Chapter 2 Introduction of Mobile Multihop Relay System (MMR)
2.4 Overview of the Medium Address Control Layer
2.4.7 Handover
Handover mechanism handles mobile station switching from one base station to another. There are two basic types: soft handover and hard handover.
In soft handover, it is used in voice-centric cellular networks such as GSM or CDMA system. It uses a make-before-break approach whereas a connection to the target BS is
established before a MS leaves an ongoing connection to a BS. When used for non-real-time data traffic (Hypertext Transmission Protocol, File Transfer Protocol), soft handover will result in lower spectral efficiency because this type of traffic is burst and does not require continues handover from one BS to another.
In hard handover, connectivity with a BS is ended first before MS switches to another BS. This is a break-before-make approach. Hard handover is more bandwidth efficient than soft handover, but it causes longer delay.
In Relay system, there are two handover scenarios: (1) Intra- virtual cell (VC) handover: In Intra-VC handover, MS will not be aware the mechanism so the mechanism can be performed by resource allocation procedure. (2) Inter- virtual cell (VC) handover: Inter-VC handover procedure needs to communicate with MR-BS to another. Therefore, the MS will be aware the process. In the implementation complexity and the backward compatibility points of view, decentralized intra-VC handover and centralized inter-VC handover would be a preferred handover solution for 802.16j [32].
CHAPTER 3
SIMULATION SETUP
In this chapter, the IEEE 802.16j system level simulation platform will be described.
The details of system architecture and simulation parameters are going to be explained. Then, the Link Budget, such as path loss and shadow fading, is set to be referred for IEEE 802.16j standard. The basic radio resource management (RRM) is mentioned in this chapter, such as Adaptive Modulation and Coding (AMC), scheduling controls, handoff method, and the traffic models of simulation is showed in this chapter. Finally, simulation architecture is also introduced.
3.1 The architecture of mobility platform
In the simulation of a mobile relay system, interference is an important element that determines performance in real cellular system. The interference effects need to be considered into the simulation. Now building a multi-cell environment in the simulation platform in which the inter-cell interferences can be considered into the performance analysis. Hence, consider the two tier interference per cell, and there are nineteen cells in the platform in order to approximate the real cell coverage with a hexagon. The system structure in Figure 3-1, we can see only center cell have two tier interference cells, as the result, consider the wrap around BSs architecture (Figure 3-2) to meet the requirement of interference calculation. After wrap around, the statistic simulation result of nineteen cells will be more meaningful.
Figure 3-1 Cell structure of system simulation
Figure 3-2 Example of Wrap around
The cell radius is set to 1 km [32]. This approximate cell coverage is a result from a plan of link budget. The total cell bandwidth is set to 10 MHz [1]. In the simulation platform, the three sectors per cell are adopted (Figure 3-3). In each sector, it has the different antenna direction and a regular pattern of deployment.
Figure 3-3 Example of sector deployment
In the simulation platform, the distance between the base station and relay station was 2/3 cell radius, and the angle between the two relay stations was used circumferential angle over the amount of relay stations. For example, the three relay stations deployment scheme were followed the base station antenna direction of the sector (Figure 3-4). The six relay stations deployment scheme were placed in the medium of the base station antenna direction and base station sector boundary (Figure 3-5).
Regarding another number of relay stations like four, five, seven, eight, etc, those are
unfairness placement in sector’s point of view. As a result of the issue, the placement scheme we only focused on three, six and nine relay stations.
Figure 3-4 Example of 3RSs deployment.
Figure 3-5 Example of 6RSs deployment.
Reuse factor 1 can be used in this simulation, and the deployment for reuse factor 1 is shown as Figure 3-6. If reuse factor 3 (Figure 3-6) was adopted, the total system bandwidth is three times of the reuse factor 1. Therefore, the total system bandwidth of reuse factor 3 is equal to 30 MHz.
Figure 3-6 Cell plane for reuse factor 1 and 3
The setting of the antenna pattern is referred to 3GPP’s model [33] as following.
2
The sector and relay stations architecture in IEEE802.16e/j system can reduce the transmission power of BS/RS antenna and inter-cell interference. The total bandwidth of a cell is divided equally into three parts, as the result, the intra-cell interference for sectors and relay stations are not need to be considered. But it still has a small part of inter-cell interference in different sectors and relay stations of distinct BSs caused by subcarrier permutation. This characteristic is very difficult to simulate, so the platform was assume a sector or relay station would produce interference to other sectors or relay station which have the same direction.
3.2 The architecture of frame transmission
In this thesis, the performance analysis focus on the downlink and it apply to OFDMA on PHY layer with TDD mode. The IEEE802.16j can support asymmetric access zone and relay zone in downlink and uplink transmission of TDD mode for which the system can adjust the ratio according to the traffic loading of downlink and uplink transmission.
The frame length in the simulation is set to 5 ms, and the frame structure we used is 1024-FFT OFDMA downlink carrier allocations – PUSC mode defined in the standard [1]. The carrier distribution is shown in Table 3-1. In the 1024 subcarriers, only 720 subcarriers carry data information and other subcarriers are used for guard band, pilot and DC subcarrier.
Table 3-1 1024-FFT OFDMA downlink carrier allocations with PUSC
Subcarrier types Number
Total subcarriers 1024
DC subcarriers 1
Guard subcarriers 92 (Left), 91 (Right)
Sub-channels 30
Data sub-carriers within each sub-channel 24
The symbol length and the number of OFDMA symbols are define in WiMAX Forum [34][35]. In 10MHz bandwidth and 1024 FFT size, the symbol period should be 102.9 μs and there will be 48 OFDMA symbols per frame. In our simulation, we assume it has the different ratio of downlink and uplink. It means that if we take off the overhead control message that included Preamble, FCH, Downlink map, Uplink map,
and we use PUSC, one slot is consisted of two/three symbols in downlink/uplink. As the result, 24 OFDMA symbols are assigned to downlink subframe, 18 OFDMA symbols are assigned to uplink subframe, 5 OFDMA symbols are assigned to the control message, and the lest one is assigned to the TTG. Therefore, the resources will have 12 OFDMA slots for data transmission in every downlink subframe, and 10 subchannels per sector (30 subchannels /3 sector = 10).
To reduce the complexity of implement platform, we assume R-RTG (RS receive/transmit transition gap) was zero, that is signify that the base station can transmit the data to the relay station in the R-RTG. In the 12 OFDMA slots, we will divide two parts: 7 OFDMA slots for access zone, 5 OFDMA slots for relay zone.
Hence, there will be 70 resource units per frame per sector (3 sector model) and 50 resource units per frame per relay station (3RSs model) to transmit data and messages.
In 6 relay stations, it will allocate the half resources, which are 25 resource units per frame per relay station.
3.3 Link budge
The link budget settings of downlink transmission in our simulation are as far as possible to match the IEEE 802.16e/j real environment.
3.3.1 Antenna parameter
In [32] [33][36], it makes deployments scenario assumptions for 802.16e/j, like Table 3-2. In our simulation we adopt the outdoor vehicular scenario, which the BS transmitted power is 46 dBm, the BS antenna gain is 17 dBi, the MS antenna gain is 3dBi on downlink transmission.
About the relay station setup, it was fix deploy in the simulation, so we adopt the outdoor to indoor condition, that is transmitted power is 36 dBm, the antenna gain is 17 dBi. And in IEEE802.16j Evaluation Methodology [36], the channel models need to consider the antenna height, like the BS antenna height is 30m, the RS antenna height is 15m. About the MS antenna height is 1.5m which was defined in [36].
The BS back off which is used to avoid the RF circuit working in the non-linear region due to the peak to average power ratio (PAPR) of OFDM system is 5dB. The common usage value of thermal noise density is -173.93dB/Hz[37]. The receiver noise figure of MS is 9dB [38].
Table 3-2 Link Budget Parameter of 802.16e/j system
Indoor Outdoor to indoor Outdoor vehicular
BS Transmit power 27 dBm (0.5 W) 36 dBm (4 W) 46 dBm (40 W)
MS Transmit power 17 dBm 17 dBm 27 dBm
BS antenna gain 6 dBi 17 dBi 17 dBi
MS antenna gain 0 dBi 0 dBi 3 dBi
BS antenna height 15 m 30 m
3.3.2 Fading effect
In wireless channel, the transmitted signal will suffer the fading issue which might significantly cause distortion to the signal. The fading effect can be categorized into three types: path-loss, shadow fading, and fast fading (multi-path and Doppler effects).
Path-loss and shadow fading are large scale fading, and fast fading belongs to small scale fading. In the simulation, we only consider the large scale fading: Path-loss and Shadow fading. The fast fading will be in the future works.
3.3.2.1 Path loss
The path-loss mode is relative to the distance between transmitter and receiver. The increasing distance will cause the more attenuation of transmitted signal strength. In IEEE802.16j Evaluation Methodology [36], it provides several path-loss models as Table 3-3, and Table 3-4. In the simulation, the cell radius is 1km and the signal transmission in 2~11 GHz is non-line of sight (NLOS), and we also need to consider the relay station antenna placement in ART (Above Roof Top) or BRT (Below Roof Top). For this reason, we choose the two scenarios adopted in our simulation.
In BS to RS link is Type-D: Macro-cell suburban, ART to ART, LOS. In BS to MS link and RS to MS link are Type-E: Macro-cell, urban, ART to BRT, NLOS.
TABLE 3-3 SUMMARY TABLE OF PATH-LOSS AND SHADOW FADING STANDARD DEV TYPES FOR IEEE802.16J RELAY SYSTEM
TABLE 3-4 RELATIONSHIP BETWEEN PATH-LOSS AND USAGE MODELS
Here, the usage models were already described in section 2.2.2 Relay Stations to Usage, those are:
Ⅰ. Fixed Infrastructure Usage Model
Ⅱ. In-Building Coverage Usage Model
Ⅲ. Temporary Coverage Usage Model
Ⅳ. Coverage on Mobile Vehicle Usage Model
3.3.2.2 Shadow fading
The main reason for shadow fading is from the shelters situation in which there might be buildings, shelters, mountains on the transmission path. According to the test result of the real wireless environment, we could know the variance of shadow fading is a log-normal distribution statistically. The standard deviation of this distribution is
based on the simulation environment. In our simulation, we use 8dB for BS and RS to MS and 3.4dB for BS to RS for the standard deviation [36]. When the user is fixed, the shadow fading effect will not alter. On the other hand, the shadow fading changes with different locations at the mobile user. Since the simulation time between two time points is only 5ms. When the MS moved at the 30kmph, it only moved 4.17cm in every 5ms. It is too short to cause significant change of received signal strength for MS even in high mobility environments. It means that shadow fading must be correlative in consecutive simulation points. Hence, the correlation model, called Gudmundson’s correlation model [39] is adopted. The formula of correlation model is shown as following. micro environments are 200m, 50m, and 5m, respectively. In our simulation, we use 5m in our platform. In Figure 3-7, we present the flow of signal-to-interference and noise ratio (SINR) computation. In fact, we should compute carrier-to-interference and noise ratio (CINR) instead of SINR, but the MS of 802.16 systems with PUSC or FUSC mode only report the sum of received CINR of each subcarrier. Therefore, the SINR is equal to CINR under these conditions. The MS parameter about the mobility model is introduced as following. The MS speed is 30 km/hr probability to change direction is 0.2 when position update. The update time was by every frame length.
Maximum angle for direction update is 45∘ .
Figure 3-7 Example of SINR computation
3.4 Basic radio resource management
This section was purposed of RRM is to improve efficiency and guarantee QoS.
In the performance analysis, the basic RRM controls are described as following.
Power control: The BS/RS transmit signal with maximum fixed power 46/36 dBm.
The power of each subcarrier is the same.
Rate control (AMC): Adaptive Modulation Coding scheme is important to meet the
required target bit error rate when transmitted power is fixed. In IEEE802.16 standard, only QPSK, 16-QAM, 64-QAM modulation schemes are available, and the coding scheme adopted in the platform is convolution code with 1/2 code rate. From standard, it can compute the carried bits per resource unit are 48, 96, 144 bits in order when using QPSK, 16-QAM, 64-QAM.Channel assignment: The OFDMA resource units are two dimensional structures:
frequency and time slot. The basic principle for downlink channel assignment in IEEE802.16 is frequency first. When resource allocation is performed, the subchannel will be assigned to a serving user first, if resource allocation in current slots were exhausted subchannels, then restart assignment from first subchannel of next time slot.
Subcarrier permutation: The distributed permutation is used in this platform. To
reduce the complexity of simulation computation, we do not use the original permutation method. The approximated permutation is to dispread each interference resource units to the whole frame.Scheduling method: In the platform, we only implement simple algorithm which is
Round Robin, Proportional Fair, Early Deadline First, Max CINR. But we only apply EDF algorithm in our performance analysis.Handoff method: In this thesis, handoff is not a weight-bearing point. So, Hard
handoff is used. This method is “Break-Before-Make”.ARQ retransmission: As mentioned in 2.4.4, simple BS/RS ARQ is implemented.
The available maximum retransmission times were depending on different service. It had three times for HTTP and FTP service, and one time for VoIP. In the platform, we used the “End to End” model, but we still need to consider the two link probability of the error occur: BS to RS and RS to MS. The error PDU will be retransmitted before exceeding lifetime or retransmission time.
Path selection: In Figure 3-8, the relay station placement was to show how many
path we can select. The user can get the signal from (1) BS only [R3], (2) Two hops [R2], (3) Three hops [R1].Figure 3-8 Example of Path selection
The BS needs to decide one path to user. In the platform, we should compute the packet lifetime in the scheduling, and the lifetime could express the every link channel condition (C1, C2, C3, C4, C5). The radio link R1 to Rp, it will dominate by the minimum channel condition: Ci, when “i” is channel link index. By every radio link: Rp, p is path selected, and the base station will choose the best radio link: RFP
provides the user transmission information. Where RFP is final path, c is min i. The formula of path select model is shown as following:
R1=arg min(C1,C2,C3) i
R2=arg min(C1,C5) i
R3=arg min(C4) i
…….
Rp=arg min(C1,C2,…,Ci) i
RFP=Max(Rpc)=Max(R1c,R2c,R3c…Rpc) (3-3)
In our platform, the path selection was only considered two hops (Figure 3-9), so the channel conditions are only three: BS to MS, BS to RS, RS to MS.
Figure 3-9 Simple Path selection
Hence, we got the three lifetimes data, and make up a simple truth table (Table 3-5). It has four link conditions, and those can separate two results: BS to MS and RS to MS.
The result is come from the formula which describe above. As a result, the path was selected.
TABLE 3-5 RELATIONSHIP BETWEEN PATH-LOSS AND USAGE MODELS
In [40], the author provided a function to evaluate the path selection, it trusted the relative radio resource and sum of the product of the cost function, and use it to decide the path. In our platform, the path selection method was base on the packet lifetime planning. Hence, it could easily implement the scheme into other scheduling models.
In [41] [42], they provided a concept that is symmetric and asymmetric path selection.
The symmetric scheme [41] is the user (BS or MS) used the same relay station or the same path in transmit or receive. The asymmetric scheme [42] is the downlink message direct transmit from base station to mobile station, but in the uplink the user
(MS) was used the relay station transmit to the base station. The concept might good for us in the future work.
3.5 Traffic model
There are three traffic models in our platform: VoIP (Voice Over Internet Protocol), FTP (File Transfer Protocol), HTTP (Hypertext transmission protocol). The traffic model implementation was followed the IEEE802.16m Evaluation Methodology Document (EMD) [9]. We consider single and mix traffic in our simulation, and the mix traffic were used the three traffics which had different ratio. About the mix traffic (Table 3-6) [9][10], we did not implement Gaming Model and Near Real Time Video Streaming Model. We transform the percentage into reasonable parts: VoIP: FTP:
HTTP=30%: 10%: 20%50%: 16%: 34%.
TABLE 3-6 TRAFFIC MIXES
3.6 Platform architecture
To sum up the above introduction, the platform was consisted of the infrastructure, interface, radio resource management, and library (Figure 3-10). In the infrastructure, we start to the “sim.c”, which include the whole flow. The “bs.c”, “rs.c”, “ms.c” were include the function initial and function enable. The generation message was saved in the interface, the method can provide convenient data exchange like the Figure 3-10.
After infrastructure and interface, the radio resource management was including the three parts: Handoff Scheduling Packet arrange. After the “Packet Arrange”, if the simulation time was over, we will get the simulation data.
Figure 3-10 Platform architecture
Finally, table 3-7is to summarize all the setting mentioned in this chapter for our simulation platform.
TABLE 3-7 PARAMETERS SETTING IN SIMULATION PLATFORM
Parameters Value/Comment
Cell layout Hexagonal grid, 19 cells (wrap around)
Sectors per cell 3
Frequency reuse factor 1x1, 1x3
Available bandwidth 10MHz in 1x1, 10/3 and 10*3 MHz in 1x3 reuse Antenna pattern (Θ3dB , Am) (70, 20 dB), according to [33]
Beamwidth 120°[33]
Antenna bore-sight gain 3 dB[33]
Cell radius 1 km[32]
Transmitter/Receiver Downlink (from BS to MSs and RSs , RS to MSs)
Duplex TDD mode
DL/UL subframe ratio 1:1
Frame length 5ms, according to [1]
Frame structure 1024-FFT OFDMA downlink carrier allocations with PUSC, according to [1]
OFDMA symbol length 102.9 μs, according to [34][35]
OFDMA symbols per slot 2 symbols
BS Tx power 46dBm (40 Watt), according to [32]
BS Antenna gain 17 dBi, according to [32]
BS back off 5 dB, according to [37]
RS Tx power 36dBm (4 Watt), according to [32]
RS Antenna gain 17 dBi, according to [32]
RS back off 5 dB, according to [37]
Thermal Noise Density -173.93 dB/Hz, according to [37]
MS Noise Figure 9dB, according to [35]
BS and RS to MS Pathloss model Type-E: PL(d)=38.4+35log10(d) dB for 50m < d < 5km[36]
Shadow fading model Log-normal distribution with STD=3.4,8dB and Gudmundson’s correlation model, according to [36]
Mobility model MS speed : 30 km/hr
Probability to change direction : 0.2 Max. angle for direction update : 45o
BS Power contol Max power
AMC QPSK+CC 1/2, 16-QAM+CC 1/2, 64-QAM+CC
1/2, according to [1]
Channel assignment Frequency first, according to [2]
Scheduling control Early Deadline First (EDF)
Handoff Hard handoff
Traffic model FTP, VoIP, HTTP [9]
CHAPTER 4
SIMULATION RESULT
In this chapter, the performance analyses with different frequency reuse factors and with and without relay station are presented. The simulation result can be classified into two sections based on different traffic types: Real-Time-Service, and Mixed traffic. In our simulation, we assume each user served by base station and relay station is perfectly traced. It means that the serving user is enhanced with maximum boresight gain of antenna pattern. In each section, the performances of real time services and mixed traffic are demonstrated include packet droop rate, the system throughput, and AMC usage are discussed.
4.1 Real Time Service
In this section, the performance analysis of real-time service is presented. When users are large, the frame will be more likely fully utilized, the permutation effect will be
In this section, the performance analysis of real-time service is presented. When users are large, the frame will be more likely fully utilized, the permutation effect will be