Simulation of IEEE 802 . 16 j Mobile WiMAX Relay Network to Determine the Most Efficient Zone to Deploy Relay Station

Wireless technologies usage is rapidly expanding and WiMAX (Worldwide Interoperability for Microwave Access) technology is more widely used. The new WiMAX standard first uses relay stations (RS) for better service quality. Published works analyses relay stations influence to the quality of service, as well as analyses relay stations resource distribution and management, creating network topology for better service quality [1-4]. However there are no specific studies done to determine most efficient zone to deploy RS. To achieve this IEEE 802.16j simulation was made with NCTUns a network simulator and emulator that directly use’s real-life TCP/IP protocol stack and applications to generate accurate simulation results. This papers goal is to determine which modulation zone is the most efficient for relay stations to be deployed to achieve maximal network throughput. This paper also analyzes the number of subscriber stations impact to the network throughput with slot load in one frame and optimal relay stations number connected to one base station to ensure the highest quality internet connection.


Introduction
Wireless technologies usage is rapidly expanding and WiMAX (Worldwide Interoperability for Microwave Access) technology is more widely used.The new WiMAX standard first uses relay stations (RS) for better service quality.Published works analyses relay stations influence to the quality of service, as well as analyses relay stations resource distribution and management, creating network topology for better service quality [1][2][3][4].However there are no specific studies done to determine most efficient zone to deploy RS.
To achieve this IEEE 802.16j simulation was made with NCTUns a network simulator and emulator that directly use's real-life TCP/IP protocol stack and applications to generate accurate simulation results.This papers goal is to determine which modulation zone is the most efficient for relay stations to be deployed to achieve maximal network throughput.This paper also analyzes the number of subscriber stations impact to the network throughput with slot load in one frame and optimal relay stations number connected to one base station to ensure the highest quality internet connection.

Simulation of IEEE 802.16j mobile WiMAX relay network
As the extension of the current standards (IEEE802.16dand IEEE802.16e),IEEE 802.16j aims at defining the multi-hop relay specification including the MAC and the physical (PHY) layers (Fig. 1) [4].
WiMAX uses adaptive modulation coding AMC, and the optimal modulation is automatically selected depending on the signal quality (Fig. 1).For example, if the relay station or subscriber station is far away from the nearest base station the connection is guaranteed to it, but with low-level modulation, that is the maximum speed is slowing down.It is investigated when signal is modulated with QPSK (Quadrature Phase-Shift Keying) and QAM (Quadrature amplitude modulation) modulation [5].[4,5] According to the newest baseline document [4], two modes, non-transparent mode and transparent mode, are specified in table 1 to support those application scenarios.IEEE 802.16j is a new standard and no such products are available yet in the market for researchers to evaluate its performances.NCTUns [6][7][8] is a powerful tool for simulations and emulations.It has two unique features.First, it uses the real-life TCP/IP (or UDP/IP) protocol stack in the Linux kernel to conduct simulations and emulations.Second, it can run up any real-life application programs on simulated nodes during simulation to generate realistic network traffic in simulations.These capabilities enable NCTUns to generate high-fidelity simulation results and evaluate the performances of real-life applications under various network conditions.It supports both the transparent mode and non-transparent mode defined in the IEEE 802.16j standard.
During simulation, channel model was developed.Simulation was made in non-transparent mode.Additional condition was made that the station isn't exposed to a signal fading and that there is no additional signal interferences in its route like tall buildings, forests and etc. Stations signal transmission power was set at 30 dBm.Data network for sending and receiving bandwidth has been allocated: 70% uplink and 30% downlink.
The second table shows the adjusted OFDMA (Orthogonal Frequency-Division Multiple Access) parameters (that we use in simulation) in NCTUns program.Corrections of these parameters were made that were in line with the European Union.COST (COperation européenne dans Le Domaine de la recherche Scientifique et Technique) 231 HATA empirical channel model was used in this simulation.It contains corrections for urban, suburban and rural (flat) environments.COST 231 HATA is expected owing to the mobile scenario for which this model is most appropriate [9].
Third table shows corresponding modulation type with data rate transfer (Mbps) without relay stations when bandwidth is 10MHz.

Results and discussions
Relay station was placed in QPSK ½, QPSK ¾ and 16QAM ½ zones in order to examine throughput increases during simulation.Information of throughput increase is shown in Fig. 2, Fig. 3 and Fig. 4, by changing relay stations place ant modulation type.Changing relay stations place in QPSK ½ modulation zone throughput increase was 25% between base station and subscriber station (Fig. 2).When relay station is operational in 16QAM ½ zone, by changing the position of relay station data throughput increases up to 5% (Fig. 4).
The obtained results show that the maximum throughput increase is in QPSK ½ modulation zone.
Instant throughput increase is in QPSK ½ zone when deploying RS.Accordingly in QPSK ¾ throughput increase is only after 22% RS distance form BS and 16QAM ½ throughput increase is only after 27% RS in those zones.This is because QPSK ½ zone is furthest from the BS.
802.16j network simulation was carried out by changing the frame duration from 10 ms to 20 ms in order to analyze the impact of transmitted signal to the network throughput.Simulation consisted of one base station, one relay station and 40 subscriber stations.Simulation was carried out in QPSK ½ zone, because in this area there is largest throughput increase using relay station.Subscriber stations received data, but didn't send any.Influence of subscriber stations number to the slot load in one frame is shown in Fig. 5. Using a 20 ms frame rate slot load in one frame increases by 18%.This increase is due that in order to transmit information base station uses more resources in access and retransmission areas.
Subscriber stations number impact to the network throughput was investigated and its dependence is shown in Fig. 6.Network throughput decreases proportionally with the increasing slot load in one frame.
Impact of relay stations to network throughput with 40 connected subscribers stations that were randomly deployed in the base stations coverage area regardless of modulation zones in which subscriber stations will operate.Fig. 7 shows relay stations impact to the network throughput.Connecting 4 relay stations to one base has the most efficiency, because then the network throughput rate stabilizes.This happens because when using four relay stations they cover the whole territory of the base station.Installing more relay stations there is no data transfer rate increase.

Conclusions
After simulation with NCTUNs, the results shows that the maximum throughput increase is in QPSK½ modulation zone.Deploying RS in that zone throughput rate increases to 25%.When relay station operational in QPSK¾ zone, by changing the position of relay station throughput increases up to 7.2%.When relay station is operational in 16QAM½ zone throughput increases up to 5%.
Instant throughput increase is in QPSK ½ zone when deploying RS.Accordingly in QPSK ¾ throughput increase is only after 22% RS distance form BS and 16QAM ½ throughput increase is only after 27% RS in those zones.This is because QPSK ½ zone is furthest from the BS.
Using a 10 ms frame transfer rate, with 40 affiliated subscriber stations, slot load in one frame increases by 8%.Using a 20 ms frame rate slot load in one frame increases by 18%.
Using a 10 ms frame transfer rate, when 40 subscriber stations connected to the network, throughput drops from 7.5 Mbps to 5.2 Mbps.Network throughput decreases by 30%.Using 20 ms frame rate throughput drops from 8 Mbps to 6 Mbps.Network throughput decreases by 25%.
Network throughput decreases proportionally with the increasing slot load in one frame.
Connecting 4 relay stations to one base has the most efficiency, because then network throughput rate stabilizes.This happens because when using four relay stations they cover the whole territory of the base station.Installing more relay stations there is no data transfer rate increase.

Fig. 3 .
Fig. 3. Maximum network throughput increase in QPSK 3/4 zoneWhen relay station was operational in QPSK ¾ zone, by changing the position of relay station throughput increases up to 7.2% (Fig.3).

Fig. 5 .
Fig. 5. Number of subscriber stations and slot load in one frame Using a 10 ms frame transfer rate, with 40 affiliated subscriber stations, slot load in one frame increases by 8%.Using a 20 ms frame rate slot load in one frame increases

Fig. 6 .
Fig. 6.Number of subscriber stations and network throughput Using a 10 ms frame transfer rate, when 40 subscriber stations connected to the network, throughput drops from 7.5 Mbps to 5.2 Mbps.Network throughput decreases by 30%.Using 20 ms frame rate throughput drops from 8 Mbps to 6 Mbps.Network throughput decreases by 25%.Network throughput decreases proportionally with the increasing slot load in one frame.Impact of relay stations to network throughput with 40 connected subscribers stations that were randomly deployed in the base stations coverage area regardless of modulation zones in which subscriber stations will operate.Fig.7shows relay stations impact to the network throughput.

Table 2 .
The OFDMA parameter values used in the simulations