However, the above studies and designs for URLLC simply follow Shannon’s capacity, i.e., are not taking the impact of the finite blocklength into consideration. It is more accurate to incorporate FBL coding assumptions into the analysis and design of URLLC networks with low- latency constraints. In such an FBL regime, data transmis- sions are no longer arbitrarily reliable. Especially when the blocklength is short, the error probability becomes sig- nificant even if the rate is selected below the Shannon limit. Taking this into account, an accurate approximation of the achievable coding rate under the finite blocklength assumption for an additive white Gaussian noise (AWGN) channel was studied in [17, 18]. Following the accurate FBL performance model, multiple user scheduling  and power allocation  of an OFDMA network has been studied. More recently, the FBL throughput  and energy efficiency  of power-domain NOMA networks have been discussed. However, the FBL performance and optimal resource allocation for code-domain NOMA net- works, e.g., SCMA, have not been addressed so far.
In wireless communication, a node may broadcast informa- tion through the electromagnetic (EM) waves to all of its neighboring nodes. At the same time, a node may receive several signals simultaneously sent from its neighbors. Due to the additive nature of the EM waves, information cannot be recovered from these scrambled signals correctly without appropriate protocols. This is a problem called multipleaccess interference (MAI). A similar problem is illustrated in the pioneering work of Gupta and Kumar , from which it can be concluded that the capacity of wireless ad hoc networks is constrained by the mutual interference of con- current transmissions among nodes (i.e., the MAI problem). When the number of nodes in a distributed ad hoc network gets larger, information is transmitted through a “multihop” method from the source node to the sink nodes. As a result, the opportunity of such a problem is additionally increased. Hence, many researchers attempt to find new approaches to boost the network capacity. Networkcoding (NC)  is a new method in information theory which allows nodes to
Free space optical network (FSO) is a promising technology, which has advantages such as low cost, free of license and also high speed broadband provision . It is applicable in many areas such as last mile, off shore and rural areas. Also it can be utilized in military and inter- satellite applications. Nonetheless, the system performance can be degraded due to different types of impairments. Absorption and scattering cause significant signal loss. Also, scintillation is the most destructive phenomenon which restricts the system throughput  .
In this section, we show that the proposed modulation and coding scheme is viable even if the real-world con- ditions induce some deviations from the system model assumed in this paper. From a practical point of view, we would be interested mainly in the situation, where a direct channel between a source and its desired destination is present (see Fig. 16). Even though this “direct” channel can be relatively weak, it inevitably affects the destina- tions’ HSI observations. In addition to this, we analyze the system where source phase pre-rotation is switched off (or ineffective due to rapid channel dynamics), and thus the relay faces a multiple-access channel with a vary- ing source channel phase offset. This assumption is valid for a practical system where the applicability of source phase pre-rotation is limited due to the unavailable ideal feedback channel or because of the undesirable delay
However, in , a network-coded operation is always needed in moderate-to-high signal-to-noise ratio (SNR) regime regardless of the channel qualities of its user broadcast phase. Actually, the broadcast phase can be seen as a multiple-access channel that consisted of two users and one destination in . According to the multiple-access channel capacity region , if all of the uplink channels are good enough, the destination can decode each user’s information correctly with only one time slot. Otherwise, only the user whose chan- nel quality cannot support its current transmission rate is helped by the relay nodes to communicate with the destination. That is, one extra network-coded opera- tion of both users’ messages is not always necessary by the cooperative scheme of  in moderate-to-high SNR regime.
appropriately determined set of rates, which uses successive interference cancelation to resolve packet collision due to wireless broadcast. When the number of transmission rates at each node is equal to the number of users, the achievable total throughput was shown to be at least a constant fraction of the centralized multipleaccess channel sum rate in slotted Aloha type networks. To facilitate practical protocol design, we also studied the case when only a limited number of transmission rates is available at each node. A game-theoretic framework was proposed to achieve the desired throughput optimal equilibrium in the absence of centralized knowledge of the total number of users. We studied the design of random access games, characterized their equilibria, studied their dynamics, and proposed distributed algorithms to achieve the equilibria. Lastly, we considered secure communications in networks with erasure and unequal link capacities in the presence of a wiretapper. For the case when the location of the wiretapped links is known, we have derived the secrecy capacity region. For the case when the location of the wiretapped links is unknown, we proposed several achievable strategies. We showed that unlike the case of equal link capacities, the secrecy capacity when the location of wiretapped links is known and unknown are generally unequal. We also showed that computing the secrecy capacity for both cases are NP-complete.
Modern Communication is intended to achieve greater heights for providing best services to the users. Due to heavy demand of bandwidth on remote access sites, opt- ical code division multipleaccessnetworks need to be improved. Erbium-doped fiber amplifier is very useful for providing various properties in optical communica- tion as it provides higher level of compatibility with less loss with better gain while communication and also re- duces noise parameters. EDFA process has been used for the cancellation of noise and to provide gain with coup- lers along with encoder and decoders in the network.
The source encoding assisted multipleaccess (SEAMA) protocol exploits source encoder characteristics for voice/da- ta integration in a wireless network . SEAMA increases network utilization while it provides the desired QoS for applications. At the application layer, SEAMA employs an embedded multistate (multirate) voice encoder. An embed- ded voice encoder has the property that a truncated ver- sion of its output bit stream can be used to generate a coarser description of the original input signal. SEAMA as- signs bandwidth to ongoing calls based on the state of the encoders (state of the calls). SEAMA resolves overflows by selectively dropping packets from the embedded bit stream of some calls, according to an optimal scheduling pol- icy. As a result, SEAMA achieves a significant gain in net- work utilization, for example, a 100% gain compared to a circuit-switched network and at least a 20% compared to PRMA . Moreover, the quality of the voice tra ﬃ c degrades gracefully by increasing the number of admitted calls. At the network layer, and similar to PRMA, SEAMA is packet switched for both voice and data and it incorpo- rates an FMB, that is, the movable boundary is updated every frame.
The adjective “static” in the terms above stresses the fact that, while the configuration ε varies, the local encoding kernels remain unchanged. The advantage of using a static linear dispersion, broadcast, or multicast in case of link failure is that the local operation at any node in the network is affected only at the minimum level. Each receiving node in the network, however, needs to know the configuration ε before decoding can be done correctly. In real implementation, this informa- tion can be provided by a separate signaling network. In the absence of such a network, training methods for conveying this information to the receiving nodes have been proposed in .
The main advantages of MC-CDMA include efficiency and flexibility in spectrum usage. Other users are allowed to use the spectrum or subcarriers in the MC- CDMA design if it is not used by the current user. This technique is robust to frequency selective fading because the symbol period is larger than the delay spread resulting in reduced Inter symbol Interference.
In this paper, we studied the throughput and average packet decoding delay (APDD) performance of S-IDNC in broadcasting a block of data packets to wireless receivers under packet erasures. By using a random graph model, we showed that the throughput of S-IDNC decreases with increasing an number of receivers. By introduc- ing the concept of perfect S-IDNC solution, we proved the NP-hardness of APDD minimization. We derived an upper bound on APDD and showed that minimiz- ing the IDNC solution size can effectively reduce APDD. By applying stochastic shortest path method, we showed that it is intractable to make optimal coding decisions in the presence of random packet erasures. We then
investigate the potentiality to establish this channel in an asymmetric manner in the sense that the group members merely negotiate a common encryption key (accessible to attackers) but hold respective secret decryption keys. We introduce a new class of GKA protocols which we name asymmetric group key agreements (ASGKAs), in contrast to the conventional GKAs. A trivial solution is for each member to publish a public key and withhold the respective secret key, so that the final ciphertext is built as a concatenation of the underlying individual ones. However, this trivial solution is highly inefficient: the ciphertext increases linearly with the group size; furthermore, the sender has to keep all the public keys of the group members and separately encrypt for each membere. We are interested in nontrivial solutions that do not suffer from these limitations. Group key agreement (GKA) is another well-understood cryptographic primitive to secure group-oriented communications. A conventional GKA allows a group of members to establish a common secret key via open networks. However, whenever a sender wants to send a message to a group, he must first join the group and run a GKA protocol to share a secret key with the intended members. More recently introduced asymmetric GKA in which only a common group public key is negotiated and each group member holds a different decryption key.
for the two-terminal network where the two source sequences are separately encoded and the decoder combines the two encoded messages to losslessly reproduce both of the two source sequences . Gray and Wyner found an exact single-letter charac- terization for both lossless and lossy rate regions on a related “simple network” . Ahlswede and K¨orner derived a single-letter characterization for the two-terminal net- work where the decoder needs to reconstruct only one source sequence losslessly ; that characterization employs an auxiliary random variable to capture the decoder’s incomplete knowledge of the source that is not required to reconstruct. Wyner and Ziv derived a single-letter characterization of the optimal achievable rate for lossy sourcecoding in the point-to-point network when side information is available only at the decoder . Berger et al. derived an achievable region (inner bound) for the lossy two-terminal sourcecoding problem in . That region is known to be tight in some special cases . Heegard and Berger found a single-letter characterization by using two auxiliary random variables for the network where side information may be absent . Yamamoto considered a cascaded communication system with multi-hop and multi-branches . For larger networks, Ahlswede et al. derived an optimal rate region for any networksourcecoding problem where there is one source node that observes a collection of independent source random variables, all of which must be reconstructed losslessly by a family of sink nodes ; Ho et al. proved the cut-set bound is tight for multi-cast network with arbitrary dependency on the source random variables ; Bakshi and Effros generalized Ho’s result to show the cut-set bound is still tight when side information random variables are available only at the end nodes .
As the number of syndrome equations for an entire OFB encoded sequence is large, the syndrome decoder operates on the segments of the received codeword, in a sequential manner. The decoding algorithm consists of two steps: error localization and error amplitude estimation. The error local- ization procedure determines the positions in which errors have occurred by inspecting the syndromes. Due to quantiza- tion noise, syndromes have nonzero values, even in absence of channel noise. The localization procedure therefore has to distinguish between the changes of syndrome values due to quantization noise and that due to impulse errors. This can be done for example by thresholding the syndrome values . However, better results can be achieved by using more sophisticated methods such as methods based on the hypoth- esis testing theory [1, 18, 19].
decoding of such packets. As a first step towards such goal, we limit ourselves to broadcast erasure channels, but emphasize that the ideas can be extended to other more complicated scenarios. We also consider the class of instantaneously decodable networkcoding schemes, in which each coded transmission contains at most one new source packet that a receiver has not decoded yet. The rationale is that in an order- insensitive application, any innovative packet that cannot be decoded immediately incurs a unit of delay. Obviously, one other source of delay is when a coded packet does not contain any new information for a receiver and hence, is not innovative. A similar definition of the decoding delay was first considered in , where the authors presented a number of heuristic algorithms to reduce order-insensitive decoding delay. In this context, our main contributions are the following.
fading channel environment, and can only be viewed as flat fading non-orthogonal CDMA channels. The THP designs for frequency-selective fading channels are presented in [LD04, LDg04, LDj04]. In this thesis, we first illustrate the principle of THP for the simple case of single-path channels with additive white Gaussian noise (AWGN) and flat Rayleigh fading channels. Then we develop two specific THP designs for frequency-selective fading channels, THP with PreRAKE combiner (PreRAKETHP) and Multipath Decorrelating THP (MDTHP). The PreRAKETHP and MDTHP incorporate Tx-based diversity combining techniques in different ways. In PreRAKETHP, the MAI cancellation is followed by the pre- RAKE combining [EN95]. While this precoder is the optimal THP design for multipath channels, it requires high computational complexity, since its MAI cancellation filters depend on the rapidly time variant mobile radio channel coefficients and need to be updated frequently. In MDTHP, the diversity combining is incorporated into the MAI cancellation, and the precoding filter is independent of the channel. Thus, MDTHP is simpler than PreRAKETHP, and results in moderate bit error rate (BER) loss.
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We have proposed and investigated a novel multiple-access scheme, namely the TH/MC-CDMA scheme, which is based on the TH, MC modulation, and CDMA techniques. From our analysis and results, we find that TH/MC-CDMA is capable of providing a range of advantages. First, TH/MC-CDMA signals have a flat Gaussian noiselike PSD over probably a very wide bandwidth. The PSD outband sidelobes decrease much more rapidly than that of a corresponding single-carrier TH-CDMA signal. Second, in TH/MC-CDMA, due to the TH characteristic, each subcarrier is only activated for a fraction of the frame time duration. Hence, the nonlinear fluctuation due to the high peak-to-average factor in conventional MC systems can be significantly mitigated. Furthermore, in TH/MC-CDMA receiver, the processing rate is on the order of the TH rate R h .