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PBB Wireless Communication System using VHF-band

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1. Overview and goal of project

    For the effective use of spectrum, terrestrial “analog” TV broadcasting service using total 370 MHz bandwidth in VHF/UHF bands terminated on July 24th, 2011 in Japan. Currently, terrestrial “digital” TV broadcasting service using 240 MHz bandwidth (470 - 710 MHz) in UHF band is available. Therefore, the digitalization has made the spectrum of total 130 MHz bandwidth newly available for other applications. Four (4) wireless systems, multimedia broadcasting for cell phone-type devices, public broadband (PBB) system, ITS, and cellular systems, were decided to be newly introduced by using total 130 MHz bandwidth as shown in Fig.1.

Fig1 Spectrum Reorganization
Fig. 1 Spectrum Reorganization in VHF and UHF Bands
Associated with Digitalization of TV Broadcasting

    Public broadband (PBB) wireless communication system is one of the most important applications. In Japan, before July 24th, 2011, the wireless communication systems for public organizations (police agency, fire department and so on) were only for voice communication and small data communication because the allocated bandwidth for the public organizations was narrow. In the spectrum reorganization associated with the digitization of broadcasting, wide bandwidth of 32.5 MHz (170–202.5 MHz) was allocated for the PBB system. In addition, this frequency band has good propagation characteristics (lower propagation/ diffraction loss, smaller Doppler shift etc.) compared to higher frequency band (UHF and higher).

    The PBB system is expected to be useful for rescue work because of its broadband wireless applications such as real-time high-quality video-image transmission over a long distance (several kilometers). Figure 2 shows one example of use case of the PBB system. Mobile stations near disaster sites transmit real-time high-quality video to the base station located at emergency headquarters.

Fig2 Example of Use Case

Fig.2: Example of Use Case of PBB system

    Goal of “PBB R&D project” is to realize truly useful and convenient broadband wireless communication system by using valuable VHF-band (170-202.5MHz).

2. Key technologies

  • Appropriate PHY design for uplink heavy data service in VHF-band “Broadband” “Mobile” “Communication” System in “VHF-band” has not been realized all over the world yet. Especially for public use, uplink heavy data service such as uplink video streaming should be supported. The PBB system should be designed for that purpose, taking into account the channel fading characteristics of 200 MHz band.

  • Appropriate RF component design for broadband communication in VHF-band

    Basically, size of RF components (antenna, filter etc.) becomes larger as radio frequency becomes lower. In broadband wireless communication systems using 200-MHz band, therefore, the RF components of the radio station become essentially large. To reduce the size of the radio station (especially for mobile station) is challenging subject.

    Also the total bandwidth (32.5 MHz) allocated for the PBB system is very wide for the radio frequency (200 MHz). To realize the RF components covering all over the allocated frequency band (170-202.5 MHz) is also challenging subject.

3. Research results

    Aiming at realization of useful PBB system, NICT has done a lot of R&D activities regarding many topics such as system requirement, measurement and modeling on propagation characteristics, design of physical (PHY) layers, prototyping etc.

    First of all, we conducted the field experiments (measurement of RSSI, delay profile, fading performance) by using the base station with the antenna height of 45 m as shown in Fig.3. RSSI measurement results in Fig.3 shows that the coverage is 7 to 8 km radius. Delay profile results in Fig.4 shows that the delay time of the delayed wave is at most 10 us for the distance from BS less than 5 km and at most 20 us for the distance from the BS over 5km. Fading performance results in Fig.5 shows that the time variance of the channel is relatively slow.

Fig3 Measured Results of RSSI

Fig. 3 Measured Results of RSSI

Fig4a Measured results of delay profile

(a) Measured results of delay profile (@ 1.5km away from BS)

Fig4b Measured results of delay profile

(b) Measured results of delay profile (@ 2.7km away from BS)

Fig4c Measured results of delay profile

(c) Measured results of delay profile (@ 6.1km away from BS)

Fig.4: Measured Results of Delay Profile

Fig5

Fig. 5: Measured Results of Fading Performance

    Based on the delay profile results, we developed three types of channel model for PBB system (PBB Prof. A, PBB Prof. B and PBB Prof. C) shown in Table 1.

Table 1: Channel Model for
200 MHz-band Broadband Wireless Communication System

  d<25km 25km<d<5km d>5km
  PBB Prof.A GSM Typical
Urban
PBB Prof.B PBB Prof.C IEEE802.22
Profile.A
  delay
time
relative
power
delay
time
relative
power
delay
time
relative
power
delay
time
relative
power
delay
time
relative
power
path 1 0µs 0dB -0.2 µs -3dB 0µs 0dB 0µs 0dB 0µs 0dB
path 2 0.7 µs -35dB 0µs 0dB 0.9 µs -18dB 0.6 µs -12dB 3µs -7dB
path 3 1.2 µs -26db 0.3 µs -2dB 1.7 µs -21dB 5.3 µs -25dB 8 µs -15dB
path 4 3.2 µs -23dB 1.4 µs -6dB 3.1 µs -25dB 6.2 µs -22dB 11 µs -22dB
path 5 5.5 µs -35dB 2.1 µs -8dB 3.8 µs -27dB 7.5 µs -19dB 13 µs -24dB
path 6 6.8 µs -35dB 4.8 µs -10dB 7.5 µs -20dB 19.5 µs -22dB 21 µs -19dB

    Considering the results of the field experiments and system requirements for the PBB system such as required data rate (over 500 kbps etc.), technical requirements were investigated and the results were reflected to the report published by the Ministry of Internal Affairs and Communications (MIC). Table 2 summarizes the major technical requirements for PBB system using 200 MHz band.

Table 2: Major technical requirement for PBB system in 200 MHz band

item specifiction
Multiple access / Duplex OFDMA/TDD
Channel bandwidth 5 MHz
Modulation
(BS) BPSK, QPSK, 16QAM, 64QAM
(MS,Portable BS) QPSK, 16QAM, 64QAM
Max. output power 5W ( MS,Portable BS ).20W (BS)
Max. antenna gain 10dBi
Channel plan 5MHz x 6 channel

    Based on the field experiments, we also designed the pilot symbol patterns for channel estimation to make the downlink more stable and to enhance the uplink data transmission rate. The proposed pilot symbol patterns for downlink and uplink are shown in Fig.6 (a) and (b), respectively.

Fig6 Proposed pilot symbol patterns

Fig.6: Proposed pilot symbol patterns for the PBB system

    Prototype of the PBB system with the proposed pilot symbol patterns has been developed as shown in Fig. 7. Fundamental specifications of the prototype are summarized in Table 3.

Fig7 Prototype of 200Mhz-band PBB systems

Fig.7: Prototype of 200 MHz-band PBB wireless communication systems

Table 3: Major specification of PBB prototype equipment

item specifiction
Center frequency band 175MHz~200.0MHz
Transmission power 5W
Bandwidth 5MHz
IF frequency band 20MHz
FPGA XilinxXC5VLXC330, XC5VLX220
Memory SDRAM 256 Nbps
Battery life time 1.5 hours (continuous transmission mode)

4. Standardization

    Japanese Standard-developing organization (SDO), ARIB (Association of Radio Industries and Businesses) has developed the Standard “ARIB STD-T103 Ver1.0” for 200 MHz-Band Broadband Wireless Communication Systems between Portable BS and MSs on March 28th, 2011. This standard is downloadable from the site below; http://www.arib.or.jp/english/html/overview/doc/1-STD-T103v1_0-E.pdf

    Major specifications of this ARIB Standard are summarized in Table 3. NICT has contributed to the development of this standard.

Table 4: Summary of ARIB Standard Regarding 200 MHz-Band
Public Broadband System between Portable BS and MSs
(ARIB STD-T103 Ver1.0)

  Mode1 Mode  
Base standard WirelessMAN-OFDMA (IEEE802.16e)  
This standard Subset of IEEE802.16e
(Mobile WiMAX)
Subset of
IEEE802.16e
pilot patterns changed
from IEEE802.16e to fit
200 MHz band operation
 
Major technical Requirements (described in Chapter 3 of this standard)
Chaned bandwidth
(Occupied BW)
5MHz
(4.9NHz)
base on "Technical
Requirements for Public
Broadband Wireless
Communication
System" (ORE
49_31[1])
Multiple access/
Duplex method
OFDMA/TDD
Transmit power 5W(37 dBm)
Major items of PHY and MAC of this standard (described in Chapters 4 and 5 of this standard)
FFT size 512 1024 1024  
subcarrier spacing 10.94KHz 5.47KHz 5.47KHz  
frame length 5msec 10msec 10msec  
pilot pattern
IEEE 802.16e
changed from
IEEE802.16e
 
MAC
IEEE 802.16e
 
radio of OFDM
symbol numbers in
DL and UL
35
:
12
26
:
21
35
:
12
26
:
21
9
:
38
37
:
10
23
:
24
9
:
38
 
other Mandatory items
specified by Mobile
WiMAX (HARQ etc.)
optional  
      subset of IEEE802.16e
 
    Mobile WIMAX
 
    subset of IEEE802.16e and Mobile WIMAX

    The PBB system needs to be based on OFDMA/TDD in the VHF-band as shown in Table 1. Although IEEE 802.22 system has almost the same scope (including operational frequency band) as the PBB system, the IEEE 802.22 system is between fixed stations and therefore some PHY parameters, FFT size and so on, are quite high specification for the mobile use. On the other hand, IEEE802.16-2009-based system supports wireless communication between BS and MSs. However, mainly the operational frequency band is microwave band not VHF-band. Therefore, some PHY parameters of IEEE802.16-2009 should be changed for VHF-band operation.

    ARIB has standardized two PHY modes shown in Table 4. Mode 1 is completely based on IEEE 802.16-2009 and just changes its operational frequency band. Mode 2 is a modified version of mode 1 and optimized for requirements for PBB application.

Mode 1
    Mode 1 is completely based on IEEE 802.16-2009 and just changes its operational frequency band. Mode 1 selected two types of FFT size, 512 and 1024. We can choose between two types of FFT size depending on how large delayed waves are considered. If we design a small communication zone, maximum delay time of delayed wave is smaller than 10 us according to the measurement results. In this case, 512 FFT size can be selected because the cyclic prefix length for guard interval in each OFDM symbol is 11.4 us. On the other hand, if we design a large communication zone, maximum delay time of delayed wave is larger than 10 us, in this case 1024 FFT size is appropriate because the cyclic prefix length is 22.9 us.

Mode 2
    Mode 2 is a modified version of mode 1. Major change is to change pattern of pilot symbol insertion for both downlink (DL) and uplink (UL) as shown in Fig. 6. For DL, the interval of pilot symbol insertion is reduced in comparison with mode 1. This means that DL of mode 2 is much robust against multipath fading in comparison with mode 1. Regarding the UL, the interval of pilot symbol insertion is increased in comparison with mode 2. The insertion interval in mode 1 is good for operational frequency band for IEEE 802.16 such as 2.5 GHz and so on. The operational frequency of PBB system is around 200 MHz. So even if the interval of pilot symbol insertion is increased in comparison with mode 2, the BER performance does not degrade. This means that we can enhance the uplink data rate without degrading the link quality. In the case of mode 2 and 9:38 of DL and UL ratio, 7.6 Mbps can be achieved. This rate is appropriate to transmit high definition video contents.

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