I. INTRODUCTION

Seeing the recent trends wireless technology has become an integral part of one’s life. Most of the communication and data transfer takes place wirelessly. Variety of technologies keeps on evolving to make the wireless transmissions at a high data and provide reliable service along with security. UWB is a technology that provides very high data rate transmission. UWB comprise of 3.1GHz to 10.6GHz of frequency band. The use of this frequency dates 100 years back when Marconi for the first time did impulse transmission wirelessly. This technology is based on pulse transmission wherein the width of the pulse is extremely low. The actual use of UWB stared in late 1960’s for military applications. In early 2000, the technology started shifting from military use to commercial use.

According to Federal Communication Commission, UWB is defined as any transmission scheme with percentage bandwidth greater than 20% is termed as UWB. It can provide data rate of 1Gbps for very short distance and provide minimum of 150Mbps over a considerable distance. The transmission power levels are way below the noise floor levels decided for CDMA mobile technology. FCC in USA has declared the UWB band as unlicensed band provided the signal levels parameters are maintained below satisfactory level. This has led to the popularity of UWB as no licensing is required for its use.

II. UWB TRANSMISSION AND RECEPTION

To achieve wide bandwidth in frequency domain it is necessary to have an impulse like transmission in time domain. To have an exact impulse is not possible practically so pulses of very short width of the order of 200-500ps are used. The voltage levels are as low as 500mV. Shape of the pulse play an important role in UWB transmission system. Moreover UWB is base band technology and so pulse shaping is essential so that detection at receiver end becomes easy. Since UWB technology is designed for low power applications it is essential that the pulses have low duty cycle. The duty cycle is kept varying depending upon the channel

and interference levels the duty cycle is decided. The pulses used for UWB are called mono pulses.The mono pulses are further classified into mainly three types namely, Gaussian monopulse, Scholtz mono cycle, and RZ Manchester.

Fig. 1 Monopulses[5]

As the order of Gaussian pulses increase the number of zero crossing also increase. The main modulation schemes used are pulse position modulation (PPM), pulse width modulation (PWM), On-Off Keying (OOK), and Bi-Phase Modulation (BPSK).

Fig. 2 PPM[5]

Ultra Wide Band Technology: Advantages and ChallengesKedar Trivedi13MECC23

Institute of Technology, Nirma UniversityAhmedabad-382481, Gujarat, India

Mob-+91-9924122428, Email-kedar_91@yahoo.co.in

Abstract: This paper deals with the most recent trend and successful technology to transmit data at a very high data rate and that too at very low power transmission levels. UWB abbreviated for Ultra Wide Band provide numerous advantages over existing wireless technologies but along with the benefits complexity also increases. This paper discusses the history of UWB its advantages and its limitations. It also focuses on the solutions found for overcoming the limitations.

Keywords: UWB, EUWB,

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Fig. 3 PWM[5]

Figure 2,3 and 4 shows the modulation schemes. The performance of each modulation scheme is different and unique in different environment. For AWGN channels the performance of PPM modulation scheme is much better than PWM while for multi path channel PWM provides good performance in terms of BER in comparison to PPM. BFSK also provides better performance in multipath channel in comparison to PPM and PWM. Recently, MIMO based OFDM modulation scheme is used for getting much better performance. Also, a hybrid version of PPM and BPSK is used for better performance. At receiver end rake receivers are used for coherent detection of signal. Since the signal level is very low large number of rake fingers are required to combine the numerous multi paths and get the desired signal. The design of rake receiver is a challenging task. Since UWB is also widely used in wireless sensor networks it becomes extremely essential to limit the usage of power. So non-coherent detection is more widely used wherein a reference signal is sent first and on the basis of received reference signal the channel characteristics are estimated and then the original signal is sent and received accordingly.

Fig. 4 BPSK[5]

III. CHALLENGES

There are numerous challenges in implementation of UWB. One of the major challenges is the interference of UWB system with the existing system working in the same frequency band. The interference can lead to degradation in the existing 802.11a technology which is working at 5GHz frequency. It can also interfere with the GPS system working in this frequency band. So, FCC has put restriction on the power levels of UWB so that interference can be reduced. On the other hand, design of hardware that can detect such low level signals is very challenging. Also, the hardware of UWB receivers should be such that it rejects the high power signals coming from existing systems. Figure 5 shows the hard limit placed by FCC on UWB system use. From it is clear that for 2-3GHz of band power level limit is about -75dBm. This is kept low as GPS system works in this region and also many military applications utilize this band. So to avoid interference the levels are kept very low. Another challenge is in the design of CMOS circuits. As the pulse width decreases, all signal processing and signal generating circuits must be capable enough to respond to this short pulses. So from design point of view it is difficult to develop UWB system. Another challenge is to design a notch filter that can stop the interference of existing system on to the UWB system. For example 802.11a works with a hard limit of 16dBm while for UWB it is -41dBm. Since 802.11a is narrow band it occupies 100 kHz bandwidth with centre frequency of 5GHz. So, this lead to inter symbol interference and to combat ISI data rate need to be reduced and if data rate is reduced then the purpose of UWB is not fulfilled. Another challenge is to design an omni directional antenna with such large fractional bandwidth. Since UWB is designed for remote devices the antennas cannot be very large and so required gain and cannot be provided. So microstrip antennas are used. But fractional

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bandwidth of microstrip antenna is very low. So designing a special microstrip antenna is difficult. The amplifiers used in trans-receivers should be extremely low noise as the power level of signal is way below the normal allowable noise floor level. This is again a challenging task. Moreover the sensitivity of the receiver should be extremely low. As the data rate is high the ADC and DAC must also work at Msample/s of rate and also provide high resolution. As the number of bits are increased in linear fashion the complexity of the increases exponentially.

Fig. 5 Hard Limit for UWB[1]

Figure 6 shows the entire trans-receiver system for UWB. Another challenge is to provide the same data rate as the distance between transmitter and receiver increases. Since the penetration index of high frequency EM waves is very less it cannot pass through walls of the room and windows. So, in-house communication is only possible. For long range communication multiple UWB boosters need to be used and so a hopping network can be established. Another challenge in UWB is to design variable bit rate system. As there is trade-off between data rate and distance it is essential to have variable bit rate system so that the communication link does not break all of a sudden if the receiver moves away. So a graceful degradation should be provided.

Fig. 6 UWB Trans-Receiver system

Figure 7 shows the graph of throughput versus the distance. It is clear from the graph that for wideband system throughput drastically reduces as distance increase while same iis not the case for narrow band systems.

Fig. 7 Throughput v/s Distance[1]

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IV. SOLUTION TO UWB CHALLENGES

To mitigate the drawbacks of UWB variety of solutions have been proposed. The solution not only lies in proper designing of hardware but an efficient software platform is essential for this purpose. The open software platform specially designed for UWB applications is known as EUWB. It is abbreviated for co-Existing short range radio by advanced Ultra Wide Band radio technology. For any fruitful communication along with the data some additional information is also shared like training bits, frame length information, synchronization bits, power level indication etc. These signaling bits are not required to be sent at all time. So a low data rate transmission (LDR) for such signaling bits is sufficient. But at the same time high data rate (HDR) is required for actual data transmission. Now, as the data rate increases the need for power also increase. Now keeping the power consumption as low as possible is the prime requirement of the system. So an open technology platform EUWB was designed. In this two different protocol stacks are used one for high data rate and other for low data rate. The physical layer is also different for both transmissions. Figure 8 shows the software block of UWB

Fig. 8 Physical Layer splitting[6]

From above figure it is clear the even the MAC layer for lower data rate is different to that of high data rate MAC. Error detection and correction blocks are added for HDR MAC as huge chunk of data is transmitted while for LDR on error detection is used in LDR MAC. The important issue is to properly synchronize and add signaling bits at regular interval in the data frame. So combiner must be efficient. This technique helps in improving the performance of the system. Now talking about the physical layer then differential BPSK modulation technique is used for signaling bits while for HDR OFDM based hybrid modulation scheme is used.Another method is to decompose the signal in various sub-bands so that interference due to existing systems can be reduced. The best way to mitigate interference is by the combination of UWB and cognitive radio. Cognitive radio can sense the environment and can make required changes according to real time conditions. It helps in modifying the frequency, transmission parameters, output power levels according to the channel condition. Cognitive radio adapts to

the channel by making changes in the network and physical layer. This can help in changing the operating frequency dynamically so as to reduce the interference of narrow band signals.

CONCLUSION

Thus UWB technology is of great importance in today’s world. Just by merging cognitive radio and UWB most of the problems and drawbacks of UWB can be overcome.

ACKNOWLEDGEMENT

I thank god for filling me with positivity. I thank my Sad Guru Bhagwan Pujya Morari Bapu and my parents for bestowing me with their blessings.

REFERENCES

[1] Sumit Roy, Jeff R. Foerster, V. Srinivasa Somayazulu, and Dave G. Leeper, “Ultrawideband Radio Design: The Promise of High-Speed, Short-Range Wireless Connectivity,”Proceedings of IEEE, Vol 2, 2004.

[2] Michael Sablatash, “Mitigation of Interference by Ultra Wide Band Radio into Other Communication Services: Evolution to Cognitive Ultra Wide Band Radio,” IEEE Conference Publication 2006.

[3] Fabrizio Granelli, Honggang Zhang, “Cognitive Ultra Wide Band Radio: A Research Vision and Its Open Challenges,”

IEEE, 2005.[4] David D. Wentzloff, Raúl Blázquez, Fred S. Lee, Brian P.

Ginsburg, Johnna Powell, and Anantha P. Chandrakasan “System Design Considerations for Ultra-Wideband Communication,” IEEE Communications Magazine, 2005.

[5] Ashish K. Thakre, Amol I. Dhenge “Selection of pulse for ultra wide band communication (UWB) system,” IJARCCE, 2012.

[6] Peter Jung, Dong Xu, Andreas Waadt, Alexander Viessmann, “The EUWB Open Technology Platforms for Ultra Wide Band Communications,” IEEE,2011

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