150 200 250 300 350 400 450 5000

0.5

1

1.5

2

2.5x 10

6

Number of Packets

thro

ughp

ut(K

ilo b

ytes

)

No.of Packets versus Throughput for RZ, NRZ and Manchester codes

rz

nrzmanchester

-6 -4 -2 0 2 4 6 8 10 1210

-4

10-3

10-2

10-1

100

Eb/No

BER

EB/No versus BER for RZ, NRZ and Manchester codes for 100 Packets

Manchester

NRZRZ

References

Performance Evaluation of SCM-WDM System Using Different Linecoding, Md. Shamim Reza, Md. Maruf Hossain, Adnan Ahmed Chowdhury, S. M. Shamim Reza and Md. Moshiur Rahman ,JOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 1, APRIL 2010

Properties and performance of the IEEE 802.11b complementary-code-key signal sets Pursley, M.  Royster Iv, T. Clemson Univ., Clemson, SC  IEEE Transactions on Communications,February 2009 Volume: 57 Issue: 2

IEEE Std 802.11b-1999. [Online]. Available: http://standards.ieee.org/getieee802/