Description
Date depot: 1 janvier 1900
Titre: Evaluation des Performances des Reseaux Cellulaires avec Relais
Directeur de thèse:
Philippe GODLEWSKI (LTCI (EDMH))
Domaine scientifique: Sciences et technologies de l'information et de la communication
Thématique CNRS : Non defini
Resumé:
The growing demand for mobile broadband internet and wireless multimedia applications and the
requirements for the future 4G mobile systems have pushed the search for means to improve
wireless networks throughput and coverage. In particular, it is desired that the new networks
provide very high data rate (up to hundreds of Mbit/s) in every location of the cell and to every
User End (UE), even to the ones dealing with the most unfavourable conditions, i.e. experiencing
a very high signal strength attenuation on the radio link with their service provider netowrk node,
here referred to as enhanced Node B (eNB). The solution to this problem in a traditional network
would consist in increasing the eNB density, in order to reduce the average distance between
UEs and their respective eNB. In this way signal strength attenuation problems can be partly
overcome. Nevertheless, this would imply a meaningful cost increase because of the growing number
of deployed eNBs and other expenses related to eNB site rentals and backhaul links costs for
connecting all the eNBs to the network backbone. Hence, new cost-efficient strategies based on the
introduction of Relay Nodes (RN) have been proposed.
An RN is a wireless device communicating with both its controller eNB and its controlled UE.
Its main task is to forward informations from the eNB to the UEs on the downlink, and from the UEs to the eNB on the uplink.
RNs have less intelligence than an eNB, they use a lower transmission power and
they are smaller (i.e in some cases they can be placed on lamp posts). In consequence, they appear
to be the best solution for ubiquitous high data rate throughout the cell without an excessive
increase in network costs. The use of RNs implies dealing with a multi-hop network scenario.
Hence, complexity is greater and new strategies for communication and management have to be
developed.
According to the definition given by 3GPP, RNs do not only amplify the received signal
before forwarding it to the recipient. Instead, they perform a decode-and-forward operation on the
processed signal. This minimizes the effects of thermal noise and interference on the first network
hop affecting the forwarded signal on the second hop. RNs can operate in simultaneous mode of
operation, sharing the same time-frequency resources with their controller eNB, or in division mode
of operation, using non-overlapping time-frequency resources. In this way, in-sector interference
between RNs and their respective eNB can be avoided. Division mode of operation can be achieved
e.g. by mean of Time-Division Duplex (TDD) mode, in which eNB and RNs transmit in different
time instant, or Frequency-Division Duplex (FDD) mode, in which eNB and RNs transmit using
multiple carrier frequencies.
Orthogonal Frequency Division Multiple Access (OFDMA) is the proposed modulation scheme
for relay-enhanced networks, because it optimally manages the frequency-selective fading and it is
flexible in resources allocation to UEs. In OFDMA, transmission of data is achieved by distributing
the symbols to be transmitted on several adjacent narrow-band subcarriers, and regulating the
data rate on each subcarrier according to the channel conditions at the subcarrier frequeny. Bad
channel conditions force to transmit at a low data rate, while high data rates on a given subcarrier
are possible if channel conditions are good. The narrow band implies long symbol duration, easing
the problem of inter-symbol interference.
Deployment of RNs in the network is expected to produce effects in terms of coverage, capacity
and transmission power. The Signal to Interference plus Noise Ratio (SINR) decreases in the
cell with the distance from the transmitting station, because the received power from the serving
station diminishes, while the power from the interferers stations increases. The SINR gives us a
measure of the quality of the communication link, and it is bonded with the achievable data rate
per UE. If RNs are deployed the distance of the UE close to the cell border with respect to the
serving station will sensibly decrease, with a positive effect on the coverage. If the SINR ratio is
higher throughout the cell, then also the achievable data rate for users in every position of the
cell will improve, leading to an increased overall cell capacity, and hence a greater throughput.
Furthermore, as the serving station-UE distance is now smaller, the serving station transmitting
power can be decreased keeping the throughput fixed with respect to the single-hop network case.
This will have a positive influence on the energy consumption, which is an important parameter in
todays’ networks because it is related to networks costs and ecologic issues (’green networking’).
Doctorant.e: Minelli Mattia