The distributed concept for orthogonal medium access is evaluated by snapshot simulations. In a snapshot, random positions of the UEs and random channel transfer functions for the links of a cell are generated in a predefined scenario. One exemplary
scenario is considered in order to give a proof of concept and to evaluate the impact of the adaptive algorithms related to the non-adaptive ones. In a snapshot, one frame is considered. For the next snapshot, new positions of the UEs and new channel transfer functions are generated in the exemplary scenario. The definition of the exemplary scenario is given in this section.
The deployment of the APs is illustrated in Fig. 6.1. The cell has the size of three neighbored hexagons. There are one BS and two RSs. The deployment is kept rather simple, but it covers various possible applications of RSs as introduced in Section 1.1. For instance, the deployment is covered in which
• a small number of RSs are deployed since RSs are used to transmit in coverage holes,
• a large number of RSs are deployed but only two RSs are transmitting within the frame considered in the snapshot.
In each snapshot, the UEs are assigned to the BS or an RS by a simple method taken from [MKWK07a] and explained in the following. Three assumptions are made for the assignment of the UEs. These assumptions are only valid for the assignment in order to keep the method rather simple: The power is assumed to be allocated uniformly among all resource blocks. An omnidirectional antenna pattern is assumed since the antenna gain as a result of the design of grids of beams and allocation of resource blocks is not known yet. Spatial multiplexing is neglected for simplicity. Based on these assumptions, the Signal-to-Noise Ratio (SNR) value averaged over all resource blocks is determined for each link. The SNR value is a random variable since a statistical model is applied for the channel modelling. Shannon’s channel capacity formula [Bla84] and its extensions to two-hop connections [Doh04] are applied to find the BS or the RS promising the highest user rate. The UE is assigned to the BS or RS promising the highest data rate. In the exemplary scenario, UEs are randomly placed such that each postion is equally probable. The dotted area depicted in Fig. 6.1 represents the area in which a UE is most likely assigned to the BS. In the other area, a UE is most likely assigned to the RS given by the shortest distance.
The scenario is further defined by the parameters listed in Table 6.1. The parameters are not conforming to a standard, but specify a general OFDMA-based network repre- senting basic features of a network according to IEEE 802.16 [IEE04] or LTE [3GP06]. These parameters are valid for the whole Section 6.2 if not stated differently. A band- width as supported in [IEE04, 3GP06] is chosen. The number NSC of subcarries in a time-frequency unit and the number F of time-frequency units in a subframe are
6.2 Distributed Concept for Orthogonal Medium Access 107
RS RS
BS
Figure 6.1. Deployment and assignment in the evaluation scenario.
chosen with respect to the coherence bandwidth of the channel as claimed in Section 2.3. A delay spread of the channel which is less than 5 · 10−6 s is assumed. The number S of slots is chosen with respect to the coherence time as claimed in Section 2.3 if pedestrian users and a carrier frequency of 5 GHz are assumed. The power Pt of the AP t and the noise power given in Table 6.1 are chosen according to [BHIT05]. The set E of bits per OFDM symbol is chosen such that a wide SINR range is covered. The SINR γ required to transmit bits is taken from Shannon’s capacity formula [Bla84] and given by
γ, r = 2− 1 (6.1)
for each receiving station r. Note that these values are chosen for simplicity. A perfect code in terms of the channel capacity and a bit error probability of zero is assumed. In order to analyze the performance in a system with a non-perfect code and a bit error probability larger than zero, the set E and the corresponding SINR values and bit error probability values are found by link level analysis as given for IEEE 802.16 and LTE in [Hoy04] and [SEW07], respectively. The APs are equipped with a UCA. The UCA consists of 12 dipoles. Only the azimuth is considered. As already assumed in Section 4.2.2, the beamforming vectors used to design the beams are chosen according to [LL00]. The pattern of the beam corresponding to the main lobe direction of 180◦ is illustrated in Fig. 4.1. The beams corresponding to the main lobe directions 240◦ and 300◦ are directed to the RSs. The channel models include a model for fast fading, slow fading and pathloss. The models have been developed within the research project EU IST-4-027756 WINNER II [IST] and are documented in [IST07b]. The BS-to-RS links
Table 6.1. Parameter setting in the evaluation scenario for the orthogonal medium access. Parameter Value bandwidth 5 MHz number NSC of subcarriers 4 in a time-frequency unit number F of time-frequency 64
units per subframe
number S of slots per frame 100
power Pt of BS and RS 35 dBm
noise power -102 dBm
set E of bits per OFDM symbol {0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8}
antennas at BS and RS UCA, 12 elements
beam type Chebyshev, 20 dB side lobe attenuation
main lobe direction 0◦, 30◦, 60◦, ..., 330◦
channel model BS to UE and RS to UE WINNER Chanel Model C2 NLOS [IST07b] channel model BS to RS WINNER Chanel Model B5a [IST07b]
outer radius of hexagons 150 m
default value for the number NUE 20 of UEs
default value for the number Gt 3 of beams in the grids of beams
default value for the minimum 10 bits/slot user rate Rmin,0,r
are modeled as LOS links between two fixed stations deployed over the rooftops in a typical urban area. The other links are modeled as Non-Line-Of-Sight (NLOS) links from a fixed station to mobile UEs in a typical urban area. The number NUE of UEs, the number Gt of beams in the grids of beams and the minimum user rate Rmin,0,r are varied in the following sections. If the impact of one of these parameters is evaluated, the other two parameters are chosen according to the default values given in the table if not stated differently. Note that the minimum user rate is only considered if the analysis reflects the objective of (P2) given as the maximization of the the sum rate subject to minimum user rate values.
In this thesis, the data rate, user rate and sum rate values are given in bits per slot. For instance, the data rate of the link (t, r) is measured by counting all the bits transmitted by AP t to receiving station r during a frame and by dividing by the number S of slots in a frame. Note that the representation in bits per slot allows a more general representation than in bits per second since the guard intervals introduced by the OFDM transmitters and removed by the OFDM receivers can be neglected.
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