777 research outputs found
Dispensing with channel estimation: differentially modulated cooperative wireless communications
As a benefit of bypassing the potentially excessive complexity and yet inaccurate channel estimation, differentially encoded modulation in conjunction with low-complexity noncoherent detection constitutes a viable candidate for user-cooperative systems, where estimating all the links by the relays is unrealistic. In order to stimulate further research on differentially modulated cooperative systems, a number of fundamental challenges encountered in their practical implementations are addressed, including the time-variant-channel-induced performance erosion, flexible cooperative protocol designs, resource allocation as well as its high-spectral-efficiency transceiver design. Our investigations demonstrate the quantitative benefits of cooperative wireless networks both from a pure capacity perspective as well as from a practical system design perspective
POWER ALLOCATION ALGORITHM FOR MIMO BASED MULTI-HOP COOPERATIVE SENSOR NETWORK
Cooperative transmission is a new breed of wireless communication systems that enables the cooperating node in a wireless sensor network to share their radio resources by employing a distributed transmission and processing operation. This new technique offers substantial spatial diversity gains as the cooperating nodes help one another to send data over several independent paths to the destination node. In recent times, an extensive effort has been made to incorporate these systems in the future wireless networks like LTE (Long Term Evolution), IEEE 802.16j (Mobile Multi-hop Relay (MMR) Networks) and IEEE 802.16m (Mobile WiMAX Release 2 or WirelessMAN-Advanced). But, there are few technical issues which need to be addressed before this promising technique is integrated into future wireless networks. Among them, managing transmission power is a critical issue, which needs to be resolved to fully exploit the benefits of cooperative relaying. Optimal Power Allocation, is one such technique that optimally distributes the total transmission power between the source and relaying nodes thus saving a lot of power while maintaining the link quality. In the first part of the thesis, mathematical expressions of the received signals have been derived for different phases of cooperative transmission. Average-Bit-error-rate (ABER), has been taken as a performance metric to show the efficiency of cooperative relaying protocols. In the second part of this Chapter, a multi-hop framework has been presented for the power allocation algorithm with Amplify-and-Forward relaying protocol. The efficiency of the power allocation algorithm has been discussed with different scenarios i.e. First for a three node (2-Hop) wireless network configuration and then for a four node (3-Hop) wireless network configuration. The transmission scenarios (2-Hop and 3-Hop) have been further categorized into multiple cases on the basis of channel quality between source-to-destination, source-to-relay, relay-to-relay and relay-to-destination links.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format
Single and multiple antenna relay-assisted techniques for uplink and downlink OFDM systems
In this paper we propose and assess the
performance of relay-assisted schemes designed for both the
uplink and downlink OFDM based systems, using efficient
distributed space-frequency block coding protocols. We
consider the use of an antenna array at the base station and a
single antenna at the user terminal. At the relay node we
consider either single antenna or an antenna array. We assume
that some of the user terminals deployed in a certain area
could act as relaying-able terminals for the communication of
other users. Two types of relay-assisted protocols are
considered: equalize-and-forward and decode-and-forward.
The optimal maximum ratio combining coefficients are derived
for the proposed relay-assisted schemes. The performance of
these cooperative schemes is evaluated under realistic
scenarios, considering typical pedestrian scenarios based on
WiMAX specifications and using channel convolutional turbo
code. The proposed schemes are also compared against the
non-cooperative OFDM based systems. +umerical results show
that the availability of antenna arrays at the relays
significantly improves the cooperative systems performance,
which outperform the non-cooperative ones in most studied
scenarios
Interference Cancellation with Beamforming and Power Control in Cooperative Networks
The interference cancellation (IC) technique for cooperative networks is investigated, if the relay and the destination are disturbed by the co-channel interference (CCI) from neighborhood system. In order to solve such interference problem, the beamforming algorithm with the appropriate weight estimation of the array smart antenna at the source (S), the power control technique at interfering source and signal combining algorithms at the destination (D). The maximum ratio combining (MRC) and the cooperative maximum ration combining (C-MRC) are used to combine the received signals arrived at D. We can also control the transmitted power at the source from interfering system, and maintain nearly the diversity gain compare with CCI and no CCI. Therefore, the proposed scheme achieves the maximum diversity gain and lower probability of error in comparison with the conventional decodeand- forward protocol (DF). And it is able to provide the power control strategy from interfering source to the relay and the destination nodes in order to achieve the minimum symbol error rate (SER) based on the experimental results from computer simulations
Relay Selection for Wireless Communications Against Eavesdropping: A Security-Reliability Tradeoff Perspective
This article examines the secrecy coding aided wireless communications from a
source to a destination in the presence of an eavesdropper from a
security-reliability tradeoff (SRT) perspective. Explicitly, the security is
quantified in terms of the intercept probability experienced at the
eavesdropper, while the outage probability encountered at the destination is
used to measure the transmission reliability. We characterize the SRT of
conventional direct transmission from the source to the destination and show
that if the outage probability is increased, the intercept probability
decreases, and vice versa. We first demonstrate that the employment of relay
nodes for assisting the source-destination transmissions is capable of
defending against eavesdropping, followed by quantifying the benefits of
single-relay selection (SRS) as well as of multi-relay selection (MRS) schemes.
More specifically, in the SRS scheme, only the single "best" relay is selected
for forwarding the source signal to the destination, whereas the MRS scheme
allows multiple relays to participate in this process. It is illustrated that
both the SRS and MRS schemes achieve a better SRT than the conventional direct
transmission, especially upon increasing the number of relays. Numerical
results also show that as expected, the MRS outperforms the SRS in terms of its
SRT. Additionally, we present some open challenges and future directions for
the wireless relay aided physical-layer security.Comment: 16 pages, IEEE Network, 201
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