Coexistence of Heterogeneous and Homogeneous Wireless Technologies in Smart Grid-Home Area Network Mohd Adib Sarijari1,2, Anthony Lo1,2, Mohd Sharil Abdullah1,2, Sonia Heemstra de Groot3, Ignas G.M.M. Niemegeers2, Rozeha A.Rashid1 1
Universiti Teknologi Malaysia, Malaysia, 2Delft University of Technology, The Netherlands, 3 Eindhoven University of Technology, The Netherlands Email: {m.a.b.sarijari, m.s.b.abdullah}@tudelft.nl,
[email protected]
Abstract— In a home environment, Smart Grid Home Area Network (SG-HAN) platform facilitates collection and delivery of power consumption information for load profiling and informed decisions on energy management. However, one of the main challenges in HAN is the overcrowded unlicensed 2.4GHz ISM frequency band, occupied by several types of radio technologies such as ZigBee, Bluetooth, and WiFi. It is crucial that, those technologies coexist peacefully to allow each user of the radio technology to fulfill their communication goals. In this paper, we present a potential coexistence scenario in SG-HAN for homogeneous and heterogeneous wireless technologies. The coexistence impact on SG-HAN performance is then modeled and analyzed. The numerical results show significant performance degradation due to the interference problems for devices in close proximity with the interference sources in a spectrum sharing environment where in the worst case scenario, SG-HAN communication is almost impossible. Keywords— Smart Grid; Home Area Network; 802.15.4; Coexistence; Interference
I.
INTRODUCTION
Smart Grid (SG)[1] is an efficient and intelligent electric energy system that has been proposed as the next-generation electrical power system. It embraces advanced technologies including communications capabilities, in every aspect of electricity generation, transmission, distribution and consumption. In a home environment, a Home Area Network (HAN) provides a communication network for the Smart Grid devices as well as for the consumer, to be able to communicate with the utilities, via neighborhood area network and wide area network [2]. One of the most important communication requirements for Smart Grid HAN (SG-HAN) is that, it has to be efficient in energy usage especially for the battery operated devices such as the in-home-display and smart appliances which use the battery as its communication module power source. In addition, SG-HAN also has to be self-organized, robust and reliable in order to provide seamless communication, plug-and-play operation and reliable connection of SG-HAN devices with the network ensuring a reliable and easy operation of SG-HAN devices to the enduser.
Therefore, it is proposed that the 802.15.4 wireless technology standard is used as the enabling technology for SGHAN communications [2]–[5]. However, because of the lowpower characteristic of the 802.15.4 standard, the communication is exposed to a harmful interference problem which is due to the coexistence, especially when operating in a home environment and using the ISM 2.4 GHz band. In this paper, we present the possible coexistence of heterogeneous and homogeneous wireless technologies in SG-HAN, and we model this coexistence using the enhanced interference modeling concept presented in [6]. Then, we analyze and discuss the impact of this coexistence to SG-HAN communication performance. The rest of the paper is organized as follows: Section II presents the possible sources of interference to SG-HAN. The interference model and mathematical analysis of the SG-HAN coexistence with others heterogeneous and homogenous wireless technologies are introduced in Section III. Section IV presents the numerical results. The related works are outlined in Section V. Finally, the conclusion is presented in Section VI. II. HETEROGENEOUS AND HOMOGENEOUS WIRELESS COEXISTENCE IN SMART GRID HOME AREA NETWORK In a Smart-Grid Home Area Network, the possible types of existing devices are: 1) smart appliances, 2) a smart meter, 3) an in-home-display, 4) distributed energy storage and generation, and 5) heating, ventilation and air conditioning devices. All these devices will have two-way communication capabilities in order to perform the SG-HAN functionalities [2] including: • Demand response: This function enables utilities to send a load control event to the SG-HAN devices through the smart meter to request a shut down or delay the operation when the power supply is at risk. This function also enables SG-HAN devices such as washing machine to operate only during the low price of electricity. • Advanced metering infrastructure: This function is used for metering purposes. In addition, it is also used by the utilities to send information to SG-HAN devices, including the real-time electricity prices.
• Distributed energy storage and distributed generation: Besides generating and storing electricity for home usage, this function also enables the electricity to be sold back to the utilities when there is an extra, especially during the high price of electricity. It is also interesting to mention that, the electric vehicle is also one type of energy storage in SG-HAN. It is proposed that the 802.15.4 wireless standard is used as the enabling technology for the two-way SG-HAN communications [2]–[5]. The global allowable operational band for this standard is the unlicensed 2.4 GHz ISM band. However, in a home area, there are also exist other systems that employ the same wireless technology (i.e. 802.15.4) and different wireless technologies (i.e. 802.11) which operate at the same frequency as SG-HAN as shown in Figure 1.
camera, pet and plant care system, baby monitoring system, light automation system, door and windows security system and human detection system. B. Coexistence of HeterogeneousWireless Technologies in Smart-Grid Home Area Network Heterogeneous wireless technologies coexistence describes the coexistence between SG-HAN and other devices which use differing wireless technologies. The green links in Figure 1 illustrate the heterogeneous wireless technologies coexistence scenarios in SG-HAN and are detailed as follows: • In home WiFi (802.11 b/g/n) equipped devices. These include a laptop, wireless mouse and keyboard, smart TV, smart phone, WiFi access point and home monitoring system such as the IP camera. • WiFi (802.11 b/g/n) network from neighboring houses. Neighboring WiFi network effect is more critical for terraced and apartment type of houses due to their close proximity with one another. • Bluetooth which possible devices include laptops, tablets and smart phones. • Microwave ovens, which typical operate at 2.4 GHz. A leakage can occur around the door and has high potential in causing interference to SG-HAN operating in close proximity and frequency with the microwave oven. • Other 2.4GHz based technologies such as baby monitoring systems, wireless mouse devices and keyboards, remote controls, medical diathermy equipment and industrial heating equipment.
Fig. 1. A Possible homogenous and heterogeneous wireless technologies coexistence scenario in Smart-Grid Home Area Network
A. Coexistence of Homogeneous Wireless Technology in Smart-Grid Home Area Network Homogeneous wireless technology coexistence takes place when SG-HAN and other devices use the same wireless communication technology, i.e. 802.15.4. The yellow links in Figure 1 illustrates the homogeneous wireless technology coexistence scenarios in SG-HAN and are detailed as follows: • Smart Utilities Network which uses the 802.15.4g standard as the communication technology. The purpose of Smart Utilities Network includes providing monitoring and control of the utility system [7] in SG Neighborhood Area Network. 802.15.4g defines a new physical layer on top of the 802.15.4. • Neighboring HAN, which uses the same standard and frequency as the SG-HAN in one home. Neighboring HAN effect is more critical for terraced or apartment type of houses where neighbors are located close to one another. • Home automation, monitoring and security systems, which use 802.15.4 wireless standard as their communication technology. This includes the wireless
III. MODELING OF HOMOGENEOUS AND HETEROGENEOUS COEXISTENCE IN SMART-GRID HOME AREA NETWORK In this section, we will analyze the performance of the 802.15.4-based SG-HAN under the coexistence of heterogeneous and homogeneous wireless technologies that operate at the same frequency, time and space. A. System Model Figure 2 shows the coexistence model in the time domain. We assume that: 1) all the wireless technology use the same (or very close) center frequency, , 2) at all time, there is at least one of the heterogeneous wireless systems, which occupies the spectrum and 3) SG-HAN devices are in a close distance with the coexisting devices. In this model, SG-HAN and homogeneous wireless technology devices operate with non-beacon mode, which utilizes the 802.15.4 unslotted carrier sense multiple access with collision avoidance (CSMA-CA) for the Medium Access Control (MAC) protocol. In 802.15.4 unslotted CSMA-CA [8], nodes that want to access the channel will first backoff for period given as, 0, 2 1 1
where is the duration for a unit backoff such that,
! "#$%&
According to [6], ! = 20 symbols, and for 802.15.4 operating in the 2.4 GHz band, "#$%& = 16 µs. Thus, = 320 µs. '( is the backoff exponent at the )* backoff and has the initial value, '+ , ' where 0 ≤ macMinBE ≤ 3. After the backoff period expires, nodes will then perform clear channel assessment (CCA) in order to check whether the channel is in IDLE state and perform transmission if so. However, if the channel is sensed to be in BUSY state, then, nodes will again backoff for -. period with an increment of BE value given as, '(/0 '( 1 1, , 2 ' 2
where aMaxBE = 5 [8] and the process will be repeated until maximum backoff limit, macMaxCSMABackoffs is reached where 0 ≤ macMaxCSMABackoffs ≤ 5. Subsequently, a failure message will be sent to the upper layer for further action.
B
45) 6 9 :;< ∙ 9(? ∙ ) ∙ @ 1 9(? A 4 AC+
where ) is the transmission time of SG-HAN, 9 :;< is the probability that the SG-HAN packet is successfully delivered, 9(? is the probability that the channel is idle from the interferers, and M = macMaxCSMABackoffs. Furthermore, TF can be calculated as: )
G 1 15 8 5 $ J !
where G is the MAC payload size of SG-HAN packet and the 15 bytes consist of 6-byte physical header, 7-byte MAC header with minimum addressing space, and 2 bytes of frame check sequence. $ J ! is the bitrate of the 802.15.4 which is equal to 250 kbps. In addition, according to Figure 2, 9(? can be defined as, 9(? K(? L MMN ∪ 'P
where (? is the idle time between two consecutive interferer transmission, MMN is the time taken to perform CCA and is equal to 8* "#$%& = 128 µs [6], [8] while E is the event that the start time of the CCA, QR)QS) falls within the period of (? . Hence, 9(? can be expressed as: 9(?
Fig. 2. Timing Model of SG-HAN Coexistence with Heterogeneous and Homogeneous Wireless System
There are three modes of CCA defined in 802.15.4 standard [8]: 1) energy detection mode, where, BUSY status is reported if the sensed energy is above the defined threshold, 2) carrier sensing mode, where, BUSY status is reported if a signal with 802.15.4 modulation and spreading characteristics is detected, and 3) the combination of both modes. In this paper, we will analyze the coexistence performance of SG-HAN communication under the first and the second modes.
In the case of CCA mode 1, where SG-HAN’s CCA is able to detect the homogenous and heterogeneous signal in the spectrum, the channel is always going to be reported as BUSY. Hence, 9(? will be approaching zero. Therefore, 45) 6 in (4) also approaches zero and subsequently causing the throughput to be almost zero. In this condition, there is almost no guarantee SG-HAN communication can be successful performed. In the case of CCA mode 2, where only homogenous signals can be detected by SG-HAN, and from Figure 2, *T?7SUATV)?SW?S?S = T7R 1 MMN
TV)?SW?S?S7SQA? = Y)TV)W 1 T7R 1 MMN Z
B. Performance Analysis According to [6], the normalized throughput, S can be calculated as: 45) 6 3 3 457 6
45) 6 is the expected value of transmission time and 457 6 is the expected value of time for one successful transmission; starting from the first backoff until the end of the packet transmission. Based on the model in Figure 2 and the concept from [6], 45) 6 can is derived as:
*T?7SUATV)?SW?S?S MMN 6 TV)?SW?S?S7SQA?
hence, 9(?
T7R 1 MMN MMN , [ )TV)W 1 T7R 1 MMN 1, T7R , [)TV)W 1 T7R 1 MMN 1,
)TV)W \ 0 )TV)W 0
)TV)W \ 0
7 )TV)W 0
where T7R is the silence period between two consecutive frames such that, 3_`3 12 ∗ "#$%& , T7R ^ d_`3 40 ∗ "#$%& ,
9 ! 3b! c 18$# !" 9 ! 3b! e 18$# !"
)TV)W is the transmission time from the homogeneous interference signal and can be calculated as: )TV)W
which caused by the changes in T7R values from SIFS to LIFS when the packet size is greater than 18 bytes. In general, it can be observed that, the bigger the homogeneous wireless technology payload, the lower the SG-HAN throughput. This is because; the channel is occupied longer by the homogeneous wireless technology devices, hence causing lesser chances for SG-HAN devices to access the channel.
YG(V)W 1 15Z 8 8 $ J !
G(V)W is the MAC payload size of the homogeneous interferer packet and the 15 bytes have a similar composition as in (5).
In addition, according to Figure 2, the 9 :;< from (4) can be calculated as: 9 :;< f ∙ 1 'J 9
where f is the probability that the packets can be transmitted and given as: f 1 1 9(? B/0 10
and PER is the packet error rate which, in referring to [8], is: 'J
l 0
0m 0n
'J 1 1 'Jk 11
S 0n ∙ ∑0n SCq1 Y S Z!
. s
pq+ RTk: r t0uv
12
'J is the bit error rate and 3_GJ is the signal to interference noise ratio that can be defined as, 3_GJ
(
)
1
V
14
where ) is the transmitted power of the SG-HAN signal and V is the noise power. In addition, ( is the power of the interferer which in this case, is the signal from the heterogeneous wireless technologies.
Fig. 3. Smart-Grid Home Area Network achievable throughput with different InterfererPayloadSize, G(V)W whenG = 50 bytes and M =4
T7R value changes from SIFS to LIFS when packet size > 18 bytes.
Furthermore, 457 6 in (1) can be calculated using (14) where, 4w x is the expected value of at )* backoff derived from (2) and (3). Therefore, S from (3) is expressed in (15) by substituting 5) 6 and 457 6 using (4) and (13) respectively. Finally, S can be written with respect to SINR as shown in equation (16) by substituting (15) with (9)-(13). From (16), the achievable throughput, J can be calculated as 3 ∗ $ J ! and are shown in the graphs of Figure 3, 5 and 6. In Figure 3, we plot four different values of homogeneous wireless technology payload size, G(V)W with SG-HAN maximum backoff limit, M set to the default value of 4 and SG-HAN payload, N is set to 50 bytes. It is observed that the SG-HAN throughput for payload size = 22 bytes (blue) and 50 bytes (green) are higher than payload size = 2 bytes (black). This is due to the characteristics of 9(? as shown in Figure 4 IV.
NUMERICAL RESULTS
Fig. 4. Probability of the channel is idle from the interferer,9(? asfunction of the homogeneous interfererpacket size, G(V)W
In Figure 5, we plot the graph of throughput as a function of SINR with all possible values of M. The payload size of SGHAN and heterogeneous wireless technology is set to 50 bytes and 22 bytes respectively. It can be seen that, for M less than 2, a higher value of SG-HAN maximum backoff limit gives a higher throughput value. This is due to the more access trials
that can be performed by SG-HAN, hence giving it higher chances in acquiring the channel for packet transmission. However, for M higher than 2, the increase of M gives a slightly lower throughput to SG-HAN due to the longer back off time that the SG-HAN needs to execute before the CCA and transmission could be performed. Therefore, the proper value of M should be chosen in order to obtain an optimum performance of SG-HAN communication. Besides M, we also could control the value of SG-HAN payload size, N in surviving the aforementioned coexistence environment. Figure 6 shows the graph of throughput versus SINR with four different values of N. It can be seen that, at a good SINR, higher value of N gives a higher throughput to SGHAN. However, this is vice versa in the low SINR condition. Therefore, the value of N should be adjusted depending on the environment that the SG-HAN operates in. It can be observed in all cases, the throughput of SG-HAN drops significantly when the SINR is lower than -2 dB and approaches zero at SINR less than -5 dB. One of the reasons for low SINR is due to the high interference signal in the environment, as can be seen in (14). In this case, the interference signal comes from the heterogeneous wireless technologies. The stronger the heterogeneous wireless technology signals are, the lesser the SG-HAN throughput. In the worst case scenario, there is almost no successful SG-HAN communication can be performed. This condition is very significant especially for devices that have both SG-HAN and heterogeneous wireless radios together such as the WiFi ready Smart TV, a laptop and the microwave oven. It is important to note that, the failure in SG-HAN communication will lead to the failure of SG-HAN devices to operate. Consequently, it is very unfortunate if the device could not operate just because it could not communicate. V.
RELATED WORKS
Besides the coexistence between 802.15.4 and WiFi, significant impact of coexistence on 802.15.4 from other wireless devices is also addressed in the literature. For example, the coexistence between 802.15.4 with microwave oven and Bluetooth is addressed in [13] and [14]. In this paper, we present the analysis of the wireless coexistence impact on the SG-HAN communication utilizing 802.15.4 technology due to the aforementioned heterogeneous and homogeneous wireless technologies. Our analysis is based on the enhanced interference modeling concept presented by [6].
Fig. 5. SG-HAN achievable throughput with different maximum number of CSMA backoff, MwhenG = 50 bytes and G(V)W =22 bytes
The impact of the 802.15.4-based system wireless coexistence has received significant interest from the research community including from academia, industries and standardization bodies especially the coexistence between 802.15.4 and WiFi. In [9], an experimental measurement is done using TelosB mote to measure the impact of the nearby WiFi access point to the packet delivery ratio (PDR) of the mote. A clear WiFi effect on TelosB mote can be seen from the result published as the PDR of the mode significantly decreases especially at the center frequency of the WiFi access point. In [10], the effect of WiFi on Body Area Network (BAN) which uses Tmote Sky as the testbed was experimentally evaluated. The result shows that the impact is more significant when the sensor node is transmitting at a lower power level. The impact of the 802.15.4 coexistence with 802.11 b/g in term of the received signal strength indicator (RSSI) had been experimentally evaluated in [11]. The work in [6] evaluated the impact of the 802.15.4WiFi coexistence through mathematical modeling, simulation and experimental setups. The results from all three evaluation methods show that the impact of this coexistence is significant. More review on the coexistence between 802.15.4 and WiFi could be found in [12].
Fig. 6. Smart-Grid Home Area Network achievable throughput with different Payload Size, GwhenG(V)W = 22 bytes and M=4
B
457 6 @ y9(? ∙ 1 9(? 3
3
AC+
A
|
B
∙ z{@ 4w x} 1 1 1MMN 1 ) ~ 1 1 9(? B/0 ∙ {@ 4w x 1 5MMN } 14 (C+
A 9 :;< ∙ 9(? ∙ ) ∙ ∑B AC+1 9(?
A A B/0 ∙ rY∑B 4w ∑B xZ 1 5MMN u AC+ 9(? ∙ 1 9(? ∙ rY∑(C+ 4w xZ 1 1 1MMN 1 ) u 1 1 9(? (C+ A f ∙ 1 'J ∙ 9(? ∙ ) ∙ ∑B AC+1 9(?
A A B/0 ∙ rY∑B 4w ∑B xZ 1 5MMN u AC+ 9(? ∙ 1 9(? ∙ rY∑(C+ 4w xZ 1 1 1MMN 1 ) u 1 1 9(? (C+ A Y1 1 9(? B/0 ∙ 1 1 1 'Jk Z ∙ 9(? ∙ ) ∙ ∑B AC+1 9(?
A A B/0 ∙ rY∑B 4w ∑B xZ 1 5MMN u AC+ 9(? ∙ 1 9(? ∙ rY∑(C+ 4w xZ 1 1 1MMN 1 ) u 1 1 9(? (C+
{1 1 9(? B/0 ∙ 1 p
0
+
S 0n ∙ ∑0n SCq1 Y S Z!
CONCLUSION
In this paper, we first identify a possible coexistence scenario with other heterogeneous and homogeneous wireless technologies and aimed to characterize the effects of interference in a SG-HAN. An analytical interference modeling and mathematical analysis for 802.15.4-based SG-HAN devices with unslotted CSMA-CA communication protocol is presented. The results indicate significant performance degradation for devices in close vicinity with the interferer sources. It is also shown that SG-HAN communication is almost impossible when the interference is strong. The failure of SG-HAN communication leads to the failure of SG-HAN devices; smart meter, IHD and smart appliances such as washing machines and refrigerators, to be functioning. Therefore, for future work, we propose a cognitive radio-based self-organized multi-channel SG-HAN communication architecture for interference mitigation solution. Future work will also include simulation and experimental tests to assess potential coexistence limits for optimal decision-making in channel selection and interference mitigation. REFERENCES
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