ZigBee Physical IEEE

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This report work started with extensive literature study in several area of ZigBee, particular in the physical aspect. This knowledge was then implemented in simulating the physical aspect of the Zigbee working in the range of 2.4 GHz. Various parameter of the physical layer are alter to demonstrate the its effect by using MATLAB(syntax code in M-file and Simulink). Results are analyzed and compared to demonstrate how the actual standard and specification are derived.

2.0 Literature Review

Zigbee is known as IEEE 802.15.4 Low-Rate Wireless Personal Area Network (WPAN) standard is part of the IEEE 802 family of digital communication standards. It is designed for low-cost, low-power applications that require relatively low data throughput which is down to an average of less than 1 bps. It is also differentiated from IEEE 802.15.1TM (Bluetooth TM) in several respects; it does not support isochronous voice, as Bluetooth does. Figure 2.0(a) shows the comparison with standard and technologies.

2.1 Applications

ZigBee technology is well suited to a wide range of energy management and efficiency, building automation, industrial, medical, home automation applications. Essentially, applications that require interoperability and/or the RF performance characteristics of the IEEE 802.15.4 standard would benefit from a ZigBee solution. Examples include:

  • Demand Response
  • Advanced Metering Infrastructure
  • Automatic Meter Reading
  • Lighting controls
  • HVAC control
  • Heating control
  • Environmental controls
  • Wireless smoke and CO detectors
  • Home security
  • Blind, drapery and shade controls
  • Medical sensing and monitoring
  • Universal Remote Control to a Set-Top Box which includes Home Control
  • Industrial and building automation

2.2 Advantages

The most appealing advantages Zigbee has is low power consumption due low duty cycle of end device that only turns on when required, uses only small capacity of battery but long operating time. It is also relatively low cost compare to other wireless network. It offered low message throughput with the size of code ranges from 32kB to 70kB and is only approximately 10% of code size used in Bluetooth technology. Other advantages are listed below:

-Large network order.

-Few QoS guarantee

-Selectable levels of security based on the Advanced Encryption Standard with

128-bit keys (AES-128) s.

2.3 Physical layer

IEEE 802.15.4/ZigBee has two available physical layers. In the 2.4-GHz band, it supports a data rate of 250 kbps; there are 16 available channels, centred at 2405 + 5k MHz, where 0 ≤ k ≤ 15. The other physical layer is a regional one, covering the 868.0 to 868.6 MHz band available in Europe and the 902 to 928 MHz band available in much of the Americas. There is a single channel in the 868 MHz band, centred at 868.3 MHz, with a BPSK data rate of 20 kbps; the standard supports a BPSK data rate of 40 kbps in the 902 to 928 MHz band, with channels centred at 906 + 2k MHz, where 0 ≤ k ≤ 9.[1]

2.4 Network topologies

To meet its wide range of potential applications, IEEE 802.15.4/ZigBee supports star, mesh, and tree networks, the latter two being multihop networks.

Multihop Network:

A multi-hop communications allows data from one device to be relayed to another device via intermediary devices. By relying on these intermediary devices, the range of a given network can be significantly increased while at the same time limiting the power consumption of each device. [2]

Star Network:

Star network topologies are commonly referred to as point-to-point and point-to-multipoint networks. This topology can be supported by either ZigBee or 802.15.4. In this network, all devices are limited to “single-hop” communications. Typical applications for star networks include garage door openers and remote controls. [2]

2.5 DSSS (Direct Sequence Spread Spectrum)

IEEE 802.15.4 uses direct sequence spread spectrum (DSSS) for robust data communications. A DSSS spectrum system is spreading baseband by directly multiplying the baseband data with the PN code sequence. At the receiver side code, synchronization is received and the signal is passed through a filter. At the next step, it is multiplied by a local version of the same PN sequence. The received data are then dispread. The signal bandwidth is reduced. An interference signal will be spread when multiplying with the dispread sequence and the power of the interference is reduced when the bandwidth are increased. The part of the interference signal that does not have the same bandwidth as the desired signal is then filter out. In this way the most of the interference power is eliminated.

2.6 Channel Access

There are two types of channel access in the IEEE 802.15.4 defined communication system. They are based on whether the devices want to retain their own exclusive time slot for communication or they have to compete with other devices for accessing the channel. The contention based allows the devices to access the channel in distributed way using CSMA - CA algorithm. With this method each devices has to scan the air medium for free channel or to compete with other devices for the channel access. With contention free method, the network coordinator decides about the channel access by using Guaranteed Time Slot (GTS) of the channel space. This method is suitable for latency sensitive devices that require short delay time and no competition for channel access.

To meet its low average power consumption goals, IEEE 802.15.4/ZigBee is capable of extremely low duty cycles — below 10 ppm [3]. The standard also supports beaconless operation — an asynchronous, asymmetrical mode supporting unslotted CSMA-CA channel access for star networks — that enables devices other than the receiving central node to remain asleep for indefinite periods, thus reducing their average power consumption still further.

2.7 Data processing

IEEE 802.15.4 2.4-GHz physical layer uses a 16-ary quasi-orthogonal signalling technique — trading signal bandwidth to recover sensitivity with coding gain. A particular 32-chip, pseudo-random (PN) sequence is used to represent four bits [3].A chip is a symbol from p-n code sequence. There are 16 chips sequence and one is defined in table below:

Information is placed on the signal by cyclically rotating or conjugating (inverting chips with odd indices) the PN sequence 8. The PN sequence is rotated in increments of four chips: symbols 0 through 7 represent rotation without conjugation; and symbols 8 through 15 represent the same rotations, but with conjugation. In this way, four bits are placed on each transmitted symbol and, because transmitted symbols are related by simple rotations and conjugations, receiver implementations can be simplified over other orthogonal signalling techniques that employ unrelated PN sequences.

Half-sine shaped Offset-Quadrature Phase Shift Keying (O-QPSK) is employed, in which the chips of even index are placed on the I-channel and the chips of odd index are delayed one-half chip period and then placed on the Q-channel. The chip rate on either the I or the Q channel is 1 Mchip/s, so the overall chip rate is 2 Mchip/s. The symbol rate is 62.5 k symbols/s, leading to a data rate of 250 kbps. [4]

3.0 Description of the Simulator

In this report, Simulink and syntax code (M-file) in MATLAB are used to generate the scatter plot. For the implementation of overall block system in Simulink model, the equivalent block diagram is as shown above. The Simulink model block consists of random integer generator, modulator and demodulator, noise channel and also error rate calculator. Simulink model effectively represent the complicated syntax code into system model block. The signal is generated by using a Random integer, the Random Integer Generator block generates uniformly distributed random integers in the range [0, M-1]. Then, the signal are modulated by OQPSK, the Offset Quadrature Phase Shift Keying (OQPSK) modulation scheme is used in this assignment. As stated in literature review, Offset Quadrature Phase Shift Keying (OQPSK) is a variant of phase-shift keying modulation using 4 different values of the phase to transmit. Addictive White Gaussian Noise is then added to the signal to simulate a practical channel which consists noise. After that, the signal is demodulated by using the same scheme-OQPSK. Result are compared with the initial signal (output from the Random Integer) to calculate the BER and SER performance of the scheme. The Discrete-Time Scatter Plot Scope block displays scatter plots of a modulated signal, which are located after AWGN block to reveal the modulation characteristics, such as channel distortions of the signal.

For M-file code, the sequence is similar as the above block diagram, the following command are to used to generated the random signal :

out = randint(m,n,rg)

The code represented above generates an m by n integer matrix of random number. Variable “rg” represent the range of random number to be generated.

The modulating part and demodulating are done by using the below code:

y = oqpskmod(x)

y = oqpskdemod(x)

To add noise to the signal:

y = awgn(x,snr,'measured')

Where x is the input signal and “snr” is the signal to noise ratio.

The scatter plot are plotted by using:

scatterplot(y)

Besides plotting the scatter plot, the BER versus SNR curve are plotted as it is important to compare different modulating scheme in term of the BER performance. This is done by using syntax code in M-file. The complete code is attached in the appendix. The following is the flow chart to describe the sequence of the code.

At the end, three graphs are generated - ideal BER characteristic, practical BER characteristic and SER characteristic.

4.0 Result and Discussion

. Dots are located at the centre of the boundary lines which is within the box or boundary line. This indicated that proper digital signal is being received. The closer the detected level/phase dots are plotted to the middle of the boundaries, the lesser noise and interference to the digital signal. However this diagram will never be obtain in practical cases. Therefore to simulate a practical real cases, Addictive White Gaussian noise must be considered, Additive Gaussian noise disturb the digitally modulated signal during analog transmission, for instance in the analog channel. Additive superimposed noise normally has a constant power density and a Gaussian amplitude distribution throughout the bandwidth of a channel. If no other error is present at the same time, the points representing the ideal signal status are expanded to form circular “clouds” as shown below.

As figure above shown, scattering of the dots indicates some marginal signal performance degradation but not enough to cause significant digital bit errors because dots are still located inside the boundary. Only dots to the edges of the quadrant or beyond represent significant degradation that results in bit errors during decoding which are shown in figure below generated by using Simulink

. In conventional QPSK, change in the input bit from 00 to 11 or 01 to 10 causes a corresponding 180° shift in the output phase therefore an advantage of OQPSK is the limited phase shift that must be imparted during modulation. The sudden phase-shifts occur about twice as often as for QPSK (since the signals no longer change together), but they are less severe. In other words, OQPSK has smaller jumps when compared to QPSK. This lowers the dynamical range of fluctuations in the signal which is desirable in engineering communications signals.

As mentioned earlier, the simulation is performed to study the performance of the design model. The study is done by observing the bit error rate (BER) with respect to the signal to noise ratio (SNR)

The simulation (Figure 4.0(f)) compares the BER versus SNR curve for different modulation scheme used. Four different modulation schemes is taken into comparison, which is the Quadrature Amplitude Modulation (QAM), Quadrature Phase Shift Keying (QPSK), Differential Phase Shift Keying (DPSK) and finally Offset Quadrature Phase Shift Keying (OQPSK) employed by Zigbee operating at 2.4GHz radio band. By comparing different modulation scheme give better insight on which modulation is better and suitable. All modulation schemes are simulated with the value M = 4, where it is effectively represented by n = 2 bits which can be calculated from the following formula below:

n = log2 (M)

The first graph below shows the theoretical bit error rate versus signal to noise ratio curve.

For practical system, white Gaussian noise is added. If comparison is made between theoretical graph and practical graph, it shows that there is not much of the different in term of the shape of the graph. ,it shows that for all the modulating scheme, the higher the SNR the lower the BER (bit error rate).The same principal are apply to the symbol error rate(SER) vs SNR(figure 4.0(g)).

For BER vs SNR, comparing the four different modulation schemes from the practical graph plotted, DPSK modulation scheme shows to have the worst bit error rate at any level of SNR, followed by QPSK and QAM having approximately similar bit error rate at any SNR level. The OQPSK tends to have the lowest bit error rate at any SNR level if compared to other modulation scheme, which is desired.

The comparison is done by taking one fixed SNR level, for example, considering the SNR level of -2. The bit error rate of OQPSK falls approximately to 10-2, which is the smallest. The QAM and QPSK have bit error rate falls within the range of 1x10-1 to 5x10-1, slightly much higher than OQPSK. The DPSK has the highest bit error rate, which is above the range 1x10-1.

2dB

Graph 4.0(h) illustrate symbol error rate (SER) versus signal to noise ratio (SNR) under the same condition for the four modulation scheme. The SER versus SNR curve has similar characteristic over the BER versus SNR curve, with OQPSK being in the best performance, followed by QPSK and QAM and DPSK having the worst performances.

Low average power is achieved with a low overall system duty cycle. However, low duty cycle must be achieved with low peak power consumption during active periods because most of the target power sources have limited current sourcing capabilities and low terminal voltage, and it is not desired to implement with complex power conditioning systems for cost and efficiency reasons. At the physical layer, the need for low duty cycle yet low active power consumption implies the need for a high data rate (to finish active periods quickly and return to sleep), but a low symbol rate (because signal processing peak power consumption is more closely tied to the symbol rate than the data rate). Therefore, this implies the need for multilevel signalling (or m-ary signalling, with m > 2), in which multiple information bits are sent per transmitted symbol. However, simple multilevel signalling, such as 4-FSK, results in a loss of sensitivity. As can be seen from figure 4.0(h), OQPSK provides a 2-dB increasein sensitivity over nearest scheme (QAM). For modulating scheme with low sensitivity, there is a need to recover the needed link margin (i.e. range) without resorting to directive antennas, the transmitted power must be increased or the receiver noise figure must be reduced, both of which can increase power consumption significantly.

5.0 Conclusion

In conclusion, IEEE 802.15.4 is still a new standard which has the potential to unify methods of data communication for sensors, actuators, appliances, asset tracking devices and so on. It offers the means to build a reliable and affordable network backbone that takes advantage of battery-powered devices communicating at low data rates. In addition the complexity and cost of the IEEE 802.15.4/Zigbee-compliant devices are intended to be low. It can potentially create a whole new ecosystem of interconnected home appliances, light and climate control systems, and security and sensor sub-networks.

In this respect, the implementation of the physical layer of the IEEE 802.15.4 standard must be optimized to meet the challenging low-cost and low-power targets. From the context above, various test and evaluation on the BER versus SNR curve and scatter plot for various scheme are done on physical layer of the ZigBee, the result shows that OQPSK is the most suitable modulating scheme which fit the ZigBee (2.4 GHz band) characteristic (low power consumption, high sensitivity, high reliability and low cost ). Thus it coincides with the actual standard for Zigbee.

6.0 References

1. Sensor Technology Handbook, Page 593 Section: 22. Wireless Sensor Networks: Principles and Applications Wilson, Jon S ISBN: 0750677295, Newnes, 2004

ZigBee Specification, ZigBee Alliance, Zigbee Document 053474r05

Version1.0, 2005-06-20

2.ZigBee and 802.15.4 Solutions -http://www.silabs.com/public/documents/marcom_doc/pbrief/Microcontrollers/en/ZigBee_Brief_Web.pdf

3.RF Circuit Design Theory and Applications, Reinhold Ludwig and Pavel Bretchko Pretince Hall, ISBN 0-13-095323-7

4. Zigbee Ready RF Design -http://rfdesign.com

5. Zigbee - http://en.wikipedia.org/wiki/ZigBee

6.Edgar H. Callaway, Jr., Wireless Sensor Networks. Boca Raton, FL: Auerbach Publications, 2003, Chap. 7.

7. chipcon,http://www.chipcon.com,2006-01

8. IEEE802.15.4 and ZigBee Compliant Radio Transceiver Design

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