Novel Handoff Scheme with MIMO ◾ 169
MAHO scheme, with the hando latency that multiplexing transmission is not
implemented in traditional schemes. Although the DIH of traditional “break- before-
make” hando scheme is always above 50ms, the DIH of the proposed MAHO
scheme is almost negligible. is is because the FER is very small when the SNR at
the receiver is greater than 4dB, even for MIMO multiplexing scheme. In CBTC
communication system, the SNR at the receiver is always as high as more than
10dB to guarantee the high reliability and robustness of the train control system.
erefore, the scheme can be reliably applied in CBTC communication system.
8.5.2 Error-Free Periods of Traditional Handoff Schemes
e time intervals of the dierent hando schemes are given in Figure8.6, and the
cell layout parameters, which are from Beijing Subway Yizhuang Line, are shown
in Table8.1.
For the received signal strength-based hando scheme, the mean number of
handos is related to the hysteresis h and the receiving power threshold T. e num-
ber of handos decreases with the increase of the hando hysteresis h. However,
the big value of h will increase the delay time to initiate the hando, which aects
the transmission performance because the MS is communicating with the AP of
worse signal quality. e number of handos increases as well when the hando
threshold gets larger from −70 to −60dBm. When T is −70dBm, the number of
handos between two APs is near 1. e lower the hando threshold T, the lesser
the number of handos. is is because the lower signal strength threshold is easier
to meet so that the hando will not be triggered. However, the delay time to initi-
ate hando will also increase with the decrease of the hando threshold, and this
degrades the communication performance as well.
e number of handos in our proposed location-based MAHO scheme is the
xed 1 because the MS hands o to the candidate AP at the only xed position
between two APs. erefore, the time interval between two consecutive hand-
os is the time that the MS spends between the two hando locations in the
Communication link of
channel DoF = 8
Communication link of
channel DoF = 2
Handoff location
dD
−d
AP
1
AP
2
Figure8.6 Time interval between two handoff procedures, v = 80 km/h.
170 ◾ Advances in Communications-Based Train Control Systems
travel, which is
Dv/
. If the average speed of the train is 80
km h/
, the error-free
period in our scheme is about 27s, and for the typical value of
h =
5d
B
, the
period is about 20,7, and 5.7s with the hando SNR thresholds of −70, −65,
and −60dBm, respectively. Our location-based MAHO scheme is superior to the
existing schemes.
8.5.3 FER of Handoff Signaling with Different Data Rates
Although the data rate of the current CBTC system is less than 1Mbps, future
CBTC systems may require higher data rates. However, passenger information
system (PIS) and video services of high data rates can be applied in the current
urban railway transit communication system as well. erefore, it is valuable to
evaluate the hando performance with higher data rates.
Figure8.7 shows the FER comparisons of the hando signaling between the
MAHO and traditional hando schemes with dierent data rates, where the hand-
o hysteresis is set as 5dB and the absolute receive power threshold is set as 5dB
more than the minimum power requirements. In the simulation, we change the
packet length to 500 bytes because the data rate increases to higher levels. When
the data rate is 6.5Mbps, the FER performance of hando signaling is better than
the traditional ones for almost all inter-site distances. We can see that the FER of
MAHO signaling increases more rapidly than traditional schemes. is is because
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Inter-site distance(a)
FER
MAHO, 6.5 Mbps
Traditional HO scheme, 6.5 Mbps, hysteresis = 5 dB
1400 1600 1800 2000 2200 2400 2600 2800 3000
3200
Figure 8.7 (a) FER at 6.5 Mbps. (Continued)
Novel Handoff Scheme with MIMO ◾ 171
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Inter-site distance
FER
MAHO, 13 Mbps
Traditional HO scheme, 13 Mbps, hysteresis = 5 dB
1200 1400 1600 1800 2000 2200 2400
2600
(b)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Inter-site distance
FER
MAHO, 26 Mbps
Traditional HO scheme, 26 Mbps, hysteresis = 5 dB
800 1000 1200 1400 1600 1800
2000
(c)
Figure 8.7 (Continued) (b) FER at 13 Mbps. (c) FER at 26 Mbps. (Continued)
172 ◾ Advances in Communications-Based Train Control Systems
the FER of MAHO is aected by the FER of the data transmission multiplexed
with hando signaling, which is with lower DoF and then has more rapid increase
of the error rate. When the data rate increases to 13Mbps, the FER performance
is much closer between the MAHO scheme and traditional HO scheme although
the FER in MAHO is still a little better. When the data rate increases more, the
FER performance of traditional HO schemes is better in the case of almost all the
inter-site distances. is is because when the data rate increases, more increment of
SNR in MAHO scheme is needed than traditional hando schemes, and the FER
of MAHO scheme will be larger than traditional schemes when the receive SNR
is identical.
When the target FER of hando signaling is
10
2
−
, without AMC applied,
the maximum tolerable inter-site distances of dierent data rates are shown in
Figure8.8. We can see that when the data rate is the most possible 6.5Mbps for
CBTC service, the maximum inter-site distance between APs is 5% more than
the traditional hando schemes. It means that to meet the target FER, MAHO
scheme needs less APs for CBTC service than traditional hando schemes.
However, when the data rate goes up to 13Mbps, the dierence of the inter-site
distance will be closer. And when it is 26Mbps or higher, the maximum tolera-
ble inter-site distance in traditional hando schemes is bigger than the MAHO.
Inter-site distance
MAHO, 65 Mbps
Traditional HO scheme,
65 Mbps, hysteresis = 5 dB
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
FER
400 500 600 700 800900
1000
(d)
Figure 8.7 (Continued) (d) FER at 65 Mbps with different coverage areas.
Novel Handoff Scheme with MIMO ◾ 173
erefore, if the required data rate is more than 13Mbps, traditional hando
schemes require less APs to guarantee the FER of the hando signaling.
8.6 Conclusion
Train–ground communication is one of the key technologies for the control cen-
ter in the ground to control the train eectively in CBTC system. e WLAN
hando process has signicant impacts on the train control performance in CBTC
systems. In this chapter, we present a MIMO-assisted WLAN MAHO hando
scheme in CBTC communication system. Unlike existing hando algorithms, we
use location-based method to initiate the probe procedure. Hando signaling and
normal data packets are transmitted by dierent antennas, so that the hando
procedures can proceed without interrupting normal data transmissions. MIMO
multiplexing mode and STBC interference cancelation algorithm are applied to
the concurrent transmissions of train control information and hando signaling.
Simulation results show that the proposed MAHO scheme can reduce the hando
latency substantially, and the error-free period is also much larger compared with
the existing schemes. When the data rate is no more than 13Mbps, the FER of the
hando signaling is also lower than traditional hando schemes.
6.5 13.0 19.5 26.0 39.0 52.0 58.5 65.0
0
500
1000
1500
2000
2500
3000
Data rate (Mbps)
Inter-site distance (m)
MAHO scheme
Traditional handoff scheme
Figure8.8 Maximum inter-site distance to meet the HO FER.
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