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Downlink Packet Transmission Control in Soft Networks Abubaker Khumsi

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Downlink Packet Transmission Control in Soft Networks Abubaker Khumsi
Downlink Packet Transmission Control in Soft Handoff Status on CDMA Wireless IP Networks
125
Downlink Packet Transmission Control in Soft
Handoff Status on CDMA Wireless IP
Networks
Abubaker Khumsi , Kazuo Mori, and Hideo Kobayashi, Non-members
ABSTRACT
A soft handoff scheme is very helpful to minimize
the service disruption in CDMA wireless IP networks.
In the conventional soft handoff scheme, the downlink
throughput performance decreases due to fading fluctuations, this is because the base station transmits
packets to mobile stations even in bad channel conditions. This paper presents a new transmission control scheme for the downlink IP packet transmissions
where the system throughput could be efficiently improved. The proposed scheme focuses on the fact that
the IP packets do not have stringent delay requirements, it aims to support the guaranteed delivery of
IP packets along the network.
handoff scheme which uses Site Selection Diversity
Transmission Power Control (SSDT) is proposed in
[3]. However, In the conventional soft handoff scheme,
the base station keeps transmitting packets even in
bad channel conditions which leads to the degradation of the system performance.
The objective of this paper is to improve the system capacity with maintaining the minimum service
disruption during handoffs of the mobile stations in
downlink IP packet transmissions on CDMA wireless
IP networks. This can be achieved by delaying the
packet transmission from base stations to mobile stations till better channel conditions during the soft
handoff mode.
Keywords: Soft handoff, IP packets, CDMA, cellular system
2. CELLULAR SYSTEMS AND TRANSMISSION POWER CONTROL
1. INTRODUCTION
In cellular system, the handoff is a process that allows a mobile station’s session in progress to continue
without interruption when a mobile station moves
from one cell to another. One of the factors affecting
the Quality of Service (QoS) in the wireless IP networks is the service disruption during handoffs of the
mobile station [1]. The packet loss or packet error
over wireless links during handoff would cause significant throughput degradation. However, realizing
an IP wireless network introduces many challenges
including the soft handoff.
The soft handoff allows a mobile station to communicate with multiple base stations simultaneously. Although the soft handoff is an effective way to increase
channel capacity, reliability, and coverage range of
the Code Division Multiple Access (CDMA) systems,
but it has some disadvantages especially in downlink channels. One of the main problems is that
the simultaneously multiple base stations transmission will cause an increase of the interference that
affects other radio links, and consequently limits the
downlink capacity [2]. To avoid this problem a soft
04PSI21: Manuscript received on January 15, 2005 ; revised
on June 10, 2005.
The authors are with the Department of Electrical and Electronic Engineering, Faculty of Engineering Mie University
Kamihama-cho 1515, Tsu-shi, Mie, 514-8507 Japan. Tel: +8159-231-9740, Fax: +81-59-231-9740 E-mail: {[email protected],
[email protected], [email protected]}elec.mie-u.ac.jp
In a DS/CDMA system, Transmission Power Control (TPC) is a vital necessity for system operation.
The capacity of a DS/CDMA cellular system is interference limited since the channels are separated
neither in frequency nor in time, and the cochannel
interference is inherently strong. A single mobile station exceeding its required transmitted power could
inhibit the communication of all other mobile stations. The TPC schemes have to compensate not only
for signal strength variations due to the varying distance between base station and mobile but must also
attempt to compensate for signal strength fluctuations typical for a wireless channel, these fluctuations
are due to the changing propagation environment between the base station and the mobile station. In the
uplink, the TPC serves to alleviate the nearfar effect.
The use of the TPC is not limited to the uplink,
but is also employed in the downlink. The controlled
transmission maintaining the transmitted power level
at the minimum acceptable level reduces the cochannel interference, which translated into an increased
capacity of the system.
2. 1 Transmission power control and handoff
There are many studies regarding the downlink
performance for systems applying TPC. Gejji [4] obtained a power control law that is radial distance dependent and provides uniform service to all mobile
stations for the downlink transmission power control in CDMA cellular systems. This TPC method
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ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.3, NO.2 AUGUST 2005
is regarded as open-looped. Open-loop TPC mechanisms cannot overcome the fast fading effect which
is arisen by multi-path fading. There are closed-loop
TPC methods proposed to track and compensate the
fast multi-path fading. Ariyavisitakul and Chang [5]
studied a closed-loop signal to interference power ratio (SIR) based power control mechanism that the
base station increases its transmitted power by an
amount of unit if the received SIR is lower than a
threshold value, otherwise it decreases its transmitted
power by an amount of unit. Chang [6] proposes two
closed-loop downlink TPC mechanisms. One is the
centralized SIR balancing transmission power control
method; this method is a base controlled scheme, the
other is the distributed SIR based transmission power
control method. In the all previous methods the effect of soft handoff has not been considered.
The transmission power control together with soft
handoff determines the feasibility of the DS/CDMA
cellular system and is crucial to its performance. Accordingly, the multiple site transmission the straightforward realization of downlink TPC with fast power
adjustment in soft handoff mode is that each active
base station modifies its output power equally in accordance with TPC commands that are sent by the
mobile station. The TPC commands transmitted by
the mobile station requests a decrease or increase in
the base station output power according to the reception quality.
Though this scheme can enhance the system capacity, the required multiple site transmission for soft
hand-off increases the downlink interference that will
affects other radio links [7]. Moreover, the transmission power of active base stations will become imbalanced due to errors in receiving TPC commands.
TPC commands reception errors cannot be avoided
in practice.
To avoid these problems Furukawa [3] proposed
a closed-loop form of downlink transmission power
control, called site selection diversity TPC (SSDT).
We refer to this scheme as conventional soft handoff scheme throughout this paper. It is explained in
Section 3.
3. SOFT HANDOFF IN DOWNLINK IP
PACKET TRANSMISSION
3. 1 Conventional soft handoff scheme
In the conventional soft handoff scheme SSDT, the
mobile station selects three base stations from the
service area which have a minimum average attenuation. These base stations are called active base
stations and the base station which has the minimum
average attenuation between the three selected base
stations defined as the selected base station. The
number of active base stations to be selected depends
on the soft handoff margin (sho-m). In this selection criterion, the difference of the minimum average
attenuation corresponds to each active base station
should be less than the soft handoff margin. The soft
handoff margin is a parameter that controls the area
of the soft handoff region. The mobile station periodically chooses one of the active base stations which
has the minimum instantaneous attenuation to the
mobile station as a transmitting base station.
The disadvantages of this method is that in bad
channel conditions the larger transmitted power is required to compensate the fading. As a result, the
downlink interference increases. Since the prevailing voice services in conventional cellular system, the
base station tries to maintain the connection of the
mobile station even in bad channel conditions, causing dramatic degradation of the system performance.
4. PROPOSED TRANSMISION CONTROL
SCHEME
In the proposed transmission control scheme, if the
channel condition is bad due to fading fluctuations
during soft handoff mode, the base station intentionally delays the packet transmission until better channel conditions are available. In other words, the base
station sends the packets only when the instantaneous
attenuation from the transmitting base station is less
than a threshold value which is given by the following
equation:
T hresholdvalue = Ave − atten + ∆p [dB]
(1)
Where ∆p[dB] is the controlled parameter in dB, and
Ave − atten [dB] is the average attenuation of the
selected base station. This scheme is described in
Fig.1.
In the soft handoff mode, the mobile station first
selects three base stations which have minimum average attenuations (active base stations). Afterwards,
the mobile station periodically chooses one of the active base stations which has the minimum instantaneous attenuation to the mobile station as a transmitting base station (a) as shown in Fig.1. The transmitting base station changes as fast as TPC signal
transmission period. In the case of the instantaneous
attenuation greater than a threshold value (b), the
base station delays the packet transmission {b}. In
the case of the instantaneous attenuation less than
a threshold value (c), the base station calculates the
transmitted power {c}. The base station transmits
the packets (d).
In this scheme, during the soft handoff mode the
concerned mobile station sends commands to the active base stations to choose the base station that has
the minimum instantaneous attenuation for the mobile station. The corresponding base station controls
its own power to keep the signal quality received in
a connecting mobile station at a constant level. The
output power of the other nonselected, active base
stations is turned down to a minimum level (zero
level in this paper). Accordingly, only one base sta-
Downlink Packet Transmission Control in Soft Handoff Status on CDMA Wireless IP Networks
127
5. 1 Signal to interference power ratio
Active base stations
The average received power of a packet transmitted by the j −th base station with transmission power
of P tx(j) and received at the i−th mobile station can
be expressed by the following equation:
Prx (i, j) = Ptx (j).
{c}
(c)
(b)
(d)
(a)
SIR(i, j) =
Mobile station (Ms)
Fig.1: Proposed soft handoff scheme
tion among the active base stations provides adequate
power to the connected mobile station.
By applying this proposed transmission control
scheme the system throughput could be efficiently
improved. The tolerable delay time would not affect IP packets transmission when these packets are
not supporting real time applications. Although this
scheme is intended to support applications, which do
not have stringent delay requirements, it can be used
for real time applications, because the proposed control scheme is limited to during soft handoff mode.
The computer simulation results which compare our
proposed scheme with the conventional soft handoff
scheme are presented in section 6.
5. SYSTEM MODEL
The service area consists of 27 hexagonal cells, as
shown in Fig.2 with the assumption that the base
stations are located at the centers of the cells and
broadcast a pilot signal with constant transmission
power. The mobile stations are uniformly distributed
across the cells and straightly move with a constant
speed in a random direction which follows a uniform
distribution. The base stations transmit packets by
DS/CDMA and the mobile stations receive the packets that satisfy the required SIR. The spreading sequences used in arriving packets do not collide. Radio channels suffer from propagation loss, shadowing
fluctuation that has a log-normal distribution with a
standard deviation of σsh [dB], and fading fluctuation
of which received power follows exponential distribution. In this model, the TPC technique is assumed
to compensate the propagation loss, shadowing fluctuation, and fading fluctuation. TPC is assumed to
be perfect without errors.
(2)
Where S(i, j) is the shadowing fluctuation in the path
between the j − th base station and the i − th mobile
station, d(i, j) is the distance between them, and α
is the propagation loss coefficient. The Signal to interference power ratio SIR(i, j) can be calculated by
the following equation:
{b}
Selected base station (Bs)
10S(i,j)/10
d(i, j)α
Gp .Prx−f (i, j)
Iint ra−f + Iint er−f
(3)
Where P rxf (i, j) is the instantaneous received power
from the j − th base station, and Gp is the processing gain. Iint ra−f and Iint er−f are the instantaneous
received power of intra-cell and intercell interference
respectively. They are given by the following equations:
Iint ra−f = (1 − F o)(1 − RA (i))Prxto−f (i, j) (4)
Iint er−f =
N
Prxto−f (i, n)
(5)
n=1
n=1
Where Prxto−f (i, j) is the instantaneous received
power of total transmitted signal from j −th base station, and N is the number of base stations within the
service area. RA (i) is a ratio of transmission power
for the i − th mobile station to the total transmission power. F o is an orthogonality factor defined as
the fraction of total received power that will be experienced as intra-cell interference due to multi-path
propagation. The F o is 1.0 for perfect orthogonality
and 0.0 for non-orthogonality. It depends on the radio
channel model (i.e., number of multi-path rays) and
has been evaluated in [8]. For example, F o is 0.6 for a
10 ray channel in a macrocell vehicular environment.
When the received signal level is constant over the
duration of a packet, the packet error rate Pe (i, j) can
be approximated by [9]
0; SIR(i, j) ≥ SIRreq
Pe (i, j) =
(6)
1; otherwise
Where SIRreq is that required for a mobile station
to receive packets correctly. The validity of this assumption is shown in [10].
Packets are correctly received at the mobile station
when their Pe (i, j) = 0.
5. 2 Traffic model
Base stations periodically generate packets for
each mobile station at intervals which follow an exponential distribution with an average of T int, which
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ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.3, NO.2 AUGUST 2005
Service area
Fig.2: Service area
is the average packet generation interval. The number of mobile stations per each cell is given by
T int ∗ G/T slot. Here T slot is the slot duration and
G is the offered load which is the average number
of generated packets per slot duration per cell [packets/slot/cell]. After the packets are transmitted, they
disappear regardless of whether they were successfully received.
5. 3 Performance measures
The downlink throughput is evaluated by computer simulation. The throughput is defined as
N suc/N slot, where N suc is the total number of
successfully received packets at the destinations and
N slot is the total number of observed slots. The delay performance is also evaluated by computer simulation. The transmission delay is defined as the selection delay plus retransmission delay. Where the selection delay equals to the packet transmission time
minus the packet generation time, and the retransmission delay is given statistically by (N tx/N suc −
1) ∗ T rtx, where N tx is the number of the transmitted packets, Nsuc is the number of the successfully
received packets and T rtx is the average interval of
retransmission. The movement interval is defined as
the interval in which we renew the locations of the
mobile stations in the simulation. Other simulation
parameters are shown in Table 1.
Table 1: Simulation parameters
Number of cells (N)
27
Cell radius
500 [m]
Propagation loss coefficient (α)
3.5
Standard deviation of shadowing (σsh)
7.0 [dB]
Spreading factor (SF)
16.0
Standard deviation of TPC errors
0.0 [dB]
perfect
Downlink orthogonality factor (F)
0.6
Required SIR (SIRreq)
5 [dB]
Slot duration (Tslot)
1.0 [ms]
Average of generation interval (Tint)
25 [slots]
Average interval of retransmission (Trtx)
10 [slots]
Mobile speed
20 [m/s]
Movement interval
10 [slots]
Control parameter (∆p)
-3, -1, 0, 1
and 3 [dB]
Soft handoff margin (sho-m)
3, 6, 10
and 20 [dB]
where ∆p [dB] is -3, -1, 0, 1, and 3 dB respectively,
and for the conventional soft handoff scheme where
the base station sends the packets regardless of the
channel conditions. The soft handoff margin (shom) is set to 3 dB. In this figure we observe a better
throughput performance for the proposed transmission control scheme over the conventional soft handoff scheme. Figure 4 shows the downlink throughput
performance with the same conditions as in Figure
3 but the soft handoff margin (sho-m) is set to 6
dB. From these two figures it can be observed that
the throughput performance for the proposed transmission control scheme is better than that for the
conventional soft handoff scheme. Besides the best
performance is achieved when ∆p [dB] is equals to 1
dB for both values of soft handoff margins.
Figure 5 shows the maximum downlink throughput performance versus the soft handoff margins for
the pro-posed transmission control scheme when ∆p
[dB] equals to -3, -1, 0, 1, and 3 dB. It is noticed that
when the soft handoff margin (sho-m) increases the
throughput performance increases because the handoff region becomes bigger and accordingly, the number of the transmitted packets in bad channel conditions is reduced.
6. 2 Transmission delay
6. PERFORMANCE ASSESSMENT
This section presents the computer simulation
results to verify the effectiveness of the proposed
method.
6. 1 Throughput performance
Figure 3 shows the downlink throughput performance for the proposed transmission control scheme
Figure 6 illustrates the transmission delay performance for the proposed transmission control scheme
when ∆p [dB] is set to -3, -1, 0, 1, and 3 dB respectively and for the conventional soft handoff scheme.
The soft handoff margin (sho-m) is set to 3 dB. Figure
7 shows the transmission delay performance in same
conditions as in Figure 6, but the soft handoff margin (sho-m) is set to 6 dB. From these two figures we
note that at low traffic load, the transmission delay
Downlink Packet Transmission Control in Soft Handoff Status on CDMA Wireless IP Networks
129
Transmission delay [slots]
Downlink throughput [packets]
100
1.2
1.0
0.8
0.6
Proposed scheme,
Proposed scheme,
Proposed scheme,
Proposed scheme,
Proposed scheme,
Conventional scheme
0.4
p= 0 dB
p= 1 dB
p= 3 dB
p= -1 dB
p= -3 dB
0
0
1
2
3
4
5
Downlink offered load G [packets/slot/cell]
p= 0 dB
p= 1 dB
p= 3 dB
p= -1 dB
p= -3 dB
1
2
3
4
5
Downlink offered load G [packets/slot/cell]
Fig.6: Transmission delay (sho-m =3dB)
Fig.3: Throughput performance (sho-m = 3dB)
Downlink throughput [packets]
Proposed scheme,
Proposed scheme,
Proposed scheme,
Proposed scheme,
Proposed scheme,
Conventional scheme
1
0.2
is a little larger in the case of the proposed transmission control scheme and smaller at higher traffic
load and depending on the value of ∆p [dB]. Figure
8 shows the transmission delay performance at maximum throughput versus the soft handoff margins for
the proposed transmission control scheme at ∆p [dB]
equals to 0, and 1 dB. It is noticed that the transmission delay is quite small, and the transmission delay is
nearly constant regardless of the soft handoff margin.
1.6
1.4
1.2
1.0
0.8
p= 0 dB
Proposed scheme,
p= 1 dB
Proposed scheme,
p= 3 dB
Proposed scheme,
p= -1 dB
Proposed scheme,
p= -3 dB
Proposed scheme,
Conventional scheme
0.6
0.4
6. 3 Overall performance
0.2
0
1
2
3
4
5
6
Downlink offered load G [packets/slot/cell]
Fig.4: Throughput performance (sho-m = 6dB)
2.2
Maximum downlink throughput
10
2.0
From the previous considerations on the basis of
computer simulation results, we note that when ∆p
[dB] is 1 dB the throughput performance is maximum while the corresponding transmission delay is
almost low for all traffic loads. The previous observations reveal that we can increase the throughput
performance by delaying the IP packets transmission
till better channel conditions are achieved. Moreover,
at high traffic load this proposed scheme can also be
efficient for IP packets which are sensitive to time delay because the transmission delay is smaller at most
traffic load.
1.8
7. CONCLUSION
1.6
This paper presents a new transmission control
scheme to support IP packets transmission which
does not have stringent delay requirements. The
downlink throughput and delay performance were
evaluated by computer simulations, and we observed
a better throughput performance for the proposed
transmission control scheme over the conventional
soft handoff scheme. We noted that at low traffic
load the transmission delay was a little larger in the
case of the proposed transmission con-trol scheme and
smaller at higher traffic load. These ob-servations reveal that the throughput performance would be increased by delaying the IP packets transmission till
better channel conditions are achieved. Moreover,
1.4
1.2
p= 0 dB
p= 1 dB
1.0
p= -1 dB
p= 3 dB
p= -3 dB
0.8
2
4
6
8
10
12
14
16
18
20
22
Soft handoff margin [sho-m] dB
Fig.5: Maximum throughput verses soft handoff
margin (sho-m) dB
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ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.3, NO.2 AUGUST 2005
Transmission delay [slots]
100
10
p= 0 dB
p= 1 dB
Proposed scheme,
Proposed scheme,
Proposed scheme,
Proposed scheme,
Proposed scheme,
Conventional scheme
p= 3 dB
p= -1 dB
3
5
p= -3 dB
1
0
1
2
4
6
Downlink offered load G [packets/slot/cell]
Transmission delay at maximum
throughput [slots]
Fig.7: Transmission delay (sho-m = 6dB)
100
p= 0 dB
p= 1 dB
NO.8, pp.1546-1554. Agust 2000.
[4] R.R. Gejji, “Forward link power control in CDMA
cellular systems,” IEEE Trans. Veh. Technol.,
vol.41, pp.532-536, Nov. 1992.
[5] S. Ariyavisitakul and L.F Chang, “Signal and
interference statistics of a CDMA system with
feedback power control,” IEEE Trans. Commun.,
VOL. COM-41, no.11, pp.1626-1634, Nov.1993.
[6] Chung-Ju Chang and Fang Ching Ren, “Centralized and Distributed Downlink Power Control
Methods for a DS/CDMA Cellular Mobile Radio System,” IEICE Trans. Commun., vol. E80-B,
No.2, pp.366- 371, Feb. 1997.
[7] M. Soleimanipour and G.H. Freeman, “A realistic
approach to the capacity of cellular CDMA systems,” in Proc. VTC96, Apr. 1996. pp.1125-1129.
[8] ETSI/SMG2, “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT candidate submission,” submitted to ITU-R TG 8/1 as a candidate for IMT-2000, Oct. 1998.
[9] K. Toshimitsu, T. Yamazato, M. Katayama, and
A. Ogawa, “A novel spread slotted Aloha system
with channel load sensing protocol,” IEEE J. Sel.
Areas Commun., vol.12, no.4, pp.665-672, May
1994.
[10] K. Mori, T. Kobayashi, Yamazato, and A.
Ogawa, “Service fairness in CDMA cellular packet
systems with site diversity reception,” IEICE
Trans. Commun., vol.E82-B, no.12, pp.1964-1973,
Dec. 1999.
10
2
4
6
8
10
12
14
16
18
20
22
Soft handoff margin [sho-m] dB
Fig.8: Transmission delay at maximum throughput
versus soft handoff margin (sho-m)
this new proposed transmission control scheme could
also be efficient for IP packets which are sensitive to
time delay because the transmission delay is smaller
at most of traf-fic load.
Abubaker Khumsi received his B.E.
degree in Electrical and Electronic engineering from Alfateh University, Libya,
in 1991 and received his M.E. degree
in Electrical and Electronic Engineering from Mie University, Japan, in 2004.
From 1993 to 1998, he was a project engineer at Waha oil company in Libya.
In 1998 he joined the higher institute of
Electroncs in Libya as an assistant lecturer till the year 2000. He is currently
working toward the Ph.D. degree in Systems Engineering at
Mie University, Japan. His research interests include wireless
IP networks.
References
[1] Eunsoo Shim, Hung-yu Wei, Yusun Chang, and
Richard D. Gitlin, “Low Latency Handoff for
Wireless IP QoS with Neighbor Casting”, Proc.
Of ICC 2002, CD-ROM, May 2002.
[2] A.J.Viterbi, A. M. Viterbi, K. S. Gilhousen, and
E. Zehavi. “Soft handoff extends CDMA cell coverage And increase reverse link capacity,” IEEE
J. Select. Areas Commun. Vol. 12, pp. 1281-1288.
Oct.1994.
[3] Hiroshi Furukawa, Kojiro Hamabe, and Akihisa Ushirokawa. “SSDT Site Selection Diversity
Transmission Power Control for CDMA Forward
Link,” IEEE J. Select. Areas Commun. VOL.18,
Kazuo Mori received the B.E. Degree
in computer engineering from Nagoya
Institute of Technology, Japan, in 1986
and received the Ph.D. degree in information electronics engineering from
Nagoya university, Japan in 2000. In
1986, he joined the Hypermedia Research Center, SANYO Electric Co.,
Ltd. And was engaged in research and
development on Telecommunication systems. From 1995 to 2000, he was a research engineer at YRP Mobile Telecomunications Key Technology Research Laboratories Co., Ltd., where he was engaged
in research on mobile communication systems. Since 2000, he
has been an Associate Professor of the Department of Electrical and Electronic Engineering at Mie University, Japan.
Downlink Packet Transmission Control in Soft Handoff Status on CDMA Wireless IP Networks
His research interests include mobile communication systems,
CDMA schemes, radio packet communications , and teletraffic
evaluation. Dr. Mori received the excellent Paper Award from
IEICE, Japan in 2002.
Hideo Kobayashi received the B.E.,
M.E., and Dr.E. degrees in 1975, 1977
and 1989, respectively from Tohoku University. He joined KDD in 1977, and engaged in research on digital fixed satellite and mobile satellite communication
systems. From 1998 to 1990, he was
with INMARSAT as a Technical Staff
and involved in the development of future INMARSAT systems. Since 1998
he has been a Professor of Mie University. His current research interests include mobile communications and wireless LAN systems. Dr. Kobayashi is a member
of IEEE.
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