WO2016178607A1

WO2016178607A1 - Inserting and extracting control data replacing cyclic prefix samples

Info

Publication number: WO2016178607A1
Application number: PCT/SE2015/050502
Authority: WO
Inventors: Gunther Auer; Miguel Berg; Tsao-Tsen Chen; Elmar Trojer
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-05-06
Filing date: 2015-05-06
Publication date: 2016-11-10

Abstract

In a communication line between access nodes, like the CPRI between BBU and RRH for instance, the standard LTE signal conveyed can be punctured to transmit additional non-standard LTE control data between the two nodes. At the receiving node, the signal is demultiplexed: control data are extracted and the LTE signal is reconstructed. Here it is proposed to replace the Cyclic Prefix (which is of no use for the cable transmission between BBU and RRH since there is almost no delay spread on the transmission line) by control data. The control data signal consists in an auxiliary OFDM signal with a power of two IFFT, ie number of samples replacing the Cyclic Prefix.

Description

INSERTING AND EXTRACTING CONTROL DATA REPLACING

CYCLIC PREFIX SAMPLES

TECHNICAL FIELD

The invention relates to methods, modulators, demodulators, computer programs and computer program products for inserting and extracting control data replacing cyclic prefix samples over a point-to-point medium.

BACKGROUND

The topology of cellular networks increases in complexity. For instance, remote radio heads can be used in locations where traditional deployment with antennas being co-located with the base stations is not ideal. For example, such a distributed transmission system is particularly useful to increase coverage and reduce handovers in areas with a large concentration of subscribers, such as in office buildings, shopping centres, train stations, airports, arenas, etc. However, in distributed transmission systems for cellular networks, it is often necessary to convey control data between system components that is not related to the transmitted LTE (Long Term Evolution) signal and is not defined in standards documentation. Such control data consumes additional resources over the communication link between the components, which implies greater link capacity requirements.

SUMMARY

It would be of great benefit if control data not related to the transmitted cellular network signal could be transmitted alongside with the cellular network signal without consuming additional link capacity. According to a first aspect, it is presented a method for inserting control data in a modulator. The method is performed in the modulator and comprises the steps of: obtaining control data samples; writing the control data samples in a data symbol at positions intended to be populated by cyclic prefix samples, the cyclic prefix samples being copies of a subset of main samples of the data symbol; and providing the data symbol comprising the control data samples on the output for transmission to a demodulator over a point-to-point medium.

The step of writing the control data samples may comprise replacing previously written cyclic prefix samples with the control data samples.

The step of writing may comprise writing the control data samples at all positions of the data symbol which are intended to be populated by cyclic prefix samples.

The method may further comprise the step of: writing control data cyclic prefix samples at a subset of positions intended to be populated by cyclic prefix samples, which are not populated by control data samples, the control data cyclic prefix samples being copies of a subset of the control data samples.

The point-to-point medium maybe a cable. The data symbol may be an orthogonal frequency division multiplex, OFDM, symbol in accordance with Long Term Evolution, LTE.

According to a second aspect, it is presented a modulator for inserting control data. The modulator comprises: a processor; and a memory storing

instructions that, when executed by the processor, cause the modulator to: obtain control data samples; write the control data samples in a data symbol at positions intended to be populated by cyclic prefix samples, the cyclic prefix samples being copies of a subset of main samples of the data symbol; and provide the data symbol comprising the control data samples on the output for transmission to a demodulator over a point-to-point medium. The instructions to write may comprise instructions that, when executed by the processor, cause the modulator to replace previously written cyclic prefix samples with the control data samples. The instructions to write may comprise instructions that, when executed by the processor, cause the modulator to write the control data samples at all positions of the data symbol which are intended to be populated by cyclic prefix samples. The modulator may further comprise instructions that, when executed by the processor, cause the modulator to write control data cyclic prefix samples at a subset of positions intended to be populated by the cyclic prefix samples, which are not populated by control data samples, the control data cyclic prefix samples being copies of a subset of the control data samples. The point-to-point medium may be a cable.

In the instructions to receive, the data symbol maybe an orthogonal frequency division multiplex, OFDM, symbol in accordance with Long Term Evolution, LTE.

According to a third aspect, it is presented a modulator comprising: means for obtaining control data samples; means for writing the control data samples in a data symbol at positions intended to be populated by cyclic prefix samples, the cyclic prefix samples being copies of a subset of main samples of the data symbol; and means for providing the data symbol comprising the control data samples on the output for transmission to a demodulator over a point-to-point medium.

According to a fourth aspect, it is presented a computer program for inserting control data, the computer program comprising computer program code which, when run on a the modulator cause the modulator to: obtain control data samples; write the control data samples in a data symbol at positions intended to be populated by cyclic prefix samples, the cyclic prefix samples being copies of a subset of main samples of the data symbol; and provide the data symbol comprising the control data samples on the output for transmission to a demodulator over a point-to-point medium. According to a fifth aspect, it is presented a computer program product comprising a computer program according to the fourth aspect and a computer readable means on which the computer program is stored.

According to a sixth aspect, it is presented a method for extracting control data in a demodulator. The method is performed in the demodulator and comprises the steps of: receiving a data symbol, from a point-to-point medium, the data symbol comprising main samples and control data samples; extracting the control data samples from the data symbol; replacing the control data samples in the data symbol with cyclic prefix samples being copies of a subset of the main samples; and providing the data symbol comprising the cyclic prefix samples for transmission over a wireless medium to one or more mobile terminals.

In the step of receiving, the data symbol may further comprise cyclic prefix samples, being copies of a subset of the main samples; and wherein the step of replacing may comprise replacing the control data samples with cyclic prefix samples such that all cyclic prefix samples of the data symbol is a set of samples being contiguous copies of the main samples.

In the step of receiving, the data symbol may comprise only main samples and control data samples. In the step of receiving, the data symbol may comprise control data cyclic prefix samples being copies of a subset of the control data samples, and wherein the method may further comprise the step of: extracting the control data cyclic prefix samples.

The point-to-point medium maybe a cable. In the step of providing, the data symbol may be an orthogonal frequency division multiplex, OFDM, symbol in accordance with Long Term Evolution, LTE.

According to a seventh aspect, it is presented a demodulator for extracting control data. The demodulator comprises: a processor; and a memory storing instructions that, when executed by the processor, cause the demodulator to: receive a data symbol, from a point-to-point medium, the data symbol comprising main samples and control data samples; extract the control data samples from the data symbol; replace the control data samples in the data symbol with cyclic prefix samples being copies of a subset of the main samples; and provide the data symbol comprising the cyclic prefix samples for transmission over a wireless medium to one or more mobile terminals.

In the instructions to receive, the data symbol may further comprise cyclic prefix samples, being copies of a subset of the main samples; and wherein the instructions to replace may comprise instructions that, when executed by the processor, cause the demodulator to replace the control data samples with cyclic prefix samples such that all cyclic prefix samples of the data symbol is a set of samples being contiguous copies of the main samples.

In the instructions to receive, the data symbol may comprise only main samples and control data samples.

In the instructions to receive, the data symbol may comprise control data cyclic prefix samples being copies of a subset of the control data samples, and wherein the demodulator may further comprise instructions that, when executed by the processor, cause the demodulator to extract the control data cyclic prefix samples.

The point-to-point medium maybe a cable.

In the instructions to provide, the data symbol may be an orthogonal frequency division multiplex, OFDM, symbol in accordance with Long Term Evolution, LTE. According to an eighth aspect, it is presented a demodulator comprising: means for receiving a data symbol, from a point-to-point medium, the data symbol comprising main samples and control data samples; means for extracting the control data samples from the data symbol; means for replacing the control data samples in the data symbol with cyclic prefix samples being copies of a subset of the main samples; and means for providing the data symbol comprising the cyclic prefix samples for

transmission over a wireless medium to one or more mobile terminals.

According to a ninth aspect, it is presented a computer program for extracting control data. The computer program comprises computer program code which, when run on a demodulator cause the demodulator to: receive an orthogonal frequency division multiplex, data, symbol, from a point-to-point medium, the data symbol comprising main samples and control data samples; extract the control data samples from the data symbol; replace the control data samples in the data symbol with cyclic prefix samples being copies of a subset of the main samples; and provide the data symbol comprising the cyclic prefix samples for transmission over a wireless medium to one or more mobile terminals.

According to a tenth aspect, it is presented a computer program product comprising a computer program according to the ninth aspect and a computer readable means on which the computer program is stored.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a schematic architecture diagram illustrating an environment where embodiments presented herein can be applied; Fig 2 is a schematic diagram illustrating the physical resources for downlink communication in LTE;

Fig 3 is a schematic diagram of a system illustrating a transmitter and a receiver over a point-to-point medium according to one embodiment; Fig 4A-B are schematic diagrams illustrating various configurations of cyclic prefix and control data according to various embodiments;

Fig 5 is a schematic diagram illustrating a modulator of Fig 3 according to one embodiment;

Fig 6 is a schematic diagram illustrating a demodulator of Fig 3 according to one embodiment;

Figs 7A- B are flow charts illustrating embodiments of methods for inserting control data performed in the modulator of Fig 3;

Figs 8A-B are flow charts illustrating embodiments of methods for extracting control data performed in the demodulator of Fig 3; Fig 9 is a schematic diagram showing some components of an embodiment of the modulator of Fig 3 according to one embodiment;

Fig 10 is a schematic diagram showing some components of an embodiment of the demodulator of Fig 3 according to one embodiment;

Fig 11 is a schematic diagram showing functional modules of the software instructions of the modulator of Fig 3 or Fig 9 according to one embodiment;

Fig 12 is a schematic diagram showing functional modules of the software instructions of the demodulator of Fig 3 or Fig 10 according to one

embodiment; and

Fig 13 shows one example of a computer program product comprising computer readable means. DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description. Fig l is a schematic architecture diagram illustrating an environment where embodiments presented herein can be applied. A cellular communication network 9 comprises a number of remote radio heads (RRHs) 8a-c for installation in locations where traditional deployment with antennas being co-located with the base stations is not ideal, e.g. to increase coverage and reduce handovers in areas with a large concentration of subscribers, such as in office buildings, shopping centres, train stations, airports, arenas, etc.

It is to be noted that, while the embodiments presented herein are described as implemented using LTE (Long Term Evolution) and/ or W-CDMA

(Wideband Code Division Multiplex), any applicable communication standard may be used, such as any one or a combination of LTE-SAE (Long Term Evolution - System Architecture Evolution), GSM (Global System for Mobile communication), EDGE (Enhanced Data Rates for GSM Evolution), GPRS (General Packet Radio Service), CDMA2000 (Code Division Multiple Access 2000), or any other current or future wireless network, such as LTE- Advanced, as long as the principles described hereinafter are applicable.

A radio base station (RBS) 5 here comprises one or more baseband modules (BB) 1. The baseband modules BB can be handled by digital units (DUs) in the RBS 5, where each DU can handle one or more BB modules 1. A combiner 6 is used in the uplink to combine uplink signals from a plurality of remote radio heads 8a-c and forward data to the baseband module 1. In downlink, the combiner 6 may function as a splitter, providing downlink signals from the baseband module 1 to each one of the connected remote radio heads 8a-c. The combiner 6 is also known as an indoor radio unit (IRU). It is to be noted though that the combiner may also be provided outdoors whenever appropriate. The combiner 6 is in this way a link for a number (in this example three) of remote radio heads 8a-c. In this way, the radio base station 5 is a link for uplink and downlink communication for the remote radio heads connected to the combiner 6. One function of the radio base station 5 is to function as a digital unit (DU), using the one or more baseband module 1, for processing uplink and downlink signals in the digital domain. The radio base station 5 is also connected to a core network 3. The core network 3 provides central functions and connectivity to external networks 7 such as the Internet and other cellular communication networks.

The remote radio heads 8a-c connected to the combiner 6 can be part of a single radio cell or they can form part of two or more different cells. Antennas do not need to be included in this embodiment of the radio base station 5 or the combiner 6, as the remote radio heads 8a-c provide the antennas for the wireless link 4 to one or more wireless devices 2. The wireless link 4 provided by the remote radio heads 8a-c includes both downlink (DL) communication to the wireless devices 2 and uplink (UL) communication from the wireless devices 2. The term wireless device is also known as mobile communication terminal, user equipment (wireless device), station (STA), mobile terminal, user terminal, user agent, machine-to -machine devices etc., and can be, for example, what today is commonly known as a mobile phone or a

tablet /laptop with wireless connectivity or fixed mounted terminal. In radio communication systems, the data is transmitted and received over the air at a specific radio frequency- either the same for transmission and reception or on separate frequencies. This is often called the radio frequency (RF) or the carrier frequency.

There are many different carrier frequencies, depending on regional spectrum allocation and spectrum license rights. To create a common radio implementation supporting this variety of carrier frequencies, a second set of frequencies is used herein, denoted the Intermediate Frequency (IF), which is used for communication on the cables between the combiner 6 and the remote radio heads 8a-c. It is to be noted that the processing of uplink and downlink signals in the combiner and the remote radio heads 8a-c do not need to occur in the digital domain and can be (but do not need to be) performed completely in the analogue domain.

The remote radio heads 8a-c convert from IF to RF for downlink

transmission and from RF to IF for uplink reception. Conversely, the combiner 6 converts from digital BB to IF for downlink transmission and from IF to digital BB for uplink reception.

By using IF instead of RF over the cables between the combiner 6 and the remote radio heads 8a-c, cheaper, widely deployed electrical cables can be used, such as Ethernet LAN cabling. In this way, existing indoor cabling can many times be reused during installation, which significantly saves cost, installation time and complexity. Optionally, the remote radio heads 8a-c are also powered over the respective cables.

The transmission and reception is under the control of the MAC (Media Access Control) scheduler in the baseband module 1. The MAC scheduler informs what transmissions should be made and informs, via the downlink signaling, the wireless devices when to transmit and on which frequency and power.

The link between the combiner 6 and the baseband module 1 utilises a digital signal interface, such as CPRI (Common Public Radio Interface).

It is to be noted that, although Fig 1 shows the baseband module 1 connected to one combiner 6, each baseband module 1 can be connected to several combiners over separate respective links. It is to be noted that while the embodiment of Fig 1 shows three remote radio heads la-c, there may be fewer or more remote radio heads connected to each combiner 6.

In the environment of Fig l, there is a need for control data (CD) to be signalled between the baseband module ι and the combiner 6, between the combiner 6 and the remote radio heads 8a-c and/ or between the remote radio heads and the baseband module l. The signalling of the control data can occur in the downlink (in a direction towards the wireless device 2) or in the uplink (in a direction towards the core network 3). It is to be noted that this control data can be separate from control data defined in the standards documentation (e.g. for LTE) and its structure can be freely designed as part of the implementation of embodiments presented herein. The control data can e.g. relate to configuration of the remote radio heads, measurements from the remote radio heads, frequency configuration for payload data over the IF interface, fault monitoring, etc.

As explained in more detail below, in a point-to-point communication link as utilised here, certain resources can be used for the control signalling which can be used for other purposes of the wireless link 4.

Fig 2 is a schematic diagram illustrating the physical resources for downlink communication in LTE (Long Term Evolution). Downlink communication is communication from a network node to a wireless device. LTE uses OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and DFT (Discrete Fourier Transform)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in Fig 2, where each resource element 35 corresponds to one

OFDM subcarrier during one OFDM symbol interval. Each resource element 35 comprises cyclic prefix section 32 and a main section 31. The main section 31 is at positions of the OFDM symbol intended to hold payload data and the cyclic prefix section 32 is at positions of the OFDM symbol intended to hold cyclic prefix samples. The intended positions for the payload data and the cyclic prefix samples are e.g. defined in LTE standards. The purpose of the cyclic prefix section 32 is to compensate for distortions (e.g. due to

multipath) of a radio channel, and retains orthogonality between OFDM subcarriers.

It is to be noted that in the description below, OFDM symbols are used to exemplify data symbols. However, the embodiments can equally be applied for any data symbols that employ cyclic prefixes.

Fig 3 is a schematic diagram of a system illustrating transmitter and a receiver over a point-to-point medium according to one embodiment.

A transmitter 38 and a receiver 39 are connected over a point-to-point medium 19. It is to be noted that the transmitter 38 and the receiver 39 are defined here as such in relation to the point-to-point medium 19. In other words, the transmitter 38 can also be a receiver for other links and the receiver 39 can be a transmitter for other links.

The point-to-point medium 19 is used for communication between only two entities and can e.g. be a CPRI link between the baseband module 1 and the combiner 6 of Fig 1 and/or an IF link over Ethernet between the combiner 6 and one or more of the remote radio heads 8a-c of Fig 1. Alternatively or additionally, the link can be a link between the baseband module 1 and one or more of the remote radio heads 8a-c of Fig 1. The point-to-point medium 19 has a direction of communication from the transmitter 38 to the receiver 39 as indicated by the arrow and can be used for uplink or downlink. Optionally, two separate links are used to provide both uplink and downlink

communication.

Hence, the transmitter 38 can e.g. be the baseband module 1, the combiner 6 or one or more of the remote radio heads. Analogously, the receiver 39 can e.g. be the baseband module 1, the combiner 6 or one or more of the remote radio heads, as long as there is a point-to-point medium to the transmitter 38. Specifically, the point-to-point medium is not a link between a radio base station and a wireless device. In this way, redundant resources which are assigned to the radio interface can be utilised here for control data. Such redundant data can be part of the cyclic prefix, whereby some of the positions intended for cyclic prefix samples instead carry control data samples. The position of the control data samples in the resource element needs to be known by both the transmitter 38 and the receiver 39. This can be pre-configured in both ends or signalled using another control channel.

Optionally, the transmitter 38 comprises a modulator 10 and a Digital to Analogue (D/A) converter 11 if the point-to-point medium 19 is an analogue communication link such as IF. If the point-to-point medium 19 is a digital communication link, such as CPRI, then the D/A converter 11 is not needed. An input 18 is used to receive main data MD and control data CD. The data provided on the input 18 is synchronised in terms of the start and the end of an OFDM symbol. Optionally, the receiver 39 comprises an Analogue to Digital (A/D) converter 21 which converts the signal received over the point-to-point medium 19 to digital representation if the point-to-point medium is an analogue

communication link such as IF. If the point-to-point medium 19 is a digital communication link, such as CPRI, then the A/D converter 21 is not needed. A demodulator 20 extracts the control data CD and modifies the signal such that the transmission signal Tx output from the receiver is suitable for transmission over the air. It is to be noted the demodulator may, but does not need to, demodulate other parts than the control data of the signal received over the point-to-point medium 19. Fig 4A-B are schematic diagrams illustrating various configurations of cyclic prefix and control data. When data for cellular networks are transmitted in a point-to-point communication link, embodiments herein utilise cyclic prefix samples for control data which can be used for other purposes of the wireless link. For instance, the resource element of Fig 2 comprises only main samples 31 and cyclic prefix samples 32. A main purpose of the cyclic prefix is to manage the effects of time dispersion of signals. When the transmission occurs over the point-to-point link as shown in Fig 3, the time dispersion is significantly less. Hence, the inventors have realised that when applied to the particular case of point-to-point communication, some of the positions intended for cyclic prefix samples can be used for samples for control data. In this way, the control data not related to the transmitted cellular network signal in the main samples can be transmitted alongside with the cellular network signal of the main samples without consuming additional link capacity. As explained in more detail below, when the control data samples are extracted on the receiver side, the control data is replaced with the cyclic prefix samples to get a sufficient amount of cyclic prefix samples to allow wireless transmission. This replacement is not complicated as long as the main samples have been received correctly since the cyclic prefix samples are copies of main samples.

Hence, no computationally expensive signal processing is needed to insert and extract the control data, nor to restore the OFDM signal to be

transmitted by reinserting the original cyclic prefix samples. In this way, the need for a parallel link to convey control data is eliminated, and/ or insertion of guard bands to separate the control data from the transmitted signal is avoided.

Looking now to Fig 4A, this illustrates a resource element being transmitted between the transmitter 38 and receiver 39 of Fig 3 over the point-to-point medium 19. Here, in a subset of the positions intended for cyclic prefix samples 32, control data samples 33 are carried instead. In other words, the main samples 31 are intact. After reception at the receiver 39, the control data samples 33 are extracted and then replaced by cyclic prefix samples to provide the resource element in accordance with LTE standards. The control data 33 is at the front of the resource element 35, whereby the residual cyclic prefix samples 32 are provided between the main samples 31 and the control data 33. Even when reduced in number, the residual cyclic prefix samples 32 provide robustness against channel distortions when sending the signal over the point-to-point medium. The residual cyclic prefix 32 should be longer in time than the excess delay at maximum length of the point-to-point medium. The residual cyclic prefix has the further advantage that it offers the possibility to recover the start of the OFDM symbol at the other end of the point-to-point medium.

In one embodiment, there are no cyclic prefix samples 32; instead, in all of the positions intended for cyclic prefix samples 32 of Fig 2, control data 33 samples are carried. This can be done when the time dispersion over the point-to-point medium is negligible. In Fig 4B, in a subset of the positions intended for cyclic prefix samples 32 (of Fig 2), control data cyclic prefix samples 34 are carried. The control data cyclic prefix samples are copies of a subset of the control data samples 33. In this way, there are cyclic prefix samples 32 for the main samples 31 and cyclic prefix samples for the control data samples 33. This allows (at least some) handling of time dispersion of the control data samples 33. Since the time dispersion is much less over the point-to-point medium compared to over the air cellular traffic, this can be completely sufficient for this application.

Fig 5 is a schematic diagram illustrating a modulator 10 of Fig 3 according to one embodiment. The modulator 10 comprises an inverse fast Fourier transform (IFFT) module 14, a parallel to serial converter 15, a cyclic prefix (CP) generator 16 and a control data inserter 17.

The modulator 10 is fed an input signal which comprises a number of subcarriers. These subcarriers are fed to the IFFT module 14. Furthermore, control data is received by the modulator 10 which is fed to the control data inserter 17.

The IFFT module 14 operates to convert the subcarrier inputs to time domain symbols, which are samples being complex numbers. The time domain symbols are provided by the IFFT module 14 to the parallel to serial converter 15 which serialises the time domain symbols to one stream of symbols, which are fed to the CP generator 16. The CP generator 16 generates the CP symbols l6

32, which are copies of a subset of the serialised symbols which form part of the main section. Specifically, the CP is generated by copying the last NCP symbols of the main section and adding the symbols to the beginning of the output from the parallel to serial converter 15. The control data inserter 17 then writes control data samples CD (provided externally) in at least some of the positions intended for cyclic prefix samples.

Distortions over the point-to-point medium connecting the two system components are typically less severe as those over the wireless channel.

Hence, a more spectrally efficient modulation and coding scheme maybe employed for the control data exchange. In order to facilitate channel estimation, reference symbols may be inserted together with the control data. Alternatively, the reference symbols already present in the regular transmit signal stream may be utilised to facilitate channel estimation.

On the other hand, if low decoding complexity at the receiver 39 is a priority, then a simple modulation scheme such as on-off keying or differential modulation may be utilised. For on-off keying or differential modulation, no channel estimation for control data detection is needed.

Each transmission block of control data may be encoded with a forward error correction (FEC) code with rate r to form a codeword. The robustness against transmission errors may be further increased by adding a number of bits for a cyclic redundancy check (CRC) and/or ACK/NACK

(Acknowledgement/Negative acknowledgement) bits for an automatic repeat request (ARQ) protocol.

Hence, there are no strict requirements on how the control data is

modulated. However, the control data bandwidth should be equal or smaller than the bandwidth of the OFDM signal. Moreover, since the original signal stream is OFDM, it may be efficient to use OFDM modulation for the control data as well. There is furthermore a difference between digital and analogue links. On a digital link (e.g. CPRI), it does not matter what we put instead of the CP. On an analog link, it may matter for two reasons: first, there is a risk that the control data causes interference into adjacent channels (if any). Second, if the control data bandwidth is larger than the filter applied before demodulation, some control data information will be lost.

As an example on how to fit an OFDM modulated control data (CD) signal into the cyclic prefix, consider LTE with 20 MHz bandwidth. For LTE, the duration of the useful part of the OFDM symbol is T_u = 2048 T_s « 66.7 and the cyclic prefix is TCP = 144 T_s « 4.7 μβ, where T_s is the sampling duration. Since the number of samples of the useful part of the OFDM symbol, NFFT, should be a two to the power of an integer to gain efficient IFFT and FFT, an appropriate choice for NCD maybe 128. The size of the residual cyclic prefix becomes NCP-NCD = 144-128 = 16, where NCP is the number of original cyclic prefix samples and NCD is the number of control data samples. The size of the residual cyclic prefix is then about 11% (16/144) of the original cyclic prefix length. The OFDM modulated control data signal would then employ a 128- point IFFT and use the same sampling duration T_s as the LTE signal.

As explained above, in one embodiment a cyclic prefix can be allocated to the OFDM modulated control data signal as well. In this case, the size of the residual cyclic prefix may be shrunk to accommodate a larger NCD. Given an equal length of both cyclic prefixes results in a length of 8 for the residual cyclic prefix. The length of the control data signal (including its cyclic prefix) then becomes NCD = 128+8 = 136.

Fig 6 is a schematic diagram illustrating a demodulator of Fig 3 according to one embodiment.

The demodulator 20 comprises a control data extractor 29 and a cyclic prefix (CP) replacer 26. The control data extractor extracts the control data samples (33 of Figs 4A-B) from the resource element and provides the control data samples to the device comprising the demodulator to make use of the control data.

The cyclic prefix replacer 26 then replaces the control data samples with cyclic prefix samples which were overwritten by the control data samples l8 the modulator. This replacement is not complicated as long as the main samples have been received correctly since the cyclic prefix samples are copies of main samples.

Figs 7A-B are flow charts illustrating embodiments of methods for inserting control data performed in the modulator of Fig 3. As explained above, the modulator can be included in a baseband module, combiner, remote radio head, etc.

In a write CD step 44, control data samples are written in an OFDM symbol at positions intended to be populated by cyclic prefix samples. As explained above, the cyclic prefix samples are copies of a subset of the main samples of the data symbol.. This writing can comprise replacing previously written cyclic prefix samples with the control data samples. Alternatively, e.g. when the modulator is in a baseband module, the cyclic prefix samples are only written at the positions (33 of Figs 4A-B) where control data will not be inserted. In such a case this step does not replace any cyclic prefix samples with control data.

In one embodiment control data samples are written at all positions of the OFDM symbol which are intended to be populated by cyclic prefix samples. In another embodiment, only a strict subset of the positions intended for cyclic prefix samples are populated with control data samples.

In a provide step 46, the OFDM symbol comprising the control data samples is provided on the output for transmission to a demodulator over a point-to- point medium. The point-to-point medium is a transmission medium between two entities and can e.g. be a cable such as an electric cable or a fibre optic cable.

Looking now to Fig 7B, only new or modified steps compared to the method illustrated by the flow chart of Fig 7A will be described.

In a write CP for CD step 45, control data cyclic prefix samples, which are copies of a subset of the control data samples, are written at a subset of positions intended to be populated by cyclic prefix samples. However, these positions are not populated by control data samples. This corresponds to Fig 4B and is explained in more detail above.

Figs 8A-B are flow charts illustrating embodiments of methods for extracting control data performed in the demodulator of Fig 3.

In a receive symbol step 50, an OFDM symbol is received from a point-to- point medium. The OFDM symbol comprises main samples and control data samples. In one embodiment, the OFDM symbol further comprises cyclic prefix samples, being copies of a subset of the main samples. In another embodiment, the OFDM symbol comprises only main samples and control data samples.

The point-to-point medium is a transmission medium between two entities and can e.g. be a cable such as an electric cable or a fibre optic cable.

In an extract CD step 52, the control data samples are extracted from the OFDM symbol.

In a replace with CP step 54, the control data samples in the OFDM symbol are replaced with cyclic prefix samples being copies of a subset of the main samples.

In one embodiment, each control data sample is replaced with a respective cyclic prefix sample such that all cyclic prefix samples of the OFDM symbol are a set of samples being contiguous copies of the main samples.

In a provide step 56, the OFDM symbol comprising the cyclic prefix samples is provided for transmission over a wireless medium to one or more mobile terminals. For example, the OFDM symbol can be an OFDM symbol in accordance with Long Term Evolution, LTE.

Looking now to Fig 8B, only new or modified steps compared to the method illustrated by the flow chart of Fig 8A will be described. In this embodiment, the OFDM symbol comprises control data cyclic prefix samples being copies of a subset of the control data samples.

In an extract CP for CD step 53, the control data cyclic prefix samples are extracted. Fig 9 is a schematic diagram showing some components of an embodiment of the modulator of Fig 3. A processor 60 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit etc., capable of executing software instructions 66 stored in a memory 64, which can thus be a computer program product. The processor 60 can be configured to execute the method described with reference to Figs 7A-B above.

The memory 64 can be any combination of random access memory (RAM) and read only memory (ROM). The memory 64 also comprises persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

A data memory 69 is also provided for reading and/ or storing data during execution of software instructions in the processor 60. The data memory 69 can be any combination of random access memory (RAM) and read only memory (ROM).

The modulator further comprises an I/O interface 67 for communicating with other external entities, e.g. over a point-to-point interface to a demodulator. Optionally, the I/O interface 67 also includes a user interface. Fig 10 is a schematic diagram showing some components of an embodiment of the demodulator of Fig 3. A processor 70 is provided using any

combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit etc., capable of executing software instructions 76 stored in a memory 74, which can thus be a computer program product. The processor 70 can be configured to execute the method described with reference to Figs 8A-B above.

The memory 74 can be any combination of random access memory (RAM) and read only memory (ROM). The memory 74 also comprises persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

A data memory 79 is also provided for reading and/ or storing data during execution of software instructions in the processor 70. The data memory 79 can be any combination of random access memory (RAM) and read only memory (ROM).

The demodulator further comprises an I/O interface 77 for communicating with other external entities, e.g. over a point-to-point interface to a modulator. Optionally, the 1/ O interface 77 also includes a user interface.

Fig 11 is a schematic diagram showing functional modules of the software instructions of the modulator of Fig 3 or Fig 9 according to one embodiment. The modules are implemented using software instructions such as a computer program executing in the modulator. The modules correspond to the steps in the methods illustrated in Figs 7A-B.

A CD obtainer 82 corresponds to step 42. A writer 84 corresponds to steps 44 and 45. A provider 86 corresponds to step 46.

embodiment. The modules are implemented using software instructions such as a computer program executing in the demodulator. The modules correspond to the steps in the methods illustrated in Figs 8A-B. A receiver 93 corresponds to step 50. A CD extractor 94 corresponds to step 52. A CP extractor 95 corresponds to step 53. A replacer 96 corresponds to step 54. A provider 97 corresponds to step 56.

Fig 12 shows one example of a computer program product comprising computer readable means. On this computer readable means a computer program 91 can be stored, which computer program can cause a processor to execute a method according to embodiments described herein. In this example, the computer program product is an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. As explained above, the computer program product could also be embodied in a memory of a device, such as the computer program product 64 of Fig 9 or the computer program product 74 of Fig 10. While the computer program 91 is here schematically shown as a track on the depicted optical disk, the computer program can be stored in any way which is suitable for the computer program product, such as a removable solid state memory, e.g. a Universal Serial Bus (USB) drive.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. A method for inserting control data in a modulator (10), the method being performed in the modulator and comprising the steps of:

obtaining (42) control data samples (33);

writing (44) the control data samples (33) in a data symbol (30) at positions intended to be populated by cyclic prefix samples (32), the cyclic prefix samples (32) being copies of a subset of main samples (31) of the data symbol; and

providing (46) the data symbol (30) comprising the control data samples (33) on the output for transmission to a demodulator (20) over a point-to-point medium.

2. The method according to claim 1, wherein the step of writing the control data samples (33) comprises replacing previously written cyclic prefix samples with the control data samples.

3. The method according to claim 1 or 2, wherein the step of writing (44), comprises writing the control data samples at all positions of the data symbol which are intended to be populated by cyclic prefix samples.

4. The method according to any one of the preceding claims, further comprising the step of:

writing (45) control data cyclic prefix samples (34) at a subset of positions intended to be populated by cyclic prefix samples (32), which are not populated by control data samples (33), the control data cyclic prefix samples (34) being copies of a subset of the control data samples (33).

5. The method according to any one of the preceding claims, wherein the point-to-point medium is a cable.

6. The method according to any one of the preceding claims, the data symbol is an orthogonal frequency division multiplex, OFDM, symbol (30) in accordance with Long Term Evolution, LTE.

7. A modulator (10) for inserting control data, the modulator (10) comprising:

a processor (60); and

a memory (64) storing instructions (66) that, when executed by the processor, cause the modulator to:

obtain control data samples (33);

write the control data samples (33) in a data symbol (30) at positions intended to be populated by cyclic prefix samples (32), the cyclic prefix samples (32) being copies of a subset of main samples (31) of the data symbol; and

provide the data symbol (30) comprising the control data samples (33) on the output for transmission to a demodulator (20) over a point-to-point medium.

8. The modulator (10) according to claim 7, wherein the instructions to write comprise instructions (66) that, when executed by the processor, cause the modulator to replace previously written cyclic prefix samples with the control data samples.

9. The modulator (10) according to claim 7 or 8, wherein the instructions to write comprise instructions (66) that, when executed by the processor, cause the modulator to write the control data samples at all positions of the data symbol which are intended to be populated by cyclic prefix samples.

10. The modulator (10) according to any one of claims 7 to 9, further comprising instructions (66) that, when executed by the processor, cause the modulator to write control data cyclic prefix samples (34) at a subset of positions intended to be populated by the cyclic prefix samples (32), which are not populated by control data samples (33), the control data cyclic prefix samples (34) being copies of a subset of the control data samples (33).

11. The modulator (10) according to any one of claims 7 to 10, wherein the point-to-point medium is a cable.

12. The modulator (10) according to any one of claims 7 to 11, wherein in the instructions to receive, the data symbol (30) is an orthogonal frequency division multiplex, OFDM, symbol in accordance with Long Term Evolution, LTE.

13. A modulator (10) comprising:

means for obtaining control data samples (33);

means for writing the control data samples (33) in a data symbol (30) at positions intended to be populated by cyclic prefix samples (32), the cyclic prefix samples (32) being copies of a subset of main samples (31) of the data symbol; and

means for providing the data symbol (30) comprising the control data samples (33) on the output for transmission to a demodulator (20) over a point-to-point medium.

14. A computer program (91) for inserting control data, the computer program comprising computer program code which, when run on a the modulator (10) cause the modulator (10) to:

obtain control data samples (33);

15. A computer program product (90) comprising a computer program according to claim 14 and a computer readable means on which the computer program is stored.

16. A method for extracting control data in a demodulator (20), the method being performed in the demodulator and comprising the steps of:

receiving (50) a data symbol (30), from a point-to-point medium, the data symbol (30) comprising main samples (31) and control data samples (33);

extracting (52) the control data samples from the data symbol (30); replacing (54) the control data samples in the data symbol (30) cyclic prefix samples (32) being copies of a subset of the main samples (31); and providing (56) the data symbol (30) comprising the cyclic prefix samples (32) for transmission over a wireless medium to one or more mobile terminals (2).

17. The method according to claim 16, wherein in the step of receiving (50), the data symbol (30) further comprises cyclic prefix samples, being copies of a subset of the main samples (31); and

wherein the step of replacing (54) comprises replacing the control data samples with cyclic prefix samples such that all cyclic prefix samples of the data symbol is a set of samples being contiguous copies of the main samples.

18. The method according to claim 16, wherein in the step of receiving, the data symbol (30) comprises only main samples (31) and control data samples (33).

19. The method according to claim 16 or 17, wherein in the step of receiving (50), the data symbol (30) comprises control data cyclic prefix samples (34) being copies of a subset of the control data samples (33), and wherein the method further comprises the step of:

extracting (53) the control data cyclic prefix samples (34).

20. The method according to any one of claims 16 to 19, wherein the point- to-point medium is a cable.

21. The method according to any one of claims 16 to 20, wherein in the step of providing (56), the data symbol (30) is an orthogonal frequency division multiplex, OFDM, symbol in accordance with Long Term Evolution, LTE.

22. A demodulator (20) for extracting control data, the demodulator comprising: a processor (70); and

a memory (74) storing instructions (76) that, when executed by the processor, cause the demodulator (20) to:

receive a data symbol (30), from a point-to-point medium, the data symbol (30) comprising main samples (31) and control data samples (33); extract the control data samples from the data symbol (30);

replace the control data samples in the data symbol (30) with cyclic prefix samples (32) being copies of a subset of the main samples (31); and provide the data symbol (30) comprising the cyclic prefix samples (32) for transmission over a wireless medium to one or more mobile terminals (2).

23. The demodulator (20) according to claim 22, wherein in the

instructions to receive, the data symbol (30) further comprises cyclic prefix samples, being copies of a subset of the main samples (31); and

wherein the instructions to replace comprise instructions (76) that, when executed by the processor, cause the demodulator (20) to replace the control data samples with cyclic prefix samples such that all cyclic prefix samples of the data symbol is a set of samples being contiguous copies of the main samples.

24. The demodulator (20) according to claim 22, wherein in the

instructions to receive, the data symbol (30) comprises only main samples (31) and control data samples (33).

25. The demodulator (20) according to claim 22 or 23, wherein in the instructions to receive, the data symbol (30) comprises control data cyclic prefix samples (34) being copies of a subset of the control data samples (33), and wherein the demodulator further comprises instructions (76) that, when executed by the processor, cause the demodulator (20) to extract the control data cyclic prefix samples (34).

26. The demodulator (20) according to any one of claims 22 to 25, wherein the point-to-point medium is a cable.

27. The demodulator (20) according to any one of claims 22 to 26, wherein in the instructions to provide, the data symbol (30) is an orthogonal frequency division multiplex, OFDM, symbol in accordance with Long Term Evolution, LTE.

28. A demodulator comprising:

means for receiving a data symbol (30), from a point-to-point medium, the data symbol (30) comprising main samples (31) and control data samples (33);

means for extracting the control data samples from the data symbol (30);

means for replacing the control data samples in the data symbol (30) with cyclic prefix samples (32) being copies of a subset of the main samples (31); and

means for providing the data symbol (30) comprising the cyclic prefix samples (32) for transmission over a wireless medium to one or more mobile terminals (2).

29. A computer program (91) for extracting control data, the computer program comprising computer program code which, when run on a

demodulator (20) cause the demodulator (20) to:

receive an orthogonal frequency division multiplex, data, symbol (30), from a point-to-point medium, the data symbol (30) comprising main samples (31) and control data samples (33);

extract the control data samples from the data symbol (30);

30. A computer program product (90) comprising a computer program according to claim 29 and a computer readable means on which the computer program is stored.