DESCRIPTION Wireless communication method and system

Technical Field

The present invention relates to wireless communication apparatus _^ method and system, where data packets are transmitted and received in the form of radio signals.

Background Art

In a wireless communication system, a data packet in the form of a radio signal is transmitted from a transmitter to a receiver. An ordinary type of data packets contains three sections; preamble, header and payload. These three sections are arranged sequentially in the packet, as shown in Fig. 2. Each section contains a data sequence, which is designed for its specific purpose. When the receiver receives the packet, the preamble section is used to do synchronization and channel estimation.

Synchronization is to obtain the timing information on how to separate individual data bits in a data sequence. Channel estimation is to estimate representative characteristics of the propagation channel between the transmitter and the receiver. Then the header section is analyzed to get the control information of the packet such as the length or the main function of the packet. Next, the payload section, which carries the real message to be

transferred from the transmitter to the receiver is demodulated. In the ordinary type of data packets, there is no overlap between any two sections.

However, in certain applications, in addition to the normal functions of a data packet, longer preamble sequence is desired. For example, in the ranging application, which is to estimate the distance between a receiver and a transmitter, the longer preamble section contributes to the improvement of accuracy in the channel estimation and ranging performance. Yet there is usually a limit on the total length of the data packet based on other system design concerns. Therefore, a new type of data packet that can accommodate longer preamble within the current data packet design is of great practical significance.

Disclosure of the Invention

In this invention, we devise a new data packet structure where the preamble sequence is multiplexed with the data sequences of the header and payload section. In other words, the preamble sequence appears not only in the regular preamble section but also in the header and payload sections as well. For example, the multiplexed preamble sequence can be orthogonal or quasi-orthogonal to the regular data sequences of the header and payload sections in code division, which is not difficult to realize in most practical systems. The advantage of the new packet design is

to significantly improve the performance of some important applications which depends on the length of the preamble sequence. Such applications include ranging and carrier sense. Ranging is to estimate the distance between a transmitter and a receiver. Carrier sense at one receiver is to determine whether the wireless channel is occupied by other receivers/users by processing its received signals.

This invention presents a wireless communication apparatus, method and system, utilizing a said new data packet structure. Wireless communication apparatus according to the present invention comprises: pulse generation means which generates a sequence of pulses according to said new data packet; and transmission means which transmits said sequence of pulses generated by said pulse generation means in the form of radio signals.

Wireless communication system for wireless communication from a transmitter to a receiver: wherein said transmitter transmits said new data packet in the form of a sequence of radio pulses, and said receiver receives said data packet.

Wireless communication method for wireless communication from a transmitter to a receiver, comprises: transmission step at which said transmitter transmits said new data packet in the form of a sequence of radio pulses, and receiving step at which said receiver receives said data packet.

Brief Description of the Drawings

FIG. 1 is a structural diagram of a communication system for illustrating the invention.

FIG. 2 is a block diagram of a transmitter.

FIG. 3 is a block diagram of a receiver.

FIG. 4 is the structure diagram of a regular data packet .

FIG. 5 is the structure diagram of the new data packet according to the invention.

FIG. 6 illustrates the pulse energy of the superimposed preamble sequence and other data sequences in the new data packet.

FIG. 7 illustrates an example of a preamble symbol which can be adopted in the new data packet

FIG. 8 illustrates an example of data symbols used in the header and payload sections of the new data packet.

FIG. 9 shows the numerical results on the extra energy consumed by the superimposed preamble sequences of the new data packet.

FIG. 10 shows the numerical results on ranging accuracy improvement by using the new data packet.

Best Mode for Carrying Out the Invention

An embodiment of the invention is described in detail by referring to the aforementioned figures.

FIG. 1 shows the system configuration of a communication system 1 according to the invention. The

communication system 1 is a system for wireless communication between the point A and B, employing the UWB (Ultra Wide Band) technology. The communication system 1 consists of a transmitter 2 sited at the point A and a receiver 3 sited at the point B.

The UWB technology is a technology for short-range radio communication, involving the intentional generation and transmission of radio frequency energy that spreads over a very large frequency range, overlapping several frequency bands allocated to existing radio communication services. The transmitter 2 and the receiver 3 can transmit and receive UWB signals, respectively. A UWB signal is defined as a radio signal with -10 dB bandwidth of at least 500 MHz or a fractional bandwidth greater than 0.2.

FIG. 2 shows the block diagram of the transmitter 2 that transmits a UWB signal.

The transmitter 2 comprises: a pulse generating section 21 which generates a basic UWB signal, a pulse shaping section 22 which shapes the UWB signal generated by the pulse generating section 21 according to certain spectral criteria, a local oscillator 23 which supplies a frequency reference signal to the mixer circuit 24, a mixer circuit

24 which performs frequency conversion on the output signal from the pulse shaping section 22 by using the reference frequency signal supplied from the oscillator 23, a filter

25 for limiting the bandwidth of the frequency-converted signal generated by the mixer circuit 24, a first amplifier

26 which amplifies the output signal from the filter 25, an antenna 27 which radiates the output signal from the first amplifier 26.

The pulse generating section 21 generates a sequence of UWB pulses as a basic UWB signal. A UWB pulse is a signal whose bandwidth is inverse of the pulse duration usually on the order of a nanosecond or a fraction of a nanosecond. The pulse sequence generated by the pulse generating section 21 is sent directly to the pulse shaping section 22.

The pulse shaping section 22 shapes the basic UWB signal generated by the pulse generating section 21 according to certain spectral requirement.

The local oscillator 23 generates a frequency reference signal for frequency conversion in the mixer circuit 24. The frequency of the reference signal may be precisely controlled by an unillustrated PLL (Phase-Locked Loop) circuit or the like.

The mixer circuit 24 converts the output signal from the pulse shaping section 22 into a desired frequency band by using the frequency reference signal generated from the local oscillator 23.

The bandpass filter 25 limits the output signal from the mixer circuit 24 within the desired frequency band by removing the unwanted spectral components beyond the desired band. The output signal from the filter 25 is directly fed to the first amplifier 26.

The first amplifier 26 amplifies the output signal from

the filter 25 and further modifies the signal in such a way that the signal spectrum is flat within the desired frequency band.

An antenna 27 radiates the output signal generated from the first amplifier 26 as radio signal waves into space which is to be received by the receiver 3 at the point B.

FIG. 3 shows the block diagram of the receiver 3, which receives the UWB signal, transmitted from the transmitter 2.

The receiver 3 comprises: an antenna 31 which captures the radio signal transmitted from the transmitter 2 in the air, a bandpass filter 32 which removes the unwanted spectral components of the output signal from the antenna 31, a low-noise amplifier (LNA) 33 which amplifies the output signal from the filter 32, a detection apparatus 34 which estimates the output signal from the LNA 33.

The antenna 31 captures the UWB radio signal transmitted from the transmitter 2 in the air, and converts the radio signal into an electrical signal.

The bandpass filter 32 limits the output signal from the antenna 31 within a desired frequency band by removing unwanted spectral components beyond the desired bandwidth.

The LNA 33 amplifies the output signal from the filter 32 in such a manner that amplification of the noise component contained in the signal is controlled under certain level. The amplified signal generated by the LNA 33 is sent to the detection apparatus 34.

The detection apparatus 34 estimates the output signal

from the LNA 33 .

FIG. 4 illustrates the structure of a regular data packet 4. The packet 4 in the form of a radio signal can be transmitted from the transmitter 2 to the receiver 3.

The regular data packet 4 consists of three sections: the preamble section 5, the header section 6 and the payload section 7, which are arranged sequentially in the packet 4. Each section contains a data sequence which is designed for its specific purpose. When the receiver 3 receives the packet 4, the preamble section 5 is first used to perform synchronization and channel estimation.

Synchronization is to obtain the timing information on how to separate individual data bits in a data sequence, and channel estimation is to estimate representative characteristics of the propagation channel between the transmitter and the receiver. Then by using the information on synchronization and channel conditions obtained from the preamble section, data in the header section 6 and the payload section 7 are demodulated. In general, the header contains the control information of the packet, such as the length or the main function of the packet, and the payload carries the real message to be transferred from the transmitter.

It needs to be emphasized that in a regular data packet, the preamble, header and payload sections are arranged sequentially. In other words, there is no overlap between

data sequences of any two sections.

Fig. 5 shows the structure of the new data packet 8 according to the invention.

The main difference between the new data packet 8 in the present invention and the regular packet 7 is that an extra preamble sequence 9 is added or superimposed to the header section 6 and the payload section 7. Hence, in effect, the preamble sequence exists in every section (the preamble section 5, the header section 6 and the payload section 7) of the new data packet 8. Therefore, use of the new type of data packets can significantly improve the performance of some important applications, which depend on the length of the preamble sequence. Such applications include ranging and carrier sense. Ranging is to estimate the distance between the transmitter and the receiver. Carrier sense at one receiver is to determine whether the wireless channel is occupied by other receivers/users or not by checking its received signals. On the other hand, introduction of the superimposed preamble may introduce interference to the data demodulation of the header and payload section. In order to reduce such interference and maintain the performance of the data modulation, the power level of the superimposed preamble sequence can be tuned to be much lower that of the data sequences in the header and payload sections. In general, a power reduction of 7dB to 1OdB of the superimposed preamble is sufficient to reduce the interference to the data demodulation.

Fig. 6 illustrates an example of the new data packet 8, which can be applied to the IEEE 802.15.4a standards.

The horizontal axis in Fig. 6 represents time; while the vertical axis represents the power level of the data sequences in the present invention. The preamble sequence is constructed by repetition of the symbol si, which appears in all the three sections of the data packet. The symbol s2 is used to modulate the information data in the header and payload sections according to a specific modulation scheme such as 2PPM (Pulse Position Modulation) and 8PPM. The power level of the regular preamble sequence in the preamble section is same as the data sequence consisting the symbol s2 in the header and payload sections, yet is much higher than the power level of the superimposed preamble.

Fig. 7 provides an example of the symbol si, which contains a burst of pulses and a long silent period, i.e, empty period. Since the UWB signals are of our primary interest, there is some constraint placed on the bandwidth or the fractional bandwidth of the pulse. The pulse burst is designed according to some conventional codes, for example the code cl in Fig. 5. The pulse shape of the code cl is the root cosine waveform with bandwidth of 500MHz and time duration of 2ns. The pulse burst is constructed according to the ternary code with length 31, specifically, cl=[+ 0 + 0 0 0 - + - + + 0 0 + + 0 + 0 0 - 0 0 0 0 - 0 + 0 - -], where the signs "+" and "-" in the code cl are represented by the pulse with opposite polarity, and symbol "0" means no pulse

is transmitted during that time interval, as shown in Fig. 7

Fig. 8 provides an example of the symbol s2 with data modulation scheme 2PPM (Pulse Position Modulation) .

With 2PPM, there are two symbols with equal time duration L, which represent binary data "0" and "1". The total time duration L is divided into two time slots, say til and tl2. For the binary data "0", the time slot til is occupied by a pulse burst and a silent period, while the time slot tl2 is completely empty. In contrast, for the binary data "1", the time slot til is empty, and the time slot tl2 is occupied by a pulse burst and a silent period. The pulse bursts contained in the binary symbols "0" and "1" should be same. The pulse burst is designed according to some code, denoted by c2. One requirement of the code c2 is that it has to be orthogonal or quasi-orthogonal to the code cl which is adopted in symbol si. For example, given the code cl is the ternary code with length 31, the code c2 can be the Walsh code with code length 8, in particular, c2= [+ - - + - + + -]. Same as in code cl, the signs "+" and "-" in the code c2 are represented by the pulse with opposite polarity.

In what follows, we show the effectiveness of the new data packet by examining several numerical results.

First, the system parameters adopted in the numerical examples is described. The ternary code with code length 31, as shown in Fig. 7, is used as the code cl for modulating the pulse burst in the preamble symbol si. The Walsh code

with length 8, as shown in Fig. 8, is used as the code c2 for modulating the pulse burst in the data symbol s2 with the modulation scheme 2PPM of the header and payload section. The root cosine waveform with bandwidth of 500MHz and time duration of 2ns is taken as the basic pulse constructing both pulse bursts cl and c2. The time durations of the symbol si and s2 are lus and 0.5us, respectively. The preamble sequence consists of repetition of the symbol si, as shown in Fig. 6. The header and payload sections contain two components: one is a sequence of data symbols representing regular data information of header and payload; the other component is the preamble sequence of repetition of si. Three time durations of the preamble section, i.e., 500us, lms, 4ms, and the header and payload section of 256us are adopted.

The power level of the superimposed preamble sequence is much lower than that of regular preamble sequence and the data sequence in the header and payload section. About 7- 15dB reduction is examined in the following numerical results. The advantages of reducing the power level of the superimposed preamble are in three folds: first, the superimposed preamble will not cause interference to the data demodulation of the header and payload sections. Second, the resulting energy consumption is negligibly small. Third, although the energy level of the superimposed pulse is low, performance gain is guaranteed by exploiting the repetition the preamble symbol si. Therefore, in summary, the new data

packet structure can greatly improve the performance of the applications depending on the preamble sequence while maintaining the regular functions of a data packet.

Next, we present two numerical results regarding the new data packet.

Fig. 9 shows the ratio of the extra energy consumed by the superimposed preamble, compared with the total energy used by the whole data packet, versus the ratio between the pulse energy of the data information (pulse _da ta) and that of the superimposed preamble (pulse _pre- . _in -. _d ata) , which is denoted by P. The horizontal axis in Fig. 9 represents pulsedata/ pulsepre-m-data (=P) in DB; while the vertical axis represents the ratio of energy used in superimposed preamble. The plots in Fig. 9 illustrate three time durations of the preamble section (500us, lms, 4ms).

It is seen that the extra energy consumption is very low, less than 5% for P >8dB. Therefore, compared with the regular data packet without the superimposed preamble, only limited amount of increase in power is needed to transmit the new data packet.

Fig. 10 shows the improvement of ranging accuracy versus the energy ratio P with 4 ,8, 16 of the preamble symbol si used for ranging in the preamble section. The horizontal axis in Fig. 10 represents pulsβdata/pulsepre-m-ciata (=P) in DB; while the vertical axis represents accuracy improvement in ranging.

Ranging is to estimate the distance between a

transmitter and a receiver by processing the received data packet transmitted from the transmitter. Here the improvement of ranging accuracy is approximated by assuming a coherent ranging scheme. The relation that the variance of a ranging estimate is proportional to 1/R, where R is the signal-to-noise ratio, is adopted. Let the variance of a ranging estimate based on the preamble section be _(jχ , and that based on the superimposed preamble be _(j2 . It can be shown that the smallest variance of the ranging estimate by combining the above two estimates is Q- , which is smaller than either _(jχ or _(j2 . The improvement of

the ranging accuracy is given by It is observed from Fig. 10 that significant accuracy improvement, about 50%, can be obtained in most of cases examined. This is because a relatively small number of si's in the preamble section, e.g., 4, 8 or 16, can be utilized for ranging, compared with much larger number of si's in the superimposed preamble, e.g., over 256 si's with in the header and payload section of 256us.

Industrial Applicability

Compared with existing data packet, the new data packet according to this invention can achieve much better performance for some important applications depending on the preamble of their data packets, while maintaining most

functions of a regular data packet.