GB2537149A

GB2537149A - Method and apparatus for wireless communication in a multiuser environment

Info

Publication number: GB2537149A
Application number: GB1506036.1A
Authority: GB
Inventors: Wen Lei; Razavi Razieh; Xiao Pei; Ali Imran Muhammad; Tafazolli Rahim
Original assignee: University of Surrey
Current assignee: University of Surrey
Priority date: 2015-04-09
Filing date: 2015-04-09
Publication date: 2016-10-12
Also published as: GB201506036D0

Abstract

The disclosed system applies low density spreading using sparse signature patterns of mainly zeros to input data. The spread chips are applied to sub-carriers of a multicarrier system. The sub-carriers are modulated on to the channel using filter bank multicarrier (FBMC), possibly using IOTA (Isotropic Orthogonal Transform Algorithm) pulse shaping. The system can generate LDS and then apply selection criterion in order to apply only suitable signatures. The selection parameters may include a minimum cycle length and a maximum degree in a factor graph.

Description

Method and Apparatus for Wireless Communication in a Multiuser Environment

Technical Field

The present invention relates to wireless communication in a multiuser environment. In particular, the present invention relates to the use of low density signatures.

Background of the Invention

Orthogonal Frequency Division Multiplexing (OFDM) has been the dominant /0 technology for broadband multi-carrier communications. However, the performance of conventional multiple-access OFDM-based systems degrades dramatically under overloaded conditions, even when sophisticated multi-user detection (MUD) is adopted.

rg To address this problem, low-density signature (LDS) techniques have been previously developed to enable OFDM and Code Division Multiple Access (CDMA) systems to operate in overloaded multiuser environments with performance that is close to a single user bound. However, further improvement would be desirable in order to increase the system capacity, by enabling more users to send and receive transmissions simultaneously.

The invention is made in this context.

Summary of the Invention

or According to a first aspect of the present invention, there is provided a method of transmitting a signal in a multiuser environment, the method comprising: applying a Low Density Signature (LDS) to a modulated data stream, to obtain an LDS modulated data stream; generating in-phase and quadrature signal components from the LDS modulated data stream; obtaining an LDS Filter Bank Multi-Carrier (FBMC) signal by passing the in-phase and quadrature signal components through a filter bank configured to apply pulse shaping; and transmitting the LDS-FBMC signal. The method may be used in an overloaded multiuser environment in which the number of users k is greater than the number of subcarriers N. In some embodiments, Isotropic Orthogonal Transform Algorithm (IOTA) pulse shaping may be applied by the FBMC aa nr filter bank.

The low density signature can be configured such that in a factor graph representation in which each factor node represents a received chip and each variable node represents a user, different factor nodes and/or different variable nodes have different degrees.

The method can further comprise determining the low density signature to be applied to the modulated data stream by: randomly generating a first vector and first permutation pattern for defining an LDS spreading matrix; determining one or more parameters of a factor graph representing the LDS spreading matrix; in response to the one or more parameters satisfying a selection criterion, applying the low density signature by using the first vector and first permutation pattern; and in response to the one or more parameters failing the selection criterion, repeatedly randomly generating a second vector and/or second permutation pattern until an LDS spreading matrix is obtained for which the one or more parameters satisfy the selection criterion, and applying the low density signature by using the obtained second vector and/or second permutation pattern.

The one or more parameters can include a minimum cycle length of the factor graph representing the LDS spreading matrix, and the selection criterion maybe satisfied if the minimum cycle length is higher than a threshold minimum cycle length.

The one or more parameters can include the degree of each node in the factor graph representing the LDS spreading matrix, and the selection criterion may be satisfied if the degree of each node is less than a threshold maximum degree.

-0or In some embodiments, the length vk of the first or second vector to be used in applying the low density signature is less than the total number of subcarriers, N, and applying the low density signature comprises: zero-padding said first or second vector by adding N-vk zeroes; and applying the low density signature to the modulated data stream by applying the first or second permutation pattern to the zero-padded vector.

According to a second aspect of the present invention, there is provided a method of estimating data symbols for a I& user from a plurality of received subcarrier signals in a multiuser environment, the method comprising: passing each of the received subcarrier signals through a Filter Bank Multi-Carrier FBMC filter bank configured to apply pulse shaping; detecting chips from each of the received subcarrier signals; obtaining a Low Density Signature LDS spreading matrix for the kill user; and applying a message -3 -passing algorithm to estimate the data symbols transmitted by the 0, user, based on the detected chips and a factor graph representing the LDS spreading matrix for the kill user. In some embodiments, IOTA pulse shaping may be applied by the FBMC filter bank.

A computer-readable storage medium can be arranged to store computer program instructions which, when executed by one or more processors, perform any of the methods disclosed herein.

According to a third aspect of the present invention, there is provided apparatus for transmitting a signal in a multiuser environment, the apparatus comprising: a Low Density Signature LDS unit configured to apply a low density signature to a modulated data stream and output an LDS modulated data stream; means for generating in-phase and quadrature signal components from the LDS modulated data stream; a filter bank configured to apply pulse shaping to the in-phase and quadrature signal components and output an LDS-FBMC signal; and an output configured to send the LDS-FBMC signal to an antenna for transmission. in some embodiments, the filter bank can be configured to apply IOTA pulse shaping.

The apparatus can further comprise: an LDS generator configured to randomly generate a first vector and first permutation pattern for defining an LDS spreading matrix; and a factor graph analyser configured to obtain one or more parameters of a factor graph representing the LDS spreading matrix, and to determine whether the one or more parameters satisfy a selection criterion, wherein in response to the one or more parameters satisfying the selection criterion, the factor graph analyser is configured to control the LDS unit to apply the low density signature by using the first vector and first permutation pattern, and wherein in response to the one or more parameters failing the selection criterion, the factor graph analyser is configured to control the LDS generator to repeatedly randomly generate a second vector and/or second permutation pattern until an LDS spreading matrix is obtained for which the one or more parameters satisfy the selection criterion, and to control the LDS unit to apply the low density signature by using the obtained second vector and/or second permutation pattern. -4 -

The one or more parameters can include a minimum cycle length of the factor graph representing the LDS spreading matrix, and the factor graph analyser can be configured to determine that the selection criterion is satisfied if the minimum cycle length is 5 higher than a threshold minimum cycle length.

The one or more parameters can include the degree of each node in the factor graph representing the LDS spreading matrix, and the factor graph analyser can be configured to determine that the selection criterion is satisfied if the degree of each node is less than a threshold maximum degree.

The LDS generator can be configured to generate the first and/or second vector with a length Vk less than the total number of subcarriers, N, and the LDS unit can be configured to apply the low density signature to the modulated data stream by zero-padding said first or second vector by adding N-vk zeroes, and applying the first or second permutation pattern to the zero-padded vector.

According to a fourth aspect of the present invention, there is provided apparatus for estimating data symbols for a kth user from a plurality of received subcarrier signals in a multiuser environment, the apparatus comprising: a FBMC filter bank configured to apply pulse shaping to each of the received subcarrier signals; a detection arrangement configured to detect chips from each of the received subcarrier signals; a Low Density Signature LDS unit configured to obtain a LDS spreading matrix for the kth user; and a data symbol estimator configured to estimate the data symbols for the kth user from the detected chips by applying a message passing algorithm to estimate the data symbols based on a factor graph representing the LDS spreading matrix for the kth user. In some embodiments, the filter bank can be configured to apply IOTA pulse shaping.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates apparatus for transmitting data across a plurality of subcarriers, according to an embodiment of the present invention; Figure 2 schematically illustrates the sequence of operations carried out in the LDS unit 35 of Fig. 1, according to an embodiment of the present invention; -5 -Figure 3 schematically illustrates apparatus for generating low density signatures, according to an embodiment of the present invention; Figure 4 schematically illustrates the FBMC-IOTA unit of Fig. 1, according to an embodiment of the present invention; Figure 5 schematically illustrates a factor graph representing a low density signature spreading matrix, according to an embodiment of the present invention; Figure 6 schematically illustrates apparatus for receiving data from a plurality of users across a plurality of subcarriers, according to an embodiment of the present invention; Figure 7 is a graph showing relative performance of various signal transmission schemes according to embodiments of the present invention, in comparison to conventional LDS-OFDM; Figure 8 is a flowchart showing a method of generating and transmitting an LDS-IOTA signal, according to an embodiment of the present invention; Figure 9 is a flowchart showing a method of determining an LDS spreading matrix to be 15 applied to a modulated data stream, according to an embodiment of the present invention; and Figure ro is a flowchart showing a method of estimating data symbols for a kiln user from a received signal in a multiuser environment, according to an embodiment of the present invention.

Detailed Description

Embodiments of the invention provide methods and apparatus for transmitting and receiving multicarrier signals over a wireless communication channel in a multiuser environment. Referring now to Figs. 1 to 3, apparatus for transmitting data across a plurality of subcarriers is schematically illustrated, according to an embodiment of the present invention. In mobile telecommunications embodiments the apparatus of Figs. to 3 may be included in a User Equipment (15E) device, for example a mobile phone handset, tablet computer or wearable electronic device. However, embodiments of the present invention are not limited to use in telecommunications networks, and may be 30 used in any application where wireless communication with multiple users is required.

As shown in Fig. 1, the apparatus is configured to generate a number N of subcarriers s, to sn and transmit the subcarriers over a wireless communication channel, with each subcarrier being transmitted at a different frequency. For each of the N subcarriers, the apparatus comprises a corresponding LDS unit 102, Filter Bank Multicarrier (FBMC) unit 103, and antenna 104. In the present embodiment the FBMC unit 103 is -6 -configured to apply Isotropic Orthogonal Transform Algorithm (IOTA) pulse shaping, and will hereinafter be referred to as a FBMC-IOTA unit 103. However, in other embodiments the FBMC unit 103 can be configured to use a different type of pulse shaping other than IOTA, including but not limited to half cosine, Root Raised Cosine (RRC), Extended Gaussian Functions (EGF), or physical layer for dynamic access (PHYDAS).

The apparatus further comprises an LDS control unit 105 configured to provide each LDS unit 102 with a low density signature. In the present embodiment, the LDS control io unit 105 is configured to receive the low density signature from the access point to which the UE is connected. That is, in the present embodiment the access point is configured to generate low density signatures for each connected UE, and to transmit the generated low density signatures to the connected UEs.

A modulated data stream 101 including the user data is initially generated in a conventional manner, by modulating a carrier signal with the data to be transmitted. Each LDS unit 102 is configured to apply the low density signature received from the LDS control unit 105 to the modulated data stream 101. The stream outputted by the LDS unit 102 is hereinafter referred to as an LDS modulated data stream. The LDS modulated data stream outputted by each LDS unit 102 is received by the corresponding FBMC-IOTA unit 103, which includes a filter bank configured to apply IOTA pulse shaping to the LDS modulated data stream. The signal outputted by the FBMC-IOTA unit 103, which is hereinafter referred to as an LDS-IOTA signal, is then transmitted by an antenna 104 as one of the subcarrier signals. -0or

By utilising a low density signature together with IOTA pulse shaping, embodiments of the present invention provide multicarrier transmission schemes that offer improved spectral efficiency in comparison to conventional schemes such as CDMA and OFDM, and which perform well under overload conditions. Specifically, the spectral efficiency is improved since the IOTA pulse shaping provides good time and frequency localization properties, allowing Inter-Symbol Interference (ISI) and Inter-Carrier Interference (IC1) to be avoided without the use of the Cyclic Prefix (CP) required in conventional OFDM. The use of low density signatures enables performance close to the single user bound to be achieved in a multiuser environment, even in overload conditions. -7 -

The application of the low density signature to the modulated data stream will now be described with reference to Fig. 2, which schematically illustrates the sequence of operations carried out in the LDS unit 102. In the present embodiment the LDS unit 102 is configured to receive a vector vk and a permutation pattern ak = lak(1), ak(2), zuk(N)} from the LDS control unit 105. Together, the vector and permutation pattern define a spreading matrix that identifies the chips on which the kth user will spread their data at any given point in time. In the present embodiment the UE for the 1st user is illustrated, that is, in the present embodiment k=1.

/o First, the data symbols from the modulated data stream are spread over a plurality of chips as denoted by the vector vk. The number of chips is determined by the density factor;1; of the low density signature, which is the ratio of the number of subcarriers used by the kth user to the total number of subcarriers N. In general, yk can be equal to or less than 1. In the present embodiment the density factor k is less than 1, and zero padding is performed by adding (N-vk) zeroes at the end of the signature for the kth user. As a result of zero padding, the total processing gain of the system becomes N even when the data symbols for the 0' user are spread across fewer than N chips. After spreading and zero padding have been performed, the permutation pattern 51k is applied to the zero-padded signature. However, in other embodiments yk may be equal 20 to one and zero-padding may be omitted. When;lc = 1, the data symbols for the kth user will be spread over all Nsubcarriers.

Apparatus for generating low density signatures is schematically illustrated in Fig. 3, according to an embodiment of the present invention. The apparatus may, for example, -0or be included at an access point in a mobile telecommunications network, for generating low density signatures for a plurality of connected UEs. The generated low density signatures can then be signalled to each connected UE.

The apparatus 300 of the present embodiment comprises an LDS generator 301 configured to randomly generate the vector vk and permutation pattern Thk for defining an LDS spreading matrix. Although in the present embodiment the vector and permutation pattern are randomly generated, in another embodiment the value of the vector and/or permutation pattern may be selected from a plurality of predefined permissible values. -8 -

The apparatus 30o further comprises a factor graph analyser 302 configured to determine a factor graph representation of the LDS spreading matrix defined by the randomly-generated vector and permutation pattern, and to determine the degree of each node in the factor graph. The degree of each node is equal to the number of edges connecting that node to other nodes in the factor graph. The factor graph analyser 302 is further configured to compare the degree of each node to a threshold maximum degree. If the degree of any node is higher than the threshold maximum degree, the factor graph analyser 302 controls the LDS generator 301 to randomly generate a new vector and/or permutation pattern.

A high degree distribution among nodes in the factor graph results in a scheme that is more robust to interference on a small number of subcarriers, by increasing the diversity. When executing a message passing algorithm at a receiver for multiuser detection (MUD), the more edges that are connected to a chip/variable node, the more reliable the information that can be utilized when processing that node. A higher density of chip/variable nodes, that is, a higher degree distribution, in general leads to better performance due to larger diversity gain. However, a higher density of chip/variable nodes increases the calculation cost and receiver complexity. Imposing a threshold maximum degree ensures that message passing can still be performed efficiently at the receiver for MUD.

Once a combination of vk and ak has been obtained for which each node in the factor graph satisfies the threshold maximum degree requirement, the factor graph analyser 302 is configured to determine the shortest cycle in the factor graph. A cycle is a finite set of connected edges that starts and ends at the same node, and which satisfies the condition that no node except the initial node and final node appears more than once. The shortest cycle in the factor graph of a low density signature can also be referred to as the girth of the low density signature. The factor graph analyser 302 is further configured to compare the girth to a threshold minimum cycle length. If the girth is less than the threshold minimum cycle length, the factor graph analyser 302 controls the LDS generator 301 to randomly generate a new vector and/or permutation pattern. Imposing a minimum cycle length avoids low density signatures being used which have short cycles, since a short cycle can reduce the efficiency of message passing due to self-propagated information transmitting to the same node in very limited iterations. -9 -

As a result of the above-described checks, the apparatus 300 of the present embodiment obtains a random vector and permutation pattern combination for which the factor graph has a girth greater than a threshold minimum value, and in which each node has a degree less than a threshold maximum value. As explained above, this ensures efficient operation of a message passing algorithm executed at a receiver for MUD.

In the present embodiment, the randomly-generated vector and permutation pattern are accepted if they satisfy the minimum girth and maximum degree criteria. However, /0 in other embodiments different selection criterion may be used when determining whether the generated vector and permutation pattern are acceptable, or whether a new vector and/or permutation pattern needs to be generated and tested.

By only allowing combinations of vectors and permutation patterns to be used which satisfy certain criteria, a minimum level of performance can be guaranteed by avoiding combinations which could increase the time taken to perform MUD at the access point. Nonetheless, in some embodiments a randomly-generated vector and permutation pattern may be used without applying any selection criterion, such that the values of the vector and permutation pattern applied by a UE are truly random.

Referring now to Fig. 4, the FBMC-IOTA unit 103 of Fig. 1 is schematically illustrated according to an embodiment of the present invention. The FBMC-IOTA unit 103 includes real and imaginary processing branches, which can also be referred to as in-phase and quadrature branches respectively. The FBMC-IOTA unit 103 comprises a complex signal generator 401 and a filter bank 402. The complex signal generator 401 can comprise any suitable means for generating the in-phase and quadrature signal components from the LDS modulated data stream. An Inverse Fast Fourier Transform (IFFT) is then used to convert from the frequency domain to the time domain, and the filter bank 402 is then used to apply Inverse Orthogonal Transform Algorithm IOTA pulse shaping to the transformed in-phase and quadrature signal components.

Referring now to Fig. 5, a factor graph representing a low density signature spreading matrix is schematically illustrated, according to an embodiment of the present invention. Each factor node 501 represents a chip received on one of the subcarriers, and each variable node 502 represents the estimated data symbol transmitted by a user. Where a factor node 501 is connected to a variable node 502 by an edge 503, this -10 -indicates that a data symbol for that particular user is transmitted on that particular chip. In the present example eight users U, to Us transmit data symbols across four chips C, to C4, corresponding to a system loading of 200%. However, these values are merely exemplary and in other embodiments there may be different numbers of users and chips.

In some embodiments of the present invention, the low density signature is configured such that in a factor graph representation in which each factor node represents a received chip and each variable node represents a user, different factor nodes and/or different variable nodes have different degrees. For example, in the scheme shown in Fig. 5, the first, third, fifth and sixth users (U,, U3, U5, U6) each spread their data symbols across two chips, whilst the second, fourth, seventh and eighth users (U2, U4, U7, US) each spread their data symbols across three chips. This arrangement can be referred to as variable degree distribution. The degree distribution of the variable nodes in Fig. 5 can be written as 0.5x2 + o.5x3, indicating that 5o% of the variable nodes have 2 connections, and 5o% of the variable nodes have 3 connections. Similarly, the degree distribution of the chip nodes in Fig. 5 can be written as x5, indicating that every chip node has 5 connections.

By allowing low density signatures with variable degree distributions to be used, embodiments of the present invention can increase the choice of available LDS schemes and enable further optimisation by selecting the scheme which offers the best performance for particular channel conditions.

Referring now to Fig. 6, apparatus for receiving data from a plurality of users across a plurality of subcarriers is schematically illustrated, according to an embodiment of the present invention. The receiver, for example an access point in a wireless telecommunications network, comprises a plurality of antennas 601 each arranged to receive one of the subcarrier si to sn through a MIMO wireless communication channel, a filter bank 602. The receiver further comprises apparatus for estimating the data symbols transmitted by each user, from the received subcarrier signals.

The apparatus for estimating the data symbols comprises a FBMC filter bank 602 configured to apply pulse shaping to a received subcarrier signal, and a detection 35 arrangement 603 configured to detect chips from each of the received subcarrier signals. In the present embodiment IOTA pulse shaping is applied, but in other embodiments a different type of FBMC filter may be used, as described above. In the present embodiment, each antenna is provided with an IOTA demodulator which includes a real (1) branch and an image (Q) branch. The real and image branches for each antenna receive data on a total of N subcarriers. The detection arrangement 603 is configured to perform serial-to-parallel conversion on the N subcarrier data received from the real and image branches of each antenna, and distribute the N subcarrier data to N real-branch detectors and N image-branch detectors as shown in Fig. 6 (DET 1 to DET N), according to the subcarrier index. In this way detection is performed separately for each of the N subcarriers, thereby reducing calculation complexity. The detection arrangement 603 is further configured to perform parallel-to-serial conversion and pass the detected chips to the next processing stage.

As shown in Fig. 6, the apparatus further comprises a data symbol estimator 605 configured to receive the detected chips from the detection arrangement 603, and a Low Density Signature LDS unit 604 configured to obtain a LDS spreading matrix for the 0' user. The data symbol estimator 6435 is configured to estimate the data symbols for the kill user from the detected chips by applying a message passing algorithm to estimate the data symbols based on a factor graph representing the LDS spreading matrix for the kth user. The LDS unit 604 may include the LDS generating apparatus shown in Fig. 3 for locally generating the low density signatures for the plurality of users. Alternatively, the low density signatures may be generated remotely and transmitted to the LDS unit 604.

Referring now to Fig. 7, a graph is illustrated showing relative performance of various signal transmission schemes according to embodiments of the present invention, in comparison to conventional LDS-OFDM. The bit error rate performance of three LDS-IOTA schemes with different degree distributions, as shown below in Table 1, is compared with that of LDS-OFDM and conventional OFDM/IOTA in multi-antenna transmissions over ITU Pedestrian Channel B. In Fig. 7, the bit error rate for each 3o scheme is plotted as a function of the energy per bit to noise power spectral density ratio, Eb/No.

In the present example, the FFT size is 64, the system loading is 200% and the number of antennas on the receiver is 4. For conventional OFDM, Welch-bound-equality (sequences that meet equality in the Welch's lower bound on total squared correlation) is used, and maximum likelihood (ML) detection is employed. As shown in Fig. 7, each -12 -of the three LDS-IOTA schemes (1, II, III) achieve superior performance over both LDS-OFDM and conventional OFDM/IOTA under overloaded conditions. Moreover, LDS-IOTA outperforms LDS-OFDM by about 1.2 -2 decibels (dB) in the medium-tohigh signal to noise ratio (SNR) region. Therefore, LDS-IOTA not only increases spectral efficiency, but also improves energy efficiency.

Table 1: Example LDS-IOTA schemes Scheme Degree distribution Girth Chip node Variable node I 0.vc4 + 0.7x5+ 0.2.X6 0.5X5 + 0.5X6 6 II x5 x5 4 III 0.03x4 + 43.94.x/ + 0.03x6 0.7x4 + 0.3x5 8 Referring now to Fig. 8, a flowchart showing a method of generating and transmitting io an LDS-IOTA signal is illustrated, according to an embodiment of the present invention. The flowchart corresponds to the method performed by the apparatus shown in Fig. 1.

First, in step S8oi a low density signature is applied to a modulated data stream, to obtain an LDS modulated data stream. Then, in step S8o2 in-phase and quadrature signal components are generated from the LDS modulated data stream. Next, in step 5803 an LDS-IOTA signal is obtained by passing the in-phase and quadrature signal components through an FBMC-IOTA filter bank configured to apply IOTA pulse shaping. The LDS-IOTA signal is then transmitted in step 5804. Although in the present embodiment Isotropic Orthogonal Transform Algorithm (IOTA) pulse shaping is used, in other embodiments a different type of pulse shaping other than IOTA may be applied, including but not limited to half cosine, Root Raised Cosine (RRC), Extended Gaussian Functions (EGF), or physical layer for dynamic access (PHYDAS).

As explained above, the combination of low density signatures with IOTA pulse shaping provides improved performance in comparison to conventional schemes such as OFDM and LDS-OFDM.

Referring now to Fig. 9, a flowchart showing a method of determining an LDS spreading matrix to be applied to a modulated data stream is illustrated, according to -13 -an embodiment of the present invention. The flowchart corresponds to the method performed by the apparatus shown in Fig. 3.

First, in step S901 a first vector and first permutation pattern for defining an LDS spreading matrix are randomly generated. Then, in step S9o2 a factor graph representation of the LDS spreading matrix is obtained based on the generated vector and permutation pattern. Next, in step S9o3 it is checked whether there are any nodes in the factor graph with degrees higher than the threshold maximum value. If any nodes have degrees higher than the threshold, the process returns to step S9ci1 and generates a new vector and/or permutation pattern.

Once a combination of vector and permutation pattern has been obtained which satisfies the degree distribution selection criterion, then in step S9o4 the minimum cycle length (girth) is determined. In step S9o5 it is checked whether the minimum cycle length is below the threshold minimum value. If the girth is less than the threshold minimum value, the process returns to step S9o1 and randomly generates a new vector and/or permutation pattern.

Once a combination of vector and permutation pattern has been obtained which satisfies both the degree distribution selection criterion in step S9o3 and the girth selection criterion in step S9o5, then in step S906 the vector and permutation pattern are determined to be acceptable. The vector and permutation pattern may then be transmitted to each of the K users.

-0or Referring now to Fig. to, a flowchart showing a method of estimating data symbols for a kth user from a received signal in a multiuser environment is illustrated, according to an embodiment of the present invention. The flowchart corresponds to the method performed by the apparatus shown in Fig. 6.

First, in step Sloo1 each received subcarrier signal is passed through a FBMC filter bank configured to apply pulse shaping. In the present embodiment IOTA pulse shaping is applied, but in other embodiments a different type of FBMC filter may be used, as described above. Then, in step S1002 the received chips are detected from the received subcarrier signals. Next, in step Sloo3 an LDS spreading matrix for the kill user. Then, in step S1oo4 a message passing algorithm is applied to estimate the data symbols transmitted by the kJ' user, based on the detected chips and a factor graph representing the LDS spreading matrix for the kth user.

Whilst certain embodiments of the invention have been described herein with reference 5 to the drawings, it will be understood that many variations and modifications will be possible without departing from the scope of the invention as defined in the accompanying claims.

Claims

-15 -Claims 1. A method of transmitting a signal in a multiuser environment, the method comprising: applying a Low Density Signature LDS to a modulated data stream, to obtain an LDS modulated data stream; generating in-phase and quadrature signal components from the LDS modulated data stream; obtaining an LDS-Filter Bank Multi-Carrier FBMC signal by passing the in-to phase and quadrature signal components through a FBMC filter bank configured to apply pulse shaping; and transmitting the LDS-FBMC signal.
2. The method of claim 1, wherein the low density signature is configured such that in a factor graph representation in which each factor node represents a received chip and each variable node represents a user, different factor nodes and/or different variable nodes have different degrees.
3. The method of claim 1 or 2, further comprising determining the low density signature to be applied to the modulated data stream by: randomly generating a first vector and first permutation pattern for defining an LDS spreading matrix; determining one or more parameters of a factor graph representing the LDS spreading matrix; in response to the one or more parameters satisfying a selection criterion, applying the low density signature by using the first vector and first permutation pattern; and in response to the one or more parameters failing the selection criterion, repeatedly randomly generating a second vector and/or second permutation pattern until an LDS spreading matrix is obtained for which the one or more parameters satisfy the selection criterion, and applying the low density signature by using the obtained second vector and/or second permutation pattern.
4. The method of claim 3, wherein the one or more parameters includes a 35 minimum cycle length of the factor graph representing the LDS spreading matrix, and the selection criterion is satisfied if the minimum cycle length is higher than a threshold minimum cycle length.
5. The method of claim 3 or 4, wherein the one or more parameters includes the 5 degree of each node in the factor graph representing the LDS spreading matrix, and the selection criterion is satisfied if the degree of each node is less than a threshold maximum degree.
6. The method of claim 3, 4 or 5, wherein the length vk of the first or second vector o to be used in applying the low density signature is less than the total number of subcarriers, N, and applying the low density signature comprises: zero-padding said first or second vector by adding N-vk zeroes; and applying the low density signature to the modulated data stream by applying the first or second permutation pattern to the zero-padded vector.
7. The method of any one of the preceding claims, wherein the FBMC filter bank applies isotropic Orthogonal Transform Algorithm IOTA pulse shaping.
8. A method of estimating data symbols for a kth user from a plurality of received subcarrier signals in a multiuser environment, the method comprising: passing each of the received subcarrier signals through a Filter Bank Multi-Carrier FBMC filter bank configured to apply pulse shaping; detecting chips from each of the received subcarrier signals; obtaining a Low Density Signature LDS spreading matrix for the kth user; and or applying a message passing algorithm to estimate the data symbols transmitted by the kth user, based on the detected chips and a factor graph representing the LDS spreading matrix for the kth user.
9. A computer-readable storage medium arranged to store computer program 30 instructions which, when executed by one or more processors, perform the method of any one of the preceding claims.
10. Apparatus for transmitting a signal in a multiuser environment, the apparatus comprising: a Low Density Signature LDS unit configured to apply a low density signature to a modulated data stream and output an LDS modulated data stream; -17 -means for generating in-phase and quadrature signal components from the LDS modulated data stream; a Filter Bank Multi-Carrier FBMC filter bank configured to apply pulse shaping to the in-phase and quadrature signal components and output an LDS-FBMC signal; 5 and an output configured to send the LDS-FBMC signal to an antenna for transmission.
11. The apparatus of claim 10, wherein the low density signature is configured such that in a factor graph representation in which each factor node represents a received chip and each variable node represents a user, different factor nodes and/or different variable nodes have different degrees.
12. The apparatus of claim 10 or 11, further comprising: an LDS generator configured to randomly generate a first vector and first permutation pattern for defining an LDS spreading matrix; and a factor graph analyser configured to obtain one or more parameters of a factor graph representing the LDS spreading matrix, and to determine whether the one or more parameters satisfy a selection criterion, wherein in response to the one or more parameters satisfying the selection criterion, the factor graph analyser is configured to control the LDS unit to apply the low density signature by using the first vector and first permutation pattern, and wherein in response to the one or more parameters failing the selection criterion, the factor graph analyser is configured to control the LDS generator to repeatedly randomly generate a second vector and/or second permutation pattern until an LDS spreading matrix is obtained for which the one or more parameters satisfy the selection criterion, and to control the LDS unit to apply the low density signature by using the obtained second vector and/or second permutation pattern.
13. The apparatus of claim 12, wherein the one or more parameters includes a minimum cycle length of the factor graph representing the LDS spreading matrix, and the factor graph analyser is configured to determine that the selection criterion is satisfied if the minimum cycle length is higher than a threshold minimum cycle length.
14. The apparatus of claim 12 or 13, wherein the one or more parameters includes the degree of each node in the factor graph representing the LDS spreading matrix, and -18 -the factor graph analyser is configured to determine that the selection criterion is satisfied if the degree of each node is less than a threshold maximum degree.
15. The apparatus of claim 12, 13 or 14, wherein the LDS generator is configured to generate the first and/or second vector with a length vk less than the total number of subcarriers, N, and the LDS unit is configured to apply the low density signature to the modulated data stream by zero-padding said first or second vector by adding N-vk zeroes, and applying the first or second permutation pattern to the zero-padded vector.o
16. Apparatus for estimating data symbols for a kth user from a plurality of received subcarrier signals in a multiuser environment, the apparatus comprising: a Filter Bank Multi-Carrier FBMC filter bank configured to apply pulse shaping to each of the received subcarrier signals; a detection arrangement configured to detect chips from each of the received subcarrier signals; a Low Density Signature LDS unit configured to obtain a LDS spreading matrix for the I& user; and a data symbol estimator configured to estimate the data symbols for the kth user from the detected chips by applying a message passing algorithm to estimate the data symbols based on a factor graph representing the LDS spreading matrix for the kth user.