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					IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 48, NO. 4, JULY 1999                                                                      1099

              A Movable Safety Zone Scheme in Urban
                 Fiber-Optic Microcellular Systems
          Ho-Shin Cho, Member, IEEE, Sang Hyuk Kang, Member, IEEE, and Dan Keun Sung, Member, IEEE

   Abstract—We consider an urban fiber-optic microcellular sys-                sharing them among multiple antenna ports. The CS controls
tem in which a cigar-shaped cell consists of several microzones               connections between its channel elements and antenna ports
with their own antenna sites connected to a central station                   by utilizing SDS and dynamically assigns traffic channels
through optical fibers. To increase the efficiency of radio re-
sources and reduce unnecessary handoffs between microzones,                   according to traffic demands. Ichikawa et al. [8] and Morita et
we propose a movable safety zone scheme. A safety zone is a                   al. [11] showed that the centralized channel control with SDS
virtually guarded area that does not permit cochannel interfer-               improves system performance in terms of blocking probability,
ence. Outside the safety zone, cochannels can be reused. The                  handoff failure probability, and so on.
safety zone can move under the condition that its user does
                                                                                 In conventional systems, CS can assign a traffic channel to
not meet cochannel interference as he moves to an adjacent
microzone. Considering user mobility characteristics in the cigar-            only one user, even if there are some users beyond the cochan-
shaped cell, we analyze and evaluate the proposed system in terms             nel interference, because it has one channel element per traffic
of intracell and intercell handoff rates, blocking probability,               channel. Low transmitter power from low-height antenna ports
intracell call-dropping probability, and channel reuse parameter.             makes cochannel distances shorter in urban microcellular
The proposed system can handle a traffic capacity of about
12 Erlangs for seven traffic channels under a call blocking
                                                                              environments. Therefore, CS coverage may include several
probability of 1% and generates a negligible number of intracell              cochannel distances. In order to increase the efficiency of radio
handoffs compared with those of intercell handoffs.                           resources, we need to reuse traffic channels in CS coverage.
  Index Terms—Cigar-shaped cell, fiber-optic, intracell handoff,
                                                                              We can consider a CS with multiple channel elements per
microzone, movable safety zone, urban microcell.                              traffic channel for the reuse of channels. However, this channel
                                                                              reuse may inherently generate internal handoffs in the CS
                                                                              coverage [12].
                          I. INTRODUCTION                                        In this paper, we propose a movable safety zone scheme

R     ECENTLY, a rapid increase in mobile communication
      demands has required an increase in the capacity of
cellular systems, especially in urban areas. To augment system
                                                                              with which a CS can concurrently assign a traffic channel to
                                                                              multiple users having no cochannel interference each other
                                                                              and can reduce internal handoffs among its antenna ports. The
capacity, microcellular systems with a radius of a several                    safety zone is a virtually guarded area that does not permit
hundred meters have been introduced. However, these micro-                    cochannel interference. In other words, cochannels can be
cellular systems require a large number of base stations (BS’s)               reused outside the safety zone. If a user moves to an adjacent
and may generate frequent handoffs.                                           antenna port area without causing cochannel interference, he
   Fiber-optic microcellular systems integrated with such tech-               can continue to use his current channel through a new antenna
nologies as subcarrier multiplexing (SCM) and spectrum de-                    port switched by an SDS, as shown in [13]. At that time, his
livery switching (SDS) have been extensively proposed and                     safety zone is newly formed from his new antenna port area,
investigated to solve the problems encountered in microcellu-                 which means his safety zone follows him. This scheme reduces
lar systems [1]–[11]. We can implement low-cost small-sized                   internal handoffs among CS antenna ports.
BS’s with easy channel control schemes by installing all                         In the urban fiber-optic microcellular system considered in
channel elements including user data processing functions such                this paper, simplified antenna ports called micro-base stations
as a modulator/demodulator, a channel encoder/decoder, and                    (M-BS’s) are set up along streets, and a cigar-shaped cell is
an interleaver/deinterleaver in a central station (CS) and by                 formed with a group of M-BS’s. This is because radio signals
   Manuscript received May 8, 1997; revised August 14, 1997. This work was    propagate along the line-of-sight (LOS) path and sharply atten-
supported in part by the Korea Science and Engineering Foundation under       uate in other directions due to the shadows of neighboring tall
Grant 95-0100-15-01-3 and in part by the Samsung Advanced Institute of        buildings [14]–[21]. In the urban microcellular environment,
   H.-S. Cho was with the Department of Electrical Engineering, Korea         we consider mobility characteristics of pedestrians, such as
Advanced Institute of Science and Technology (KAIST), Taejon 305-701,         walking along the street and turning or crossing over to an
Korea. He is now with the Electronics and Telecommunications Research         adjoining street at intersections. We obtain channel holding
Institute (ETRI), Taejon 305-350, Korea (e-mail: chohs@etri.re.kr).
   S. H. Kang is with the Department of Electronics Engineering, University   time in the urban cell and then describe our proposed system
of Seoul, Seoul 130-743, Korea.                                               using a continuous time Markov chain model. Based on the
   D. K. Sung is with the Department of Electrical Engineering, Korea         model, we evaluate the performance of the proposed system
Advanced Institute of Science and Technology (KAIST), Taejon 305-701,
Korea.                                                                        in terms of intercell and intracell handoff rates, call blocking
   Publisher Item Identifier S 0018-9545(99)05786-2.                           probability, intracell call-dropping probability, and channel
                                                           0018–9545/99$10.00 © 1999 IEEE
1100                                                          IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 48, NO. 4, JULY 1999

                                                                   microzone at              , then his safety zone is formed to
                                                                   include all microzones within a cochannel interference from
                                                                   microzone . If he moves to an adjacent microzone                 at
                                                                                  , then microzone           becomes free from the
                                                                   cochannel interference of channel and microzone
                                                                   belongs to the range of the cochannel interference. At that
                                                                   time, if channel is not yet occupied at microzone                 ,
                                                                   then he can continue to use channel through the antenna port
                                                                   of microzone         , and his safety zone boundary also moves
                                                                   by one microzone in the direction in which he is moving,
                                                                   as shown in Fig. 3(a). On the other hand, if channel has
                                                                   already been occupied at microzone                  at            ,
                                                                   the user can no longer use channel because of cochannel
                                                                   interference. If an available channel exists within a new safety
                                                                   zone centering at microzone             , an intracell handoff is
                                                                   initiated, as shown in Fig. 3(b). If there is no available channel,
                                                                   the call is terminated.
                                                                      Fig. 4 shows a block diagram of the proposed system to
                                                                   support the movable safety zone scheme. A CS needs several
                                                                   channel elements per traffic channel in order to assign the same
                                                                   channel concurrently to multiple users having no cochannel
Fig. 1. Urban fiber-optic microcellular system.                     interference each other. When user             calls user , user
                                                                     ’s messages are routed from his channel element to the
                                                                   designated E/O converter connected to his destination (user
reuse parameter.                                                     ’s microzone) by SDS. At the M-BS of user , the optical
  The rest of this paper is organized as follows. In Section II,   signal transmitted from the CS is converted into an electrical
an urban fiber-optic microcellular network is introduced, and       signal by an O/E converter and then is radiated from the
the movable safety zone concept is proposed. In Section III,       antenna of the M-BS. Conversely, the radio signal received
the proposed system is analyzed using a continuous Markov          at the M-BS of user           can be processed in the reverse
chain model. In Section IV, some numerical results are taken.      direction. User       can concurrently communicate with user
Finally, conclusions are drawn in Section V.                          with another channel element of the same traffic channel if
                                                                   this communication does not yield cochannel interference to
                II. SYSTEM DESCRIPTION AND                         user     or user .
   In this section, we introduce an urban fiber-optic microcel-                         III. SYSTEM ANALYSIS
lular system, as shown in Fig. 1. We consider a grid-structured      In order to analyze the proposed movable safety zone
urban area with horizontal and vertical streets. M-BS’s are set    scheme, we make the following assumptions.
up in the middle of every street block, and the coverage of
                                                                     • Users make new calls at uniformly distributed random
M-BS is called a microzone. A group of rectilinear M-BS’s
                                                                       points in the cigar-shaped cell.
forms a cigar-shaped cell. M-BS’s are connected to their CS
                                                                     • New call generations follow a Poisson process with rate
with optical fibers. The CS controls intracell handoffs between
M-BS’s as well as intercell handoffs, and dynamically assigns
                                                                     • Call holding time       is exponentially distributed with
traffic channels with SDS.
                                                                       mean         .
   The basic concept of the proposed movable safety zone
                                                                     • For both new calls and handoff calls, dwelling time in
scheme is as follows. Each communicating user has his own
                                                                       one microzone,         has a -stage Erlang distribution
safety zone consisting of a group of microzones within a
                                                                       with mean           . Then, the total dwelling time in
range of cochannel interferences from his current microzone.
Occupied channels in a safety zone can be reused outside               microzones         is another Erlang-distributed random
the safety zone. If a separation of        microzones on a line        variable, and its probability density function         is
is required to avoid cochannel interference, the safety zone           given by
is maximally composed of               microzones, as shown in
Fig. 2(a). In this case, we call the value of      the minimum                                                                    (1)
cochannel distance. Fig. 2(b) and (c) shows some safety zones
limited by the cigar-shaped cell. We assume that the size of a       • The distributions of       in all microzones are indepen-
cigar-shaped cell ( ) is much larger than the maximum size             dent and identical.
of the safety zone (          ).                                     • No priority is given to intercell handoff calls.
   Fig. 3 illustrates the operation of the movable safety zone       • Minimum cochannel distance is equal to two for mathe-
scheme. Let a mobile user initiate a call with channel at              matical simplicity.
CHO et al.: MOVABLE SAFETY ZONE SCHEME IN MICROCELLULAR SYSTEMS                                                                                  1101



Fig. 2. Shapes of the safety zone: (a)   1 + n  j  M 0 n, (b) j > M 0 n, and (c) j < 1 + n.


Fig. 3. Movable safety zone concept: (a) the current channel can be used at microzone  j+1  because the channel is not occupied at microzone   j+n+1
and (b) an intracell handoff is required because the current channel is occupied by someone at microzone j+n+1      .

In this analysis,   and    denote the number of microzones                      a user needs an intracell handoff. Let    ,      , and     be the
and channels of a cigar-shaped cell, respectively.                              dwelling times from a channel occupancy to a turning handoff,
                                                                                a straight handoff, and an intracell handoff, respectively. Then
                                                                                we can express         as
A. Channel Holding Time
  Channel holding time         is the time duration between a
channel occupancy and its channel release. Fig. 5 shows four
cases of channel release: (a) a user completes his call; (b) a
user turns at the intersection (a turning handoff); (c) a user                  Since in our proposed system,            is expected to be much
moves straight to an adjacent cell (a straight handoff); and (d)                larger than other variables ,         , and     , we approximate
1102                                                                IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 48, NO. 4, JULY 1999

Fig. 4. System architecture supporting the movable safety zone scheme.

Fig. 5. Four cases of channel release.

(2) by                                                                   where     the probability of straight movement without turning
                                                                         at intersections. From (1)

This approximation will be justified by simulation later in this                                                                          (5)
   Let       and      be the number of microzones that a
user passes before a turning handoff and a straight handoff,             Similarly, the cumulative distribution function of     is
respectively. Then the cumulative distribution function of    ,
        , is obtained as


                                                                   (4)   From the three cumulative distribution functions of         ,     ,
                                                                         and    , the cumulative distribution function of  ,               ,
CHO et al.: MOVABLE SAFETY ZONE SCHEME IN MICROCELLULAR SYSTEMS                                                                    1103

Fig. 6. Comparison between approximated analytic results and simulation results for mean channel holding time,    [
                                                                                                                 E Tch   ]   =   100 s,
M = 10; Ps = 0:5; r = 5.

is obtained by                                                     From (10), when                 , (           ), the channels
                                                                   of the cigar-shaped cell are classified into two sets (   and
                                                                         ) according to their reuse factor
                                                                                  channel               if
                                                                                  channel               if

                                                                   for all                    . Therefore, the number of channels
                                                                   in      and        is given by

Finally, the mean channel holding time can be expressed as                                                                         (12)

                                                                      The cigar-shaped cell has symmetry with respect to its
                                                                   center. From this property, we can assign a position number
                                                                   to each microzone with the following position function
   Fig. 6 shows good agreements in expected channel holding
times between approximation results from (3) and simulation
results. Therefore, the approximation of (3) is acceptable.                                                                        (13)

B. Probabilities                                                   where is the microzone number (            ). For microzone
                                                                    , we name the neighboring microzones          ,       , and
  Let the system state represent the number of users being                , as shown in Fig. 7. Under the condition that the
served in a cigar-shaped cell. Then the maximum value of ,         minimum cochannel distance is equal to two, the number of
     , is given by                                                 cases that a given channel is being used in      microzones
                                                             (9)   among       microzones at an arbitrary instant, denoted by
                                                                            , is obtained as
where       means a smallest integer larger than or equal to .
  Let     be the reuse factor of channel , representing the                                                                        (14)
number of users using channel . We consider a channel
assignment policy that keeps the difference of reuse factors
                                                                   Let     ,    , and     be the number of cases that a given
        for                       as small as possible. Under
                                                                   channel belonging to      is used in       ,         , and
the policy, we assume that
                                                                           among all the cases that the channel is used at
                                                            (10)   microzones, and      is the number of the cases that the
1104                                                            IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 48, NO. 4, JULY 1999

                                                                     inequality condition of (10), we can obtain the probability that
                                                                     a new call generated at microzone is blocked at



                                                                     Even if a new call occupies a channel successfully, it may
Fig. 7. Naming scheme of  zone;  zone; and 
 zone: (a) p M=2 for
                 d e
M , even number, p M=2 for M , odd number and (b) p > M=2 for M ,    experience a forced termination or forced channel changing
                 d e
even number, p > M=2 for M , odd number.                             within the cigar-shaped cell because available channels in its
                                                                     moving safety zone change over time. For example, when a
channel is used in both          and       . Then, the values        call moves from microzone           to microzone , if its current
                                                                     channel is being used in microzone               , the moving call
of     ,     ,    , and       at each microzone are given by
                                                                     cannot keep its channel because of cochannel interference.
(15)–(17), with (17) given at the bottom of the page.
                                                                     In this case, if there is any other channel available in the
   1)                                                                safety zone composed of microzone              , microzone , and
                                                                     microzone         , the channel is switched to the available
                                                                     channel; this is called an intracell handoff. If there is no
                                                                     available channel in the safety zone, the call is dropped; this
                                                             (15)    is called an intracell call dropping.
                                                                        Let      and       be the intracell call-dropping probabilities
                                                                     when a user using channel (                 ) moves to microzone
                                                                        from its         and           at                 , respectively.
                                                                     Then        and       can be derived as (19) and (20), given at
                                                                     the bottom of the next page, where

   3)                       , see (17).
  In the proposed system, a new call generated at microzone
  is blocked if each of       channels is being used at least
once in        ,       , or          of microzone . From the

CHO et al.: MOVABLE SAFETY ZONE SCHEME IN MICROCELLULAR SYSTEMS                                                                  1105

                                                                     Similarly, the probability that a call generated in microzone
                                                                     experiences at least one intracell handoff (     ) is given by

In (19) and (20),       represents the steady-state probability.
Similarly, let      and        be the intracell handoff proba-                                                                  (24)
bilities when a user using channel (                   ) moves
to microzone from its             and          at              ,
respectively. Then       and        can be derived as (21) and     where                       and       are shown in (25) and
(22), given at the bottom of the next page.                        (26), given at the bottom of the next page.
   We can obtain the probability (23) that a call generated in
microzone is eventually dropped within the cell                    C. Call Arrival Rate
                                                                      Let       and        be the probabilities that a call generated
                                                                   in microzone tries a turning handoff and a straight handoff,
                                                                   respectively. Then        and       can be derived by (27) and
                                                                   (28), given at the bottom of the next page.
                                                                      New call rate and turning-handoff call rate are identical over
                                                                   all microzones. However, straight-handoff calls are generated
                                                                   only at the ends of the cigar-shaped cell, i.e., microzone 1
                                                                   and microzone      . Therefore, the call arrival rate of each
                                                                   microzone is written as
In (23)                                                                                                          or


                                                                   We assume that straight-handoff calls and turning-handoff calls
                                                                   occur according to Poisson processes with rates       and     ,
                                                                   respectively. The     and      can be given by



and         and         are the probabilities of moving toward
the nearer and farther cell boundaries at the channel occupancy    Finally, the total arrival rate of new calls, turning-handoff
point, respectively.                                               calls, and straight-handoff calls is approximated by a Poisson


1106                                  IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 48, NO. 4, JULY 1999

Fig. 8. State transition diagram.

process with the following rate:           D. Limiting Distribution of System State
                                             Considering Poisson call arrivals, generally distributed ser-
                                    (31)   vice time, and a finite system capacity      , we can represent






CHO et al.: MOVABLE SAFETY ZONE SCHEME IN MICROCELLULAR SYSTEMS                                                                           1107

                                     TABLE I
                        LIST   OF   SYSTEM PARAMETERS

                                                                    Fig. 11. Blocking probability and the probability that a call is eventually
                                                                    dropped within a cell.

Fig. 9. Blocking probability at each state.

                                                                    Fig. 12. Channel reuse parameter.


Fig. 10.   Handoff call rates.

                                                                      The blocking probability in microzone                  (     ) can be
the proposed system as an                     loss system. The
                                                                    obtained as
limiting distribution of the system state is identical to that of
an                loss system due to the insensitivity property
[22]. The state transition diagram for the                   loss
model representing the proposed system is shown in Fig. 8. In
this figure,        denotes the conditional blocking probability     where         is the blocking probability in microzone            at state
at state . Solving the balance equations, we can obtain the          .
steady-state probability
                                                                                        IV. NUMERICAL EXAMPLES
                                                                       We now evaluate the proposed system in terms of blocking
                                                                    probability, intracell call-dropping probability, and cell capac-
                                                                    ity. We use a numerical method to calculate the system state
                                                                    probability. The parameters used in our analysis are listed in
                                                            (32)    Table I.
1108                                                                 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 48, NO. 4, JULY 1999

Fig. 13.   Comparison between the systems with and without the movable safety zone scheme.

   Fig. 9 shows the blocking probability of new call at each                We define a channel reuse parameter        at              , as
state. When a cell has seven channels, i.e.,                 , the
conventional cellular systems without channel reuse within the                                                                       (34)
cell usually have the blocking probability of one at              ,
while the proposed system yields the blocking probability
about 10         at the same state. Even if the system state              The channel reuse parameter represents the average number
                                                                          of channel reuses in a cell. Fig. 12 shows the channel reuse
increases up to 13, we can keep the blocking probability
                                                                          parameter for varying the offered traffic load. We can see that
below 1%.
                                                                              increases as the offered traffic load increases. This implies
   The proposed system may generate intracell handoff calls.              that the system capacity is expandable up to              due to
Fig. 10 shows the handoff call rate versus the new call rate              channel reuse as the offered traffic load increases. For example,
     for three different types of handoff calls. We can observe           the channel reuse parameter is about 1.6 at an offered load of
that the intracell handoffs rarely occur. For example, the                about 11.6 Erlangs, corresponding to the acceptable blocking
intracell handoff call rate is about 0.0004 for           , which         probability level of 1%, as shown in Fig. 11. This implies that
corresponds to only one intracell handoff among 500 new calls.            the system can handle 1.6        7 channels.
In addition, the intracell handoff call rate is much lower than              No movable safety zone means no channel reuses like
turning-handoff call rate which is dominant in total intercell            conventional systems. In other words, the channels of CS can
handoff call rate of turning-handoff and straight-handoff calls.          be used only one time in a CS coverage (a cigar-shaped cell
From this result, we can infer that the effect of intracell               in this paper) consisting of multiple microzones. On the other
                                                                          hand, however, it guarantees no intracell handoff resulting in
handoffs is negligible.
                                                                          no intracell call dropping. The blocking probability of the
   Fig. 11 shows the blocking probability       and the probabil-
                                                                          system without the movable safety zone scheme is definitely
ity that a call is eventually dropped within a cell   for varying         higher than that of channel reuse system with the movable
the offered traffic load.      is given by                      and        safety zone scheme. In order to evaluate the two systems,
     is similarly written as                      . The proposed          we use the weighted sum                 . As shown in Fig. 13,
system can accommodate about 12 Erlangs under a blocking                  simulation results are higher than analytical results for the
probability of 1% for           , while under the same condition,         movable safety zone scheme because (10) underestimates the
the conventional cellular systems handle about 2.64 Erlangs.              blocking and call-dropping probabilities. Although we take
This means that system capacity increases by                              into account the difference between simulation and analytical
       . The probability          is smaller than             . In        results, we can improve of QOS with the movable safety zone
the analysis of quality of service (QOS), the weight on                   scheme.
forced terminations may be ten times larger than that on
new call blockings [23]. The effect of the intracell call                                       V. CONCLUSIONS
dropping on QOS in the proposed system is negligible because                We proposed a movable safety zone scheme in urban
                    .                                                     fiber-optic microcellular systems and described a new CS
CHO et al.: MOVABLE SAFETY ZONE SCHEME IN MICROCELLULAR SYSTEMS                                                                                          1109

architecture for managing the scheme. We also evaluated the                            environments at 900 MHz, and 6 GHz,” IEEE Trans. Veh. Technol., vol.
proposed urban fiber-optic microcellular system in terms of                             43, pp. 762–766, Aug. 1994.
                                                                                [18]   A. J. Goldsmith and L. J. Greenstein, “A measurement-based model for
the intra and intercell handoff rate, the blocking probability,                        predicting coverage areas of urban microcells,” IEEE J. Select. Areas
the intracell call-dropping probability, and the channel reuse                         Commun., vol. 11, pp. 1013–1023, Sept. 1993.
                                                                                [19]   L. R. Maciel and H. L. Bertoni, “Cell shape for microcellular system in
parameter by considering user mobility characteristics and cell                        residential and commercial environments,” IEEE Trans. Veh. Technol.,
shapes in urban areas.                                                                 vol. 43, pp. 270–278, May 1994.
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                                                                                       “Microcellular propagation characteristics for personal communications
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 [1] T. E. Darcie, “Subcarrier multiplexing for multiple-access lightwave                              Ho-Shin Cho (S’92–M’99) received the B.S., M.S.,
     networks,” IEEE J. Lightwave Technol., vol. LT-5, pp. 1103–1110, Aug.                             and Ph.D. degrees in electrical engineering from the
     1987.                                                                                             Korea Advanced Institute of Science and Technol-
 [2] M. Shibutani, T. Kanai, K. Emura, and J. Namiki, “Feasibility studies on                          ogy (KAIST), Taejon, Korea, in 1992, 1994, and
     an optical fiber feeder system for microcellular mobile communication                              1999, respectively.
     systems,” in Proc. ICC’91, pp. 1176–1181.                                                            Since March 1999, he has been a Senior Member
 [3] T. S. Chu and M. J. Gans, “Fiber optic microcellular radio,” IEEE Trans.
                                                                                                       of Research Staff at the Electronics and Telecommu-
     Veh. Technol., vol. 40, pp. 599–606, Aug. 1991.
 [4] C. Harvey, I. C. Symington, and D. J. Kirsten, “Cordless communica-                               nications Research Institute (ETRI), Taejon, where
     tions utilizing radio over fiber techniques for the local loop,” in Proc.                          he has conducted studies on performance analysis
     ICC’91, pp. 1171–1175.                                                                            and test of base stations for IMT-2000. His re-
 [5] S. Rozental, J. Griffin, and W. Gillie, “Optical fiber in the customer                              search interests include mobility modeling, resource
     access network,” Telecom. J. Australia, vol. 42, no. 2, pp. 13–21, 1992.   management, traffic analysis, and intelligent antenna scheme in wireless
 [6] K. Kumoziki, “A fiber-optic passive double star network for microcellu-     communication systems.
     lar radio communication systems applications,” IEICE Trans. Commun.,
     vol. E76-B, no. 9, pp. 1122–1127, Sept. 1993.
 [7] R. Ohmoto, H. Ohtsuka, and H. Ichikawa, “Fiber-optic microcell radio
     systems with a spectrum delivery scheme,” IEEE J. Select. Areas                                      Sang Hyuk Kang (S’92–M’98) received the B.S.,
     Commun., vol. 11, pp. 1108–1117, Sept. 1993.                                                         M.S., and Ph.D. degrees in electrical engineering
 [8] H. Ichikawa and M. Ogasawara, “A centralized control microcell radio
                                                                                                          from the Korea Advanced Institute of Science and
     system with spectrum delivery switches,” IEICE Trans. Commun., vol.
                                                                                                          Technology (KAIST), Taejon, Korea, in 1990, 1992,
     E76-B, no. 9, pp. 1115–1121, Sept. 1993.
 [9] H. Ohtsuka, R. Ohmoto, H. Ichikawa, M. Ogasawara, and T. Murase,                                     and 1997, respectively.
     “A subcarrier transmission approach to microcellular system,” in Proc.                                  In September 1997, he joined the faculty of
     ICC’92, pp. 82–86.                                                                                   the University of Seoul, where he is currently a
[10] H. Ichikawa, T. Murase, and K. Morita, “Dynamic channel assignment                                   Lecturer in the School of Electrical Engineering.
     using sub-carrier multiplexing techniques in microcellular system,” in                               His research interests include traffic engineering in
     Proc. VTC’92, pp. 645–648.                                                                           B-ISDN, performance/reliability analysis of com-
[11] K. Morita and H. Ohtsuka, “The new generation of wireless communi-                                   munication systems, mobile communications, and
     cations based on fiber-optic technologies,” IEICE Trans. Commun., vol.                                intelligent networks.
     E76-B, no. 9, pp. 1061–1068, Sept. 1993.
[12] J. C.-I. Chuang, “Performance issues and algorithms for dynamic
     channel assignment,” IEEE J. Select. Areas Commun., vol. 11, pp.
     955–963, Aug. 1993.
[13] W. C. Y. Lee, “Applying the intelligent cell concept to PCS,” IEEE         Dan Keun Sung (S’80–M’86) received the B.S. degree in electrical engineer-
     Trans. Veh. Technol., vol. 43, pp. 672–679, Aug. 1994.                     ing from Seoul National University, Seoul, Korea, in 1975 and the M.S. and
[14] J. B. Andersen, T. S. Rappaport, and S. Yoshida, “Propagation mea-         Ph.D. degrees in electrical and computer engineering from the University of
     surements and models for wireless communications channels,” IEEE           Texas, Austin, in 1982 and 1986, respectively.
     Commun. Mag., vol. 33, pp. 42–49, Jan. 1995.                                  From May 1977 to July 1980, he was a Research Engineer with the Elec-
[15] S. Y. Tan and H. S. Tan, “Propagation model for microcellular com-         tronics and Telecommunications Research Institute, where he was engaged
     munications applied to path loss measurements in Ottawa city streets,”     in research on the development of electronic switching systems. In 1986, he
     IEEE Trans. Veh. Technol., vol. 44, pp. 313–317, May 1995.                 joined the faculty of the Korea Advanced Institute of Science and Technology
[16] U. Kauschke, “Propagation and system performance simulations for the       (KAIST), Taejon, Korea, where he is currently a Professor in the Department
     short range DECT system in microcellular urban roads,” IEEE Trans.         of Electrical Engineering. His research interests include ISDN switching
     Veh. Technol., vol. 44, pp. 253–260, May 1995.                             systems, ATM switching systems, wireless networks, and performance and
[17] V. Erceg, A. J. Rustako, and R. S. Roman, “Diffraction around corners      reliability of systems.
     and its effects on the microcell coverage area in urban and suburban          Dr. Sung is a member of IEICE, KITE, KICS, and KISS.