WO2008062163A2

WO2008062163A2 - Improved methods and apparatus for transmitting and receiving data

Info

Publication number: WO2008062163A2
Application number: PCT/GB2007/004400
Authority: WO
Inventors: Mustafa Kubilay Gurcan
Original assignee: Imperial Innovations Limited
Priority date: 2006-11-24
Filing date: 2007-11-16
Publication date: 2008-05-29
Also published as: WO2008062163A3; GB0623481D0

Abstract

A method of transmitting data, the method comprising: receiving a block of input data from a data source, the block comprising a plurality of digits of data; processing said block of input data to form a plurality of sub-blocks of data; distributing the plurality of sub-blocks of data to each of a corresponding plurality of channel coders to produce a plurality of channels of sub-block data; in each of the said channels: repeating each digit of sub-block data a plurality of times, spreading the repeated data by multiplying by a spreading code sequence, and modulating the data and introducing a phase angle; summing the data from each of the said channels to produce output data; and transmitting the output data.. The method is particularly applicable, but by no means limited, to Code Division Multiple Access (CDMA) systems. Also provided are methods of receiving data, and transmitter and receiver apparatus.

Description

IMPROVED METHODS AND APPARATUS FOR TRANSMITTING AND RECEIVING DATA

This invention relates to the transmission and reception of data. It is particularly applicable, but by no means limited, to Code Division Multiple Access (CDMA) systems.

Background to the Invention

There is a desire in communication systems to increase the capacity of the channels (links) comprising the system. Capacity increase should be taken to mean increasing the rate at which data can be transmitted for a given specified error probability, or conversely, reducing the error probability when operating at a specified data rate.

In Wideband Code Division Multiple Access (W-CDMA) systems, the usable data rate carried over each link depends on the signal-to-noise ratio (SNR) at the output of the single-user receiver. The SNR is determined by the transmission power and the level of noise received from other transmitters around the receiver, and also the distance between the transmitter and receiver. Contemporary approaches to W-CDMA data transmission usually allocate a fixed-length spreading code sequence to each user to provide the highest data rate for a given total transmission power for the W-CDMA system.

Recent attempts to increase capacity include the high-speed downlink packet access scheme which is described in Reference 1. The essential elements of this system are shown in Figures 1 and 2. At the transmitter (Figure 1) of the scheme described in Reference 1, binary data from the data sources 1 appears at 11, after which it is encoded using channel encoders 2 of a type well known in the art. Data from the channel coder is fed at 12 to a serial-to-parallel converter 3. In the serial-to-parallel converter unit 3, successive blocks of k binary digits are taken at 12 and are fed at 13 to an M-ary signal generator 4. The term "M-ary", as used herein, is well known in the art and refers to M- level signals for use in modulation, with M being the order of modulation, as those skilled in the art will appreciate. The M-ary signal generator 4 produces at its output 14 a signal which generally can take one of 2* different values. These signals may for example be voltage values. The signals appearing at 14 are then fed to a spreading code sequence unit 5 which operates in a manner that is well known to those skilled in the art of spread spectrum and CDMA systems. The signals appearing at 15 are then power amplified by power transmission control units 6. Finally, the N signals appearing at 16 are added together in an adder unit 7 prior to transmission over the communication channel. The signals are transmitted to the communication channel at 17. It will be appreciated that pass-band modulation and demodulation may be involved and Figures 1 and 2 represent the equivalent baseband schemes for such systems.

At the receiver (Figure 2) the signals received from the channel at 17 are fed to de-spreading units 8 which act in a manner that is well known to those skilled in the art of spread spectrum systems. These units have the effect of isolating the signals on the separate data channels, and at 28, M-ary signals corresponding to noise-corrupted versions of those at 14 are obtained. These signals are then fed at 28 to an M-ary-to-binary decoder decision device 9, and the block of k binary digits appearing at 29 are then fed to a channel decoder 10. After decoding in the channel decoding unit, data appearing at 30 corresponding to that appearing at the output 11 of the data source 1 are obtained. In the scheme described in Reference 1, the capacity of the N channels comprising the system is improved by adjusting the powers P_n i = l, , N; generated and applied by the power control units 6 at the transmitter to produce the signal at 16. The powers P₁ are adjusted in accordance with the well-known inverse channel signal-to-noise ratio power allocation method described in References 2 and 3, or its iterative version as described in Reference 4. The aim of these algorithms is to obtain a target signal-to-interference-noise ratio (SNIR) in order to meet a given error rate whilst maximising the rate at which data can be transmitted. Although some improvement in capacity is achieved, the degree of improvement is severely limited, due mainly to the fact that, as M-ary signalling is involved, a one bit per symbol epoch increase in data rate requires a doubling of the transmitted power.

Summary of the Invention

A first aspect of the present invention provides a method of transmitting data as defined in Claim 1 of the appended claims. By using parallel channels in this manner, better capacity may be achieved than with previously known M-ary signalling.

A second aspect of the invention provides a method of transmitting data as defined in Claim 17 of the appended claims.

A third aspect of the present invention provides a method of receiving data as defined in Claim 19 of the appended claims.

A fourth aspect of the present invention provides a method of de-correlating data as defined in Claim 23 of the appended claims.

A fifth aspect of the present invention provides a signal produced by a method in accordance with the first or second aspects of the invention. A sixth aspect of the present invention provides transmitter apparatus configured to implement a method in accordance with the first or second aspects of the invention.

A seventh aspect of the present invention provides receiver apparatus configured to implement a method in accordance with the third or fourth aspects of the invention.

An eighth aspect of the present invention provides a telecommunications system comprising a transmitter and a receiver in accordance with the sixth and seventh aspects of the invention respectively.

Preferable, optional, features are defined in the dependent claims.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:

Figure 1 illustrates the transmitter of a high-speed downlink packet access scheme known from the prior art (Reference 1); Figure 2 illustrates the receiver of a high-speed downlink packet access scheme known from the prior art (Reference 1);

Figure 3 illustrates the transmitter of a system according to an embodiment of the present invention;

Figure 4 illustrates a random phase inversion and modulation unit usable in the transmitter of Figure 3;

Figure 5 illustrates the receiver of a system according to an embodiment of the present invention, being operable with the transmitter of Figure 3;

Figure 6 illustrates a demodulator usable in the receiver of Figure 5; Figure 7 illustrates a phase correction unit usable in the receiver of Figure 5; and

Figure 8 illustrates a possible de-correlating scheme usable in the receiver of

Figure 5.

Detailed Description of Preferred Embodiments

The present embodiments represent the best ways known to the applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved.

An illustrative example of one embodiment of the invention is described herein with reference to Figures 3 to 8. The overall transmitter system is illustrated in

Figure 3 and the corresponding overall receiver is illustrated in Figure 5. The present embodiment significantly improves the communication capacity over schemes previously known in the art, such as that described above in connection with Reference 1.

The transmitter

At the transmitter of the system N data sources are considered, where N is an integer and may be 1. Each data source may correspond to a separate user. The operations performed on data from each source are similar and for purposes of illustration, consideration will be restricted to the method of operation as applied to data source 1.

At 51, binary data is taken from the source 41 in blocks. The blocks may contain L_* R digits. These digits are then fed to a serial-to-parallel converter 42 (e.g. a multiplexer) which divides the sequence of L» R digits into L separate sub-blocks each consisting of R digits. L is chosen to be slightly higher than the spreading factor of the spreading signal (discussed below in connection with the spreading unit 45). R is the total number of bits, L» R, divided by L, where the total number of bits is a fixed parameter determined by the system.

The first sub-block of R digits is fed at 52,1 to a channel encoder 43 of a type well known in the art. The second sub-block of R digits is fed at 52,2 to a second channel encoder which may be the same as 43. Likewise, the remaining sub-blocks, each of R digits, are fed to corresponding channel encoders. From the point of view of operation, each of the sub-channels 1, ...., L functions in the same way and hence, from hereon, consideration will be devoted to sub-channel 1.

After channel encoding, the binary digits are fed at 53 into a rate reduction unit

44, which repeats each of the binary digits appearing at 53 a given number of times, say Q-I, so the total number of digits corresponding to a single input digit at 53 is Q . The number Q may be the same as or different from L, with

P » L

Q being chosen so that it is slightly greater than ^desired , where P_deήred is the

"Total power level which provides an acceptable quality of transmission over the link, and P_total is the total power assigned to the user.

These signals which appear at 54 are then each fed to a spreading code sequence generator 45 where, in a manner well known to those familiar with spread spectrum systems, each repeated digit is multiplied by the same spreading sequence. It will be appreciated that the spreading code sequence differs for each of the L channels employed by the user, and also differs from user to user. The outputs from spreading code sequence generator which appear at 55, "the chips" as they are known in the art, are then fed to a random phase insertion and modulation unit 46. The random phase insertion and modulation unit 46 is shown in Figure 4 and now will be described. The chip signals at 55 corresponding to a given data bit appearing at the output from 54 are operated on by the unit shown in Figure 4. All the chip signals of the spreading sequence associated with a data bit are then each multiplied respectively in 80 and 81 by the cosine and the sine of a random phase angle θ_{j w} which is in the range 0 to 2π radians , where i = !, ■■■, L denotes the sub-channel number and w = l, --, Q denotes the number of each digit in the repeated sequence appearing at 54. The successively generated phase angles are statistically independent one from another. The outputs from 80 and 81, which appear at 90 and 91, are fed to multipliers 82 and 83 which act as double-sideband suppressed-carrier amplitude modulators that are well known in the art. The angular frequency ω_c is the carrier frequency at which it is desired to operate the system. The output from the modulators which appear at 92 and 93 are then added in 84 to produce an output at 56 which is fed to a power control unit 47 as shown in Figure 3. The power control unit 47 may be a power amplifier that subjects signals appearing at 56, corresponding to R data bits, to power amplification.

To arrive at the power level P₁ to be used at the transmitter, the following procedure may be adopted. The signal-to-noise-ratio SNIR is measured at 73 in the receiver and is reported back to the transmitter. The transmitter calculates the power to be used during the following frame by multiplying the target (desired) SNIR by the power used during the previous frame and by dividing the result by the measured SNIR. As those skilled in the art will appreciate, a "frame" is a group of digits which appear at the output of the rate reduction unit 44. This power control technique may be used in telecommunications systems other than CDMA, and need not necessarily be used in connection with the rate reduction, phase insertion, modulation and signal spreading techniques described above.

The signals of the L channels appearing at 57 are then added together in an adder 48, prior to transmission over the channel 58. The output from the other

N-I data sources are also added prior to transmission, or at a common processing receiver to the summed output from the first data source. The phases of each of the N-I other users would be generated in the same way as for the first user and it would be arranged for them to be statistically independent from one to another.

The receiver

Figure 5 shows an illustration of the receiver of the system, operable with the transmitter described above. At 58, signals are received from the channel and are fed to a demodulator 60, which is also shown in Figure 6, and which is of a type well known in the art. In the demodulator 60, carrier recovery is employed to obtain the carrier frequency ω_c and hence cosω_ct and smω_ct signals for use in demodulation. The carrier may be recovered using well- known techniques such as those involving the use of pilot signals. The signals received at 58 are multiplied in 100 and 101 by cosω_ct and smω_ct and signals at the output of these multipliers are then fed at 110 and 111 into low-pass filters 102 and 103. The outputs 112 and 113 are baseband signals that are then sampled in 104 and 105 at the chip rate for further processing in the phase correction unit.

The sampled signals that are obtained from 104 and 105 are fed at 71a and 71b respectively to the phase correction unit 61, which is also shown in Figure 7. In the phase correction unit 61 the signal received from 71a is multiplied in unit 120 by cos0_iiW and the signal received from 71b is multiplied in unit 121 by smθ_{t w} . The signals from 120 and 121 are then added in unit 122 to obtain the output at 72.

It will be appreciated that the corresponding values of θ_{i w} used in the phase insertion and modulation unit 46 at the transmitter and those values used in the phase correction unit 61 at the receiver will be identical. This can be achieved, for example, by storing the θ_{i w} values as a sequence of random numbers in a memory unit at the transmitter and storing the same sequence of random numbers in a memory unit at the receiver, with it being further arranged for corresponding successive selections from the two memory units to be the same.

The signals from the phase correction unit appearing at 72 are then fed to a bank of de-spreading units 62, with each of the dispreading units operating as an inverse of the spreading code sequence generator units 45 employed at the transmitter, in a manner that is well known to those skilled in the art of spread spectrum communications.

At the output 73 of the de-spreader 62, signals are fed to a so-called de- correlating unit 63. The signals appearing at 73 correspond to the repeated noise-corrupted versions of the signals appearing at 54 in the transmitter. The de-correlating unit 63 acts so as to weight each member of the noise corrupted set of signal samples by a pre-selected value (weight) and it then brings them into time alignment and sums them.

Figure 8 shows one possible embodiment of a de-correlating scheme employed by the de-correlating unit 63. For convenience, the weighting, or multiplying, factors CC₁, , a_Q , may be chosen to be unity. The signal-to-interference- noise ratio at the outputs 74 is the SNIR for the binary signalling associated with each sub-channel. For a given error rate the required signal-to- interference-noise ratio is significantly less than that required in Reference 1. For M-ary signalling of a type known in the art (Reference 1) to operate at the same bit error rate a higher SNR would be required.

At the output 74 of the de-correlating unit 63, signals are fed to a decision making detector 64, which may be a standard binary detector. The output 75 from the decision-making detector 64 is then fed to a channel decoder 65, which provides an output signal 76. The channel decoder 65 is the decoding unit for a specific encoder unit 43. The detected signal at 75 comprises the data containing errors, but which is at a higher data rate than the decoded data appearing at 16. The output data at 76 is at the original data rate from the source at 52,1 but it is comprised of data that has been error corrected.

It will be appreciated that each user divides his total power allocation between the L sub channels, and therefore the signal-to-interference ratio (SNIR) will be significantly reduced over that which would be obtained if the user had devoted all of the allocated power to a single channel.

However, the SNIR reduction per channel mentioned above is offset by the use of the phase insertion and phase correction. The effect of the combined use of the phase insertion unit 46 and the phase correction unit 61 is such that the signals corresponding to a data digit and its repeats are added in 63 in a coherent (voltage) manner, whilst the interference from a user's other channels

2, , L and from other data sources (2, ...., N) are added on an incoherent

(power) basis within channel 1, and likewise for channels 2,...., L, and for other users. It will be appreciated that the binary data rate for each sub-channel is — bit per

input binary digit, assuming that the channel encoder is a rate 1 encoder. It will be further appreciated, however, that there are L sub-channels for each user

and the effective data rate for each user is therefore— binary digits per input

epoch. If overall the same total power is used as in the M-ary system described

earlier by way of reference 1, then the corresponding data rate — can be made

to be greater than data rate k of the M-ary scheme.

Evaluation of the M-ary system of Reference 1 and an embodiment of the present invention have been carried out, the results of which are shown in Table 1. Theoretical bit error rates based on SNIR values were used in these evaluations. From Table 1 it is clear that a significant improvement in capacity can be obtained with the present embodiment.

Table 1

Thus, the invention described and illustrated by way of examples enables an improvement in capacity to be obtained. Using parallel binary channels in the manner described provides better capacity than with previously known M-ary signalling. Rate reduction per channel, when combined with the use of parallel channels, makes it possible to obtain an increase in total capacity. Applications

The techniques and embodiments described above are suitable for the transmission of data in a mobile network, e.g. in a 3G CDMA network. It should be noted, however, that their application is not limited to CDMA, and could, for example, be used in encryption devices or modulators for non- CDMA applications.

Moreover, although the present examples refer to the transmission of parallel channels of binary data, it should be noted that the principles described herein may also be used to transmit parallel channels of data other than binary - for example, M-ary data.

Technical construction

The "units" in the transmitter, such as the channel encoder 43, the rate reduction unit 44, the random phase insertion and modulation unit 46, the spreading unit 45, the power control unit 47 and the adder 48, may be provided as separate pieces of equipment or discrete components or circuits that are communicatively connected in order to enable the signal processing methods described herein to be performed. Alternatively, two or more of the "units" may be integrated into a single piece of equipment, or provided as a single component or circuit. In further alternatives, one or more of the "units" may be provided by a computer processor programmed to provide equivalent functionality.

Similarly, the "units" in the receiver, such as the demodulator 60, the phase correction unit 61, the de-spreading unit 62, the de-correlating unit 62, the decision making detector 64, and the channel decoder 65 may be provided as separate pieces of equipment or discrete components or circuits that are communicatively connected in order to enable the signal processing methods to be performed. Alternatively, two or more of the "units" may be integrated in a single piece of equipment, or provided as a single component or circuit. In further alternatives, one or more of the "units" may be provided by a computer processor programmed to provide equivalent functionality.

In some instances, the sequence of the units in the transmitter or the receiver may be changed, as those skilled in the art will appreciate.

Summary A method has been described for improving the capacity of communication channels. A user's incoming block of data is taken and is divided into L sub- blocks which are transmitted in parallel. For each parallel sub-block of data, a different spreading code sequence is allocated together with a fraction of the available total power. This results in a reduced SNIR at the output of the receiver for each of the L spreading code sequences compared with the SNIR at the output of the single spreading code receiver. Binary digit repetition and random phase insertion is introduced for each binary digit at the transmitter for each of the L sub-channels to reduce the effective data rate for each sub-channel with overall rate recovery being obtained by the use of L parallel sub-channels.

At the receiver a digit combining method is used which makes use of random phase insertion at the transmitter, and phase correction at the receiver, to improve the SNIR for each of the L receivers to a level that is acceptable.

The corresponding transmitter and receiver operations in the invention provide an increase in the capacity (e.g. by a factor of approximately 3 to 6), compared with the capacity that can be obtained when a user is allocated a single spreading code sequence for the same amount of transmission power. References

1. 3GPP, "High speed downlink packet access" , 3GPP TS 25.308 version

7.0.0, Release 7 http://webapp.etsi.org/action/PU/20060704/ts_125308v070000p.pdf 2. A. Goldsmith, "Wireless Communications", Cambridge University

Press 2005, pp 466-469. 3. A. Goldsmith, "The capacity of downlink fading channels with variable rate and power", IEEE Transactions on Vehicular Technology, 46:569-

580, 1997. 4. S. Ulukus and R.D. Yates, "Adaptive Power control and MMSE

Interference Suppression", ACM Wireless Networks, Vol. 4, pp. 489-

496, 1998.

Claims

1. A method of transmitting data, the method comprising: receiving a block of input data from a data source, the block comprising a plurality of digits of data; processing said block of input data to form a plurality of sub- blocks of data; distributing the plurality of sub-blocks of data to each of a corresponding plurality of channel coders to produce a plurality of channels of sub-block data; in each of the said channels: repeating each digit of sub-block data a plurality of times, spreading the repeated data by multiplying by a spreading code sequence, and modulating the data and introducing a phase angle; summing the data from each of the said channels to produce output data; and transmitting the output data.

2. A method as claimed in Claim 1, wherein successively generated phase angles are statistically independent from one another.

3. A method as claimed in Claim 1 or Claim 2, wherein the phase angle is randomly generated.

4. A method as claimed in Claim 1 or Claim 2, wherein the phase angle is determined according to a predefined rule.

5. A method as claimed in any preceding claim, wherein the phase angle is in the range 0 to 2π radians.

6. A method as claimed in any preceding claim, wherein the step of processing said block of input data to form a plurality of sub-blocks of data is performed by a serial-to-parallel converter.

7. A method as claimed in Claim 6, wherein the serial-to-parallel converter comprises a multiplexer.

8. A method as claimed in any preceding claim, wherein the data source is a single data source.

9. A method as claimed in any of Claims 1 to 7, wherein the data source is one of a plurality of data sources.

10. A method as claimed in Claim 9, wherein each of the plurality of data sources is a separate user.

11. A method as claimed in Claim 9 or Claim 10, wherein the output data from each of the data sources are summed prior to transmission.

12. A method as claimed in any of Claims 9 to 11, wherein the phase angles of the data from the different data sources are statistically independent from one another.

13. A method as claimed in any preceding claim, wherein the number of data repeats varies from block to block, or from sub-block to sub-block, for a given data source or user.

14. A method as claimed in any preceding claim, wherein the number of data repeats per block or sub-block varies from data source to data source or from user to user.

15. A method as claimed in any preceding claim, further comprising, in each channel, the steps of: processing the data to form a plurality of frames; measuring at a receiver a first signal-to-noise ratio of a first frame transmitted at a first transmit power; and transmitting a subsequent frame at a second transmit power, the second transmit power being determined by a function of a target signal- to-noise ratio, the first transmit power, and the first signal-to-noise ratio.

16. A method as claimed in Claim 15, wherein the second transmit power is determined by multiplying the target signal-to-noise ratio by the first transmit power and dividing by the first signal-to-noise ratio.

17. A method of transmitting data, the method comprising: processing the data to form a plurality of frames; transmitting a first frame at a first transmit power; measuring at a receiver a first signal-to-noise ratio of the first frame; and transmitting a second frame at a second transmit power, the second transmit power being determined by a function of a target signal- to-noise ratio, the first transmit power, and the first signal-to-noise ratio.

18. A method as claimed in Claim 17, wherein the second transmit power is determined by multiplying the target signal-to-noise ratio by the first transmit power and dividing by the first signal-to-noise ratio.

19. A method of receiving data, the data comprising spread data at various phase angles, the method comprising: demodulating the data; processing the data to produce a plurality of sub-channels of data; and in each of the said sub-channels: bringing the data into phase, de-spreading the data, and de-correlating the data.

20. A method as claimed in Claim 19, wherein de-correlating the data further comprises weighting the data.

21. A method as claimed in Claim 20, wherein weighting the data comprises multiplying the data by a predetermined value.

22. A method as claimed in any of Claims 19 to 21, wherein the received data comprises data transmitted according to a method as claimed in any of Claims 1 to 17.

23. A method of de-correlating data, the method comprising the steps of: bringing the data into time alignment; and summing the data.

24. A method as claimed in Claim 23, further comprising the step of weighting the data, the step of weighting the data being performed either before bringing the data into time alignment or before summing the data.

25. A method as claimed in Claim 24, wherein the step of weighting the data comprises multiplying the data by a predetermined value.

26. A method as claimed in any preceding claim, performed in a code division multiple access telecommunication system.

27. A method as claimed in any preceding claim, wherein the data is binary.

28. A signal produced by a method as claimed in any of Claims 1 to 18, or as claimed in Claim 26 or Claim 27 when dependent on any of Claims 1 to 18.

29. Transmitter apparatus configured to implement a method as claimed in any of Claims 1 to 18, or as claimed in Claim 26 or Claim 27 when dependent on any of Claims 1 to 18.

30. Receiver apparatus configured to implement a method as claimed in any of Claims 19 to 25, or as claimed in Claim 26 or Claim 27 when dependent on any of Claims 19 to 25.

31. A telecommunications system comprising a transmitter as claimed in Claim 29 and a receiver as claimed in Claim 30.

32. A method substantially as herein described with reference to and as illustrated in any combination of Figures 3 to 8 of the accompanying drawings.

33. Apparatus substantially as herein described with reference to and as illustrated in any combination of Figures 3 to 8 of the accompanying drawings.