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Implementation of Non-Orthogonal Multiplexing for Narrow Band Power Line Communications

Itziar Angulo, David de la Vega, Amaia Arrinda, Igor Fernandez

Dept. Communications Engineering Univ. of the Basque Country (UPV/EHU)

Bilbao, Spain E-mail: itziar.angulo@ehu.eus

Iñigo Berganza Iberdrola S.A. Bilbao, Spain

Asier Llano, Txetxu Arzuaga LV Products

ZIV Metering Zamudio, Spain

Abstract—This paper explores the possibilities of Layered Division Multiplexing (LDM), a non-orthogonal multiplexing technique, for its application in Narrow Band Power Line Communications (NB-PLC). Both theoretical calculations and simulation results are included based on PoweRline Intelligent Metering Evolution (PRIME) in CENELEC A-band. However, improvements presented in the paper can be directly applied to any NB-PLC system based on Orthogonal Frequency Division Multiplexing (OFDM). LDM proves to be a promising technology for future PLC applications, based on obtained results.

Keywords—LDM; PLC; non-orthogonal multiplexing; OFDM; PRIME

I. INTRODUCTION

Layered Division Multiplexing (LDM) is a non-orthogonal multiplexing technique, now included as part of ATSC 3.0, the latest version of the Digital Terrestrial Television (DTT) standard used mostly in North America and South Korea [1]-[3]. LDM has demonstrated improved efficiency in the frequency use, with respect to the classical multiplexing techniques such as Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM) [4]. It is based on power division multiplexing, where the 100% of the bandwidth is used 100% of the time [1]-[3].

Beyond broadcasting, LDM is a general purpose multiplexing technique that can be applied to other communication systems. This paper analyzes the possibilities of LDM for Narrow Band Power Line Communications (NB-PLC). Although the analysis could be applied to any NB-PLC system based on Orthogonal Frequency Division Multiplexing (OFDM), this paper is specifically focused on PoweRline Intelligent Metering Evolution (PRIME) [5]. In the literature, the advantages of LDM when compared to the classical multiplexing techniques TDM or FDM have been already demonstrated for other uses [2],[4],[6]; in the case of PRIME, the numerical results of the comparison will depend on the selected configurations for both LDM and TDM/FDM when applied to PRIME.

This paper is organized as follows. Section II introduces the LDM technology. Section III discusses some application scenarios for the LDM technology applied to NB-PLC. Section IV describes the implementation of the LDM

technology in PRIME v1.3.6, with both theoretical and simulation results. Finally, Section V summarizes the main conclusions of the paper.

II. LAYERED DIVISION MULTIPLEXING

A. Main features

LDM consists in sending two (or more) data streams, with a power difference between them of ∆ dB, calculated as the power of the upper level minus the power of the lower level. For the case of two data streams, the Lower Layer (LL) signal is injected ∆ dB below the Upper Layer (UL) signal (see Fig. 1).

The UL signal and the LL signal might be dedicated to independent services, and might have different characteristics in terms of robustness or data rate. For instance, the UL signal may include more robust coding techniques with lower data rates (difficult reception conditions, critical services or network management data, for example), while the LL signal may include less robust coding techniques with higher data rates (data services for better reception conditions or additional services). The distribution of power for each layer is completely configurable, but there is always a compromise between the selected injection level and the minimum Signal to Noise Ratios (SNRs) required for accessing each layer.

Fig. 1. Graphical representation of a LDM signal in the frequency domain

IEEE ISPLC 2017 1570329662

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B. LDM transmitter

As explained before, the UL and LL provide independent services that may have different characteristics, since they have their own Bit-Interleaved Coded Modulation (BICM) modules at the transmitter. Then, they are combined according to the injection level ∆, which determines how the total transmission power is distributed between the two layered signals [6].

Assuming that the total transmitted power is distributed as PT = PUL + PLL, then

10

1

1 10UL TP P

10

10

10

1 10LL TP P

where PUL and PLL are the power levels assigned to the UL and LL respectively. For instance, a 4 dB injection level indicates that the UL signal is transmitted with the 72% of the total power, whereas the LL signal is transmitted with the remaining 28% [6].

Fig. 2 shows the implementation of the LDM technology in the transmission block diagram of PRIME, and Fig. 3 shows an example of an LDM constellation formed from two PRIME signals.

C. LDM receiver

In order to access the LL signal, one of the most common approaches is to cancel the UL signal in the receiver. To do so, a decoding block and a re-modulation/cancellation path are needed in the receiver, whereas the alignment and equalization parts will be the same for both layers. Although this cancellation process involves additional complexity to perform channel decoding and re-encoding, it provides the most reliable UL signal estimate in most cases [6].

Fig. 4 shows the implementation of the cancellation process in a reception block diagram of PRIME. It should be noted that for UL-only reception, no changes are needed in the receiver and thus, the addition of LDM technology would be fully backwards compatible with a single layer (UL only) receiver.

Depending on the configuration of the LDM signal and, consequently, the corresponding working range of SNRs, it is possible to implement a low complexity LDM receiver that is able to decode the LL without cancellation. This significantly reduces the complexity of the receiver, since it is no longer needed to regenerate the UL signal to access the LL.

Fig. 2. Block diagram of a PRIME PLC transmitter with Layered Division Multiplexing technology.

Fig. 3. Example of a LDM constellation for PRIME (a) UL DBPSK (b) LL D8PSK (c) Combined LDM constellation.

(a) (b) (c)

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Fig. 4. Block diagram of a PRIME receiver with LDM technology applying cancellation.

On the contrary, this multiplexing technique involves a potential inter-layer interference. Hence, in LDM, the LL signal acts as an interference source to the UL, reducing its noise tolerance capacity. Moreover, assuming a fixed total transmission power, adding the LL signal will also reduce the transmission power of the UL. Therefore, there is a two-fold impact on the UL signal, when it is compared to a non-LDM transmission: the LL signal acts as a noise interference source, and the transmission power is reduced [7]. As a consequence, both effects will reduce the SNR in the receiver. Due to the spectral nature of the OFDM modulation used in PRIME, it can be considered that the LL acts as Additive White Gaussian Noise (AWGN) for the UL.

Being SNRUL and SNRLL the SNR of the UL and LL in single layer mode respectively, and ∆ is the injection level, all expressed in dB [7], the SNRLDM,UL (the SNR of the received LDM signal to fulfill a certain SNR of the UL) is given by (3)

10 10, 10log 1 10 10log 1 10

ULSNR

LDM UL ULSNR SNR

The lower layer, assuming that the cancellation noise is negligible, is only affected by the power division between layers, which is represented by the injection level (Δ). Therefore, the SNRLDM,LL, this is, the SNR of received LDM signal to fulfill a certain SNR of the LL, is given by (4),

10, 10log 1 10LDM LL LLSNR SNR

As shown in equations (3) and (4), the variation in SNR depends mainly on the difference in power between UL and LL, this is, on the injection level (Δ). A high value for this parameter will provide a lower interference of LL on UL, but on the contrary, a low value of Δ will enable a more successful

access to LL. Therefore, a tradeoff value of Δ that fulfills both conditions must be selected.

As a result, the SNR threshold required in the receiver for a proper reception will be determined by the characteristics of the multiplexing technique. For this reason, the SNR threshold required for UL and LL must be calculated in order to evaluate the feasibility of implementing this non-orthogonal multiplexing technique.

III. USE CASES FOR LDM-PRIME

The use of LDM technology in PRIME communications provides a higher data rate in the same frequency bandwidth, and additionally, a lower number of collision occurrences in the MAC layer, due to the simultaneous transmission in two or more layers, which allows a lower use rate of the transmission channel for the same data throughput. Moreover, it allows a clear separation of different types of data or addressees of the information by means of the use of the LDM layers.

For all these reasons, LDM technology may be used in PRIME communications mainly in those situations where tight requirements of data throughput or parallel transmission are required, either of different types of data within the same subnetwork, or between different groups of devices within the same subnetwork. In such situations, the data streams associated to each purpose can be distributed to the UL and LL, based on their different sensitivity levels. As a general basis, the UL may carry regular services of the PRIME network, mainly metering data or traffic control data, while the LL may contain data of services associated to good quality links or services that can afford being re-transmitted in the UL in case there is a transmission error in the LL.

This section describes some use cases where the LDM technology can be clearly applied to PRIME communications in a smart metering network in order to provide additional functionalities or to improve PRIME specifications.

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A. Optimization of the control traffic

The Keep Alive control packet (ALV), as it is described in the PRIME specification, is basically a short message sent by the Base Node individually to each Service Node to check if they are connected as an active device to the PRIME subnetwork. Service Nodes must reply to the ALV requests in order to be considered by the Base Node within the subnetwork topology. Moreover, Service Nodes operating in Switch mode relay the ALV packets to the Service Nodes located downstream.

The periodicity of the ALV requests by the Base Node and the corresponding responses by the Service Nodes is determined by the Base Node, individually for each Service Node, by means of a timer that is reset with every new ALV packet. A very long period between consecutive ALV requests may not be able to provide an updated list of the active Service Nodes of the subnetwork, while an excessively short period may generate a great additional traffic load within the subnetwork. Therefore, the goal of each Base Node is to achieve an updated list of the active Service Nodes with a reasonable increase of traffic load. The configuration of the ALV traffic has a significant influence in this issue, mainly in PRIME subnetworks composed of a large number (hundreds) of Service Nodes, as this is the most used type of control traffic, regardless of the existence of data traffic.

The LDM technology can reduce the percentage of the time occupied by ALV control traffic in two ways: first, the use of LDM, if successful, will reduce the number of time slots for ALV control traffic; and second, in any case, it will provide useful information about the state of the transmission channel for the proper configuration of ALV traffic.

Regarding the first issue, two ALV packets can be sent simultaneously to different Service Nodes using the UL and the LL of LDM. This represents a saving of the time of the channel occupation, mainly in the initial steps of the link connection, or even more, if the LL is configured with a higher order constellation.

Moreover, the transmission of simultaneous ALV packets in both layers of LDM can provide information about the state of the channel to get each Service Node. Hence, if the Base Node does not receive the answer of the ALV request sent in the LL after a certain period of time, the Base Node will assume the targeted Service Node was not reached in the LL due to channel conditions, and the next ALV for that Service Node will be sent in the UL. This information allows the optimization of the configuration of the ALV traffic, by adjusting the periodicity of the ALV requests to each Service Node, and therefore, avoiding traffic overloads and lacks of information about the subnetwork topology. This is particularly useful in the initial steps of the network configuration and when the channel conditions of the link with a Service Node change.

B. A new transmission scheme for data and control traffic

An extension of the previous scenario is the use of a systematic, simultaneous transmission scheme for data and control packets, e.g., GPDU’s (Generic MAC Protocol Data Units), which would include control packets in the UL and

data traffic in the LL. This would allow a significant increase of the traffic capacity of the PRIME subnetwork.

As real deployments of PRIME subnetworks usually use the most robust transmission scheme as a conservative measure against rapidly changing channel conditions, this could be made backwards compatible by using the same modulation and coding schemes in both UL and LL. As a result, though the SNR threshold levels would be higher for both layers, the data throughput would be doubled.

A modified version of this choice is the simultaneous transmission of data and control traffic in the LL, while the UL transmits a Beacon PDU.

C. Firmware upgrade

The firmware upgrade of the Smart Meters in a PRIME network is a highly demanding task, as it requires a high data throughput during a reasonably long time. A multiplexing techniques as LDM can be particularly useful in this scenario, as the firmware upgrade may progress in parallel with other operations.

The firmware upgrade is designed as a high packet error rate transmission, which is efficient in mass data recovering. As a general rule, the firmware upgrade may be executed as a background medium-term task, while the main operations in the network are being carried out. By means of LDM, the LL may be used for firmware upgrading, while the UL may be used for usual metering and control data. This way, the firmware upgrade will not disturb the normal operation of the PRIME network.

D. Information of link quality and network topology

PRIME specification is based on an adaptive master-slave topology in order to best adapt to the varying features and requirements of the PLC channel. PRIME uses CSMA-CA as medium access protocol, but no specific routing protocols are defined. So, the topology of each subnetwork is managed and updated by the Base Node, based mainly on the ALV control packets.

Due to the limited bandwidth and the hostile, changing channel conditions, it would be extremely useful for the Base Node to have a detailed knowledge of channel conditions for each Service Node within the subnetwork available, as it would allow adapting the routing of the messages to these conditions and avoiding packet losses.

The use of LDM could be used to have additional, more detailed information and metrics about link quality and topology. This specific information could be sent by Switch and Terminal Nodes to the Base Node on the LL, without disturbing the normal PRIME communications. As a result, the Base Node could take more appropriated decisions about the network topology, the Switch nodes that are required in different scenarios and the more efficient routing protocols.

E. Dedicated link to connect Base Node to network backbone

In certain installations of Secondary Substations it may be difficult to have connectivity to the backbone. A common

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solution consists of installing the necessary equipment for backbone connectivity in a near separated room, while the Base Node remains connected next to the Secondary Substation. This solution requires a dedicated link from the Base Node to the equipment for backbone connectivity, in order ensure the connection. This dedicated link requires the use of the channel during a high percentage of the time, in order to transmit upstream the metering data of the whole subnetwork.

In this scenario, the LDM technology may be an efficient solution, by providing a dedicated link in the LL for this specific requirement, without generating any overhead to the main communication, which is allocated in the UL.

IV. LDM IMPLEMENTATION FOR PRIME V1.3.6.

In order to test the feasibility of the LDM technology applied to PRIME, a full transmission and reception chain was implemented in Matlab, according to the schemes shown in Fig. 2 and Fig. 4.

A. Signal configuration

In order to analyze a real scenario, two requirements were applied to the signal configuration: first, backwards compatibility with current Smart Meters; and second, the use of the most extensively used configurations of PRIME signals, in terms of coding and modulation techniques.

The first requirement implies the use of version 1.3.6 of PRIME, as this is the version used by millions of already deployed Smart Meters. Therefore, both UL and LL signals are configured as PRIME v1.3.6 signals (see Fig. 2 and Fig. 4).

For the second requirement, DBPSK, DQPSK and D8PSK modulation schemes are considered in the analysis, and only convolutional coding will be considered, as non-coded options have not been used in Smart Metering deployments due to their lower robustness. Considering that the UL signal is affected by the injection of the LL signal, for the UL the most robust configuration defined in PRIME v1.3.6 is considered, i.e. DBPSK with convolutional coding (DBPSKF). For the LL signal, DBPSK, DQPSK and D8PSK with convolutional coding (DBPSKF, DQPSKF, D8PSKF) are evaluated.

B. Theoretical calculations

As a first step, the SNR thresholds of the LDM signal to receive the UL and LL have been theoretically calculated for the selected configurations. In these calculations, the SNR thresholds for different PRIME configurations in single layer (without implementing LDM), were obtained from [8]. These SNR thresholds were calculated for a reference for good reception equal to a Frame Error Rate FER=10-2, with a payload of 256 bytes, for a flat channel response and AWGN.

Considering all the previous assumptions, the SNRLDM,UL and SNRLDM,LL thresholds that fulfill the above-mentioned reference values for good reception have been calculated from equations (3) and (4) for different injection levels. These threshold values are shown in Table I.

Results show how the SNR threshold required for good reception is very dependent on the injection level. Hence, higher values of injection level imply a decrease of SNRLDM,UL threshold and an increase of the SNRLDM,LL threshold. The reason of this aspect is that the LL acts as Additive White Gaussian Noise (AWGN) for the UL, and consequently, a higher injection level implies a reduction of noise for the UL.

C. Simulation results

Simulations considering different modulation schemes and injection levels were developed to evaluate the influence of the configuration of UL and LL on the performance of LDM technology. The behavior of both signal layers in terms of FER vs. SNR was obtained, using 10,000 PHY Protocol Data Units (PPDUs) in the simulations. As a representative result, Fig. 5 and Fig. 6 show results from simulations for an injection level of 6 dB, as this value has been found as a tradeoff for a proper access to both UL and LL.

1) Upper Layer performance Fig. 5 shows the performance of the UL for the selected

configurations of the LL and an injection level of 6 dB. According to the theoretical calculations, and comparing results from Table I and Fig. 5, the performance of the UL highly depends on the injection level, but not on the type of modulation used for the LL. As shown in Fig. 5, the difference in the threshold values for different modulation schemes in the LL is approximately 1 dB, while a variation in the injection level from 6 dB to 9 dB generates a variation in the SNR threshold of 3.1 dB (see Table I).

The simulation results show that the degradation of the UL, when the LL is DBPSKF modulated, corresponds with the value obtained in Table I. However, the degradation of the UL when the LL is DQPSKF or D8PSKF modulated is lower than the theoretical threshold (approximately 1 dB). This is probably due to the characteristics of differential modulation, and it opens the door to exploring new ways of injecting the LL signal.

2) Lower Layer performance Fig. 6 shows the performance of the selected

configurations of the LL for an injection level of 6 dB. As shown in the figure, the simulation results match theoretical calculations for single layer, confirming that the cancellation noise is negligible, as it was described in Section II.C and assumed to obtain equation (4). In other words, provided that the reception of the LL fulfills the SNRLDM,LL threshold, the quality of the data transmitted by LL is not degraded by the use of LDM technology.

TABLE I. THEORETICAL SNR THRESHOLDS FOR THE SELECTED LDM-PRIME V1.3.6 CONFIGURATIONS [5]

SNR thresholds for different injection levels (dB)

SNRLDM,UL SNRLDM,LL

DBPSKF DBPSKF DQPSKF D8PSKF

∆ = 6 dB 9.3 11.0 14.0 18.5

∆ = 7 dB 7,8 11,8 14,8 19,3 ∆ = 8 dB 6,8 12,6 15,6 20,1 ∆ = 9 dB 6,2 13,5 16,5 21,0

5

0.0001

0.001

0.01

0.1

1

4 9 14 19

FER

SNR (dB)

DBPSK_F LL

DQPSK_F LL

D8PSK_F LL

Fig. 5. Performance of the UL (DBPSK_F) as a function of the modulation of the LL, ∆ = 6 dB

0.0001

0.001

0.01

0.1

1

4 9 14 19

FER

SNR (dB)

DBPSK_F

DQPSK_F

D8PSK_F

Fig. 6. Performance of the LL as a function of the modulation used in this layer, ∆ = 6 dB

V. CONCLUSIONS

In this paper, Layered Division Multiplexing (LDM), a non-orthogonal multiplexing technique, is applied to NB-PLC. This technique allows the transmission of separated data streams in layers of different power level within the same bandwidth. This technology provides a higher data rate, and additionally, a lower number of collision occurrences in the MAC layer. As a result, the use of the available spectrum is more efficient and it is possible to clearly discriminate different types of data in separated layers. On the contrary, this multiplexing technique requires higher SNR threshold values for good reception.

Results demonstrate that the LDM technology presents some qualitative advantages when compared to the classical multiplexing techniques TDM/FDM. First, to obtain the same aggregated data rate, LDM requires a lower number of PPDUs and less overhead (since the preamble is common for both layers). Second, the two services are independent in the PHY layer, which allows a clear differentiation of traffic for different types of equipment or users.

LDM is particularly useful in NB-PLC communications in those situations where tight requirements of data throughput or parallel transmission are required. As a general basis, the UL

may carry regular metering services, while the LL may contain additional data linked to good quality links. Accordingly, LDM technology can be applied to optimize the control traffic load, to obtain information of link quality and network topology, to develop the firmware upgrade process without disturbing the metering communications, or even to provide a solution to specific needs, such as a dedicated link to connect Base Node to the network backbone, as it is described in the paper. More advanced configurations, such as new transmission schemes for data and control traffic, or the insertion of Beacon PDUs are also possible.

As a proof of concept, the study analyzes the requirements for implementing LDM in a real NB-PLC scenario: PRIME v1.3.6, with coding and modulation schemes currently used and full backwards compatibility. With this purpose, a full transmission and reception LDM/PRIME chain was implemented in Matlab, which allows to evaluate the feasibility of this technology and to obtain the SNR thresholds required for good reception. Improved results could be obtained if no fully backwards compatibility is applied; for example, new mechanisms to improve robustness, such as new coding or repetition techniques, might be considered in LL or UL signals, or other modulation schemes could be analyzed. This way, the required SNR thresholds would be lower, improving the efficiency of the LDM system. The conclusions of the study can be applied to any NB-PLC system based on Orthogonal Frequency Division Multiplexing (OFDM).

Acknowledgment This work has been financially supported in part by the

University of the Basque Country (UFI 11/30) and by the Basque Government (IT-683-13 and Elkartek program).

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[3] Advanced Television Systems Committee, “ATSC Standard: Physical Layer Protocol (A/322)”, Doc. A/322, September 2016.

[4] D. Gomez-Barquero, O. Simeone, "LDM Versus FDM/TDM for Unequal Error Protection in Terrestrial Broadcasting Systems: An Information-Theoretic View," in IEEE Transactions on Broadcasting, vol. 61, no. 4, pp. 571-579, Dec. 2015.

[5] PRIME Technical Working Group. Specification for PoweRline Intelligent Metering Evolution 1.3.6.; Prime Alliance: Brussels, Belgium, 2012.

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