Chapter 3 QoS_GTE Scheduling Scheme
3.3 TC Decision Algorithm
* arg max{ m}
m M
m S
∈
=
\ { *}
M =M m
Figure 3.3. The flow chart of SO_based resource allocation algorithm.
3.3 TC Decision Algorithm
Since using relaying may cause resource inefficiency, how to reuses the resource is becoming a critical topic to enhance the system throughput. The TC decision algorithm aims to find which RSs can transmit at the same time using the same resource. Therefore, the original occupied resource can be reserved to other transmitters so that using resource efficiency can be improved, which compensates the drawback of using relaying.
After the SO_based resource allocation algorithm, which RSs are active in the frame has already been decided. Denote I={1,…,i} is the set of active RSs. i is the index of RS. We have to build a concurrency candidate set Ai directing at each active RSi. Any active RSa whose angle separated from RSi is not less than 120∘is included into Ai. In other words, RSa is the concurrency candidate of RSi. For an example, in Figure(a), assume all RSs are active so the concurrency candidates for RS1 are RS5, RS6, RS7, RS8 and RS9, that is, A1={5,6,7,8,9}. Notice that there are at most five concurrency candidates for RSi.
After candidate selection, picking up the exact concurrency RSs from the candidate set is followed. If Ai includes the two candidate RSs which are 120∘separated from RSi, the set α120 is includes these two RSs and these two RSs are chosen as concurrency RSs to transmit with RSi at the same time. If α120= , φ which means the two 120∘RSs are not active at the same time, there is only one candidate RSa in Ai with maximum traffic load ∆ selected as concurrency RS a a i for RSi. As the example illustrated above, RS5 and RS9 are both in Ai so they are concurrency RSs for RS1. The transmit time T for the transmit concurrency RSs is decided by the maximum transmission time of transmit concurrency RSs.
120° 120°
(a)
Downlink frame
Time Sub-channel
TB TRa
RS_
RS_a RS_iRS_k j BS ...
TR
TRi TRj
(b)
(c)
Figure 3.4.Transmission concurrency example. (a) candidate selection (b) an OFDMA frame before concurrency (c) an OFDMA frame
after concurrency
Figure(b) shows an OFDMA frame before concurrency and suppose RSa, RSi, and RSj can transmit concurrently. After transmission concurrency, as Figure(c), RSa, RSi, and RSj are combined together to transmit, which makes original-occupied resources released. The transmission time T is set to be TRa since TRa is the maximum transmission time among TRa, TRi and TRj. The released resources can be utilized again and the assignment of these resources is controlled by SO_based resource allocation algorithm. The detail flow chart of TC decision algorithm is described in Figure. The loop between the SO_based resource allocation algorithm and the TC decision algorithm will never stop until there is no resource released after the TC.
if Ai =φ
if
α
120 =φ
if I =φ
max{ }
i
i a
a A
a = ∈ ∆
120
ai =α
\ { }i I =I a
Figure 3.5. The flow chart of TC decision algorithm.
Chapter 4
Simulation Results and Discussions
In the simulations, the WiMAX relay-assisted network is set to be compatible with IEEE 802.16 standard. System parameters are listed in Table 4.1, and scalable parameters in physical layer are modified according to the suggested values in [18].
The OFDMA system is based on 5 MHz bandwidth and the frame duration is 5 ms.
The number of sub-carrier is equal to the FFT size, 512, but only 384 sub-carriers are used for data transmission, while the others are used for pilot channel or guard channel. The cell size is set to be 1500 m and the RS coverage is 500 m. BS power is 25 W and each RS power is 5 W and equal power is supposed for each sub-channel.
Table 4.1. System Parameters
Sampling frequency spacing 11.16 kHz
OFDMA symbol duration 100.8 µs
Useful symbol time 89.6 µs
Guard time 11.2 µs
Number of data subcarriers 384
Number of subchannels 24
Number of data subcarriers per subchannel 16 Number of OFDMA symbol for downlink
transmission per frame 24
Thermal noise density (N0) -174 dBm/Hz
Power of BS 25 W
Power of RS 5 W
4.1 Manhattan Street Like Environment
A Manhattan street scenario is considered. The width of each street is 30 meters.
The separation of two adjacent parallel streets is 200 meters. The probability of being LOS and NLOS are given as:
( )
The models of path loss and shadowing are defined differently with respect to the transmitter and receiver pair, BS to MS, BS to RS, and RS to MS, which has already mentioned in (1), (2), (3) in Section 2.4. The corresponding path loss parameters for BS to MS channel, BS to RS channel, and RS to MS channel corresponding path loss parameters are listed in Table 4.2 and Table 4.3. The MS position is distributed uniformly and randomly. By snap-shot method, the MS position will be re-generated after every 10 seconds so that channels also vary after each 10 seconds. Channels are assumed to be fixed within 10 seconds.
Table 4.2. Path-loss and shadow fading parameters Path loss
Table 4.3. Path-loss parameters of channel from RS to MS Path-loss
4.2 Traffic Models and QoS Requirements
The QoS requirements and detailed parameters for the four traffic models are given in this section. Each voice user is modeled as an ON-OFF model, in which the length of ON period and OFF period follows the exponential distribution with mean 1 second and 1.35 seconds, respectively [14]. During ON period, the mean data rate is 12.2 kbps. The video streaming traffic consists of a sequence of video frames which are emitted regularly with interval 100 ms. the video traffic model parameters are listed in Table 4.4.4, where the source data rate is 64 kbps.
Table 4.4. Video streaming traffic model parameters [15]
Parameters Value Parameters Value
T 100 ms M s 250 bytes
N 8 αi 1.2
αs 1.2 K i 2.5 ms
K s 40 bytes M i 12.5 ms
Parameters for HTTP traffic model are given in Table 4.5 [15]. Note that each HTTP packet shall be smaller than 1500 bytes. As to FTP traffic, the size of each file is modeled as truncated lognormal distribution with mean 2 MB, standard deviation 0.722 MB, and a maximum value 5 MB. The arrival interval between two sequential files is exponentially distributed with mean 180 seconds. The parameters of FTP are defined in Table 4.6 [15].
Table 4.5. HTTP traffic model parameters Parameters Value Parameters Value
σm 25032 bytes M e 2M bytes
µm 10710 bytes αNe 1.1
m s 50 bytes K Ne 2
M s 2M bytes MNe 53
σe 126168 bytes λr 0.033
µe 7758 bytes λP 7.69
m e 50 bytes L 1500
Table 4.6. FTP traffic model parameters Parameters Value
σf 0.722M bytes µ f 2M bytes M f 5M bytes
λr 0.006
Table 4.7 lists the QoS requirements of each traffic types and the he transmission time for a frame is 5 ms. In addition, the minimum required transmission rate of HTTP users (in non-real-time services) is slightly larger than the arrival rate of HTTP traffic in page download. This means transmission rate for download of a web page is guaranteed and hence the response time of a web browser is small.
Table 4.7. The QoS requirements of each traffic type
Traffic Type Requirement Value
required BER 10-3
maximum delay tolerance 40 ms (8 frames) voice
(real-time)
maximum allowable dropping ratio 1%
required BER 10-4
maximum delay tolerance 10 ms (2 frames) video
(real-time)
maximum allowable dropping ratio 1%
required BER 10-6
HTTP
(non-real-time) minimum required transmission rate
100 kbps (500 bits per frame)
FTP
(best-effort) required BER 10-6
4.3 Performance Evaluation
In two-hop relay network, it is proven that the least maximum path loss (LMP) relaying node selection scheme [5] has the highest probability in selecting the good channel, which is based on a route that has the lowest bottleneck (in terms of path loss). Let PLn1, PLn2 denote the path loss in dB associated with the first and the second hop, respectively, along the nth route. Then the selected route, rs is determined as follows:
( )
arg min max pathloss of the first hop , pathloss of the second hop .
s n RSs
r = ∈
We make some modification on LMP scheme in the simulations. That is, after choosing a two hop route rs, it has to compare its bottleneck path loss again with the direct path pathloss. The route with lower bottleneck path loss is finally chosen.
In the simulations, the number of user is composed of four service traffic types with equal percentage. We define the traffic load as the ratio of the total average rate
of all users over the system maximum transmission rate. The maximum transmission rate is achieved when Q users are multiplexed for each sub-channel and the highest modulation order is used for all users. It is equal to 11.74 Mbps in the simulation environment of this thesis. Note that the average arrival rate of voice, video, HTTP, and FTP users is equal to 5.2 Kbps, 64 Kbps, 14.5 Kbps, and 88.9 Kbps, respectively.
Thus, the traffic load varies from 0.15 to 0.9 as the number of users varies from 40 to 240.
For fair comparison, the scheduling algorithm of each relay scheme is SO_based resource allocation algorithm. The level of t(x), denoted as U*, is equal to 6. The threshold value Sth is set to be the average of maximum and minimum Sm of HTTP traffic (Sth=1.0714). Although HTTP is the third priority traffic, HTTP should also own a service right to guarantee the QoS as possibly as it can when HTTP is facing some kind of time emergency. This is why we set Sth as the average of maximum and minimum Sm of HTTP traffic.
4.3.1 Performance Evaluation on Relay schemes
In this simulation, twelve RSs are surrounding the BS uniformly in a circle and RSs are at the cell radius of three fourths. While the desired site location of three-fourth cell radius is not exactly at an intersection, the RS would be placed on the nearest intersection from the desired site. In the following discussions, we compare the proposed QoS_GTE with LMP, LMP with TC, and without relay (w/o Relay), respectively.
Figure shows the modulation order distribution on radio links for TT_based path selection algorithm, LMP path selection scheme, and w/o Relay technique. We can observe that the probabilities of QPSK for TT and LMP are both much lower than w/o
w/o Relay. Moreover, the relay 1_hop links and 2_hop links are almost used in 64_QAM. These is because channels from BS to RSs are usually LOS which causes lower path-loss and RSs re-transmit the information received from BS which can strengthen the transmitted signal power. Therefore, relaying SINR at the receiver would be larger than the no relaying SINR and the modulation order of relay schemes is intuitively higher than the without relay case.
Figure is the average modulation order on system. The overall path modulation order and transmission efficiency are considered to obtain the average modulation order on system. The average overall modulation order of a relay path is the average of its 1_hop link and 2_hop link. However, as using a relay path, it takes two transmission times, which are transmission time from BS to RS and the time from RS to MS, to send the same information. The re-transmission reduces the system efficiency half. Therefore, the average modulation order on system is half of the average overall modulation order when using relay path. Since paths in w/o relay are all direct paths, the average links modulation order for w/o Relay is the same as the average modulation order on system. From Figure we can observe that LMP average system modulation order is smaller than w/o Relay case, which is the contrary of Figure. It has been mentioned that the efficiency of using relay is half of without relay due to the re-transmission in relay path. From this point of view, it can be said that using relay improperly is not good for the system performance. Compare the average system modulation order of two relaying schemes. TT is more than LMP.
It is because that TT decides the path by transmission time, which consists of whole path of path loss, shadowing, and interference, rather than according to the path loss of one hop only. TT selects a path more soundly and accurate in system than LMP does.
0
Figure 4.1. Modulation Order Distribution on Radio Links
0
Figure 4.2. Average Modulation Order on System
0 QoS_GTE w/o TC decision algorithm, LMP with TC decision algorithm (LMP+TC), LMP only, and without relay (w/o Relay) scheme. QoS_GTE outperforms the other four schemes. The TT chooses the most efficient and good channel condition path. By means of choosing the minimum transmission time path, the system resource using efficiency is taken into account. No matter the direct path or the relay path, it is the most suitable path for system not only due to its higher modulation order. The TC decision algorithm make the several RSs transmit concurrently so that the resource can be fully used. Since the SO_based resource allocation is used, it not only guarantees QoS but also maximizes the throughput. Since the LMP scheme selects the path only based on the bottleneck path-loss, it does not consider the overall path situation and neglects the effects on system while using relay path. This is why the performance of TT scheme is better than LMP scheme.
The throughput of LMP and TT are less than w/o Relay when the traffic load is below 0.75. This is because that twelve RSs share the resource in TDMA mode. In TDMA mode, each RS transmits the information in different symbol time in spite of the usage of sub-channels. If the sub-channels in a symbol time for a certain RS cannot be used entirely, the spare resource is not allowed to be used by other RSs, which results in resource consuming. This is why the relay schemes with good channel quality but poor throughput performance. Therefore, TC is undoubtedly the main factor to increase the system throughput in using relay. By several RSs transmitting concurrently, TC advances the system efficiency and improves the system throughput.
As the traffic load keeps increasing, the throughputs of these five algorithms start to decrease. LMP, LMP+TC, w/o Relay begin to fall down when traffic load is 0.6 and QoS_GTE and TT decline when the traffic load is 0.75. The reason of throughput reducing is that resource is used for QoS guarantee. When the traffic becomes heavy, more urgent packets are waiting to be transmitted. Using SO_based resource system is able to served more users which delay the dropping point of the throughput performance.
Figure(a) and (b) depict the voice and video packet dropping rate, respectively.
The voice packet dropping rates are almost zero despite the varying of traffic load. As the voice packets are urgent, they have the highest Sm and SO_based algorithm will
belongs to voice packets users prior. On the other hand, the video packet dropping rates are almost zero for light traffic load. The video packet dropping rate of QoS_GTE and QoS_GTE w/o TC start to increase when the traffic load is 0.75 and exceed the maximum allowable dropping rate (1%) at traffic load with 0.9. For LMP and LMP+TC, their video packet dropping rates increase for the traffic load higher than 0.6 and exceed the maximum allowable dropping rate at traffic load with 0.9. For the case of w/o Relay, its video packet dropping rate increases for traffic load higher than 0.45 and exceeds the maximum allowable dropping rate at traffic load with 0.6.
Since a voice packet has higher priority than video one, Sm of a voice packet is higher than the one of a video packet when they are in the same urgent situation. By much better than LMP so the video packet dropping rates of LMP and the case of w/o Relay are much higher than the one of TT.
Figure illustrates the non-guaranteed ratio for HTTP traffic. Unlike the real time traffic, packets of HTTP users will not be dropped but still waiting for service when they cannot reach the minimum transmission rate. In Figure, the guaranteed ratios for HTTP traffic are almost the same when the traffic load is light. As the traffic load becomes higher, the non-guaranteed ratio of w/o Relay rises steeply and those of TT, TT+TC keep low. Since we set Sth equal to the average of HTTP maximum and minimum Sm, about half of the HTTP packets with Sm lower than Sth is probably not be served priory. Only when HTTP packet is very urgent and its Sm is higher than Sth,
the HTTP packet may have a chance to be served at the first stage of SO_based
algorithm. Moreover, Sm of voice packets and video packets are usually greater than HTTP packets’. Urgent HTTP packets also have to be waiting for service until real time services are almost done. This is why the non-guaranteed ratio maintains in 2%
at light traffic load. As traffic load grows, the non-guaranteed ratio increases severely because the resource is allocated to the real time service to avoid larger dropping rate.
0
(b) Video Packet Dropping Rate
Figure 4.4. Packet Dropping Rate of Real Time Services
0
Figure 4.5. Non-guaranteed Ratio of HTTP User
Figure shows the average transmission rate of FTP. Since the Sm of FTP packets are all set to be 0.25 and this value is lower than Sth, FTP packets will be transmitted only in best effort manner. The FTP transmission rate is nearly the same at light traffic load. As the traffic load increases, the average transmission rate increases until a
condition is good. As a result, the system throughput can be enlarged by these high modulation order FTP users. Besides, the FTP traffic arrival rate is almost half of the total traffic arrival rate in our simulation which may cause FTP dominates the throughput as long as the FTP packets are served. This is why the trend of Figure is the same as Figure.
Figure 4.6. Average Transmission Rate of FTP
4.3.2 Station Performance Evaluation on Location of Relay
To evaluate the impact of the RS location, we compare the performances for three RS locations: one is at two thirds of the cell radius, another is at three fourths of the cell radius, and the other is at the seven eighths of the cell radius. Also, if the desired site is not located also an intersection, the RS is placed on the nearest intersection from the desired site location. Both QoS_GTE and LMP+TC are used in the
simulation. SO_based resource allocation algorithm is adopted as the scheduling for LMP+TC.
Figure depicts the system throughput for three distinct RSs locations in QoS_GTE and in LMP+TC, respectively. At the light traffic load, the performance order of three cases has no regularity. When the traffic load is at 0.3, the case of 2/3 is the best.
When the traffic load is 0.45, the case of 7/8 is the best. This is because there is enough resource for low traffic load; therefore, the location of RS does not affect the system performance. On the contrary, as the traffic load increases, the throughput performance from best to worst is the case of 3/4, the case of 2/3 and the case of 7/8, which is clearer in LMP+TC scheme. For the case of 2/3 radius, RSs are so close to each other that each RS coverage is overlapped and this results in reducing the service
When the traffic load is 0.45, the case of 7/8 is the best. This is because there is enough resource for low traffic load; therefore, the location of RS does not affect the system performance. On the contrary, as the traffic load increases, the throughput performance from best to worst is the case of 3/4, the case of 2/3 and the case of 7/8, which is clearer in LMP+TC scheme. For the case of 2/3 radius, RSs are so close to each other that each RS coverage is overlapped and this results in reducing the service