Analytical Estimation of Packet Delays in Relay-based IMT-advanced Networks Daniel C. Schultz, Ralf Pabst, Bernhard Walke RWTH Aachen University, Faculty 6, Chair of Communication Networks (ComNets), Aachen, Germany e-mail: {dcs|pab|walke}@comnets.rwth-aachen.de Abstract— The paper shows that relays as inherent technology of IMT-Advanced system can provide low delays as required for future multi-media services. The results shown prove that the proposed relaying scheme is able to keep relay requirements on the air interface and to support delay sensitive high interactive services. The analytical results are validated by system level simulations, which show that the relay might become even more important for real scenarios in order to connect the User Terminal (UT) on the uplink. I. I NTRODUCTION Fourth-Generation (4G) radio system such as investigated in the EU funded IST WINNER project aim at the provision of ubiquitous broadband access for nomadic users. It is well known that the range of a 4G broadband radio interface will be limited by the high attenuation at the envis- aged carrier frequencies around 2.4 GHz and beyond 3.4 GHz as identified by the World Radiocommunication Conference (WRC) 2007. Further the rather limited amount of spec- trum identified for broadband radio communication will force mobile network operator to deploy an increased number of site with limited transmission power owing to in order to achieve the necessary capacities per area element especially in densely populated urban areas. Fixed layer-2 Relay Node (RN) appear to be the most promising technology to overcome the unfavorable radio propagation conditions and bring the capacity available at the Base Station (BS) into the area cost efficiently [1][2]. Relays will be cost efficient due to their deployment flex- ibility and the missing fixed, either by cable, fibre wire or microwave, backbone connection. Thus they provide the opti- mal means for initial network roll out of broadband wireless systems. On the other hand RNs need extra radio resource to connect to the backbone network. That these resources are justified in terms of capacity has been proven in several papers [3]. Next to the achievable throughput, delay is an important Quality of Service (QoS) measure for future broadband internet services. To achieve a good insight in the delay characteristics the Complementary Distribution Function (CDF) of delay in a relay based OFDMA system will be determined. The basis for the analytical delay calculation will be the WWI New Radio (WINNER) Medium Access Control (MAC) super frame as briefly introduced in the following section. In Section III for the delay calculation in MAC frame based Orthogonal Frequency Division Multiple Access (OFDMA) systems will be derived and calculated analytically. In Sec- tion IV the calculated results are validated by means of system level simulations. II. WINNER MAC SUPER- FRAME STRUCTURE In the EU funded WINNER project an OFDMA based MAC frame has been developed (for further details see [4] and [5]). The WINNER MAC frame concepts has large similarities with the currently developed 3rd Generation Partnership Program (3GPP) Long Term Evolution (LTE) OFDMA protocol as well as with the approach followed be mobile WiMax. Figure 1 for the Time Division Duplex (TDD) mode the super frame consists of a set of eight sub-frames. Each sub- frame is further divided into an Downlink (DL) and an Uplink (UL) slot. In the configuration assumed for this paper each slot consists of 15 OFDM symbols and is 0.3372 ms plus 8.4 μs duplex guard time long. resulting in a MAC frame duration of 0,6912 ms. The frames in the Frequency Division Duplex (FDD) mode are of similar length, but subdivided in the frequency domain for UL and DL operation. III. DELAY CHARACTERISTICS OF THE TDD MAC SUPER- FRAME To achieve a good insight in the delay characteristics the CDF of delay will be determined. To better classify the results for the multi-hop scenario the results for the 1-hop scenario will be presented firstly as reference. A. One hop Delay TDD Figure 2 shows an extract of the investigated MAC Super Frame (SF), which is composed of a number of MAC Frames each again subdivided into DL and UL phase. For the investigation of delay characteristics the traffic flows are not fully loaded in order to avoid an impact of scheduling strategies and buffer waiting times. The calculation is based on a given constant packet arrival rate with equidistant packet interarrival times t DL−Rate and t UL−Rate for the DL and UL direction respectively. The total delay t DL−Delay (i) of packet i on the DL is calculated as follows: t DL−Delay (i)= t DL−W ait (i)+ t DL−trans (i)+ +p ret−SH ∗ t ret−SH (1) with the waiting time t DL−W ait (i)=(n − i) ∗ t DL−Rate (2) 978-1-4244-1645-5/08/$25.00 ©2008 IEEE 2411