SPREAD SPECTRUM TRANSMISSION SYSTEM WITH REDUCTION OF INTERSYMBOL INTERFERENCE

Abstract: In a spread spectrum communication system a transmitter uses a spreading codeword to obtain spreaded digital symbols from source symbols. These spreaded digital symbols are transmitted to a receiver which uses de-spreading means to obtain the originally transmitted source symbols. A problem in spread spectrum transmission system is that due to multipath transmission so-called intersymbol interference can occur. This intersymbol interference causes an increased bit error rate. In the transmission system according to the present invention it is proposed to use a de-spreading codeword which is shorter than the spreading codeword, so no overlap of signals originated from different symbols with the despreading codeword occurs. Preferably, the spreading codeword comprises a concatenation of the despreading codeword and a preamble. This preamble can be a cyclically permutated part of the despreading codeword.

Description: SPREAD SPECTRUM TRANSMISSION SYSTEM WITH REDUCTION OF INTERSYMBOL INTERFERENCE The present invention relates to a spread spectrum transmission system comprising a transmitter with spreading means for deriving a spreaded digital signal from a digital source signal according to a spreading code word, the transmitter comprises transmit means for transmitting means for transmitting the spreaded digital signal to a receiver via a transmission medium, the transmission system comprises a receiver with receive means for receiving the spreaded digital signal from the transmission medium, the receiver further comprises de-spreading means for deriving a replica of the digital input signal from the spreaded digital input signal. The present invention also relates to a transmitter, a receiver, spreading unit, a despreading unit, a communication method, a transmission method and a receiving method. A transmission system according to the preamble is known from US patent No. 5,970, 588. In recent years, the use of spread spectrum systems in mobile telephony has increased rapidly. In a spread spectrum transmission system a digital input signal of a given symbol rate is multiplied with a codeword from a spreading code to obtain a spreaded digital signal having a substantially higher symbol rate. The spreaded digital signal is modulated on a carrier and transmitted to a receiver which demodulates its input signal to obtain the spreaded digital signal. A replica of the source signal is obtained by de-spreading the spreaded digital signal by multiplying it with a de-spreading code word. In mobile telephone systems different spreading code words are used for different subscriber stations. The spreading code words used by different subscriber stations are chosen to be orthogonal to avoid interference between the subscriber stations. It is observed that in mobile telephony systems spread spectrum communication can be used for the uplink (from mobile terminal to base station) and for the downlink (from base station to mobile terminal). A problem with spread spectrum systems in mobile telephony is the occurrence of intersymbol interference due to multipath transmission. Multipath transmission occurs when the radio signal is received via a direct path and one or more indirect path which involve reflection. The arrival time of a radio signal received via the direct path differs from the arrival time of a radio signal received via an indirect path. Due to this time difference, the signal correlated with the de-spreading codeword contains not only a contribution of the present symbol to be detected, but also contributions of preceding and/or subsequent symbols. This intersymbol interference can lead to an increased error rate. The object of the present invention is to provide a spread spectrum transmission system according to the preamble, in which the intersymbol interference due to multipath transmission is decreased. To achieve said objective, the spread spectrum transmission system according to the invention is characterized in that the de-spreading means comprise correlation means for correlating the spreaded digital signal with a de-spreading codeword which is shorter than the spreading codeword. By using de-spreading codewords shorter than the spreading codewords for detecting the symbols, the overlap between the de-spreading codeword and the contributions of previous and/or subsequent codewords will decrease. This results in a decreased amount of intersymbol interference. An embodiment of the invention is characterized in that the spreading codeword comprises the de-spreading codeword and a plurality of added symbols. A suitable choice for the spreading codewords is to use the de-spreading codeword with a number of added symbols. These added symbols reduce the above mentioned overlap and still maintains a spreaded digital signal that can be detected easily. A further embodiment of the invention is characterized in that the added symbols comprise a part of the de-spreading codeword. Using a part of the de-spreading code results in the spreading code results into an decreased sensitivity for intersymbol interference. The present invention will now be explained with reference to the drawings. Fig. 1 shows a block diagram of a transmission system according to the invention in which a rake receiver is used. Fig. 2 shows a spreaded signal comprising three multipath components in a prior art transmission system. Fig. 3 shows a spreaded signal comprising three multipath components according to the present invention. Fig. 4 shows a graph of the bit error rate as function of the energy per bit for a prior art transmission system and a transmission system according to the present invention. In the spread spectrum transmission system according to Fig. 1 , a plurality of signals, each being intended for a different receiver, are applied to a corresponding encoder 4 • • ■ • 12 in a transmitter 2. Normally, the encoders 4 • • • • 12 comprise a combination of a source encoder for compressing the signals to be transmitted and a channel encoder for enabling transmission errors to be detected and corrected.. The source encoders can comprise a speech encoder, an audio encoder and/or a video encoder. The channel encoder can e.g. be convolutional encoders or block encoders. The output symbols of the encoder 12 are applied to a first input of the spreading means, which are a multiplier 14. The multiplier 14 multiplies each output symbol of the encoder 12 with a spreading code word in order to obtain the spreaded digital signal having a substantially higher symbol rate that the output signal of the encoder 12. Signals intended for other receivers are also encoded, and spreaded by spreading means according to a spreading codeword. However the spreading codewords used for different receivers are substantially orthogonal, in order to enable the corresponding receivers to extract the proper signal from its input signal. The outputs of the multipliers 6 • • • • 14 are applied to an adder 10 which combines the different spreaded digital signals into one signal. An output of the adder 10 is connected to an input of a modulator 18 that modulates the combined spreaded digital signals on a carrier with a suitable frequency. The output of the modulator 18 is connected to an antenna 20 for transmitting the modulated signal to receivers 22, 24 and 28. The signal transmitted by the antenna 20 can reach the receivers 22, 24 and 26 via a direct path and via one or more indirect paths due to reflections against objects such as buldings, brigdges and mountains. The presence of this multi-path transmission results into intersymbol interference. In the receiver 28, a signal provided by an antenna 26 is applied to a demodulator 30. The demodulator 30 demodulates the received signal and derives a replica of the combined spreaded digital signals. Said replica of the combined spreaded digital signal is applied to a cascade connection of a plurality of delay elements 32, 34 and 36. The output signal of the demodulator 30 is also applied to a first input signal of a first branch 38 , also called finger, of a rake receiver 41. An output of the first delay element 32 is connected to a second finger 40 of the rake receiver 41. An output of the delay element 34 is connected to a third finger of the rake receiver 41 and the output of the delay element 36 is connected to an input of a fourth finger of the rake receiver 41. In each of the fingers 38, 40, 42 and 44 of the rake receiver, the input signal is multiplied with a de-spreading codeword corresponding to the spreading codeword associated to the receiver 28. According to the present invention the length of the de-spreading codeword is smaller than the length of the spreading codeword, in order to reduce the intersymbol interference when multipath transmission occurs. According to the present invention, the spreading codeword can comprise the de-spreading codeword with a preamble attached to it. This preamble can be a fixed preamble, but preferably this preamble is a cyclic continuation of the spreading codeword. This means that when a preamble of N symbols is needed, the preamble comprises the N last symbols of the spreading codeword in reversed order. Each of the fingers 38, 40, 42 and 44 of the rake receiver is arranged for receive one of the multipath components of the received signal. Therefore it receives from a synchronization and channel estimation unit 49 a de-spreading sequence which is delayed over an appropriate time. Furthermore, the signal in each finger of the rake receiver 41 is multiplied with a value which corresponds to its strength. This is done to enable so-called maximum ratio combining of the output signals of the fingers 38, 40, 42 and 44 by means of an adder 48. It is well known that by use of maximum ratio combining an optimum performance of a rake receiver is obtained. The construction of the synchronization and channel estimation unit 49 is well known to those skilled in the art. It is observed that the de- spreading codeword to be used in the unit is the same as is applied to the fingers 38, 40, 42 and 44. The output of the adder 48 is applied to an input of a decoder 50. The decoder 50 first performs channel decoding and performs subsequently the source decoding, in order to obtain a replica of the signal applied to the input of the transmitter 2. In Fig. 2, three components 52, 54 and 56 of the input signal of the receiver 28 are shown in case a spreading code according to the prior art is used. The present spreading code can e.g. be a maximum length LSFR code which is obtained by using a shiftregister with a suitable feedback network. These components 52, 54 and 56 are received via different transmission paths having different signal delay values. In the prior art rake receiver, the received signal is multiplied by the de-spreading codeword in three of the four fingers of the rake receiver 41. In this case the de-spreading codeword is equal to the spreading codeword "+1,+1,+1,-1,+1,-1-1". The alignment of the de-spreading codeword in the first finger 38 corresponds to the non-hatched part of component 52. The alignment of the de-spreading codeword in the second finger 40 corresponds to the non-hatched part of component 54 and the alignment of the de-spreading codeword in the third finger 42 corresponds to the non-hatched part of component 56. For the signal at the output of the i th finger F, can be written: F 0 = (7 - h 0 +h 2 )-b 0 - (h^h^-l In (1) h, is the strength of the signal component received in the i" 1 finger of the rake receiver, and b k corresponds to the symbol values represented by the input signal. b 0 is the current symbol value and b.j and bi are the previous and the next symbol values. From (1) it can be seen that in the output signal of all the three fingers a contribution of other symbols (bj, b.j) different from the present symbol bo is present. This (undesired) contribution is called intersymbol interference. This intersymbol interference results in an increased bit error rate. In Fig. 3 three components 58, 60 and 62 of the input signal of the receiver 28 are shown in case a spreading code according to the present invention is used. The spreading codeword according to the present invention can comprise a maximum length LSFR code with a preamble attached to it. In the example according to fig. 3, the spreading codeword is equal to "-1,-1,+1,+1,+1- 1,+ 1,-1,-1" comprising the basic codeword "+1,+1, +1,-1, +1,-1,-1" with the preamble "-1,-1" This preamble comprises the final part of the basic codeword in a reversed order. The components 58, 60 and 62 are received via different transmission paths having different signal delay values. In the rake receiver according to the present invention, the received signal is multiplied by the de-spreading codeword in three of the four fingers of the rake receiver 41. In this case the de-spreading codeword corresponds to the basic codeword "+1,+1, +1,-1, +1,-1-1"". The alignment of the de-spreading codeword in the first finger 38 corresponds to the last seven symbols in the non hatched part of component 58. The alignment of the de- spreading codeword in the second finger 40 corresponds to the last seven symbols in the non- hatched part of component 60 and the alignment of the de-spreading codeword in the third finger 42 corresponds to the last seven symbols in the non-hatched part of component 62. For the signal at the output of the i* finger F, can be written: F 1 = (7 -h 1 -h 0 -h 2 )-b 0 ( 2 ) F 1 = (7 -h 2 -h 0 -h 1 )-bo From (2) it can be seen that the output signals F 0 , Fj and F 2 do not comprise any intersymbol interference anymore. This results in an decreased symbol error rate at high signal to noise ratios. Fig. 4 shows the bit-error rate as a function of the signal to noise ratio defined by the ratio of the energy per bit E b and the spectral density of the noise. These function is calculated for a basic spreading sequence with a length of 15 symbols. It is assumed that the delay profile E[h-h * ] is represented by 1, 0.5, 0.25. Graph 64 shows the bit error rate for a narrow band system in which no spreading is used. Graph 66 shows the bit error rate for a system according to the prior art. From Fig. 3 it can be seen that the spread spectrum system according to the prior art shows a better performance for all signal to noise ratios. However it can be seen that the bit arroe rate in the spread spe4ctrum system according to the prior art has a bit error floor around 5-10 * . Graph 68 shows the bit error rate for a transmission system according to the present invention. From graph 68 it can be seen that for high signal to noise ratios, the bit error rate in the system according to the present invention is substantially lower for signal to noise ratios above 15 dB, and slightly higher for signal to noise ratios above 15 dB. It is conceivable that the use of a prefix according to the invention is only activated when the signal to noise ratio is above a given threshold value.

Claims: CLAIMS: 1. Spread spectrum transmission system comprising a transmitter with spreading means for deriving a spreaded digital signal from a digital source signal according to a spreading code word, the transmitter comprises transmit means for transmitting means for transmitting the spreaded digital signal to a receiver via a transmission medium, the transmission system comprises a receiver with receive means for receiving the spreaded digital signal from the transmission medium, the receiver further comprises de-spreading means for deriving a replica of the digital input signal from the spreaded digital signal, characterized in that the de-spreading means are arranged means for despreading the spreaded digital signal by using a de-spreading codeword which is shorter than the spreading codeword. 2. Transmission system according to claim 1, characterized in that the spreading codeword comprises the de-spreading codeword and a plurality of preamble symbols. 3. Transmission system according to claim 2, characterized in that the preamble of the spreading codeword comprises a part of the de-spreading codeword. 4. Transmitter with spreading means for deriving a spreaded digital signal from a digital source signal according to a spreading code word, the transmitter comprises transmitting means for transmitting the spreaded digital signal, characterized in that the spreading codeword comprises a basic spreading codeword and a plurality of preamble symbols. 5. Transmitter according to claim 4, characterized in that the preamble of the spreading codeword comprises a part of the basic spreading codeword. 6. Receiver with receive means for receiving a spreaded digital source signal, being spreaded according to a spreading codeword, the receiver further comprises de- spreading means for deriving the digital source input signal from the spreaded digital signal, characterized in that the de-spreading means are arranged means for despreading the spreaded digital signal by using a de-spreading codeword which is shorter than the spreading codeword. 7. Receiver according to claim 6, characterized in that the spreading codeword comprises the de-spreading codeword and a plurality of preamble symbols. 8. Receiver according to claim 7, characterized in that the preamble of the spreading codeword comprises a part of the de-spreading codeword. 9. Spreading means for deriving a spreaded digital signal from a digital source signal according to a spreading code word, characterized in that the spreading codeword comprises a basic spreading codeword and a plurality of preamble symbols. 10. De-spreading means for deriving a digital source input signal from a spreaded digital signal being spreaded according to a spreading codeword, characterized in that the de- spreading means are arranged for despreading the spreaded digital signal by using a de- spreading codeword which is shorter than the spreading codeword. 11. Communication method comprising a deriving a spreaded digital signal from a digital source signal according to a spreading code word, transmitting the spreaded digital signal via a transmission medium, the transmission method further comprises receiving the spreaded digital signal from the transmission medium, deriving a replica of the digital input signal from the spreaded digital signal, characterized in that the communication method comprises despreading the spreaded digital signal by using a de-spreading codeword which is shorter than the spreading codeword. 12. Transmission method comprising deriving a spreaded digital signal from a digital source signal according to a spreading code word, transmitting the spreaded digital signal, characterized in that the spreading codeword comprises a basic spreading codeword and a plurality of preamble symbols. 13. Receiving method comprising receiving a spreaded digital source signal, being spreaded according to a spreading codeword, de-spreading means for deriving the digital source input signal from the spreaded digital signal, characterized in that the de-spreading means are arranged means for despreading the spreaded digital signal by using a de-spreading codeword which is shorter than the spreading codeword.

Inventors: LINNARTZ JOHAN P M G, LINNARTZ, JOHAN, P., M., G.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H04B1
IPC codes: H04B1, H04Q7