Noman SHABBIR, Muhammad A. FAWAD, Muhammad N. IQBAL, Junaid ZAFAR GC University Lahore, Pakistan. doi:10.15199/48.2015.04.29 OFDMA Based Power Line Communication for Smart Grids Abstract. This paper investigates the design and simulation of Orthogonal Frequency Division Multiple Access (OFDMA) communication system using Narrowband-Power Line Communication (NB-PLC) for Smart Grids (SG) communication through extremely difficult channel environments such as multipath and noise. In OFDMA, sub-band carrier allocation is performed using contiguous, distributed and sorted sub-band carrier allocations and Bit Error Rate (BER) performance of this system is enhanced by these techniques, which are analyzed in this research. Simulation results indicates that sorted sub-band carrier allocation scheme gives better performance than contiguous sub-band carrier allocation scheme. Streszczenie. W artykule zbadano system komunikacyjny typu OFDMA wykorzystujący algorytm NB-PLC w zastosowaniu do sieci typu Smart Grid pracujący w trudnych warunkach zaszumienia. Do alokacji nośnej międzypasmowej wykorzystano Bit Error Rate BER algorytm. System komunikacji dla sieci typu Smart Grid bazujący na ,metodzie OFDMA. Keywords: OFDMA, PLC, Smart Grids Słowa kluczowe: OFDMA, Smart Grid, komunikacja Introduction The traditional grid is an out-of-date infrastructure and lacks the pervasive electricity control features, which results in an inefficient power delivery. Therefore, there is a strong need to develop grids that can provide reliable methods of energy generation, efficient transmission, distribution, and control mechanism between utilities/customers. This emerging next generation of power grid is called Smart Grid (SG). The SG incorporates, within a physical power system, Information and Communication Technologies (ICT) that enhances grid efficiency and provides wide range of customer oriented services [1,2]. An important part of SG is a robust and ubiquitously available communication network that will enable various devices connected to the SG to exchange vital information at all times. The utilization of modern communication technologies for information exchange in SG enables electric utilities to make best possible decisions for efficient power generation, transmission and distribution. Both wired and wireless communication technologies have been proposed in literature for data communication in SG [2,3,4]. Recently, researchers have shown greatest interest to the use of in-place electricity power cables that have sufficient potential to serve for data communication in SG. The reason behind this motivation is that Power Line Communication (PLC) eliminates the need for installing new infrastructure for communication and without any doubt, power lines are the most extensive and ubiquitous network. PLC is the best option for smart grid communication because it is pervasive and permeant [5]. PLC even offers an opportunity to electric utilities to have their own communication infrastructure. Therefore, it is expected that PLC will be a dominant technology in an overall hybrid communication networks that will be utilized in future for SG communication connectivity. SG applications such as AMR are usually low data rate for which Narrowband power line communication is a suitable technology. The power lines possess several challenges that should be taken into account to reliably communicate over them. The high background/impulsive noise, multipath propagation, frequency selective fading, and varying grid topologies cause fading of the communication signal in the PLC. These channel impairments for communicating over Low Voltage Power Lines Communication (LV-PLC) are welldescribed by the multipath signal propagation model. This research focuses on the problem of low data rate Automatic Remote Metering (AMR) communication between the customer premises and the data concentrator placed on the transformer using smart meters. Although Narrowband-Power Line Communication (NB-PLC) systems are well suited for AMR, but due to the harsh characteristics in power lines, major concerns exist on the reliability of NBPLC. Orthogonal Frequency Division Multiple Access (OFDMA) based communication system is proposed to be a solution to the most of the problems in NB-PLC systems. Survey of the Related Work OFDMA subcarrier allocation can be generalized, adjacent (contiguous) or distributed. As evident by names, in adjacent subcarrier allocation contiguous or continuous groups of subsets are allocated to the users. While in distributed algorithm random mode is adopted and the resultant sub-band offers frequency diversity. These subsets ensure better performance with frequency selectivity. Different OFDMA based algorithms are presented in literature [6]. Power line communication is noisy, robust and frequency selective and for this OFDMA is preferred. In Fischer’s algorithm, author has proposed a loading algorithm for discrete multi-tone transmission over channels with inter-symbol interference [7]. The beauty of Fischer’s algorithm is this that is well suited for high datarate transmission over copper wires. Ming have altered Fischer’s algorithm to implement bit and power allocation in sub-bands rather than subcarriers, which resultantly minimizes signalling overhead. Two or more sub-carriers are grouped to form sub-bands and simulations results disclose that signalling overhead can be reduced by 75 % for 512 sub-carrier OFDM systems [8]. Srinivasa in [9] developed a simpler model for narrow band power line communication in smart grids which is a statistical-time-varying model. Results show that appropriate carrier allocation provides dependable and uniform admittance to all units/meters in chorus indifferent from their physical location. Each smart meter is evaluated through BER to transmitted power to reach at maximum efficiency. A simulation models show that total channel power of one Watt for 20 meters is achievable with the capacity of few Kbps per meter [9]. It is advocated that power line communication has extraordinary potential that can provide up-to 1 Gbps by increasing bandwidth of present power line communication technology up-to 100 MHz combined along with new modulation schemes. The considered communication system is having multiuser and multi-cellular structure. The concept behind the radio system and power line communication is same. OFDMA communication System is proposed by the author of [10], as compared to CDMA, as a solution for throughput problems that are usually observed in power line communication. PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 91 NR 4/2015 131 Research Question & Problem Statement The existing network of power lines is most effective, reliable and cost effective for smart grid applications. The transmission of power and digital data on the existing power lines is not a child work. The unfriendly features of power lines offer restrictions for reliable communication. Notwithstanding, the present improvements in the field of digital communication with modulation schemes has inspired the scientists to utilize power line communication for this unique task. Narrow band power line communication is a sub-division of power line communication that is capable of low data rate communication. Most of the smart grid applications are low data rate applications thus making narrow band power line communication more suitable for this purpose. There are many challenges in power line utilization in the form of frequency selective fading, time varying noise, high attenuation resulting in signal fading and impedance mismatch. In the presence of these stumbling blocks OFDMA is suggested as a solution to the aforementioned problems. Carrying the argument further, the characteristics of OFDMA exhibits appropriate behaviour when compared to other schemes for power line communication. Problem Solution In this research, a smart grid system is designed that consists of four meters. The rest of the parameters are defined in Table 1. Two OFDMA based algorithms are used for simulations, which are as follows: A. Contiguous / Adjacent Sub-band Allocation Algorithm (CSAA): Many continuous groups of sub-carriers are tied up to form a sub-band. These types of sub-carriers are assigned to offer multiuser diversity. This multiuser diversity is very suitable for static users because the static user is relatively constant in channel response experience. Following is detail of this OFDMA based algorithm. The total available spectrum contains the entire of subcarriers. The sub-carriers are arranged in ascending order of their frequencies such as first subcarrier occupies the lowest spectrum and subcarrier NC contains the highest spectrum. All subcarrier indexes are contained in set φ = {1,2,3,4….NC}. All the subcarriers NC are divided in NB sub-bands. Thus (1) NCB = NC . NB Number of bits for every OFDM symbol to transmit is assumed as: (2) Rtot = bave . NC Here, bave is the average spectral efficiency mandatory for the system. th The gain of the channel k subcarrier is represented as hk. The average channel gain of the subcarriers in a specific sub-band is evaluated as B. Selected (Sorted) Sub-band Carrier Allocation Algorithm (SSCA): High BER is caused due to few subcarriers which are badly affected by frequency selective fading. To solve this issue sorted sub-band carrier allocation is adopted so that we can leave out such subcarriers. In selected sub-band carrier allocation sub-bands are arranged in ascending order. Resultantly, only those subcarriers are sent to users those have better channel gains. The channels with frequency selectivity show better results under this algorithm. The algorithm is explained as follows. The spectrum comprises of NC subcarriers All of the NC subcarriers are arranged according to their frequencies in ascending direction, such as, subcarrier one lies at the lowest spectrum, while the subcarrier NC possesses the highest spectrum. All of the sub-carriers which are arranged present in the set φ = {1, 2, 3, 4 ….NC}. The sub-carriers are organized on the bases of their channel gains hk. This arranged Sub-carrier keys set is denoted as {a1, a2, a3, a4… aNC} such as ha1≤ ha2 ≤…. ≤ haNC. Assemble these arranged NC subcarriers into NB subbands. For Nm consumers with diverse channel circumstances which they are experiencing from transmitter to the receiver (local grid station), allocate the first sorted sub-band to the user. Leave the other sub-bands and transmit the user data over the sorted ones. Until all users are assigned a sub-band repeat the process. Simulation Results Performance of these algorithms will be gauged for OFDMA communication system for information transmission. In PLC, each user experiences different multipath delays due to different number of connections, joints, cable length and some other on ground factors. Similarly, the performance of contiguous sub-band allocation algorithm reduces as number of subcarriers per sub-band increases. The bit error rate of four meters under OFDMA communication system is shown in figures below. Table 1. Parameters of OFDMA Communication System Parameters Type/Value Communication Method OFDMA Modulation Scheme 16-QAM Number of Carriers 32 Carrier Spacing 400 KHz Frequency Range 3-500 KHz FFT Size 256 Length of Cyclic Prefix 16 Channel LV-PLC Access Channel -3 LV link BER Specification 10 (3) For Nm meters with unalike channel circumstances which they are facing from meter to the data concentrator, allocate the first sub-band regardless of its value of Hi to the ith meter (where i = 1, 2, 3, 4… Nm ). Transmit the data of meter over the given sub-band and leave the rest of sub-bands. All users are allocated a sub-band by repeating the last two steps. 132 Fig 1. BER of all meters in CSAA PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 91 NR 4/2015 Fig. 1. shows the BER of meter 1 to 4 is 10-3 and Eb/N0 for these meters remain 10.7, 10.5, 16.5 and 27.5 dBs respectively. The results reveal that channel conditions for meter four are worse as compared to rest of the three meters. Therefore Eb/N0 for meter four is the highest. The analytical (speculative) and simulated probability of Bit Error for OFDMA based communication system using contiguous sub-band allocation for meter one shown in Fig. 2. However, Eb/N0 of meter 2, 3 and 4 remains 10.5 dBs, 12 dB, and 17.5 dB. Thus results tell us that the conditions for meter four are even not better this time. However, improvement in performance is gauged. Fig. 5. BER of meter four using CSAA Fig 2: BER of meter one using CSAA The analytical (theoretical) and simulated probability of Bit Error for OFDMA based communication system using contiguous sub-band allocation for meter two, three and four are presented in Fig. 3, 4 and 5 respectively. Fig. 6. BER of all meters using SSCA Now the individual BER of all the meters and the simulated and theoretical difference is depicted in graphical representation in Fig. 7, 8, 9 and 10. Fig 3: BER of meter two using CSAA Fig. 7. BER for meter 1 using SSCA Fig 4: BER of meter three using CSAA Now selected/sorted sub-band algorithm is used to observe the behavior of these meters. The simulated results -3 are shown in the figure below. In Fig. 6., BER of 10 is achieved for meter one while its Eb/N0 remains 10.5 dB. -3 Moreover bit error rate of meter 2, 3 and 4 remains 10 . For meter 1 and 2 there isn’t any significant difference between the theoretical and simulated results it because of low attenuation and less multi-paths. Whereas for meter 4 has greater variation in simulated and theoretical results due to high attenuation and more multi-paths. This algorithm reduces the difference between the theoretical and simulated results and produces better results as compared to contiguous sub-band carrier algorithm. This improvement is understandable for channels those have characteristics of frequency selectivity. PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 91 NR 4/2015 133 both the algorithm. For future work, some more algorithms such as distributed sub-band allocation algorithm and hybrid sub-band allocation algorithm can be experimented for more reliable and swift communication for smart grids by utilizing OFDMA based communication system. Furthermore, for the improvement in BER performance in OFDMA communication system, some other approaches such as Least Mean Square Error (LMSE) and Maximum Like-hood Sequence Estimation (MLSE) can be experimented for improved digital signal processing. REFERENCES Fig. 8. BER for meter 2 using SSCA Fig. 9. BER for meter 3 using SSCA Fig. 10. BER for meter 4 using SSCA Conclusion In comparison of both algorithms, the sorted sub-band carrier allocation shows better performance due to better selection of subcarriers that are to be transmitted between user and local grid. The result of MATLAB simulation shows that attenuation will be up-to 10 dB for meters with less than 10 paths while for meters with 15 and above paths it may increase up-to 42 dB and deep notches are observed in the graphical representation. These notches clearly indicate the impedance mismatch. Probability of error is least when sorted sub-band carrier algorithm is used as compared to adjacent sub-carrier algorithm in OFDMA communication system. However, sorted sub-band algorithm is complex to implement and more time consuming as compared to other algorithm. It is because of the extensive sorting of the subcarriers. 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Noman Shabbir, Dept. of Electrical Engineering, GC University, Katchehry Road, Lahore, 54000, Pakistan. E-mail: [email protected]; Engr. Muhammad Atif Fawad, Dept. of Electrical Engineering, GC University, Katchehry Road, Lahore, 54000, Pakistan. E-mail: [email protected]; Engr. Muhammad Naveed Iqbal, Dept. of Electrical Engineering, GC University, Katchehry Road, Lahore, 54000, Pakistan. E-mail: [email protected]: Dr. Engr. Junaid Zafar, Dept. of Electrical Engineering, GC University, Katchehry Road, Lahore, 54000, Pakistan. E-mail: [email protected] PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 91 NR 4/2015
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