Coexistence Issues Of Heterogeneous Communication Networks In Smart Grid

Smart grid is an intelligent power network that merges various advanced technolo-gies in power, communication, and control. It is featured by its two-way flows of elec-tricity and information amongst all the grid components, based on which optimal powergeneration and consumption is realized in the whole grid. The transition from traditionalpower grid to smart grid is inevitable for the power industry, and this transition process isaccelerated with the development of communication technologies.Two-way real-time data transmission is the key in realizing smart grid, so the perva-sive communication network is one of the basic architectures for smart grid. Consideringthe deployment characteristics of grid components, none of the existing communicationtechnology can independently achieve this goal, so the basic communication architectureof smart grid must be a heterogeneous and hierarchical communication network architec-ture. Specially, for the neighborhood area and home area with a relatively complex nodesdeployment, low-cost heterogeneous wireless communication network architecture is theonly choice, so IEEE made a new wireless network standard, i.e., the SUN(Smart UtilityNetwork) standard, for the data transmission in smart grid neighborhood areas. However,the SUN is designed to operate on the license-exempt bands on which other neighbor-hood and home area wireless networks also exist, which obviously leads to the channeloverlapping, thus coexistence issues caused by channel contention between the hetero-geneous networks is particularly prominent. Therefore, how to evaluate the coexistenceperformance in a more precise way and establishing more effective coexistence mecha-nisms are the problems that must be investigated and solved in the design of smart gridheterogeneous communication networks. Accordingly, this dissertation chooses WLANas a typical coexisting system of SUN, and mainly focus on their coexistence issues.Firstly, the analytical model is used to identify that the channel separation and nodedistance are the key factors for the coexistence performance, which provides a theoreticalbasis for the coexistence mechanisms proposed below. Bit error rate(BER) and packet er-ror rate(PER) are selected as the basic indicators for coexistence performance. The BERcalculation model for coexistence interference analysis is firstly established, in which pathloss model is used to indicate the node distance relations and power factor is used to indi-cate the central frequency relations and power spectrum destiny relations of the interferingand desired signals. It is proved that increasing the frequency separation between the co-existing systems can reduce their mutual interference. Then, based on the traditional PERcalculation model, the security distances between SUN and WLAN/Zig Bee in differentcoexistence scenarios are summarized.However, the traditional PER calculation model can not express the transmissioncharacteristics of the coexisting systems, so PER values obtained by the traditional modelare not precise. Considering this, the packet collision idea is presented for PER calcu-lation. According to the timing relations between their packet transmissions, the basicpacket collision model for SUN and WLAN is firstly built, in which the desired packetis classified into two parts, i.e., the collided part and non-collided part. Compared withthe traditional PER calculation model, the basic packet collision model is closer to ac-tual data packet transmission situation in the case of mutual interference, and thus theaccuracy in PER calculation is better than the traditional method. But the basic packetcollision model is then proved not suitable for the multi-interference situation, so an in-terference number based segmentation algorithm for the desired packet is proposed, inwhich different segment is related to different BER value. The average length of eachsegment of the desired packet is derived from the perspective of probability theory. Basedon the proposed segmentation theory, a new packet collision model for multi-interferencesituation is proposed. In the new model, the BER performance of the each time segmentin the desired packet is obtained by a segmentation process, and thus a more accurate PERvalue for the desired packet can be derived, which is validated by the simulation.Considering the randomness of the interference node distribution in actual coexis-tence scenarios, a theoretical PER calculation method for the desired node is presentedunder the situation that the interference node follows spatial Poisson point distribution inthe two-dimensional plane. The theoretical upper bound of the desired node’s PER valueis obtained based on derivation of the average value of the total interference power at thedesired node. Furthermore, the implementation method for packet collision models insuch scenarios is also depicted.For coexistence mechanisms, the on-demand interference avoidance scheme is firstlyresearched. Firstly, according to the channel allocation of SUNs and WLANs, an on-demand channel agility scheme for single gateway SUNs is established, by which theSUN periodically detects the WLAN interference in its current channel and determineswhether or not to switch the channel based on the detection result. Channel switching isperformed if interference is detected. Simulation shows that this mechanism can guar-antee the performance of SUNs in coexistence environment, but the existence of PERdetection periods makes it can not respond to the WLAN interference in a timely manner.Considering this, a modified channel agility scheme is proposed. In the new scheme, theSUN router node makes LQI(Link quality indicator) detection for each received packet,which leads to a more timely response to the WLAN interference that appears in thecurrent channel. Its effectiveness is validated by simulations. Finally, aiming at the multi-gateway SUN, an on-demand channel agility scheme is also proposed, in which differentgateway nodes are assigned different SUN channels. By combining the usage of routinginformation, the channel switching of the child node is realized by changing the root gate-way. However, the specific performance of this scheme needs to be evaluated by furtherresearch and simulations.On-demand channel agility scheme is a passive coexistence mechanism, and it cannot essentially avoid the mutual interference between SUNs and WLANs. To solve thisproblems, a dynamic spectrum sharing based coexistence scheme is proposed from theperspective of cognitive radio, in which the exchange of spectrum data is added into theneighborhood and home area networks, and the gateway node with spectrum manage-ment function assigns channels for SUN and WLAN services in an on-demand manner.So, this is an active coexistence mechanism and it can essentially avoid the channel con-tention between SUNs and WLANs. Additionally, taking account of the priority levels forthe SUN and WLAN services, fair and unfair channel assignment modes are proposed.Two-dimension Markov chain based channel state analysis models are established for thetwo types of channel assignment modes under single mapping and non-single mappingchannel relations, respectively. Based on the analysis model, the steady state probabilityfor each channel status is derived, according to which the blocking probability, drop-ping probability, throughput for SUN services are further derived as well as the systemefficiency. Simulation results indicate that, with the same parameters, the access perfor-mance for SUN services under the fair mode is better than that under the unfair mode.Meanwhile, increasing the coexistence channels or reserved channels can significantlyimprove the access performance for SUN services. It is consistent with the general rulesof resource allocation, thus the effectiveness of the analysis model is validated.