Broadband Powerline Communications Networks Design phần 6 pptx

130 Broadband Powerline Communications Networks OFDM symbols OFDM symbols f t Time slots Figure 5.4 OFDM/TDMA network based on the OFDM building an OFDM/TDMA transmission system [Lind99, WongCh99]. In this case, the network resources are divided into time slots, each of them carrying an integer number of OFDM symbols (Fig. 5.4). The length of the time slots can be fixed or variable, but the number of OFDM symbols within a time slot has to be an integer. Some of the OFDM subcarriers can fail because of the disturbances (e.g. because of the long-term narrowband noise, Sec. 3.4), or they can operate with variable data rates if bit loading is applied. In both cases, the entire network capacity changes dynamically, according to the actual disturbance conditions. An OFDM symbol includes a particular number of bits/bytes and carries a specific amount of user data payload. Thus, if the network capacity is decreased, the payload of an OFDM symbol is reduced as well. There are the following two solutions to keep the payload of an OFDM symbol constant: • There are a number of so-called “spare subcarriers” that can be used in the case of fail- ures or capacity decrease. However, if the disturbance conditions are more convenient at the moment, the spare subcarriers remains unused, which is not efficient. • The duration of OFDM symbols is dynamically changed according to the current net- work capacity and availability of the subcarriers. Thus, the duration of the OFDM symbols is varied so that an OFDM symbol always carries a fixed amount of payload bytes. However, after each capacity change, the system has to be again synchronized to adapt to the lengths of the time slots and to fit an integer number of OFDM symbols. To avoid the change of both symbol and time slot duration, the size of user data transmitted within a time slot can be variable to fit within an OFDM symbol, according to the actual network conditions and its currently available transmission capacity. 5.2.1.3 Data Segmentation The division of the transmission resources in the time domain usually causes segmentation of larger data units (e.g. IP packets) into smaller data units. This is necessary because the data has to fit into data segments carried by the time slots provided by a TDMA scheme. At the same time, the data segmentation ensures a finer granularity of the network capacity and a simpler realization of QoS guarantees. Thus, if network resources are divided PLC MAC Layer 131 into smaller accessible portions, it is easier to manage the network resources and share them between various telecommunications services, ensuring realization of their particular QoS requirements. Furthermore, the data segmentation also ensures a higher efficiency in the case of disturbances. So, if a disturbance occurs, a data segment or a number of segments is damaged, and only damaged segments should be retransmitted (e.g. by an ARQ mechanism,). Accordingly, a smaller portion of the network capacity is used for the retransmission, which improves the network utilization. On the other hand, a data segment consists in a general case of two parts; a header field and a payload field. The payload is used for storage of the user information to be transmitted over the network, and the header field consists of information needed for the control functions of the MAC and other network layers (e.g. control of data order, addressing, etc.). Therefore, the segmentation causes an additional overhead and there is a need for optimization of the data segment size, which depends on the disturbance characteristics in network. An optimal segment size can be chosen in accordance with the BER in a communica- tions system, as is presented in [Modi99]. If a network applying a perfect retransmission algorithm is considered, such as selective-reject ARQ (Sec. 4.3.4), the optimal segment size to be used in the network can be calculated according to the Eq. (5.2). S opt = −h ln(1 − p) −  −4h ln(1 − p) +h 2 ln(1 − p 2 ) 2ln(1 − p) (5.2) p – channel bit-error-rate h – number of overhead bits per segment Figure 5.5 shows the optimal segment size, depending on the BER in a network, calculated for h = 40 overhead bits (5 bytes) per segment. With an increasing BER, segments errors become more frequent, and accordingly it is often necessary to retransmit the damaged data segments. Therefore, in the case of higher BER in the network, the segment size has to be chosen to be smaller. On the other hand, larger data segments can be used in networks with lower BER. For example, in order to operate at a BER of 10 −3 asegment size of a few hundred bits should be used; e.g. about 240 bits (30 bytes). BER 2000 1600 1200 800 400 0 10 −5 10 −4 10 −3 10 −2 10 −1 Segment size/bit Figure 5.5 Optimal segment size versus BER 132 Broadband Powerline Communications Networks The size of data segment is usually chosen to ensure an efficient network operation under the worst acceptable disturbance conditions. However, the BER in a network changes dynamically, depending on several factors, such as number of active stations in the net- work, activity of noise sources in the network environment, and so on. Thus, the size of the data segments, calculated for the worst case is not optimal any more. Therefore, realization of data segments with variable size, which depends on the current BER in the network, seems to be a reasonable solutions. However, this approach causes a higher complexity for realization of such communications systems. 5.2.2 FDMA 5.2.2.1 Basic FDMA The next option for the division of the network resources into the accessible sections is to allocate different portions of the available frequency spectrum to different subscribers. This access method is called Frequency Division Multiple Access(FDMA). Similar to the orthogonality condition from Eq. (5.1), the orthogonality between different users can also be defined in the frequency range [DaviBe96]: ∞  −∞ X i (f )X j (f ) df =  1 i = j 0else (5.3) FDMA provides a number of transmission channels, representing the accessible sections of network resources, spread in a frequency range (Fig. 5.6). Each transmission channel uses an extra frequency band, within entire frequency spectrum of a transmission medium, that can be allocated to particular users and services. The data rate of a transmission chan- nel depends on the width of the frequency band allocated to the channel. Principally, the transmission channels with both fixed and variable data rates, such as the case in TDMA, Frequency bands Protection bands f t Figure 5.6 Principle of FDMA PLC MAC Layer 133 can also be realized in an FDMA system by a dynamic frequency allocation to partic- ular transmission channels. To ensure the orthogonality between individual transmission channels, a protection interval in frequency domain has to be provided between FDMA frequency bands. A big advantage of the FDMA scheme over TDMA is the robustness against nar- rowband disturbances [MoenBl01] and frequency-selective impulses. In this case, the disturbances can be easily avoided by reallocation of the existing connections from the frequencies affected by the disturbances to the available part of the frequency spectrum. The same principle can be applied for avoidance of the critical frequencies, which are forbidden for PLC because of EMC problems (Sec. 3.3). FDMA scheme can be implemented in different transmission systems, such as spread- spectrum and OFDM-based transmission systems, which are considered as suitable for realization of broadband PLC systems (Sec. 4.2). In an SS/FDMA system (combina- tion of spread-spectrum and FDMA), the transmission is organized within the frequency bands, provided by the FDMA. On the other hand, because of the specific division of the frequency spectrum in multiple subcarriers, the application of FDMA in OFDM-based transmission systems leads to an OFDMA (OFDM Access) scheme [NeePr00, Lind99, WongCh99], which is also called clustered OFDM [LiSo01]. Because of the robustness of FDMA-based schemes against narrowband disturbances, OFDMA is considered as a suitable solution for the organization of multiple access in PLC access networks. 5.2.2.2 OFDM Access According to the OFDMA scheme, the subcarriers with relatively low data rates are grouped to build up the transmission channels with higher data rates providing a simi- lar FDMA system [NeePr00, KoffRo02]. However, the protection frequency bands, which are necessary in FDMA to separate different transmission channels (Fig. 5.6), are avoided in an OFDMA system thanks to the provided orthogonality between the subcarriers, as described in Sec. 4.2.1. Each transmission channel (CH) consists of a number of subcarriers (SC), as is presented in Fig. 5.7. The subcarriers of a transmission chan- nel can be chosen to be adjacent to each other, or to be spread out in the available frequency spectrum. The transmission channels represent the accessible sections of the network resources that are established by the OFDMA scheme. So, the task of the MAC protocol is to manage the channel reallocation between a number of subscribers and different telecom- munications services. The transmission channels can be organized so as to have constant or variable data rates, which can be ensured by the association of variable numbers of sub- carriers building a transmission channel. The subcarriers can be managed in the following three ways: (a) A group of subcarriers (SC), all with a fixed data rate, form a transmission channel (CH) with a constant data rate. (b) A group of subcarriers with variable data rates (caused by bit loading, Sec. 4.2.1) form a channel. Accordingly, the channels also have variable data rates. (c) The subcarriers are grouped according to the available data rates per subcarrier, in order to build up the transmission channels with a certain data rate. The subcarrier data rates are variable, but the channel data rate remains constant. 134 Broadband Powerline Communications Networks SC1 SC2 SC3 SCk SC1 SC2 SC3 SCk SC1 SC2 SC3 SCk SC1 SC2 SC3 SCk CH1 CH2 CH3 CH n Figure 5.7 OFDMA channel structure In case A, the transmission channels have the same transmission capacity and always include the same subcarriers (Fig. 5.7). If one or more subcarriers are not available (e.g. they are defective) the transmission channel cannot be used, although other subcarriers are still available. In case B, the subcarriers of a transmission channel change their data rates according to the network and disturbance conditions (bit loading), and with it change the channel data rate, too. In case C, all available subcarriers are summarized into a number of channels with a certain (fixed or variable) transmission capacity. That means, a number of subcarriers are grouped according to their available capacity to form a transmission channel with a desired capacity. In this case, the transmission channels do not always include the same subcarriers. 5.2.2.3 OFDMA/TDMA As is mentioned above, the slotted nature of OFDM-based transmission systems leads to a logical division of the network resources in the time domain (TDMA). An OFDMA system can also be extended to include the TDMA component, which leads to a com- bined OFDMA/TDMA scheme (Fig. 5.8). In this case, the transmission channels, which are divided in a frequency range, are also divided into time slots with a fixed or vari- able duration. Accordingly, each time slot carries a data segment with a fixed or variable PLC MAC Layer 135 OFDMA channels TDMA time-slots OFDM symbols f t Figure 5.8 OFDMA/TDMA scheme size. The data segments present the smallest accessible portions of the network resources provided by the OFDMA/TDMA scheme, which are managed by a MAC protocol. Thus, in the case of OFDMA/TDMA, the MAC protocol controls access to both transmission channels and time slots. Each transmission channel consists of a number of subcarriers, which can be grouped in different ways, as is provided by the OFDMA scheme (Fig. 5.7). Accordingly, a transmission channel can include a variable number of subcarriers or a fixed number of subcarriers with variable data rates (bit loading), causing variable data rates of the transmission channel as well. On the other hand, a time slot carrying a data segment consists of a number of OFDM symbols with a certain duration and payload capacity, as is described above for an OFDM/TDMA system. In any case, the number of the OFDM symbols per time slot and per channel, which corresponds to a data segment, has to be an integer. 5.2.3 CDMA The CDMA (Code Division Multiple Access) method provides different codes to divide the network resources into the accessible sections. The data from different users is distin- guished by the specific code sequences and can be transferred over a same transmission medium, by using a same frequency band, without interferences between them. The CDMA scheme is based on the spread-spectrum principle, recently called Code Division Multiplex (CDM), and is also denoted as Spread-Spectrum Multiple Access (SSMA). In Sec. 4.2.2, we presented the spread-spectrum technique from the transmission point of view without consideration of the multiple access capabilities of the CDMA scheme. In the description below, we discuss possibilities to use the features of the spread-spectrum technique for realization of various CDMA systems. 136 Broadband Powerline Communications Networks 5.2.3.1 Principle CDMA can be realized by application of several coding methods (see e.g. [Pras98]). The most considered methods in recent telecommunications systems, such as wireless networks, are [DaviBe96, Walke99] • DS-CDMA – Direct Sequencing CDMA – based on Direct Sequence Spread Spectrum (DSSS) method, where each user’s data signals are multiplied by a specific binary sequence, and • FH-CDMA – Frequency Hopping CDMA – based on Frequency Hopping Spread Spec- trum (FHSS) method, where the transmission is spread over different frequency bands, which are used sequentially. In a DS-CDMA system, all subscribers of a network use the entire available frequency spectrum of a transmission medium. To be able to distinguish between different subscribers, data signals from different network users are multiplied by different code sequences, which are chosen to be unique for every individual user or connection (Fig. 5.9). At the receiver side, the arriving signal is again multiplied by the uniquely specified code sequence. The result of the multiplication is the originally sent data signal, which is extracted between all other data signals, multiplied by different code sequences. Thus, data signal S i (t), generated by user i, is multiplied by its corresponding code sequence C i (i) building a coded signal S i (t)C i (t), which is transmitted over a medium (e.g. wireless or PLC channel). A receiving user listens to the transmission medium and can receive coded signals generated by all network users, so-called “signal mix” S 1 (t)C 1 (t) to S n (t)C n (t), originated by application of their own codes. However, to receive and decode the original data signal S i (t), it is necessary to multiply the signal mix by the unique code sequence C i (t), which is only known or currently applied by the receiving user. To explain how it is possible to distinguish between signals from different users in a CDMA system, we present an example by considering two signals S a (t), with a bit sequence {1, 0, 1, 1} and S b (t), with {0, 1, 1, 0}, generated by two users A and B (Fig. 5.10). Both users code the bit sequence with their own code sequence C a (t), with {1, 0, 1, 0},andC b (t), with {1, 0, 0, 1}, respectively. Both code sequences are transmitted with four times higher data rates than the original user signals. After the multiplication of bit and code sequences, users A and B deliver their signal products S a (t)C a (t) and S b (t)C b (t) to a shared transmission medium. Thus, a sum signal S a (t)C a (t) + S b (t)C b (t) is received by destination users A’ and B’, which are target users Signal mix Data signal Code C i ( t ) S i ( t ) S 1 ( t )C 1 ( t ), , S i ( t )C i ( t ), , S n ( t )C n ( t ) S i ( t ) Data signal Code C i ( t ) S i ( t )C i ( t ) Coded signal Transmitter ReceiverTransmission medium Figure 5.9 Principal scheme of a DS-CDMA transreceiver PLC MAC Layer 137 t +1 −1 t +1 −1 t +1 −1 t +1 −1 t +1 −1 t +1 −1 1011 0110 1010 100 1 S a ( t ) C a ( t ) S a ( t )C a ( t ) S b ( t ) C b ( t ) S b ( t )C b ( t ) Figure 5.10 CDMA signal generation/coding – example t +1 −1 +2 −2 S a ( t )C a ( t ) + S b ( t )C b ( t ) t +1 −1 t +1 −1 1011 0110 S a ( t )S b ( t ) t +1 −1 t +1 −1 +2 +2 −2 −2 [S a ( t )C a ( t ) + S b ( t )C b ( t )] C b ( t )[S a ( t )C a ( t ) + S b ( t )C b ( t )] C a ( t ) Figure 5.11 CDMA signal decoding – example for both signals S a (t) and S b (t), respectively (Fig. 5.11). To extract the original signals from users A and B at the right receiver, target users A’ and B’ have to multiply the sum signal by code sequences C a (t) and C b (t), which are also used at the transmitters for signal coding. The result of this multiplication is original bit sequences S a (t) and S b (t) received by A’ and B’ respectively. 138 Broadband Powerline Communications Networks S i ( t ) C i ( t ) S i ( t )C i ( t )S i ( t ) C i ( t ) S 1 ( t ) C 1 ( t ) S 1 ( t )C 1 ( t ) S n ( t )C n ( t ) C n ( t ) S n ( t ) C 1 ( t ) C n ( t ) S 1 ( t ) S n ( t ) Transmission medium ReceiversTransmitters S 1 ( t )C 1 ( t ) + S i ( t )C i ( t ) + + S n ( t )C n ( t ) + Figure 5.12 A DS-CDMA system The same principle of dividing information signals of various network users can be applied if a larger number of subscribers use a same shared transmission medium. In this case, a code sequence has to be defined for every connection in the network (C 1 (t), . . . , C i (t), . . . , C n (t)), as presented in Fig. 5.12. Both transmitting and receiving participant of a connection have to use the same code sequence. If we consider communications network with a centralized structure, such as PLC access networks (Sec. 3.1), a central unit (e.g. base station) uses a number of code sequences to receive signals from different network users. The application of different codes ensures realization of a transmission channel within a CDMA system. So, the transmission channels are determined by applied code sequences providing the accessible portions of the network resources, such as the time slots in TDMA and frequency bands in FDMA schemes. As is mentioned above, a DS-CDMA system occupies the entire frequency band that is used for the transmission over a medium. On the other hand, FH-CDMA systems use only a small part of the frequency band, but the location of this part differs in time [Pras98]. During a time interval (Fig. 5.13), the carrier frequency remains constant, but in every time interval, it hops to another frequency (Sec. 4.2.2). The hopping pattern is determined by a code signal, similar as in a DS-CDMA system. Thus, the transmission channels in an FH-CDMA system are defined by the specific code as well. So, during a data transmission, a subscriber uses different frequency bands. The change of the frequency bands in the time is specified by the code sequence, allocated to th e subscriber. In a special case, if the codes allocated for the individual users always point to the same frequency band, the same users always transmit over the same frequency bands, which leads to a classical FDMA system. A further variant of CDMA schemes is TH-CDMA (Time Hopping CDMA), where the data signal is transmitted during so-called “rapid time-bursts” at time interval determined by a specific code sequence (Fig. 5.14). In a TH-CDMA system, the entire frequency PLC MAC Layer 139 Frequency Time Figure 5.13 FH-CDMA – time/frequency diagram Frequency Time Figure 5.14 TH-CDMA – time/frequency diagram spectrum is used, such as in a DS-CDMA. However, the exact time slots to be used for a particular transmission are determined by a code sequence, for example, allocated to a network user. If there is a synchronization among code sequences that one user transmits only during a particular time slot, TH-CDMA becomes a TDMA system. The variants of CDMA presented above can be combined to build up so-called “hybrid CDMA solutions”. The hybrid schemes, such as DS/FH, DS/TH, FH/TH and DS/FH/TH, can be applied to join the advantages of different CDMA variants. Furthermore, the CDMA techniques can also be combined with other multiple access schemes; for example, building a CDMA/TDMA [ChlaFa97] or a CDMA/FDMA scheme [SchnBr99]. In a CDMA/TDMA scheme, the accessible sections of the transmission resources are provided by both division [...]... easy because it is fully controlled by the base station (Fig 5. 26) In this direction, the base station transmits data to one or multiple network stations, or it broadcasts information to all network stations In any case, there are only data packets 152 Broadband Powerline Communications Networks WAN Uplink Downlink Base station Figure 5. 26 Transmission directions in a PLC access network from the base... offered network load (e.g expressed in bps) of 1 is equal to the maximum data rate in the network 1 56 Broadband Powerline Communications Networks Utilization 0.4 0.3 Slotted ALOHA 0.2 Pure ALOHA 0.1 0.0 Offered load 0.001 0.01 0.1 1 10 100 Figure 5.30 Network utilization of ALOHA protocols Substituting Eq (5. 26) and G = gT in Eq (5.28), we have finally S = Ge−2G (5.29) The random nature of the ALOHA protocol... PNS generator classes are implemented In the following, the mostly encountered ones are described; [Meel99b]: G WGN(f ) RR, WGN(t) d(t).N 0 /2 0 t 0 Figure 5. 16 Autocorrelation of the White Gaussian Noise f 144 Broadband Powerline Communications Networks N=7 Rxx (t) −1 −N t/Tc N 0 Tc +1 Xp DC = 0 t 1/Tc f −1 N.Tc Figure 5.17 Autocorrelation and the frequency occupation of a periodic sequence m-Sequence... unique advantage of the m-sequence toward all other PNS codes generators Unfortunately, its crosscorrelation is not as good as its autocorrelation Therefore, when a large number of 1 46 Broadband Powerline Communications Networks transmitters using different codes share a frequency band, the code sequences must be carefully chosen to avoid interference between users Gold Codes In spite of its best autocorrelation... different to zero takes the values from the set {−9, −1, +7} according to Eq (5.11), because t (r) = 9 according to Eq (5.13) This autocorrelation function is presented in Fig 5.23 148 Broadband Powerline Communications Networks 5.2.3.4 Capacity In TDMA and FDMA systems, network capacity is limited by used frequency spectrum determining the number of the transmission channels in time and frequency domain,... is the network capacity So, the maximum number of users in the network is inversely proportional to the required link metric (Eb /N0 ) If we consider communications system with frequency reuse, such as cellular mobile networks and broadband PLC access networks with repeaters (Sec 2.3.3 and Sec 3.1), a CDMA-based network cannot be considered as an isolated system, because it is influenced by neighboring... data rates, is firmly determined by the number of time slots or frequency bands If there are no free transmission channels in a network, new connections cannot be accepted, causing 150 Broadband Powerline Communications Networks so-called “blocking” In CDMA systems, the same situation exists if there are no free channels (codes) in the network, causing so-called “hard blocking” However, CDMA systems...140 Broadband Powerline Communications Networks in the time domain (by time slots) and division in the code domain, by allocation of code sequences Thus, a user accesses a determined time slot and applies a specific code sequence... [AkyiMc99], typical for different kinds of data transfer that are expected in the access networks, such as broadband PLC networks Unlike fixed access methods, dynamic access protocols are adequate for data transmission, and in some cases, it is also possible to ensure realization of QoS guarantees for various telecommunications services The dynamic protocols are divided into two subgroups; contention... collision resolution to reduce number of collisions in the network, f Transmission gap Data burst Allocated channel bandwidth t Figure 5.27 Bursty data traffic and fixed access strategy 154 Broadband Powerline Communications Networks Dynamic access Contention protocols Random access Arbitration protocols Dedicated access Hybrid protocols ALOHA CSMA Elimination Tokenpassing Polling Reservation Collision resolution . systems. 1 36 Broadband Powerline Communications Networks 5.2.3.1 Principle CDMA can be realized by application of several coding methods (see e.g. [Pras98]). The most considered methods in recent telecommunications. 0 f G WGN ( f ) R R , WGN (t) t d(t). N 0 /2 Figure 5. 16 Autocorrelation of the White Gaussian Noise 144 Broadband Powerline Communications Networks R xx (t) t/ T c 1/ T c f N = 7 −1 X p N . T c T c t +1 −1 DC. its crosscorrelation is not as good as its autocorrelation. Therefore, when a large number of 1 46 Broadband Powerline Communications Networks transmitters using different codes share a frequency band, the code