WO2015038070A1

WO2015038070A1 - A communication device and related method of communication

Info

Publication number: WO2015038070A1
Application number: PCT/SG2014/000429
Authority: WO
Inventors: Quee Seng Tony Quek; Marco MASO; Subhash LAKSHMINARAYANA
Original assignee: Singapore University Of Technology And Design
Priority date: 2013-09-16
Filing date: 2014-09-12
Publication date: 2015-03-19

Abstract

A communication device (300) is disclosed, which comprises a splitter module (306) configured to separate a cyclic prefix from a communications signal; an energy harvester module (308) configured to convert the cyclic prefix portion of the communications signal into a DC output; and a decoder module (310) configured to demodulate the communications signal according to a block- transmission scheme, the decoder module at least partially powered by the DC output from the energy harvesting module. A related method of communication is also disclosed.

Description

A Communication Device and Related Method of Communication

Field

The present invention relates to a communication device and a related method of communication. Particularly, but not exclusively, it pertains to a receiver for orthogonal frequency division multiplexing (OFDM) communication.

Background

In wireless communication systems, a receiver is usually driven by an external power source (e.g . a battery) , which provides electrical power to the receiver to decode information symbols received in an RF signal sent by a transmitter (and to enable other functions at the receiver). With recent advances in microwave technology and signal processing, use of wireless power transfer (WPT) has also enabled power to be wirelessly transferred from one device to the other [1 ]. The discussions below will focus on com munications carried out using the OFDM protocol, a type of block-transmission scheme.

An OFDM signal typically comprises two components: ( 1 ). a first component of the signal for carrying the information to be relayed to the receiver, and (2). a second component for mere redundancy purposes known as the cyclic prefix (CP). It is to be appreciated that the purpose of having the CP is of twofold [3]. Firstly, the CP is appended by the transmitter to accommodate the channel impulse response and reduce inter-block interference (I BI) between two adjacent OFDM blocks. Secondly, presence of the CP eliminates inter-carrier interference (ICI) between sub-carriers at the receiver and allows for simpler signal equalization procedures. Specifically, in conventional OFDM receivers, the CP is removed and discarded after the time-frequency synchronization, since the content of the CP is not useful for information retrieval purposes. A significant amount of power is however completely wasted (at an OFDM receiver) during the CP removal operation, regardless of a size difference between the CP portion and a portion of the OFDM signal carrying the information. This undesirably translates into an unavoidable spectral efficiency loss for the transmitter. As an example, consider the following : FI G. 1 shows a generic scenario 1 00 of an OFDM transmitter (OTX) 102 communicating with an OFDM receiver (ORX) 104. The communication may be carried out in time division duplex (TDD) or frequency division duplex (FDD) mode. Specifically, in this case, the ORX 1 04 is a legacy OFDM receiver 200 arranged with the associated device architecture depicted in FI G . 2. For sake of explanation clarity, the sig nal model is briefly discussed below.

Let h = [h₀ . . . h define an independent and identical d istributed Rayleigh fading channel vector, of size / + 1 taps, representing a frequency-selective channel between the said OTX 102 and ORX 104. Each transmitted OFDM symbol spans N + L symbols in the ti me domain, consisting of N data samples and L≥ I samples of cyclic prefix (CP) [2] . This is done in order to accommodate effects of the multipath propagation , inherent to freq uency-selective wireless channels. Then define s = [s_f . . . s_N ] as an A/-sized input symbol vector at the OTX 1 02 and p = [p_i . . . p_N ] as an /V-sized vector carrying the power associated to each of its symbols. ^ is defined as a unitary discrete

Fourier transform (DFT) N - ] } and

as a CP insertion matrix of size {N + L) x N, then the (N + L)-sized transm it signal is consequentially defined as: x = AF ^~' s (2)

Thus, the received signal at the ORX 1 04 may be expressed as: y = Hx + n (3) wherein n ~ CN(o, a² I_N+l ) represents an additive white Gaussian noise (AWGN) vector and H is an (N + L) x (N + L) matrix modelling the convolution of the channel from the OTX 102 to the ORX 104 with x, given by equation (4) below: h₀ 0 · ·^■ 0 h,

H = K (4)

o 0

0

0 0 h, K

Thereafter, let y = \ y represent a row vector representing the received stream.

After the synchronization process, a CP prefix removal operation is performed on y, and the first L received samples are discarded. Afterwards, a DFT yields r, /V-sized frequency domain representation of the information symbols, is defined as: r = FBy = F H AF^~ls + F n (5) wherein B = [0_Nxl I_N ] is an N x (N + L) matrix representing the CP removal operation, H is an N x (N + L) matrix defined as H_d stripped off the L first lines and F n has size N and same statistical parameters as n. A one-tap equalization and further decoding operations concludes the information retrieval process at the ORX 104. It will thus be appreciated that the content in and nature of the CP are completely neglected by the ORX 1 04 because the CP does not carry any useful information. As aforementioned, regardless of the size difference between the CP and a portion of the OFDM sig nal carrying the information, a significant amount of power (i.e. up to 20% [4]) is wasted at the ORX 04 during the CP removal operation, consequently resu lting in a spectral efficiency loss. One object of the present invention is therefore to address at least one of the problems of the prior art and/or to provide a choice that is useful in the art.

Summary

According to a 1 ^st aspect of the invention, there is provided a communication device comprising a splitter module configured to separate a cyclic prefix from a communications signal; an energy harvester module configured to convert the cyclic prefix portion of the communications signal into a DC output; and a decoder module configured to demodulate the communications signal according to a block-transmission scheme, the decoder module at least partially powered by the DC output from the energy harvesting module.

Advantageously, the proposed device is arranged to exploit the CP to extract electrical power from the communication signal (e.g. an OFDM signal), thus enabling self-sustainable wireless power transfer to be achieved from the OFDM transmission via a transmitter to the device, which consequently allows a battery life of the device to be also extendable. Hence, loss of spectral efficiency at the transmitter is beneficially transformed into a tunable energy saving for the device.

Preferably, the block-transmission scheme may be orthogonal frequency division multiplexing.

Preferably, the decoder module may be configured to use Discrete Fourier Transform for demodulating the communications signal.

In addition, the device may preferably further comprise an equalization module, a Parallel-to-Serial converter module , and a constellation demapper module for processing the demodulated commu nications signal from the decoder module, wherein the equalization module, Parallel-to-Serial converter module, and constellation demapper are at least partially powered by the DC output.

According to a 2^nd aspect of the i nvention, there is provided a method of communication comprising separating a cyclic prefix from a communications signal; extracting the energy from the cyclic prefix portion of the communications signal into a DC output; and demodulating the communications signal according to a block-transmission scheme, at least partially using the power from the DC output.

Preferably, demodulating the communications signal may include using Discrete Fourier Transform for demodulating the communications signal.

It should be apparent that features relating to one aspect of the invention may also be applicable to the other aspects of the invention.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Brief Description of the Drawings

Embodiments of the invention are disclosed hereinafter with reference to the accompanying drawings, in which:

FIG. 1 shows a typical scenario of an OFDM transmitter communicating with an OFDM receiver, according to prior art.

FIG. 2 is a block diagram of the device architecture of a legacy OFDM receiver, according to prior art.

FIG. 3 is a block diagram of the device architecture of a communication device, according to an embodiment of the invention.

FIG. 4 is a flow diagram of an exemplary method, according to the said embodiment.

FIG. 5 is a graphical comparison of channel prefix decoding and extraction operations for the legacy OFDM receiver of FIG.2 and the device of FIG. 3.

FIG. 6 is a graph showing a portio n of total decoding power required at the device of FIG. 3 that is harvested from an OFDM signal received by the said receiver as L changes, with N set to a value of 128. Detailed Description of Preferred Embodiments

1. Self-sustainable OFDM transmission - receiver architecture

A proposed communication device 300 (hereafter as device for brevity) is disclosed, according to an embodiment shown in FIG. 3. Specifically, the device 300 is configured as a receiver for a block-transmission scheme communication, e.g . the OFDM protocol. Hereafter in this embodiment, for sake of illustration, discussion of the device 300 will be made with reference to the OFDM protocol . Thus, a reference to the device 300 then includes a reference to it as an OFDM receiver (to function as the ORX 104 depicted in FIG. 1 ), but not however to be construed as limiting as such. It is to be appreciated that the device 300 is arranged to exploit the CP, an unavoidable redundancy inherent to any OFDM transmission, to enable wireless power transfer from the OTX 102 to the device 300. The device 300 comprises the following signal processing components: a time-and-frequency synchronization module 302, a Serial-to-Parallel (S/P) converter module 304, a cyclic prefix (CP) extraction module 306, an energy harvester module 308, a decoder module 31 0, an equalization module 312, a Parallel-to-Serial (P/S) converter module 314, and a constellation demapper module 316. The decoder module 310 is realised using Discrete Fourier Transform (DFT) in this case, but not limited to as such. Also, it is to be appreciated that the CP extraction module 306 replaces a CP removal module 202 typically arranged in conventional OFDM receivers (i.e. see FIG. 2) .

The time-and-frequency synchronization module 302 receives a (base-band) signal transmitted by the OTX 102, and performs necessary time and frequency synchronization processing on the sig nal before forwarding the processed signal to the S/P converter module 304. The S/P converter module 304 is configured to convert the processed signal into a p lurality of samples, being N + L, which are collectively a total number of samples in this case. Specifically, the N and L samples correspond to the information portion and CP portion of the received signal, respectively. It is to be appreciated that the L number of samples may include one CP or multiple CPs. Thereafter, via the CP extraction module 306, the N + L number of samples are separated into the two portions N and L, due to the fixed structure imposed by the OFDM transmission (as is known in the art and also from communication theory) . That is, the plurality of N + L number of samples are forwarded to the CP extraction module 306 (configured to function as a signal splitter) for separating the CP portion from the information portion of the received signal, and thereafter the N number of samples are provided to the decoder module 310 to be demodulated according to the OFDM protocol, while the L number of samples are provided to the energy harvester module 308 to be extracted for electrical power (i.e. convert the CP portion of the received signal into a DC output). The demodulated N number of samples are provided to the equalization module 312 to be appropriately equalized as required, before being combined into a single signal by the P/S converter module 314. The combined single signal is fed into the constellation demapper module 316 to be processed for generating an output bitstream corresponding to the information stored in the original received signal.

Accordingly, a related method 400 of communication using the device 300 is shown in FIG. 4, in which the said method 400 comprises separating a cyclic prefix from the signal at step 402, extracting energy from the cyclic prefix portion of the signal into a DC output at step 404, and then demodulating the signal according to a block-transmission scheme (e.g . the OFDM protocol) , at least partially using the power from the DC output at step 406.

Hence, the information portion (i.e. the N samples) of the received signal is provided to a decoding portion of the device 300, whereas the CP portion (i.e. the first L samples) is provided to a portion of the device 300 capable of extracting energy from the received signal. As a result, a size of the CP at the OTX 102 is configurable such that the CP may be exploited by the ORX 104 to extract useful power from the received signal, which is otherwise typically wasted in conventional OFDM receivers. So in this embodiment, the role of the CP in the OFDM transmission is significantly modified by beneficially transforming a spectral efficiency loss for the OTX 102 instead into a tunable power gain for the device 300. Through appropriate tuning of a group of transmit parameters, the OTX 102 is consequently able to transfer enough power to the proposed device 300 to decode all information symbols included in the received signal, zeroing the impact of the OFDM decoding on an available battery life of the device 300, and thereby yielding a self-sustainable OFDM transmission. The reason for this is a power radiated at the OTX 102 is normally about several orders of magnitude greater than a power required at the device 300 to decode an information symbol. It is to be appreciated that power consumption per information symbol using Fast Fourier transform (FFT), being the most speed and power demanding portion for OFDM reception using legacy receivers, may be made as small as a few μ\Λ/ [8].

In other words, the proposed device 300 is configured to exploit the communication structure imposed by OFDM transmission to realize wireless power transfer from the transmitter to the device 300, which (different to conventional techniques) is accomplished without requiring any significant modification of the OFDM protocol or signal processing procedures at the OTX 102. Moreover, no consequential or additional impact is caused on surrounding devices or systems which may otherwise be using other types of different communication technologies.

Now with reference to the signal model afore explained in the "Background" section, for the proposed device 300, the N number of samples forwarded from the CP extraction module 306 to the decoder module 31 0 still carry the vector r, unaltered in comparison to the legacy OFDM receiver 200 of FIG. 2. On the other hand, a new L-sized vector q, defined as: q = Qy (6) wherein Q = [l_t 0 _N J L x N + L matrix extracted the CP from y, is carried by the branches connecting the CP extraction module 306 and energy harvester module 308. Subsequently, power extracted from the CP by the energy harvester module 308 is provided to the decoder module 310, equalization module 312, P/S converter module 31 4, and constellation demapper module 316 for powering thereof, thus enabling self-sustainable OFDM transmission. A graphical comparison 500 in FIG. 5 depicts differences in terms of signal representation between channel prefi x decoding and extraction operations for the legacy OFDM receiver 200 of FIG . 2 and the proposed device 300, in which the synchronization reference provided by the time-and-frequency synchronization module 302 is also considered for sake of simplicity of the representation. 2. Performance Evaluations

Mathematical analyses have been carried out to show feasibility of the proposed device 300, and in particular, it has been found that closed form feasibility conditions are derivable. Moreover, data from numerical tests performed also confirmed the find ings of those ana lyses. Performance evaluations of the proposed device 300 for different configurations are now explained below.

According to a model typically adopted in similar stud ies [1 0] , as a reasonable first approximation , the power consum ption of circuit components (of a receiver) arranged for decoding is considered proportional to a number of information symbols included in a received sign al. Thus, let -Ρ, ε Ή ^* represent power consumed by the device 300 per decoded information symbol . Furthermore, if the energy harvester module 308 is also taken into account, it is noted that existing literature contribution on wireless power transfer typically model the RF- to-DC conversion to scavenge energy from a received sig nal a nd convert the scavenged energy into electrical energy, by means of an energy conversion efficiency ? e [0,l], as a result of the law of energy conservation [ 1 1 ], [1 2] . I n practice, β represents all possible i nefficiencies associated with the energy conversion performed by the energy harvester module 308, and (by definition) has a maximum assigned value of 1 . For the following numerical evaluations to be discussed, a conservative value for the energy conversion efficiency is assumed , i .e. β - 0.5 (which is typically made in relevant literature on this subject [1 1 ] , [12]) to frame a likely rea l istic operating scenario.

Additionally, for simplicity in representation of the evaluation results, define P_MAX = i for a normalized total transm it power budget at the OTX 1 02 and defi ne p

ζ ₌ ^Ι χ _{as a ra}tj₀ between the total t ransmit power budget at the OTX 102 and the power consumed by the device 300 per decoded information symbol . Specifically, it is assumed that the OTX 102 and the device 300 are separated by a physical distance of some tenth s of meters and the path loss factor has a generic value of about 3, and so it is th us safe to let ξ & {300,600} . For information , it is to be noted that power radiated by a base station may range from several tenths to some hundreds of Watts, depending on a number of antennas deployed at the base station . Finally, assum ing that a full self- sustainability may not always be possible for the different configurations considered in the evaluations, let p = diag(p) ^ ^ ^N"^N and define:

wherein δ represents the ratio between a total harvested power and NP_d, which is a total decoding power consumption at the device 300. For the performance evaluations, extensive Monte Carlo simulations were performed to obtain statistically relevant results in respect of the proposed device 300. This consequently allowed a reliable assessment of the merits of the proposed device 300 to be achieved for characterizing the performance. It is to be appreciated that no particular channel model was adopted in the evaluations and hence, independent and identical distributed Rayleigh fading channels of size / + 1 taps were considered, with uniform power delay profile, as afore described for the signal model explained in the ''Background" section . Specifically, each channel vector h is generated such that h ~ CN(0J_{/+ l} /(/ + 1)) .

Finally, for computational tractability, it is assumed that N = 128, and

^' 9 l_

L e TV , in accordance with the proposed for the least resource dema nd ing .128 ' 4

long term evolution (LTE) OFDM config uration [4].

_9_ J_

Now, let L vary within the aforementioned interval (i.e. z, e N ) and

128 ' 4 computed values obtained for δ are d epicted in a g raph 600 shown in FIG 6. It is observed from FI G . 6 that as L cha nges, a maxim um supported δ varies (i . e. δ' ) between a value of 0.41 (for ξ = 300) and a value of 1 .63 (for ξ = 600) , even if β has a value of as low as 0.5. It is to be appreciated that δ^' > ] simply implies that a power that may be scavenged from^' the CP is g reater than the decoding power consumption at the device 300. The results in FIG . 6 thus confirmed that values of L for which δ ' is non-negative may always be found for each of the considered configurations , showing that a partial self-sustainabil ity is always possible, regardless of the configuration adopted. In cases in which ξ ≤ 400, even though full self-sustainability is not achievable, a non-negligible amount of power may still be harvested by the device 300, and so impact of the OFDM decoding on the battery life at the device 300 may still be significantly reduced too. Conversely, for cases in which ξ > 400, full self-sustainability is achieved from the resulting OFDM transmission. Therefore, it wt^'i\ be appreciated that if ξ is of a fairly large value (i.e. greater than 400), then power scavenged from the CP is not only sufficient to compensate for power consumed during the decoding process, but is also able to provide further additional power that may be stored for later usage (if necessary), thereby further extending the battery life of the device 300.

Furthermore, from a qualitative perspective, a decreasing trend for the values of L is identifiable that allows for self-sustainability of the OFDM transmission as the value of ξ increases. This implies that more energy efficient signal processing components may be arranged at the device 300 (or having a lower distance separating the OTX 102 and ORX 04) to enable appropriate reduction of a size of the CP to achieve self-sustainability, thus allowing a higher OFDM downlink rate to be attainable. In pa rticular, if ξ > 400, a size of the CP lower than — is sufficient to achieve full self-sustainability, wherein L =— also

4 . 4 advantageously coincides as corresponding to the extended mode of the OFDM downlink transmission defined in present LTE standards [4].

3. Summary

The proposed device 300 exploits the concept of wireless power transfer (e.g. in OFDM communications) to prolong a battery life of the device 300. Particularly, the device 300 is arranged not to discard the CP of the received signal, but rather to exploit the CP to extract electrical power from the signal. This effectively enables wireless power transfer between the OTX 102 and the device 300 (functioning as the ORX 104) to be attained. According ly, loss of spectral efficiency at the OTX 102 is transformed into a tunable energy saving for the device 300. In principle, a local power source (e.g. a battery) of the ORX 104 may then at least be partially relieved of power consumed during the process of decoding an OFDM signal, and thu s beneficially extends the usable operating life of the power source. Further, appropriate tuning of a group of transmit parameters may also result in an amount of power carried by the CP to be sufficient for decoding information symbols included in the signal, but advantageously without requiring modifications to the OFDM protocol or decoding procedures at the ORX 104. Hence, the OFDM signal provides the device 300 with both the information and power necessary for retrieval thereof, enabling the OFDM transmission to be self-sustainable in terms of power consumption at the device 300.

It is appreciated that the idea of enabling self-sustainable OFDM transmission as adopted in the device architecture of the proposed device 300 is appealing in many contexts and has wide envisaged applications (i.e. not limited to a specific product). Presently, the market penetration of technologies and standards in the telecommunications sector using the OFDM protocol is growing rapidly. Particularly, a vast majority of modern communication standards adopt the OFDM protocol as the underlying technology for the physical layer (in terms of the OSI layer terminology). For exam ple, Wireless Fidelity (WiFi) technologies and many modern mobile communication standards (e.g. 3G/4G services) use the OFDM protocol. Hence, any OFDM transmission may potentially be configurable to be self-sustainable by adopting the architecture/methodology proposed in the said device 300 of FIG. 3, and also through appropriate adaptation of a size of the CP. Accordingly, usage of the device architecture in the proposed device 300 enables extension of the battery life thereof; indeed this advantage is applicable to any battery powered communication device (whether fixed or mobile) acting as a receiver for a block-transmission scheme, thus generally resulting in lower power consumption and operating costs. Operation of the proposed device 300 is independent of operating environments adhering to any specific communication standards, and thus is flexibly usable in any country/industry, where disparate communication standards/protocols may be adopted; i.e. the proposed device 300 is interoperable with any existing communication standards/protocols.

From a practical perspective, implementability of the proposed device 300 is related to technological advancements in the field of radio frequency to direct current energy conversion [5], State-of-the-art in afore said field is rapidly advancing, in which the prototyping activity is ever-growing [7] and initial commercial products/solutions are starting to appear on the market. Accordingly, it is to be appreciated that the theoretical conception of the proposed device 300 is thus an initial step towards development of a proof-of- concept, in the form of a prototype.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention. For example, the device 300 may also be suitably modified for use in non-OFDM based systems. In addition, the decoder module 31 0, equalization module 312, P/S converter module 314, and constellation demapper module 316 may at least be partially powered (rather than fully powered) by the power extracted from the CP by the energy harvester module 308.

References

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3GPP. TR 36.814, Further advancements for E-UTRA physical layer aspects, v.9.0.0. Technical report, 3GPP, March 2010.

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Claims

1 . A communication device comprising :

a splitter module configured to separate a cyclic prefix from a communications signal;

an energy harvester module configured to convert the cyclic prefix portion of the communications signal into a DC output; and

a decoder module configured to demodulate the communications signal according to a block-transmission scheme, the decoder module at least partially powered by the DC output from the energy harvesting module.

2. The device of claim 1 , wherein the block-transmission scheme is orthogonal frequency division multiplexing .

3. The device of any preceding claims, wherein the decoder module is configured to use Discrete Fourier Transform for demodulating the communications signal.

4. The device of any preceding claims, further comprises an equalization module, a Parallel-to-Serial converter module, and a constellation demapper module for processing the demodulated communications signal from the decoder module,

wherein the equalization module, Parallel-to-Serial converter module, and constellation demapper are at least partially powered by the DC output.

5. A method of communication comprising:

separating a cyclic prefix from a communications signal;

extracting the energy from the cyclic prefix portion of the communications signal into a DC output; and

demodulating the communications signal according to a block- transmission scheme, at least partially using the power from the DC output.

6. The method of claim 5, wherein the block-transmission scheme is orthogonal frequency division multiplexing .

7. The method of claim 5 or 6, wherein demodulating the communications signal includes using Discrete Fourier Transform for demodulating the communications signal.