Sharif Digital Repository

We consider an internally coded multiple access technique for Ultra Wide-Band systems which combines time-hopping (TH) and spread-time (ST) techniques using a super-orthogonal encoder. In this method, the duration of each bit is divided into Ns frames. The outputs of a superorthogonal encoder and a PN sequence specify the frame number, in which the data is transmitted in spread-time CDMA form using the second PN sequence. We consider soft decision before Viterbi decoding at the receiver. We obtain the performance of the proposed method. We also consider the uncoded ST-CDMA system and compare its performance with our method. The results indicate that our method has much more better... 

The multiuser performance of two kinds of ultra-wideband (UWB) systems, namely, multi-carrier frequency-hopping-UWB and time-hopping-UWB, is evaluated. We derive the bit error rate of these systems in both AWGN and fading channels. Our numerical results show that the TH-UWB system always outperforms the MC-FH-UWB system with non-coherent detection. However, the MC-FH-UWB system with coherent detection presents a better performance than TH-UWB in a very few cases. Then, we use a practical low-rate convolutional error correcting code, which does not require any extra bandwidth further than that needed by the uncoded scheme, to improve the system performance. The results show that the coded... 

We propose a new multiple access technique for Ultra Wide-Band (UWB) systems based on time-hopping (TH) and direct sequence (DS) spread spectrum techniques. In this method, the duration of each bit is divided into N s frames each one consisting of Nh chips. The outputs of a super-orthogonal encoder and a PN sequence specify the frame number, in which the data is transmitted in DS spread-spectrum form using the second PN sequence. We consider hard decision with threshold and soft decision before Viterbi decoding at the receiver. We evaluate the performance of the proposed method and compare it with the conventional DS-CDMA system. The results show that the proposed method has the best... 

Multirate time hopping ultra wideband (TH-UWB) communication systems are considered where each user can have several multirate services. Three methods named as parallel mapping, serial mapping with variable spreading factor, and multicode using Walsh subcode concatenation schemes are investigated. In the parallel mapping method, one kind of signature code (PN code) is used to distinguish users and their diverse services alike. For the serial mapping with variable spreading factor scheme, one signature code is assigned to each user and different services send their data in distinct frames. Whereas in the multicode using Walsh subcode method, different users and different services of the same... 

In this paper, we present the average bit error probability (BEP) of a decode-and-forward relay network employing UWB links, developed by IEEE 802.15.4a standards, between its nodes. Our methodology is based on computing the Characteristic Function (CF) of the decision variable at the destination terminal. Numerical results show that significant improvement in the BEP of impulse radio UWB system is obtained by adding a relay node. The analytical expressions are also validated by computer simulations which confirm the accuracy of the approximations used in obtaining the BEP  

In this paper, we propose a new method of signaling based on Frequency Shifted Reference (FSR) for Ultra-wideband (UWB) communication system in which the channel estimation is not required in the decoder. This signaling is more like orthogonal signaling of frequency shift keying (FSK), and has its advantage of non-coherent detection; so as a result, one of the major drawbacks of previously introduced FSR-UWB system can be eliminated. It is shown that in binary signaling the proposed method achieves similar performance as conventional Transmitted Reference UWB (TR-UWB) systems without any rate loss. Also, it is demonstrated that the proposed method outperforms the conventional FSR-UWB system... 

We evaluate the performance of the internally coded partial-band DS-CDMA scheme first introduced in [1], in a multipath Rayleigh fading channel. The scheme of [1] is based on frequency-hopping (FH) and direct-sequence (DS) spread spectrum techniques. The total bandwidth is divided into Ns subbands. The output of a super-orthogonal encoder and a pseudorandom (PN) sequence selects one subband, in which the user's data is sent in the form of DS-CDMA signal using the second PN sequence. We consider soft detection at the receiver front end. We obtain the upper and lower bounds on the bit error rate. For comparison, we also consider the conventional DS-CDMA system. It is shown that for the same... 

In this paper, we present the bit error probability (BEP) of a relay network employing UWB links between its nodes. Since UWB systems can resolve many paths and are thus rich in multipath diversity, the use of the Rake diversity combining is very effective. However, because of the complexity, it is impractical in the scenarios with portable terminals or in the networks with multiple relays. Time-reversal technique, in which the transmitting nodes can do the pre-diversity filtering of UWB signal before transmission, is known as an approach to achieve the performance equivalent to the Rake combining without increasing the receiver complexity. The performance of such a network with simple... 

In this paper, the extension of cooperative communication to the context of TH-UWB is investigated. In particular, the average bit error probability (BEP) is provided for cooperative TH-UWB systems with decode-and-forward relaying protocol. In the considered relay network, UWB links among the nodes are modeled according to IEEE 802.15.4a standards. Our methodology is based on computing the Characteristic Function (CF) of the decision variable at the destination terminal. We use Gaussian quadrature numerical method to approximate the CF of interference component appeared in decision variable term. This technique permits to predict the system performance in different IEEE 802.15.4a defined...