Airspan Networks AIRSPAN-IDR900 Indoor Data Radio (IDR) User Manual SysDescr 02

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WipLL Radio Technology Physical Layer
The WipLL system provides wireless, local-loop connectivity between the
provider’s IP-based backbone and the subscriber. This radio link is established
between WipLL transceivers located at the Base Station and subscriber sites.
This chapter discusses the following radio frequency (RF) physical layer issues
related to the WipLL system:
Frequency Hopping Spread Spectrum
Modulation
Frequency Bands
Standards Compliance
WipLL RF Antennas
Radio Planning
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2.1. Frequency Hopping Spread Spectrum
The WipLL system implements frequency-hopping code division multiple access
(FH-CDMA) spread spectrum modulation for digital signal transmission over the air
between the Base Station and the subscriber site. The WipLL system’s frequency
hopping supports a channel bandwidth of 1 MHz or 1.33 MHz, and channel spacing
of 1 MHz (or 1.75 MHz if operating in the 3.5 GHz band).
Frequency hopping is a basic modulation techniques used in spread spectrum signal
transmission. Spread spectrum enables a signal to be transmitted across a frequency
band that is much wider than the minimum bandwidth required by the information
signal. The transmitter "spreads" the energy, originally concentrated in narrowband,
across a number of frequency band channels on a wider electromagnetic spectrum.
In an FH-CDMA system, a transmitter "hops" between available frequencies
according to a specified algorithm, which can either be random or predefined (see
Figure 2-1). The transmitter operates in synchronization with a receiver, which
remains tuned to the same center frequency as the transmitter. A short burst of data
is transmitted on a narrowband signal. The transmitter then tunes to another
frequency, and transmits again. Therefore, the receiver is capable of hopping its
frequency over a given bandwidth several times a second (20 hops per second in the
WipLL system), transmitting on one frequency for a certain period of time, then
hopping to another frequency and transmitting again. The WipLL system supports a
hopping speed of 50 msec hopping intervals.
f5
f4
f3
f2
f1
10
11
12
Frequency
Each channel
is 1 MHz wide
TIME
Figure 2-1: An example of Frequency Hopping Spread Spectrum
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The advantages of implementing FH-CDMA in the WipLL system include the
following:
Frequency Hopping Spread Spectrum (FHSS) is based on interference
avoidance. Narrow band interference that does not meet the SNR blocks only a
few hops, decreasing the throughput only partially.
The required spectrum for an FHSS system is flexible in that it does not have to
be contiguous.
FHSS can coexist with other systems in the same spectrum band.
To intercept transmission, a receiver must “know” the hopping sequence
therefore, FHSS ensures security.
Frequency diversity copes with the frequency selective fading and multipath.
The RF channel obtained by the WipLL operator is divided into n 1-MHz subchannels, with center frequencies located at integer multiples of 1 MHz (see Figure
2-2). These sub-channels are organized into a set of orthogonal hopping sequences.
Several methodologies are available for creating these sequences, depending on
available spectrum and local regulations.
RF channel
Sub-channel
Assigned band
Figure 2-2: Relationship between “sub-channel”, “RF channel”, and “assigned
channel”
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Table 2-1 shows an example of six orthogonal sequences that can be derived from
seven sub-channels.
Table 2-1: Example of six orthogonal FH sequences
Sequence No.
Sub-channels (frequencies)
Up to 32 such sequences, each with up to 99 sub-channels can be pre-configured in
the WipLL ROM. An additional 32 sequences can be configured by the WipLL
operator in the RAM to provide further flexibility.
2.2. Modulation
The WipLL system is based on Continuous Phase Frequency Shift Keying (CPFSK)
modulation. Frequency Shift Keying uses m different frequencies for m symbols.
The simplest FSK is binary FSK, where 0 and 1 correspond to different frequencies:
Figure 2-3: Graph displaying different frequencies for 0 and 1 bits
FSK is similar to non-linear analogue FM, but with digital modulation.
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FSK provides the following benefits:
Non-coherent detection is possible - no carrier synchronization is required.
Immunities to non-linearity - the envelope contains no information and,
therefore, can be hard-limited; information is carried by zero crossings:
Can be used with non-linear power amplifiers
Better efficiency
The FSK phase can be discontinuous or continuous, as displayed in Figure 2-4.
Figure 2-4: FSK phase: discontinuous (left wave); continuous (right wave)
Continuous wave is more natural than discontinuous and provides the following
advantages:
Smaller bandwidth (discontinuous wave causes high frequency components)
Operates better when transmission link has non-linearities
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2.3. Frequency Bands
WipLL provides a Wireless Local Loop (WLL) solution in the following frequency
bands:
Licensed bands:
700 MHz (698 – 746 MHz)
2.5 GHz (MMDS)
2.8 GHz (TDD)
3.3 to 3.8 GHz TDD/FDD (50 or 100 MHz duplex separation)
Unlicensed bands:
ISM 900 MHz (902 MHz to 928 MHz)
ISM 2.4 GHz (TDD)
5.8 GHz (TDD)
For details on specific WipLL products, see Appendix B.
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2.4. Standards Compliance
Table 2-2 lists standards to which WipLL complies.
Table 2-2: WipLL standards compliance
Standard
EMC
Compliance
• 700 MHz: FCC part 27
• 900 MHz: FCC part 15
• 2.4 GHz: ETS 300 826; FCC part 15
• MMDS: FCC part 21
• 3.5 GHz: EN 300 385; EN 300 386-2; ETS 300 132-2
• 5.8 GHz: FCC part 15
Radio
• 700 MHz: FCC part 27
• 900 MHz: FCC part 15
• 2.4 GHz: EN 300 328-1; FCC part 15; RSS 139; Telec
• MMDS: FCC part 21
• 3.5 GHz: EN 301 253
• 5.8 GHz: FCC part 15
Safety
UL 1950, EN 60950
Environmental
ETS 300 019
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2.5. WipLL RF Antennas
WipLL provides a variety of internal antenna types as well as an option for
connecting off-the-shelf, third-party external antennas. Table 2-3 provides a general
description of the WipLL RF antenna parameters.
Table 2-3: WipLL RF antenna specification
Parameter
Description
• Integral flat-printed antenna: for BSR, PPR, SPR, and IDR devices: No
RF cable is involved for connection between BSR and SPR. The interface
between outdoor radio unit-to-indoor unit (ODU-to-IDU) is by CAT-5
cable.
Antenna type
• Integral narrow-beam antenna: for the BSR device operating in the 3.5
GHz band.
• Integral high-gain antenna: for SPR and PPR devices operating in the
3.5 GHz and 2.4 GHz bands.
Polarization
Vertical (Horizontal polarization is optional for SPR at 3.5 GHz)
ETSI compliant
EN 302 085, Class CS1 for the BSR, and TS2 for the SPR
Receive diversity
Supported in single BSR through dual integral antennas
External thirdparty antennas
(optional)
Connects to BSR, PPR, and SPR using an N-type connector. Connects to
IDR using a TNC connector. Provides further flexibility for the WipLL
operator to improve link budget or cost-effectiveness of the Base Station. For
example, an omni-directional antenna for 360º coverage can be implemented
by a single BSR.
For BSRs operating in the 700 MHz or 900 MHz bands, two N-type
connectors are provided for attaching two external third-party antennas for
dual antenna diversity at the WipLL Base Station. When operating in the 700
MHz band, the BSR is supplied with a panel-type antenna; the SPR model
with a yagi-type antenna
Notes: Devices with external antennas do not contain built-in (internal)
antennas.
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2.5.1. WipLL Internal Antenna Specifications
Table 2-4, Table 2-5, Table 2-6, and Table 2-7 list the internal antenna specifications
of the BSR, PPR, SPR, and IDR devices, respectively.
Table 2-4: BSR (Base Station) antenna specifications
Parameter
Frequency Gain
range
(dBi)
(MHz)
Beam
Polarization VSWR Impedance Frontwidth
to(ohm)
HXV
back
ratio
(degrees)
(dB)
900
MHz
902 - 928
60 x 60
Vertical
1:1.5
50
25
2.4 GHz
2,400 -2,500
11
60 x 25
Vertical
1:1.5
50
25
MMDS
2,500 - 2,690
11
65 x 22
Vertical
1:1.6
50
25
2.8 GHz
2,700 - 2,900
11
60 x 23
Vertical
1:1.5
50
25
3.x GHz
3,300 - 3,800
12
60 x 17
Vertical
1:1.5
50
25
Narrowbeam
3.x GHz
3,400 -3,700
18
16 x 18
Vertical
1:1.5
50
30
5.8 GHz
5,725 - 5,875
12
60 x 15
Vertical
1:1.5
50
25
BSR
Type
Table 2-5: PPR (Base Station) antenna specifications
Parameter
Frequency Gain
range
(dBi)
(MHz)
Beam
Polarization VSWR Impedance Frontwidth H
to(ohm)
XV
back
ratio
(degrees)
(dB)
2.4
GHz
2,400 -2,500
18
19 x 25
Vertical
1:1.6
50
28
3.x
GHz
3,400 -3,700
18
16 x 18
Vertical
1:1.5
50
30
5.8
GHz
5,725 - 5,875
12
60 x 15
Vertical
1:1.5
50
25
PPR
Type
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Table 2-6: SPR (CPE – outdoor unit) antenna specifications
SPR
Type
Freq.
range
(MHz)
Gain
(dBi)
Beam
width
HXV
(deg.
Polarization
VSWR
Impedance
(ohm)
Front
-toback
ratio
(dB)
700 MHz
710 - 716
740- 746
60 x 60
Vertical
1:1.6
50
20
900 MHz
902 - 928
60 x 60
Vertical
1:1.9
50
23
2.4 GHz
2,400 2,500
15
24 x 33
Vertical
1:1.6
50
28
High-gain
2.4 GHz
2,400 2,500
18
19 x 25
Vertical
1:1.6
50
28
MMDS
2,500 2,690
15
21 x 29
Vertical
1:1.6
50
25
2.8 GHz
2,700 2,900
15
21 x 30
Vertical
1:1.6
50
25
3.5 GHz
3,400 3,600
15
18 x 28
Vertical/
Horizontal
1:1.6
50
25
High-gain
3.5 GHz
3,400 3,600
18
16 x 18
Vertical
1:1.6
50
25
5.8 GHz
5,725 5,875
16
21 X 12
Vertical
1:1.6
50
25
Notes:
1) The SPR 700 MHz and 900 MHz models have larger dimensions than the
standard SPR models. Their dimensions are the same as that for the BSR.
2) The SPR 3.5 GHz and SPR 2.4 GHz models are available in large and
standard (smaller) dimensions (chassis). The dimensions (i.e., large or small)
affect the antenna gain.
3) The 700 MHz internal antenna covers only 710 - 716 MHz and 740 - 746
MHz (i.e. Band C). To cover the entire band of 698 – 746 MHz, an external
antenna is used (see Section 2.5.2tbd”).
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Table 2-7: IDR (CPE - indoor unit) antenna specifications
IDR
Type
Freq.
range
(MHz)
Gain
(dBi)
Beam
width
HXV
(deg.)
Polarization
VSWR Impedance Frontto(ohm)
back
ratio
(dB)
900
MHz
902 - 928
67 x 93
Vertical
1:1.9
50
-17
2.4 GHz
2,400 to
2,500
10
65 x 32
Vertical
1:1.6
50
25
3.5 GHz
3,400 to
3,600
10
65 x 32
Vertical
1:1.6
50
25
2.5.2. WipLL External Antennas
WipLL provides options to attach external antennas when operating in the 700 and
900 MHz bands.
2.5.2.1. 900 MHz
The WipLL BSR and IDR devices operating in the 900 MHz band, provide N-type
receptacles for connecting external antennas. The BSR provides two N-type
receptacles (for antenna diversity), and the IDR provides one TNC-type receptacle.
This document lists the specifications of these external antennas intended for the
BSR and IDR devices.
2.5.2.1.1. BSR External Antennas
Airspan provides the following external antennas for BSR devices operating in the
900 MHz band:
Sector antenna
Omnidirectional antenna
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2.5.2.2. Sector Antenna
This antenna is designed for best non-line of sight performance with Airspan’s BSR
operating in the 900 MHz band. Advanced features include: high gain and
mechanical downtilt.
Electrical specifications
Frequency range
870 –960 MHz
Polarization
Vertical
Gain
15.5 dBi
Half-power beam width
• H-plane:65°
• E-plane:13°
Front-to-back ratio
>25 dB
Impedance
50 .
VSWR
<1.3
Intermodulation IM3
(2 x 43 dBm carrier)
<–150 dBc
Max.power
500 W (at 50 °C ambient temperature)
Mechanical specifications
2-12
Input
7-16 female
Connector position
Bottom
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Weight
6 kg
Wind load
• Frontal: 220 N (at 150 km/h)
• Lateral: 140 N (at 150 km/h)
• Rearside: 490 N (at 150 km/h)
Max.wind velocity
200 km/h
Packing size
1422 x 272 x 160 mm
Height/width/depth
1294 /258 /103 mm
2.5.2.2.1. Omnidirectional Antenna
This antenna is designed for best non-line of sight performance with Airspan’s BSR
operating in the 900 MHz band.
Electrical specifications
Frequency range
870 – 960 MHz
Polarization
Vertical
Gain
11 dBi
Impedance
50Ω
VSWR
<1.5
Intermodulation IM3
(2 x 43 dBm carrier)
<–150 dBc
Max. power
500W (at 50 °C ambient temperature)
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Mechanical specifications
Model Type
736 347
736 348
Input
7-16 female
7-16 female
Connector position
Bottom
Top
Weight
8 kg
Radome diameter
51 mm
Wind load
210 N (at 150 km/h)
Max.wind velocity
200 km/h
Packing size
3316 x 148 x 112 mm
Height/width/depth
3033 mm
3022 mm
2.5.2.3. IDR External Antennas
Airspan provides one of the following external antennas for the IDR device
operating in the 900 MHz band:
10 dBi Panel antenna
6.5 dBi Panel antenna
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2.5.2.3.1. 10 dBi Panel
Electrical
Frequency range
902 - 928 MHz
Gain
10 dBi (min)
VSWR
1.5:1 (max)
3 dB Beamwidth
(related to vertical
polarization)
• Azimuth: 65 (typ)
Polarization
Linear (Vertical or Horizontal)
Sidelobes level
-18dB (max) @ +/-90
Cross polarization
-14dB (max)
F/B ratio
-20dB (max)
Input impedance
50 (ohm)
Input power
6W (max)
Lightning protection
Non
• Elevation: 55 (typ)
Mechanical
Dimensions (LxWxD)
305x305x25 mm (max)
Weight
1.5 kg (max)
Connector
N-Type Female
Radome
Plastic
Base plate
Aluminum with chemical conversion coating
Mounting kit
MT-120018
Environmental
Test
Standard
Duration
Temperture
Notes
Low temperature
IEC 68-2-1
72 h
-55°C
High temperature
IEC 68-2-2
72 h
+71°C
Temp. cycling
IEC 68-2-14
1h
-45°C +70°C
3 Cycles
Vibration
IEC 60721-3-4
30 min/axis
Random 4M3
Shock mechanical
IEC 60721-3-4
4M3
Humidity
ETSI EN300-2-4
T4.1E
144 h
95%
Water tightness
IEC 529
IP67
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Solar radiation
ASTM G53
1000 h
Flammability
UL 94
CLASS HB
Salt spray
IEC 68-2-11 Ka
500 h
Ice and snow
25mm radial
Wind speed survival
220 Km/h
160 Km/h
Operation
Wind load (survival):
• Front thrust
• 26.8 kg
• Side thrust
• 2.2 kg
2.5.2.3.2. 6.5 dBi Panel
Electrical
Frequency range
902-928 MHz
Gain
6.5 dBi (min)
VSWR
1.5:1 (max)
3 dB Beamwidth
• Azimuth
• 80° (typ)
• Elevation
• 80° (typ)
polarization
Linear (Vertical or Horizontal)
Cross polarization
-14dB (max)
F/B ratio
-11dB (max)
Input impedance
50 (ohm)
Input power
6W (max)
Lightning protection
NON
Mechanical
Dimensions (LxWxD)
190x190x30 mm (max)
Weight
0.7kg (max)
Connector
N-Type Female
Radome
Plastic
Base plate
Aluminum with chemical conversion coating
Outline drawing
RD41245600C
Mounting kit
MT-120018/A
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Environmental
Test
Standard
Duration
Temperture
Notes
Low temperature
IEC 68-2-1
72 h
-55°C
High temperature
IEC 68-2-2
72 h
+71°C
Temp. cycling
IEC 68-2-14
1h
-45°C +70°C
3 Cycles
Vibration
IEC 60721-3-4
30 min/axis
Random
4M3
Shock mechanical
IEC 60721-3-4
4M3
Humidity
ETSI EN300-2-4
T4.1E
144 h
95%
Water tightness
IEC 529
IP67
Solar radiation
ASTM G53
1000 h
Flammability
UL 94
Class HB
Salt spray
IEC 68-2-11 Ka
500 h
Ice and snow
25mm radial
Wind speed survival
220 Km/h
160 Km/h
Operation
Wind load (survival):
• Front thrust
• 10 kg
• Side thrust
• 1.6 kg
2.5.2.4. Considerations for WipLL 700
For most of the frequency bands, WipLL products provide a variation of models
consisting of internal and external antennas. The internal antennas of all WipLL
products, except for WipLL 700, cover all frequency bands.
WipLL 700’s internal antenna covers only Band C (i.e., 710 to 716 MHz, and 740 to
746 MHz) frequency band. Therefore, for WipLL 700, Airspan provides an external
antenna, allowing coverage in the entire 700 MHz band (698 to 746 MHz), including
the licensed A and B bands used in USA.
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2.5.2.5. External Antennas
For most bands, WipLL products allow connection of a large variety of external
antennas. However, WipLL 700 provides a limited variation of external antennas,
including, amongst others, the following:
90° panel or omnidirectional (for BSR)
14-element yagi antenna (for SPR)
These external antennas can be supplied by Airspan. The external antennas connect
to the WipLL devices by an N-type connector.
2.5.2.6. RF Planning Guidelines for Band C in FCC Market
Some operators (e.g., in the USA) have licenses for Band C (710 – 716 MHz and
740 – 746 MHz). When operating in Band C, WipLL 700 allows a maximum of four
BSRs at a Base Station (according to FCC regulations). This is to reduce RF
interference with other radio devices that may be operating in nearby frequencies.
With the 1 Msps mode, the center frequencies are 711.5, 712.5, 713.5, 714.5, 741.5,
742.5, 743.5, and 744.5. Thus, the frequency allocation for four BSRs (i.e., sectors)
is 711.5, 741.5, 714.5, and 744.5.
With the 1.33 Msps mode, the center frequencies are 712, 713, 714, 742, 743, and
744. Thus, the frequency allocation for four BSRs (i.e., sectors) is 712, 742, 714,
and 744.
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Figure 2-5: Frequency allocation in a four-sector Base Station
Radio interference may occur between the BSRs operating in the upper frequency
range (i.e., 742 MHz and 744 MHz) and the lower frequency range (i.e., 712 MHz
and 714 MHz). To overcome this interference, a 1-meter vertical separation is
recommended between the BSRs operating in the upper frequency and the BSRs
operating in the lower frequency.
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2.5.2.7. Specifications
Table 2-4 and Table 2-6 list the external antenna specifications for BSR and SPR
devices operating in the 700 MHz band.
Table 2-8: BSR 700 MHz external antennas
External
antenna type
90° panel
Omnidirectional
Parameter
Value
Frequency Range (MHz)
698 - 746
Gain (dBi)
14
Beam Width H X V (degrees)
90 x 20
Polarization
Vertical
VSWR
< 1.4
Impedance (ohm)
50
Front-to-Back Ratio (dB)
> 25
Frequency Range (MHz)
698 - 746
Gain (dB)
7.5
Beam Width H X V (degrees)
360 x 20
Polarization
Vertical
VSWR
1.5
Impedance (ohm)
50
Table 2-9: SPR 700 MHz external yagi antenna
Parameter
2-20
SPR 700 MHz
Frequency Range (MHz)
698 - 746
Gain (dBi)
13
Beam Width H X V (degrees)
32 x 32
Polarization
Vertical/horizontal
VSWR
< 1.8
Impedance (ohm)
50
Front-to-Back Ratio (dB)
> 15
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2.6. Radio Planning
WipLL radio planning can be divided into the following areas:
Main technical parameters
Coverage analysis
Interference analysis: FDD vs. TDD
Frequency allocation: Synchronized vs. Unsynchronized operation
Capacity considerations
Selecting appropriate mode of operation
Radio Planning software tool
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2.6.1. Main Technical Parameters
The main technical parameters required for RF planning for WipLL are summarized
in Table 2-10.
Table 2-10: Radio specifications
Parameter
Value
Radio Technology
FH-CDMA
Multiple Access Method
Proprietary Adaptive TDMA protocol (Preemptive
Polling Multiple Access – PPMA)
Output Power
27 dBm for all models, except for the following:
• WipLL 700: 32 dBm
• WipLL 900: 30 dBm (but when operating in
countries complying with FCC, max. is 23
dBm)
Sub-Channel Spacing
1 MHz or 1.75 MHz (1.75 MHz is possible only
for devices operating in the 3.5 GHz band)
Symbols per second (Msps)
Two modes are supported:
• 1 Msps, or
• 1.33 Msps
Sub-Channel bandwidth (measured at 20 dB
attenuation point)
1 MHz or 1.33 MHz, depending on the selected
mode
Modulation
Multilevel (2, 4, or 8) CPFSK1
Receiver Sensitivity (BER 1E-6 at 2/4/8
FSK)
-90/ -83/ -75 dBm
SNR Thresholds (BER 1E-6 at 2/4/8 FSK)
12/ 20/ 28 dB
Interference Rejection Factor for 1.33 Msps
mode (1 Msps mode):
• ± 1 MHz
• 5 dB (7 dB)
• ± 2 MHz
• 30 dB (40 dB)
• ± 3 MHz
• 52 dB (53 dB)
The intermediate 4-FSK modulation is not supported when 1.33 Msps mode is
selected
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Parameter
Value
• ± 4 MHz
• 58 dB (60 dB)
• ± 5 MHz
• 63 dB (64 dB)
Receiver Noise Figure
10 dB
2.6.2. System Coverage
System coverage includes the following:
Line of sight (LOS)
Link Budget
2.6.2.1. Line of Sight
Usually, WipLL requires the existence of a line of sight (LOS) between the base
station transmitter and the subscriber’s receiver (near line of sight [NLOS] may be
possible to a limited extent for ranges of a few hundred meters). Therefore, the
availability of LOS (clear first Fresnel Zone) should be estimated during CPE
installation or preferably during network planning. Recommended propagation
models used in coverage analysis are based on free-space propagation with
compensation for ground and irregular terrain reflections and diffraction. Specific
propagation model names vary between different software tools. The model should
also include a certain level of fade margin, as discussed in the next section.
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2.6.2.2. Link Budget
The coverage analysis of WipLL includes the analysis of the power balance between
the transmitter and the receiver, threshold considerations, margins, reserves, and
certain statistics of the system. Therefore, the lead-in reception level is measured by
the following equation:
Rx = Tx – LossTx + AntGainTx – PathLoss + AntGainRx – LossRx
Where,
Rx =
Reception level in dBm
Tx =
Transmitter power in dBm (27 dBm in the WipLL system)
LossTx =
Transmitter losses in dB (0 dB in the WipLL system)
LossRx =
Terminal receiver losses in dB (0 dB in the WipLL system)
AntGainTx =
Transmitter antenna gain
AntGainRx =
Receiver antenna gain in dBi (decibels referenced to isotropic
radiator)
PathLoss =
Propagation loss in dB
Note: Both the base station and the subscriber site can serve as transmitter or
receiver. For downlink budget, the transmitter is the base station and the
receiver is the subscriber; and vice versa for the uplink budget.
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2.6.2.2.1. Propagation loss
Propagation is the dispersal of the signal into space as it leaves the antenna. The loss
of this propagation depends on the signal path between the transmitter and the
receiver. Obstructions in the signal path such as trees and buildings can cause signal
degradation. Several models simulate signal attenuation along this path.
Propagation loss should incorporate fading margins to compensate different
phenomenon such as multipath shadowing and climatic behavior of the waves.
Based on this, the parameter path loss can be represented by
PathLoss = L + Fade Margin.
Free Space model:
Free space propagation loss is valid where the first Fresnel Zone is clear. In this
case, free space propagation loss is given by the following equation:
LFS = 32.44 + 20logd[km] + 20logf[MHz]
Fade Margin:
Fade margin further introduces fading factor to the propagation loss to cover the
different signal fading and shadow effects, as well as the degradation caused by
interferences. The fading factor depends on the time availability parameter
defined by the operator, and should be calculated according to the ITU 530
model for 99.9% availability.
For simplicity purposes, the ITU model can be replaced by a 10 dB Flat fade
margin as a rough estimation.
Rainfall:
Radio signals are attenuated by moisture in the atmosphere. The level of
attenuation varies with carrier frequency, the quantity of rainfall, and the
distance from the transmitter to the receiver. The variation of attenuation
with frequency is particularly strong and highly non-linear. At 3 GHz, the
highest attenuation is about 0.06 dB/Km; for a typical WLL path of, for
example, 6 Km, the attenuation is only 0.36 dB. Therefore, for the purpose of
link budget, we can assume that the impact of rainfall is negligible.
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2.6.2.2.2. Link Budget Results
Based on the previous formulas mentioned in the above sections, the following link
budget results can be obtained for 99.9% availability:
Modulation
Rate
(Mbps)
Range (in km)
2.4
GHz2
MMDS
(2.5
GHz)
3.5 GHz
5.8
GHz
900
MHz
700
MHz
8 FSK
3 or 4
15
4 FSK
11
11
10
11
22
2 FSK
1 or 1.33
14
14
13
11
15
28
Note: Link budget is calculated for the standard integrated WipLL antennas.
Where required, the range can be increased by the implementation of external
antennas.
2.6.3. Interference Analysis
Interference analysis should be based on parameters defined in Section 2.6.1, “Main
Technical Parameters” to determine the downlink and uplink carrier-to-interference
ratio (C/I). The C/I is a key factor in determining the supported modulation for each
link. Thresholds for C/I for the different modulations are mentioned in Section 2.6.1,
“Main Technical Parameters”.
Interference analysis depends on the duplex scheme implemented in the system.
Since WipLL supports both FDD and TDD, different considerations should be
applied.
Although the transmitter is capable of transmitting 27 dBm, in most cases the EiRP
in the ISM band is limited by local regulations. For example, ETSI limits the EiRP
to 20 dBm, FCC to 36 dBm, and TELEC to 27 dBm. The link budget calculated
here assumes no limit.
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2.6.3.1. FDD vs. TDD
Frequency Division Duplex (FDD) interference analysis is relatively simple. The
frequency separation between downlink and uplink (50 to 100 MHz) enables
Airspan to conduct downlink and uplink interference analysis independently.
However, in Time Division Duplex (TDD), since Tx and Rx are not synchronized,
cross-link interference, for example, between a transmitting BSR and an adjacent
receiving BSR, may result. It is expected that these interferences will be dominant in
the TDD mode.
The major consequence of this are the different limitations imposed on frequency
separation between adjacent BSRs:
FDD: 2 MHz
TDD: 4 MHz
The required separation is a function of the isolation between co-located antennas.
The 4-MHz separation is based on 60 dB isolation. Based on this, a simple
frequency allocation for a stand-alone cell is presented in Figure 2-6.
11
Frequency Allocation
Single Cell (TDD)
Frequency Allocation
Single Cell (FDD)
Figure 2-6: Frequency Allocation for a stand-alone cell: FDD (left) vs. TDD (right)
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2.6.4. Frequency Allocation
Frequency allocation includes the following issues:
Synchronized vs. Unsynchronized operation
Frequency Reuse
Frequency Allocation template
2.6.4.1. Synchronized vs. Unsynchronized Operation
The frequency allocation scheme in WipLL depends on the mode of operation.
Three main options exist:
Unsynchronized Frequency Hopping
Synchronized Frequency Hopping
Fix Sub-Channel Assignment
2.6.4.1.1. Unsynchronized Frequency Hopping
A scenario may exist whereby the operator does not want to synchronize base
stations due to cost or regulatory issues. For example, the FCC prohibits
synchronization between base stations when operating in the unlicensed band. In
such a scenario, the BSRs (and SPRs) are assigned pseudo-random frequency tables,
and, therefore, collisions between transceivers are possible, resulting in some level
of retransmissions. However, implementing orthogonal frequency tables (hopping
sequences) reduces collisions.
In this mode of operation, the available set of sub-channels, which must include a
prime number of sub-channels, is organized into several different hopping sequences
that are orthogonal to each other. Each BSR in the coverage area is assigned a
different sequence (until there is a need to start reusing the tables). The degradation
level in performance due to collisions will be a random number, depending on the
length of the frequency tables (the number of available sub-channels).
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Figure 2-7 displays a graph depicting the hit or blocking probability as a function of
number of interferers for different table lengths (7, 23, and 79).
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
N=7
11
13
N = 23
15
17
19
N = 79
Figure 2-7: Hit probability as a function of active interferers
Note: The probability of collisions with 20 interferers and 79 sub-channels is
only 20%.
2.6.4.1.2. Synchronized Frequency Hopping
In most scenarios, the operator synchronizes between WipLL base-stations located
in the same coverage area. This option provides the best control over intra-system
interferences.
In this mode of operation, the available set of sub-channels is arranged in a single
hopping sequence, common to all transceivers (BSRs and SPRs) in the coverage
area. Since the table ID is identical to all radios, the only parameter that needs to be
assigned is the phase, that is, the starting point within the sequence.
By selecting the sequence appropriately, the relative frequency separation between
the transceivers remains constant over time, so that interference analysis is quite
similar to any Frequency Division Multiple Access (FDMA) system.
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2.6.4.1.3. Fix Sub-Channel Assignment
In some scenarios—mainly licensed bands in which available spectrum is limited—
it is possible to create a set of “hopping” tables, each based on a single sub-channel.
Using this approach, the hopping nature of the system is actually disabled, so that
synchronization is not required. The trade-off of this approach is loss of frequency
diversity, discussed previously as a means to overcome frequency-selective fading.
Interference analysis in this mode is identical to any FDMA system.
2.6.4.2. Frequency Allocation
Figure 2-8 presents frequency allocation for three adjacent cells for the FDD and
TDD schemes. This frequency allocation is relevant only for synchronized
frequency hopping or fix sub-channel assignment options. For the unsynchronized
frequency hopping option described previously, orthogonal frequency tables are
assigned instead of specific sub-channels (or phases). As described in Section 2.6.3,
“Interference Analysis”, 60-dB isolation between co-located BSRs is assumed for
the TDD allocation.
Figure 2-8: Frequency Allocation FDD (left) and TDD (right)
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2.6.5. Capacity Considerations
This section provides high-level guidelines for evaluating WipLL capacity. This
capacity relates to the number of subscribers supported by a single BSR according to
a certain service mix. The methodology presented here provides a simplified
approach for evaluating WipLL capacity capabilities. It can be used as an initial
estimation; however, it cannot replace a more accurate capacity analysis performed
by an RF planning team.
2.6.5.1. General
The general concept for determining the number of subscribers per BSR is presented
in Figure 2-9.
Aggregate CIR / Over Subscription Factor
BSR Limit (Kbps)
Voice Calls * Call BW
Figure 2-9: Determining number of subscribers per BSR
The BSR limit is 4 Mbps; therefore, the total voice and data bandwidth must be
lower or equal to 4 Mbps.
The number of voice calls should be derived from Erlang B tables according to
voice traffic per subscriber (in erlangs), and expected blocking probability (typically
1%) parameters. The VoIP bandwidth depends on the specific codec that will be
used. Typical values are specified in the next section.
The data portion of the aggregated traffic is based on the sum of the committed
information rate (CIR) for all subscribers assigned to the BSR, divided by the
appropriate over subscription rate.
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Since voice and data services differ by their packet size and their sensitivity to delay,
their protocol efficiency can be substantially different. Therefore, when calculating
bandwidth for different applications, the gross bandwidth should be used, which
takes into account the protocol efficiency.
2.6.5.2. VoIP Bandwidth and Simultaneous Calls
The VoIP bandwidth and the number of supported simultaneous calls are a function
of the selected codec and the sample interval. Assuming a 4 Mbps gross rate, the
following numbers can be used:
Table 2-11: VoIP bandwidth (Kbps) for 4 Mbps gross rate
Codec
G.711 (64 Kbps)
G.729 (8 Kbps)
G.723.1 (5.3 Kbps)
Sample Interval
(msec)
Simultaneous Calls
20
10
40
15
20
14
40
28
30
22
60
42
If silence suppression is used, a factor of 65% should be applied on the call
bandwidth.
Note: The selection of the appropriate codec should be based on a balance
between the occupied bandwidth and the voice quality.
2.6.5.3. Data Bandwidth
It is assumed that packet size for data applications is relatively large (about 1,500
bytes), resulting in a protocol efficiency of 80%. Taking this into account, the sum
of all data bandwidth should be divided by 80% to obtain the bandwidth over the air.
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2.6.5.4. Calculation Example
This example assumes that the required services are based on voice traffic of 100
merlangs per subscriber and a CIR of 256 Kbps. In addition, in this example, a 1%
blocking is expected for voice calls, and 1:10 over subscription is expected for data.
The number of subscribers that can be supported by a single BSR (N) is equal to
Calls (traffic, Gos ) ⋅
4,000 Kbps
10% ⋅ N ⋅ 256 Kbps
≤ 4,000 Kbps
Sim _ Calls (Codec)
80%
Assume that N = 40. Therefore, the aggregate voice traffic is equal to 4 erlangs.
Using Erlang B tables with 1% blocking, 10 voice channels (calls) are required. For
G.729 with 40 msec sample interval and silence suppression, the total capacity set
for voice is obtained by 10*143 Kbps*65% = 930 Kbps. The total capacity set for
data is obtained by 40*256 Kbps*10%/80% = 1,280 Kbps. This implies that more
subscribers can be supported, since the aggregated capacity (2,210 Kbps) is lower
than the 4 Mbps limit. If N (BSRs) increases, the limit of 4 Mbps will be reached for
N = 80. (The process of finding N can be simplified by using an electronic
spreadsheet).
2.6.6. Selecting an Appropriate Operation Mode
WipLL enables the operator the flexibility to choose between several modes of
operation according to the operator’s needs. One of the optional modes relates to the
symbol rate of the modem. This section presents the main issues that should be
considered when selecting an operation mode when deploying the WipLL system.
2.6.6.1. WipLL Multiple Modes
WipLL offers the capability to select between two operating modes in terms of
symbol rate. The modem can operate in either 1 Mega symbols per second (Msps) or
1.33 Msps. The operating mode is software selectable for each BSR. The differences
between the modes of operation are related to the bit rate and the channel bandwidth.
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2.6.6.1.1. Bit Rate
The 1 Msps mode supports three levels of modulation, as presented in Table 2-12.
Table 2-12: WipLL bit rate at 1 Msps
Modulation
Bit/Symbol
Bit rate (Mbps)
8-level FSK
4-level FSK
2-level FSK
The 1.33 Msps mode supports two levels of modulation according to Table 2-13.
Table 2-13: WipLL bit rate at 1.33 Msps
Modulation
Bit/Symbol
Bit rate (Mbps)
8-level FSK
2-level FSK
1.33
Note: The differences in sensitivity and SNR values between the two modes
are negligible.
2.6.6.1.2. Channel Bandwidth
The 1.33 Msps is based on shorter symbols, with the trade-off of a wider channel
bandwidth. The 20 dB attenuation point for the 1 Msps mode and the 1.33 Msps
mode is 1 MHz and 1.33 MHz, respectively.
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2.6.6.2. System Range Considerations
System range depends on the maximum output power of the system. Different
approaches exist, depending on region and frequency band.
2.6.6.2.1. Unlicensed Bands
FCC part 15 (paragraph 247) differentiates between three types of systems:
Digital modulated
Frequency hopping
Hybrid
Limitations on Tx (transmit) power and EIRP differ on this basis. Table 2-14
summarizes the limitations for the different WipLL products.
Table 2-14: Tx power and EIRP limitations for WipLL products
Frequency
band
900 MHz
2.4 GHz
5.8 GHz
Mode
Tx power
EIRP
System type
1 Msps
15 dBm
36 dBm
Hybrid
1.33 Msps
18 dBm
36 dBm
Hybrid
1 Msps
--
36 dBm
Frequency
hopping
1.33 Msps
--
27 dBm
Frequency
hopping
1 Msps
--
36 dBm
Frequency
hopping
1.33 Msps
18 dBm
36 dBm
Hybrid
Note: For WipLL 900, the BSR’s external antenna must have a minimum cable loss
of 2.5 dB to comply with FCC’s EIRP limit of 36 dBm. EIRP is calculated as:
Max. Power Output + Antenna Gain + Cable Loss ≤ 36 dBm (EIRP)
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The table below lists examples of cable loss per cable for maximum antenna gains,
based on the formula above. Note that the EIRP is either equal to or less than 36
dBm.
Table 2-15: Example of cable loss per cable for maximum antenna gains
Cable type
BELDEN 9913
BELDEN 89907
Cable
length (ft)
Tx power
(dBm)
Cable loss
(dB)
10
18
0.6
30
18.9
100
Max.
antenna
gain (dBi)
Max. EIRP
(dBm)
1.5
18.6
18.6
36
36
21.8
4.4
18.6
36
10
19.3
1.9
18.6
36
30
22.6
5.2
18.6
36
100
23
16.3
18.6
25.3
Table 2-16, Table 2-17, and Table 2-18 present system ranges (in kilometers) for the
different frequency bands (900 MHz, 2.4 GHz, and 5.8 GHz, respectively) and
modes of operation (i.e., 1 Msps and 1.33 Msps).
Table 2-16: WipLL range at FCC limits for 900 MHz
Mode
1 Msps
1.33 Msps
Modulation
Rate
(Mbps)
Range
(Km)
8 FSK
4 FSK
10
2 FSK
24
8 FSK
2 FSK
1.33
35
Table 2-17: WipLL range at FCC limits for 2.4 GHz
Mode
1 Msps
1.33 Msps
2-36
Modulation
Rate
(Mbps)
Range
(Km)
8 FSK
4 FSK
10
2 FSK
22
8 FSK
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Mode
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Modulation
2 FSK
Rate
(Mbps)
1.33
Range
(Km)
Table 2-18: WipLL range at FCC limits for 5.8 GHz
Mode
1 Msps
1.33 Msps
Modulation
Rate
(Mbps)
Range
(Km)
8 FSK
4 FSK
2 FSK
10
8 FSK
2 FSK
1.33
Note:
Link budget is calculated for the uplink assuming free space
propagation standard integrated antenna and 10 dB fade margin.
As shown in the tables, the system range for the 1 Msps mode is approximately three
times higher than for the 1.33 Msps mode due to the 9 dB differences in Tx
(transmit) power.
2.6.6.2.2. Licensed Bands (3.5 GHz)
No distinction exists between the two modes in terms of system range.
2.6.6.3. Interference Rejection
Two types of interference should be considered:
Intra-system interference: caused by multiple WipLL transmitters
Inter-system interference: caused by external transmitters, mainly in the
unlicensed bands
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2.6.6.3.1. Intra-system Interference
As mentioned previously, 1.33 Msps mode is achieved by using wider channel
bandwidth. This results in a higher bit rate at the expense of increasing adjacent
channel interference. This difference can become critical mainly for licensed bands
in which the available spectrum might be very limited. It is beyond the scope of this
document to provide specific rules regarding the preferred mode as a function of the
available spectrum. This is due to the fact that such analysis depends on many
parameters such as the cell layout and the topography of the coverage area.
However, it should be evaluated during radio planning.
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2.6.6.3.2. Inter-system Interference
The immunity of the WipLL system to external interference is due to its spread
spectrum system. Exposure to external interference is highest when operating in the
unlicensed band. Both FCC and ETSI set limits on the spreading level that is
required by a frequency hopping spread spectrum system.
ETSI:
According to EN 300 328, FHSS modulation must use at least 20 well-defined,
non-overlapping channels separated by the channel bandwidth as measured at 20
dB below peak power.
Since WipLL’s carriers must be located at integer multiples of megahertz
(MHz), the above statement limits the channel spacing to 1 MHz for the 1 Msps
mode, and 2 MHz for the 1.33 Msps mode. This limits the number of possible
hops in the WipLL system when operating under ETSI regulations to between 20
and 80 for the 1 Msps mode, and between 20 and 40 for the 1.33 Msps mode.
FCC:
Part 15 of the FCC sets the same requirement for non-overlapping channels.
For frequency hopping systems operating in the 902 MHz to 928 MHz band with
a channel bandwidth greater than 250 KHz, the system shall use at least 25
hopping frequencies.
For frequency hopping systems operating in the 5725 MHz to 5850 MHz band,
the system shall use at least 75 hopping frequencies.
For frequency hopping systems operating in the 2400 MHz to 2483.5 MHz band,
the system shall use at least 15 non-overlapping channels.
In general, since the number of different hops is substantially lower, the capability of
WipLL to overcome interferences is reduced. In practice, ETSI allows the operator
the flexibility not to use the entire spectrum, in case, for example, a constant
interference is identified in a portion of the spectrum. This makes the problem much
less critical than in the FCC case, where the entire spectrum must be used.
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2.6.6.4. System Capacity
The modulation for each link in the WipLL system is adaptively determined
according to the signal strength and the BER. When evaluating system capacity, it
should be taken into account that the intermediate 4-FSK modulation is not
supported when operating at 1.33 Msps.
Assume a WipLL deployment where subscribers are located at various distances
from the base stations. In general, three coverage circles can be expected around
each base station:
Inner circle—supporting 8-level FSK
Intermediate circle—supporting 4-level FSK
Outer circle—supporting 2-level FSK
Let pi, pm, and po represent the percentages of subscribers located at the inner,
intermediate, and outer coverage circles, respectively. The average capacity of the
system (without taking into account advanced features like fairness) for each of the
possible modes can be given by the following formulas:
Capacity1Msps [ Mbps] = pi ⋅ 3 + p m ⋅ 2 + p o ⋅ 1
Capacity1.33 Msps [ Mbps] = pi ⋅ 4 + ( p m + p o ) ⋅ 1.33
Radio planning can provide estimations for pi, pm, and po so that the WipLL operator
can determine which option provides the highest system capacity.
2.6.6.5. Conclusion
The 1.33 Msps mode provides superior bit rate, but at the expense of a possible
reduction in radio coverage (FCC), and/or increase in interference. A certain amount
of radio planning is required to determine the preferable mode to maximize the
overall capacity of the access network.
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2.6.7. Radio Planning Software Tool
To design an optimal fixed wireless broadband network, the operator requires an RF
design tool that includes features of a geographic information system (GIS) module,
a propagation prediction module, and a fixed wireless module.
The GIS module should map data that contain or can be assigned geographic
coordinates, for example, terrain elevation, land-use classification, site locations,
design area boundaries, roads, and highways.
The propagation prediction module should provide RF propagation analysis and
predictions. This module must support features that allow the propagation model to
be calibrated using drive test measurements.
The fixed wireless module should support directional CPE antennas for coverage,
capacity and interference analysis in LOS and NLOS conditions, and adaptive
modulation.
2.6.7.1. Geographic Information Systems Database
To enable detailed and accurate propagation prediction, terrain information must be
supplemented by a Geographic Information Systems (GIS) database. Clutter
databases are typically based on Satellite or Aerial photography, depending on the
resolution of the planning. Clutter databases should include Definition of Clutter
Types and Clutter Height relative to the underlying DEM.
The resolution of data varies depending on the environment at which the planning is
targeted. Airspan recommends the following:
Rural: 10 to30 meters
Urban: 5 to 20 meters
The specific format of the database depends on the planning tool. The planning
software provider or user’s manual should be consulted to obtain the appropriate
format.
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