- Research Article
- Open Access
Pulse Interval Modulation for Ultra-High Speed IR-UWB Communications Systems
© Marijan Herceg et al. 2010
- Received: 16 February 2010
- Accepted: 21 July 2010
- Published: 5 August 2010
This paper analyzes performances of the Pulse Interval Modulation (PIM) scheme for impulse radio ultra-wideband (IR-UWB) communication systems. Due to the PIM anisochronous nature, a tap delay line (TDL) coded division multiple access (CDMA) scheme based on strict optical orthogonal codes (SOOC) is proposed. This scheme is suitable for multiuser high-speed data asynchronous transmission applications because the average symbol length is shorter than in Pulse Position Modulation (PPM) schemes and it needs only chip synchronization. The error probability over the additive white Gaussian noise (AWGN) channel is derived in the single- and multi-user environment and compared with other modulation schemes.
- Additive White Gaussian Noise
- Code Division Multiple Access
- Packet Error Rate
- Additive White Gaussian Noise Channel
- Pulse Amplitude Modulation
Trends in modern communication systems place high demands on low power consumption, high-speed transmission, and anti-interference characteristics. Therefore, impulse radio ultra-wideband (IR-UWB)  systems have recently gained increased popularity. Since IR-UWB symbols are transmitted by short pulses ( 2 ns), energy has spread over the frequency bands of up to 10 GHz. These pulses have to follow strict regulations concerning power and spectrum restrictions defined by local authorities, like the Federal Communications Commission (FCC)  in the USA. Because of power and spectral properties of the transmitted IR-UWB pulses, different types of orthogonal pulse shapes are used to provide a higher spectral efficiency [3, 4]. Derivation of the Gaussian pulse and modified Hermite pulses (MHPs), usually called Hermites , provides a wide range of various pulse combinations for IR-UWB transmission and for that reason they are most commonly used as pulse shapes. The state of art in IR-UWB systems is presented by many applicable modulation techniques like Pulse Amplitude Modulation (PAM), PPM, Pulse Shape Modulation (PSM), on-off-keying (OOK), and biphase modulation (BPM). A combination of the exposed modulation techniques (hybrid techniques) can provide system improvements in terms of the error probability, a higher data rate, a less complex receiver, or less power consumption. Many hybrid techniques for IR-UWB communication systems have been applied recently, such as Pulse Position Amplitude Modulation (PPAM) , Biorthogonal Pulse Position Modulation (BPPM) , OOK-PSM , PPM-PSM , and hybrid Shape-Amplitude Modulation [10, 11]. Regarding pulse amplitudes, positions, and shapes, three types of modulations can be distinguished. In PAM and OOK modulation information is contained in the amplitude of the signal, PPM uses the position of the pulse to convey information, whereas in PSM, information is conveyed in the shape of the pulse.
PIM was first introduced in  for wireless optical communication systems. It is interesting because it displays a higher transmission capacity by eliminating unused time chips within each symbol and does not require both chip and symbol synchronization, but only chip synchronization, since each symbol is initiated with a pulse. Different anisochronous and synchronous pulse time modulation (PTM) techniques for optical short-range wireless communications are compared in .
This paper is organized as follows. Section 2 describes basic properties of the PIM scheme. In Section 3, the proposed system model is described, while in Section 4 error performance analysis is made over the AWGN channel. Section 5 gives simulation results while some conclusions are given in Section 6.
Mappings between source bits and transmitted chips for 4-PPM and 4-PIM (Example).
where is the number of users, is time delay of the k th user (assumed to be the integer multiple of ), and is an AWGN component with zero mean and variance /2.
At the input of the receiver, is passed through the TDL decoder, where by tuning the delay elements a higher amplitude pulse can be formed by each of the pulses in the signature sequences. The decoded signal is then fed into the correlator-based matched filter which multiplies the signal by the template waveform. The decision which chip is empty and which chip contains a pulse is made on the basis of autocorrelation properties of the Gaussian monocycle at the threshold detector. At the output of the threshold detector, the transmitted PIM data encoded stream is estimated by removing redundant ( ) chips. The pulse at the start of the PIM symbol is then used to load the output of the counter in the latches and to reset the counter.
Channel model is an AWGN channel (no multipath components).
MUI is approximated as a Gaussian random variable.
There is a perfect synchronization and power control between the transmitter and the receiver.
The decision variable can then be modeled as a Gaussian random variable with mean and variance
where is the channel bandwidth and is the bit rate defined in (2), (3).
This paper proposes an anisochronous PIM scheme for IR-UWB communication systems. The basic principles and characteristics of anisochronous PIM scheme are outlined. Unlike PPM, PIM requires no symbol synchronization, which results in a much simpler receiver structure (only one correlator). The proposed multiple access method based on SOOC-TDL-CDMA allows a totally asynchronous transmission and it needs only chip synchronization which significantly reduces hardware complexity, while classical time-hopping IR-UWB needs both frame and chip synchronization which increase hardware complexity. It is shown that an increase of code weight can decrease PER at the cost of hardware complexity (more delay elements at TDL) and the influence of in both single- and multi-user environment is analyzed. The major disadvantage of anisochronous PIM techniques is that they have a variable symbol length, and hence the time required to transmit a data packet containing a fixed number of bits is not constant. Employing some form of a source coding scheme, packet length variation can be limited still maintaining the increase in information capacity over isochronous modulation techniques. Simpler receiver complexity and very high achievable bit-rates make PIM modulation very attractive for IR-UWB short-range communication systems.
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