Chapter 2 Overview of 802.16e system
2.4 Service and Data Flow
In Figure 2-14, we show the service flow of 802.16 system. IEEE 802.16 standard can support multiple services with different requirements of QoS, such as data, voice, video. The MAC layer defines QoS signaling mechanisms and functions that can control BS and MS data transmissions. The CS layer of downlink and uplink classifies the different QoS services into
UL BW
Figure 2-14 Example of 802.16 Service Flow
connections and assigns the connection with a unique connection indicator (CID). Then, the data of the connections are forwarded to the appropriate queues. On the downlink, the transmission is relatively simple because the BS is the only one that transmits during the
downlink subframe. The BS schedules the all downlink connections and the data are broadcasted to all MSs and an MS uses the DL-MAP information to picks up the data belong to it. The BS determines the number of slots that each MS will be allowed to transmit in an uplink subframe. This information is broadcasted by the BS through UL-MAP at the beginning of each frame. UL-MAP contains information element (IE), which include the transmission opportunities, i.e. the slots in which the MS can transmit during the uplink subframe. After receiving the UL-MAP, each MS will transmit data in the predefined slots as indicated in IE. The BS uplink-scheduler determines the IEs using BW-request sent from MSs to BS.
In Figure 2-15, we show the data flow of OFDMA TDD mode. The service traffic got into the MAC layer will be mapped into MAC SDUs. A MAC SDU is divided into one or more MAC PDUs or multiple MAC SDUs are packed into a single MAC PDU through the operation of fragmentation or packing. Multiple MAC PDUs may be concatenated into bursts having the same modulation and coding in either uplink or downlink directions. The bursts are mapped into OFDMA frame and transmitted after subcarrier permutation.
In this chapter, we brief introduce the IEEE 802.16e system and you can refer to [3][6][14~18] about those insufficient details. The wide bandwidth allocation and QoS mechanisms are provided in the 802.16 standard. But the details of scheduling, admission control and reservation management are left undefined and provide an important mechanism for vendors to differentiate their equipment. In this thesis, to effectively support real-time and non-real-time services, the system performance of 802.16e with different radio resource managements in scheduler controls and admission control criterion will be evaluated.
Figure 2-15 Example of Data Flow of OFDMA TDD Mode
Chapter 3
Simulation Setup
In this chapter, the IEEE 802.16e system level simulation platform will be described.
The details of system architecture and simulation parameters are going to be presented. Then, the Link Budget, such as path loss and shadow fading, is set to be suitable for IEEE 802.16e standard. The setting of basic radio resource managements, such as power control, rate control (AMC), scheduling controls, and handoff method, is showed in this chapter. Finally, the traffic models of simulation are introduced.
3.1 The Architecture of Mobility Platform
In the simulation of a mobile system, the interference from other cells is an important element that would affect the overall performance of single cell. This effect need to be considered into the simulation. When the number of simulation cells increase, it will cause high load of the simulation time and computation. Between these two tradeoffs of accurate simulation and computing cost, we consider two-tier interference cells. According to approximate the cell coverage with a hexagon, we consider nineteen cells in our simulation as shown in Figure 3-1. From Figure 3-1, we can see that only the central cell completely has two-tier interference cells, the other cells can not find out the symmetric two-tier interference cells. Even if we use nineteen cells to simulate, only the statistic simulation value of the central cell can be referred, causing a lower simulated efficiency. Hence, we adopt a wrap around technique as shown in Figure 3-2. This technique can make any of nineteen cells owns complete two-tier interference cells. The main ideal is the lacks of any two-tier interference
Figure 3-1 Cell Structure of System Simulation
Figure 3-2 Example of Wrap Around
cells of a specific cell are copies from the simulated cells which are besides the two-tier interference cells of the specific cell. Through the clever arrangement, any cell has complete, symmetric, and different two-tier interference cells. Because the cell owns whole interference after wrap around, the statistic simulation value of nineteen cells would be meaningful.
The cell radius which we set is 1 km [19]. This approximate cell coverage is a result from a plan of Link Budget. The total cell bandwidth that we choose is 6 MHz [1]. In our simulation platform, a cell is divided into three sectors as shown in Figure 3-3. Each sector has the different antenna direction and a regular pattern of deployment. The sector architecture in 802.16e system can reduce the transmission power of BS antenna and intercell interference. But it still has a small part of intercell interference in different sectors of distinct BSs due to subcarrier permutation. This characteristic is very difficult to simulate, so we assume a sector would produce interference to other sectors which have the same direction.
3-Sector Scenario
Figure 3-3 Example of sector deployment
In our simulation platform, the setting of antenna pattern uses the 3GPP’s model [20]
as shown in (1).
) or three (1x3). The total bandwidth of frequency reuse factor 1 is 6 MHz in our simulation.
Frequency reuse factor 3 need triple bandwidth, 18 MHz. The cells with frequency reuse factor 3 has longer distance between two cell used the same bandwidth than frequency reuse factor 1. Therefore, the interference in frequency reuse factor 3 scenario is lower due to stronger interference pathloss caused by longer distance. But the cost is the triple bandwidth need be used. Figure 3-4 shows the deployment of frequency reuse factor. In frequency reuse factor 1 scenario, the overall cells use the same bandwidth. In frequency reuse factor 3 scenario, only partial cells use the same bandwidth. For instance, #0 (# means number), #7, #9,
#11, #13, #15, #17 cells use the identical cell bandwidth in Figure 3-4.
Figure 3-4 Example of the Deployment of Frequency Reuse Factor
3.2 The Architecture of Frame Transmission
In this thesis, we focus on the downlink transmission and use the OFDMA technique with TDD mode. The IEEE 802.16 standard can support an asymmetric downlink and uplink transmission of TDD mode, which adjusts the ratio according to the traffic loading of downlink and uplink transmission. In our simulation, we use a simple assumption. We assume the downlink and uplink transmission have a ratio of equality and they use a half frame, respectively. In 2.3.1, we introduce the time duration of a frame. In our simulation, the frame length we used is 10 ms. The frame structure we used is 2048-FFT OFDMA downlink carrier allocations –PUSC mode defined in the standard. The carrier distribution is shown in Table 3-1. In the 2048 subcarriers, only 1680 subcarriers can carry data information and other subcarriers are used for guard band and dc subcarrier.
Table 3-1 2048-FFT OFDMA downlink carrier allocations with PUSC
Subcarrier types number
Total subcarriers 2048
DC subcarriers 1
Guard subcarriers 184 (Left), 183 (Right)
Sub-channels 60
Data sub-carriers within each sub-channel 24
The length of an OFDMA symbol is an important parameter which involves how many symbols a frame has. The IEEE 802.16 standard provides the method to calculate the OFDMA symbol time, just like (2).
BW is the nominal channel bandwidth. NFFT is the smallest power of two greater than Nused which is the number of used subcarriers includes the DC subcarrier. n is a sampling factor.
This parameter, in conjunction with BW and Nused determines the subcarrier spacing, and the useful symbol time. This value is set to 8/7. G is the ratio of CP time to “useful” time. The following values shall be supported: 1/32, 1/16, 1/8, and 1/4. From these equations, we can get the OFDMA symbol time with our settings. We use 6 MHz bandwidth and set G value equal to 1/32. The OFDMA symbol time will be 308 μs. Using this value, one subchannel has sixteen OFDMA symbols roughly per frame. Under the equal downlink and uplink transmitted ratio, a subcannel of downlink or uplink subframes owns eight OFDMA symbols, respectively. In the downlink PUSC mode, a transmission slot occupies two OFDMA symbols, so one subchannel has four slots in the downlink subframe. The total slots of a downlink subframe of PUSC mode are 240 (4*60).
3.3 Link Budget
The link budget settings of downlink transmission in our simulation are as far as possible to match the IEEE 802.16e real environment. In IEEE S802.16e-03/23 document [19], it makes deployment scenario assumptions for 802.16e, 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 SS (MS) antenna gain is 3dBi on the downlink transmission. 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 5 dB. The common usage value of thermal noise density is -173.93 dB / Hz. The receiver noise figure of MSs is 9dB [21].
Table 3-2 Link Budget Parameter of 802.16e system
Scenario Parameter
Indoor Outdoor to indoor Outdoor vehicular
BS Tx power 27 dBm (0.5 W) 36 dBm (4 W) 46 dBm (40 W)
MS Tx power 17 dBm 17 dBm 27 dBm
BS ant gain 6 dBi 17 dBi 17 dBi
MS ant gain 0 dBi 0 dBi 3 dBi
BS ant height 15 m 30 m
In wireless channel, the transmitted signals will suffer the fading effect, which can change the original signals. The fading can be divided into three type: pathloss, shadow fading, and fast fading (multipath and doppler effect). In our simulation, we only consider the pathloss and shadow fading. The fast fading will be used in the future work. The pathloss model is used to present the signal strength decreases with increasing distance between transmitter and receiver. In Winner D5.4 document [22], it provides several pathloss models, such as Table 3-3. Because the cell radius is 1 km in our simulation and the signal transmission in 2~11 GHz is non-line of sight (NLOS), the C2 scenario is more suitable and we use it in our simulation.
Table 3-3 Pathloss Model Scenarios
Scenario Path-loss [dB] Shadow fading
standard dev.
The main reason forms shadow fading is from the shelters, like buildings, or mountain, on the signal transmitted path. According to the test result of the real wireless environment, we can know the variant of shadow fading is a log-normal distribution statistically. So, we can use the log-normal distribution to produce the shadow fading effect. The standard deviation of this distribution is based on the simulation environment. In our simulation, we use 8 dB for the standard deviation [22]. 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. In the different simulation points, we can use the log-normal distribution to produce a value for the shadow fading, respectively. But this method has a problem, the time between two neighbor simulation points is very small so that the mobile user location will not change very obvious even at high mobile speed. It means the variance the shadow fading will not be large and have a correlated relationship between two neighbor time points. Hence, we use the concept of a correlation model, called Gudmundson’s correlation model [23], in (3).
(3) macro, urban macro, and urban micro environments are 200m, 50m, and 5m, respectively. In our simulation, we use 5m as our parameter. Using log-normal distribution and correlation model, the simulation can get more actual shadow fading.
TxEIRP Pathloss
Figure 3-5 Example of SINR computation
In Figure 3-5, we present the flow of signal-to-interference and noise ratio (SINR) computation. The OFDMA technique uses the multiple carrier to transmit signal and we should compute carrier-to-interference and noise ratio (CINR), not SINR. But the MSs of 802.16 system with PUSC or FUSC mode only compute and report the sum of received
CINR per carrier to BS, not individual CINR. Hence, the SINR and CINR are the same under these conditions. Finally, the mobility model we use is like below. The MS speed is 30 km/hr.
Probability to change direction is 0.2 when position update. Max. angle for direction update is 45o.
3.4 Basic Radio Resource Management
The purpose of radio resource managements is to raise the efficiency and reliability of wireless transmission. In our performance analysis, we will use the basic radio resource management method as follow.
n Power Control: In 802.16e system, the power control is not a major method to maintain the transmitted quality and is more required for MSs. So, we assume the BS use maximum and fixed power to transmit signals. The power of per subcarrier is the same.
n Rate Control (AMC): The adaptive modulation and coding scheme is a major method to keep the quality of wireless transmission. The IEEE 802.16e standard supports a variety of modulation and coding scheme, which we introduce in 2.2.3. In our simulation, we only get the link performances of BPSK, QPSK, 16-QAM, and 64-QAM modulation schemes, so we use these modulation schemes. In formal, the BPSK is not available in 802.16e OFDMA version, but we can use QPSK which repeats the data of BPSK once to transmit. The coding scheme is used to correct errors in the receiver and we use convolution code (CC) with 1/2 code rate. From 2.2.2 and 3.2, we can get one slot using BPSK, QPSK, 16-QAM, and 64-QAM with CC 1/2 can carry 24, 48, 96, 144 bits, respectively.
n Channel assignment: The OFDMA frame structure has two dimensions, the slot with two OFDMA symbols and the subchannel, for channel assignment. In our simulation, we obey the definition of 802.16e standard that we introduce in 2.2.2. The basic principles
are to segment the data after the modulation block into blocks sized to fit into one OFDMA slot, and map the slots in the subchannels with higher priority than that in the OFDMA symbols. In other words, the data mapping method is frequency first.
n Subcarrier permutation: In our platform, we use the distributed subcarrier permutation.
If we use the permutation formula and series to implement the permutation, it is too complex and low efficient in simulation computation. So, we use the statistic method to simulate the permutation effect, like Figure 2-5. The interference of one slot produced by the other cells will be dispersed to all subframe.
n Scheduling method: The scheduling control is an important radio resource management.
Many research has made about all kinds of scheduling methods. In our simulation, we choose some famous and suitable scheduling method for OFDMA techniques, such as round robin (RR), proportional fair (PF), max CINR (MC), fair throughput (FT), and early deadline first (EDF) [24][25][26][27]. The detail will be introduced as follow:
ü Round Robin (RR): With respect to Round Robin scheduling, where users are cyclically scheduled irrespective of the channel condition.
ü Proportional Fair (PF): The PF scheduler allocates the user m* who maximizes the ratio of achievable instantaneous data-rate over average received data-rate. The PF provide a good tradeoff between allocation fairness and system throughput by utilizing the multiuser diversity. The user in the OFDMA system is scheduled at frame n using
, where DRCms(n) denoting the achievable instantaneous data-rate for user m at time n on subcarrier s, Rm(n) denotes the moving average of data-rate at user m has received up to time n according to the following equation (5):
)
ü Max CINR (MC): The MC scheduler allocates the user m* who has maximum received CINR. The MC provides a good multiuser diversity, but less fairness. The user in the OFDMA system is scheduled at frame n using following function (6):
arg max{ ( ) }
, where CINRms(n) denoting the carrier-to-interference and noise ratio for user m at time n on subcarrier s.
ü Fair Throughput (FT): The FT scheduler allocates the user m* who has minimum average received data-rate and it is a special case of PF. The FT provides a good fairness, but less multiuser diversity. The user is scheduled at frame n using following function for real time services. The user is scheduled at frame n using following function (8):
m
*= arg
mmin{ DB − Age − T
t}
(8), where DB is delay bound, Age is the time that the user’s packet has stayed in the MAC layer, and the Tt is the required time to finish the transmission of the packet.
n Handoff method: In this thesis, handoff is not a weight-bearing point. So, we use the simplest method: hard handoff. This method is “Break-Before-Make”.
n ARQ retransmission: In our simulation, we only implement the ARQ retransmission and don’t use the HARQ. When the PDU is error, the ARQ retransmission will work. We don’t limit the retransmission times. The PDU can be retransmitted at all times until it is correct.
3.5 Traffic Models
In IEEE 802.16e standard, the data traffics are divided into five QoS classes, such as UGS, rtPS, ertPS, nrtPS, and BE. The details are described in 2.3.4. In general, UGS, rtPS, and ertPS are designed for real time services and the major differences among these classes are bandwidth request-and-grant on the uplink. The nrtPS and BE are devised for non-real time services. In order to reduce the simulation time and computed loading, we reasonably use the HTTP services to stand for non-real time services and the VoIP services to represent real time services on the downlink based on the previous discuss. The HTTP traffic model adopts 3GPP model [28] as shown in Table 3-4. The VoIP traffic model uses G729-1 codec [29] as shown in Table 3-5. The HTTP services use TCP/IP protocol to transmit, so the HTTP
Table 3-4 HTTP Traffic Model
events distribution number
Number of packet call requests per session Geometrically 5 Reading time between two consecutive packet call Geometrically 412
Number of packets in a packet call Geometrically 25
Time interval between two packets inside a packet call Geometrically 4/rate
Packet size Pareto A=1.1
K=81.5
packet needs to add 20 bytes TCP header and 20 bytes IP header. The VoIP services use RTP/UDP/IP protocol to transmit. The VoIP packet must add 12 bytes RTP header, 8 bytes UDP header, and 20 bytes IP header.
Table 3-5 VoIP Traffic Model
codec Framesize(byte) samples Interval(ms) Rate(bps)
G729-1 10.0 2.0 20.0 8k
Finally, we use Table 3-6 to summarize this chapter and present the arrangement of the parameter setting in our simulation platform.
Table 3-6 The Parameter Setting in Simulation Platform
Parameters Value/Comment
Cell layout Hexagonal grid, 19 cells (wrap around)
Sectors per cell 3
Frequency reuse factor 1x1 and 1x3
Available bandwidth 6 MHz in 1x1 reuse 18 MHz in 1x3 reuse
Antenna pattern 70° with 20 dB front-to-back ratio, according to [20]
Cell radius 1 km
Transmitter/Receiver Downlink (from BS to MSs)
Duplex TDD mode
DL/UL subframe ratio 1:1
Frame length 10ms, according to [1]
Frame structure 2048-FFT OFDMA downlink carrier allocations with PUSC, according to [1]
OFDMA symbol length 308μs, according to [1]
OFDMA symbols per slot 2 symbols
BS Tx power 46dBm (40 Watt), according to [19]
BS Antenna gain 17 dBi, according to [19]
BS back off 5 dB, according to [21]
Thermal Noise Density -173.93 dB/Hz, according to [21]
MS Noise Figure 9dB, according to [21]
MS Antenna gain 3 dBi, according to [19]
Pathloss model 35.0log(d[m])+31.5, 50m<d<5km, according to [22]
Shadow fading model Log-normal distribution with STD=8dB and
Gudmundson’s correlation model, according to [23]
Mobility model MS speed : 30 km/hr
Mobility model MS speed : 30 km/hr