Ultra – Wideband (UWB) has been attracting many researchers for its potential to support a high bit rate in a short range wireless communication system. In order to achieve the high bit rate, a short spreading code is often preferred to a long spreading code in a Direct sequence spread spectrum (DSSS) UWB system.
In this paper, we propose a new modulation technique based on frequency and time hopping Pulse Position Modulation (PPM) UWB which greatly reduces the Multiple Access Interference (MAI) under multiple access communication in comparison with conventional time hopping or frequency hopping schemes. The probability of error is derived for multi-user synchronous transmitter in UWB multi-path channel with Additive White Gaussian Noise (AWGN). Simulation results show that bit error probability performance of FTH – PPM UWB out performs the time – hopping pulse position modulation (TH – PPM) UWB system. It also shows that multi user capacity of FTH – PPM UWB system is much better than TH – PPM UWB system.
Ultra-Wideband
Ultra-Wideband technology [8] is a wireless transmission method that occupies bandwidth more than 20% of the center frequency or more than 500MHz of bandwidth. In UWB it is required that the data rate should be at least 110Mbps with a 10m separation between the transmitter and the receiver, and 200Mbps with a 4m separation. Since the transmitted power is strictly limited but wide bandwidth is available, UWB is suitable for a short range, high rate system such as a Personal Area Network (PAN).
1.1.1 Applications of UWB
Since UWB utilizes unlicensed spectrum with wide bandwidth, there is a large number of possible applications, such as positioning, secured transmission, and high rate data communications.
First, the very short UWB pulse causes multipath terms to be detected with high resolution. This enables very accurate positioning. A few examples will be in building object tracking, wall penetrating positioning in a hazardous environment, and fine positioning for medical purposes. Besides, geolocation devices are also being developed.
Second, UWB can be used for a high security transmission system. It is the military which was initially interested in UWB technology due to its potential for security. When operated with low power and short pulses (probably with a hopping scheme), it would be extremely difficult to intercept transmitted signals unless a receiver has the exact timing information.
Lastly, a high data rate indoor communication system has recently become an important area for UWB technology. It can provide a cheap and low power receiver at a high data rate for PAN systems. From an optimistic point of view, it will be able to supplant most present-day indoor wired and wireless communication systems. For example, a DVD player, a TV, a monitor, a computer, and all other peripherals can interconnect via UWB, eliminating most data wires. In this thesis, we consider a high rate (over 110 Mbps), indoor wireless communication system.
1.2 Time Hopping & Frequency Hopping in UWB Systems
Ultra wide bandwidth (UWB) spread - spectrum (SS) multiple access techniques have recently received considerable attention for future commercial and military wireless communication systems[3]. The report and order of the FCC (Federal Communications Commission) in the USA that allowed UWB communications systems in the 3.1-10.6 GHz range has intensified the interest especially from possible chip and equipment manufacturers. One possible application lies in Personal Area Networks (PAN), where high data rates are sent over a short distance. The FCC has imposed two restrictions on the use of the spectrum: a requirement that the transmission bandwidth is a minimum of 500MHz (though it is not completely clear over which time duration the instantaneous spectrum must fulfill that condition), and a restriction on the transmit power spectral density, namely -41.3dBm/MHz However, the FCC imposes no specific modulation or multiple-access (MA) format as long as those restrictions are fulfilled.
This fact gives a great practical as well as theoretical value to the problem of finding a good modulation and MA scheme for ultra wideband communications. This topic is also a major factor in the deliberations of the IEEE 802.15.3a [7] standardization committee, which has been established to develop an UWB system that can provide multiple piconets with 110Mbit/s at 10m distance, as well as higher data rates at smaller distances. Two candidate schemes are frequency-hopping (FH) and time hopping (TH). Recent information-theoretic results allow interesting conclusions about good spreading schemes. However, those investigations do not consider the constraints put on the signaling schemes by the FCC regulations, nor do they cover quantitatively the cases of TH and FH. It is the purpose of this paper to generalize those results to FCC compliant, TH and FH systems, and investigate the impact on system design.
1.3 Multiple Access Communication
Multiplexing is defined as the sharing of a communications channel through local combining at a common point. In many cases, however, the communications channel must be efficiently shared among many users that are geographically distributed and that
sporadically attempt to communicate at random points in time. Three schemes have been devised for efficient sharing of a single channel under these conditions; they are called frequency-division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). These techniques can be used alone or together in telephone systems, and they are well illustrated by the most advanced mobile cellular systems.
1.3.1 Frequency – Division Multiple Access
In FDMA the goal is to divide the frequency spectrum into slots and then to separate the signals of different users by placing them in separate frequency slots. The difficulty is that the frequency spectrum is limited and that there are typically many more potential communicators than there are available frequency slots. In order to make efficient use of the communications channel, a system must be devised for managing the available slots. In the advanced mobile phone system (AMPS), the cellular system employed in the United States, different callers use separate frequency slots via FDMA. When one telephone call is completed, a network-managing computer at the cellular base station reassigns the released frequency slot to a new caller. A key goal of the AMPS system is to reuse frequency slots whenever possible in order to accommodate as many callers as possible. Locally within a cell, frequency slots can be reused when corresponding calls are terminated. In addition, frequency slots can be used simultaneously by multiple callers located in separate cells. The cells must be far enough apart geographically that the radio signals from one cell are sufficiently attenuated at the location of the other cell using the same frequency slot.
1.3.2 Time – Division Multiple Access
In TDMA the goal is to divide time into slots and separate the signals of different users by placing the signals in separate time slots. The difficulty is that requests to use a single communications channel occur randomly, so that on occasion the number of requests for time slots is greater than the number of available slots. In this case information must be buffered, or stored in memory, until time slots become available for transmitting the data. The buffering introduces delay into the system. In the IS54 cellular system, three digital signals are interleaved using TDMA and then transmitted in a 30-kilohertz frequency slot that would be occupied by one analog signal in AMPS. Buffering
digital signals and interleaving them in time causes some extra delay, but the delay is so brief that it is not ordinarily noticed during a call. The IS54 system uses aspects of both TDMA and FDMA.
1.3.3 Code – Division Multiple Access
In CDMA, signals are sent at the same time in the same frequency band. Signals are either selected or rejected at the receiver by recognition of a user-specific signature waveform, which is constructed from an assigned spreading code. The IS95 cellular system employs the CDMA technique. In IS95 an analog speech signal that is to be sent to a cell site is first quantized and then organized into one of a number of digital frame structures. In one frame structure, a frame of 20 milliseconds’ duration consists of 192 bits. Of these 192 bits, 172 represent the speech signal itself, 12 form a cyclic redundancy check that can be used for error detection, and 8 form an encoder “tail” that allows the decoder to work properly. These bits are formed into an encoded data stream. After interleaving of the encoded data stream, bits are organized into groups of six. Each group of six bits indicates which of 64 possible waveforms to transmit. Each of the waveforms to be transmitted has a particular pattern of alternating polarities and occupies a certain portion of the radio-frequency spectrum. Before one of the waveforms is transmitted, however, it is multiplied by a code sequence of polarities that alternate at a rate of 1.2288 megahertz, spreading the bandwidth occupied by the signal and causing it to occupy (after filtering at the transmitter) about 1.23 megahertz of the radio-frequency spectrum. At the cell site one user can be selected from multiple users of the same 1.23-megahertz bandwidth by its assigned code sequence.
CDMA is sometimes referred to as spread-spectrum multiple access (SSMA), because the process of multiplying the signal by the code sequence causes the power of the transmitted signal to be spread over a larger bandwidth. Frequency management, a necessary feature of FDMA, is eliminated in CDMA. When another user wishes to use the communications channel, it is assigned a code and immediately transmits instead of being stored until a frequency slot opens.
1.4 Multiple Access Interference
Multiple Access Interference (MAI) is a type of interference caused by multiple cellular users who are using the same frequency allocation at the same time. In both 2G
and 3G mobile networking, each user is then given a pair of frequencies (uplink and downlink) and a time slot during a frame. Different users can use the same frequency in the same cell except that they must transmit at different times. This multiple-access interference can present a significant problem if the power level of the desired signal is significantly lower (due to distance) than the power level of the interfering user.
CONCLUSIONS
In this paper we have proposed and analyzed bit error probability performance of frequency and time hoping PPM UWB multiple access communication in IEEE P802.15 multipath channel. We have derived an expression for the bit error probability for multi-user synchronous transmitter case. It is observed that introduction of frequency hopping along with time hopping improves BER performance by an average of 4 dB. Further, doubling the number of frequency hops improves BER performance by 1 dB. The proposed technique improves BER performance without reducing the data rate.
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