Tài liệu Digital RF Transport pptx

Digitization of RF signals Source of Loss of Dynamic Range in Loss of Dynamic Range in Loss/Noise Analog RF Transport Systems Digital RF Transport Systems Optical combiner 3.0 dB loss pe

Digital RF Transport

The Advantage of Digital RF Transport for

Distributing Wireless Coverage and Capacity

Introduction Every business needs an edge Many industries—from consumer electronics to household appliances to automobiles—have successfully gained an edge with digital components and production methods supported by digital devices The drive to digital is apparent in wireless voice and data communications, too, as networks migrate toward IP-based architectures, software defined radio, and other technologies The digital edge isn’t just convenient The digital edge translates into bottom line benefits for wireless service providers

For wireless service providers, the combination of limited spectrum and deployment of spectrum-hungry data services already points to digital solutions Add in impending consolidations and capex restrictions, and wireless service providers need an edge that adds flexibility, improves quality of service, and reduces capex/operating costs That edge is found in digital methods of transporting RF signals

What is digital transport of RF?

Transporting RF signals over fiber cables provides a highly cost effective solution for distributing wireless coverage and capacity Used for both indoor and outdoor, low power and high power applications, RF transport solutions link remote antennas and cell sites to base transceiver stations

Analog and digital RF transport systems feature significant differences that impact network performance Analog RF transport systems may transport digitally modulated signals, such as TDMA or CDMA Or analog RF transport systems may include a data path for carrying alarm information In either case, the method of transporting RF signals is analog, where the RF signal is preserved

as an analog waveform throughout the transport Also, the network components required to support analog RF transport—including lasers, splitters/combiners, converters, and repeaters—are all analog devices

The Advantage of

Distributing Wireless Coverage and Capacity

Digital RF Transport

Unlike analog RF transport systems, digital RF transport

solutions involve digitization of RF signals To digitize RF

signals, continuous samples or snapshots of the RF

spectrum are taken and numerical values are assigned to

each sample, converting a continuous voltage RF signal

to a list of discrete values It is this list of discrete 1s and

0s that becomes the digitized RF signal

ADC’s patented RF digitization technology converts an

entire section of the RF spectrum to a digital bit stream,

transporting the digital bit stream over fiber, and

reconstructing the RF signal at full bandwidth at the

other end of the link, shown in Figure 1 Taking a

snapshot of the RF spectrum 71 million times per second,

this technology can fully digitize a 35 MHz section of

spectrum, which is within the Nyquist rate of 71 Mbps1

Thirty-five MHz of spectrum exceeds the spectrum

holdings of any wireless service provider in 800 MHz,

1900 MHz or UMTS networks With 14 bit samples taken

at 71 Mbps, the resulting digitized representation of the

RF signal is over 1 Gbps with dynamic range performance

that can practically exceed 70 dB over all conditions

The relationship between dynamic

range, loss, and noise in RF transport

Dynamic range is like bandwidth—more is better

Expressed in decibels (dB), dynamic range is the difference

between the strongest signal and signal just above the

noise floor

Comparing the dynamic range of analog and digital RF

transport systems is like comparing the dynamic range of

vinyl LP records and compact discs The LP replicates sounds from a musical performance using analog techniques; the compact disc, digital techniques The dynamic range of a LP record is about 50 dB For a compact disc, dynamic range is about 90 dB With a wider and more accurate dynamic range, the compact disc captures sounds on the high and low end of a musical performance—sounds that do not appear on the LP record version of the same musical performance The full depth

of cannon booms and sparkle of percussionist’s triangle are fully replicated on the compact disc These sounds are

lost on the LP With analog-formatted music, there is an incomplete replication of the original musical performance

In wireless communications, poor dynamic range is most apparent

in data communications When just several data bits of a RF signal are clipped due to a narrow dynamic range, entire blocks of information become corrupt or lost Clearly, wide dynamic range is critical as more data services consume the wireless spectrum

What limits dynamic range in an analog RF transport system? Loss and noise Every splitter, combiner, and connector adds loss And RF transport systems generate a variety of noises Every amplifier or transport medium adds noise There’s also laser shot noise, laser RIN, and thermal noise in optical devices There’s intermodulation, dispersion, and reflection There’s ambient RF noise in coax and UTP cables There is even interferometric intensity noise generated by optical transmission on fiber of longer distances Figure 2 compares the impact of loss and noise

in analog and digital RF transport systems

It is loss and noise that make digital and analog RF transport systems so different In all analog RF transport systems, loss and noise generated in the transport are directly added to the RF signal In analog systems, noise is cumulative and cannot be removed from the signal; once

RF to

Digital

Digital

to RF

Optical TX

Optical RX

fiber

Figure 1 Digitization of RF signals

Source of Loss of Dynamic Range in Loss of Dynamic Range in

Loss/Noise Analog RF Transport Systems Digital RF Transport Systems

Optical combiner 3.0 dB loss per 2:1 split None

loss in transport path

Optical transport loss 2 dB loss per 1 dB attenuation None

Optical connector loss 2 dB loss per 1 dB attenuation None

Optical transport noise 2 dB loss per 1 dB noise None

RF noise 1 dB loss per 1 dB noise 1 dB loss per 1 dB noise

Figure 2 Comparing loss of dynamic range in analog and digital RF transport systems

the noise is there, it is there for good

Unlike analog systems, noise and loss have virtually

no impact on digital RF transport because the

discrete values of the digitized RF signal and the

transport medium are fully independent In digital

terms, a noisy 1 is still a 1 and a noisy 0 is still a 0 In digital

RF transport over fiber, as long as the digital bit stream is

transported over allowed optical transport requirements—

which are less stringent than analog optical transport

requirements—RF signals are regenerated with identical

performance as the original RF signal digitized to create

the digital bit stream With digital RF transport, as long as

the signal can be detected, the digitized RF signal is

transported error free, as depicted in Figure 3

To illustrate, suppose a RF signal shows the following characteristics: the weakest signal is –110 dBm, strongest signal is –40 dBm Before transmission over an RF transport system, the dynamic range of the signal is therefore 70 dB In a digital transport system over fiber cable, the dynamic range of the digitally transported RF signal remains constant at 70 dB—irrespective of optical loss and noise generated in transport By contrast, in an analog RF transport system—where losses and noise are permanently added to the RF signal—adding 25 dB of optical loss reduces dynamic range by 50 dB, resulting in a dynamic range of 30 dB This is far below the level of acceptable network operation, between 60 dB and 80 dB See Appendix A, comparing dynamic range, for a more detailed example

Figure 3 Even as optical loss grows, digital RF transport maintains dynamic range of the signal

The cumulative effects of loss quickly degrade analog RF transport.

Loss of dynamic range inhibits the information carrying and call handling capability of analog RF

transport systems Digital RF transport systems—immune to degradation caused by noise in the

transport that affects analog RF transport systems—offer a wider dynamic range and greater

performance for wireless communications Greater dynamic range allows strong and weak signals

to be transported simultaneously, which translates into improved quality of service

The relationship between noise,

higher speeds, and signal strength

in RF transport

In wireless communications, there is a clear relationship

between signal level and bit rate—increased data rates

require increased signal Figure 4 below shows that for

every doubling of the data rate, an approximately 3 dB

increase in signal level is required For example, as a

network migrates from GSM (200 kHz) at 115 kbps to

EDGE at 384 kbps, the increased data rate requires about

an 8 dB increase in signal

The signal level corresponds to the signal to noise ratio

(SNR) SNR is the ratio of the power of the signal to the

power of the noise in the transport To maximize network

efficiency, the goal is to operate RF signals at the lowest

possible SNR, the point where the signal is only a little bit

stronger than the noise

In RF transport, there is an inverse relationship between

signal and noise As noise is added to the system, less

signal output is achieved; conversely, less noise enables a

greater signal Because digital RF transport is immune to

transport noise, a higher SNR is possible than with

analog RF transport systems With a better SNR, digital

RF transport delivers significantly better signal quality that

translates into higher data rate capabilities

With stronger RF output power on the forward path—

base transceiver station broadcasting out to the mobile

user—wireless devices can be farther away from the cell

site, effectively expanding coverage and increasing capacity by enabling additional radios With an inherently higher SNR, digital RF transport maximizes use of valuable spectrum and offsets shrinking coverage areas caused by higher speed services Dollar for dollar, higher SNR enables better utilization of network equipment by improving signal quality and expanding coverage areas

Improved performance of digital RF transport on the reverse path enables higher power of the forward path, resulting in a balanced path, something sacred to network designers Because lower noise and stronger signal— increased SNR—translate into wider dynamic range, both the more distant, weak signals and strong signals are detected on the return path of a digital RF transport system Without a balanced path, strong signals swamp the system, blocking weak signals, frustrating users, and corrupting data With higher forward path output and greater sensitivity to both weak and strong signals on the uplink, digital RF transport—with higher SNR and wider dynamic range—ensures enhanced coverage and a balanced path

Higher noise limits signal strength in analog RF transport Because digital RF transport is immune to optical transport noise, greater signal quality is achieved—enabling higher data rates, better use of spectrum, and wider coverage areas on the forward and return paths With higher SNR, digital RF transport improves QoS by maximizing coverage and preserves capital by maximizing valuable spectrum and network equipment.

-125.00 -120.00 -115.00 -110.00 -105.00 -100.00 -95.00

-90.00

Data Rate (kbps)

IS-95B 1xRTT TDMA/EDGE GSM/EDGE UMTS

IS-95B

1xRTT

1xEV-DO/DV

UMTS

Figure 4 Every doubling of the data rate requires approximately 3 dB increase in signal.

Digital delivers distance

The foolproof method for ridding an analog RF transport

system of noise is to reduce cable distance For analog RF

transport in coarse wave division multiplexing (CWDM)

applications, fiber cable distance can be limited to only 2

to 4 miles, which may mean only a mile of straight line

distance Devices such as over the air repeaters are used in

niche applications to increase distance but have significant

limitations For distance, analog RF transport systems just

can’t measure up to digital RF transport systems,

effectively imposing limits on network design and system

performance

Analog RF transport systems do have an array of methods

to compensate for noise and increase distance Automatic

Gain Control devices are essential for analog RF transport

systems, even though there is a limit on the total number

of loss/gain blocks in the system There are devices that

reduce noise by improving linearity and reducing

dispersion Analog lasers, which have a shorter lifetime

than digital lasers, can be replaced more frequently to

keep unwanted RF signal noise in check Yet all of these

work-around measures add complexity and cost to the

system And even with these measures, noise from

splitters, connectors, converters, cables and other sources

still limit the performance of analog RF transport systems

With digital RF transport, distance is virtually unlimited Fiber cable lengths of 12 miles (19.3 km) or

more are possible with no degradation of dynamic range,

no degradation of service, and with less investment in network equipment Figure 5 shows how digital RF transport maintains dynamic range over greater distances

Digital RF transport maintains dynamic range over longer distances, allowing network planners more flexibility in design This enables a centralized radio capacity network architecture, whereby antennas are installed remotely from base stations The distance limitations of analog RF transport systems are due to the basic weakness in all analog RF transport systems—the cumulative effects of noise and loss reduce dynamic range and signal strength.

Figure 5 With 1.5 dB of optical loss per mile and 6 dB CWDM loss, digital

transport of RF signals maintains dynamic range for over 12 miles.

At this rate of optical loss, analog RF transport can only maintain dynamic range for less than 4 miles.

Digital RF transport stands

for flexibility

The relative noise immunity of digital RF transport systems

delivers wider dynamic range, higher SNR, increased signal

output, and virtually unlimited distance—all of which

translates into improved quality of service in terms of

coverage, clarity, and speed Yet the key benefit of digital

RF transport lies in a factor cherished by planners,

engineers, operations personnel, and CFOs alike—

flexibility

Digital transport of RF signals is transparent to modulation

techniques ADC’s patented digital RF transport

technology supports AMPS, TDMA, CDMA and GSM And

this same technology works as networks migrate from

IS-95 to EU-DO In addition, digital RF transport is not

designed for any particular handset, any particular base

transceiver station equipment, or any specific architecture

Digital RF transport works with singlemode fiber,

multimode fiber, millimeter wave connections, and free

space optics Analog systems, by contrast, are limited to

coax or singlemode fiber cables to minimize noise and

signal reflections Choosing digital RF transport is truly a

choice for today and the future

As wireless data rates increase and coverage

areas shrink, digital RF transport offers greater

flexibility For example, base station hotels tap existing

and unused fiber cables in CATV and metro rings to

distribute wireless coverage and capacity, minimizing

capex as well as reducing annual fiber lease costs By using

technologies such as dense wavelength division

multiplexing (DWDM), CWDM, or wavelength division

multiplexing (WDM), base station hotels connect remote

antennas to base stations, mining more bandwidth from

existing spectrum and infrastructure Digital bit streams are

particularly suited

for the task because the digital RF signal and the optics are

totally independent This is not true for analog RF

transport systems where leakage from one channel to

another degrades performance of the RF signal as well as

signals in adjacent channels In fact, network planners will

find that many CATV and metro area network rings are

not set up to handle the effects of analog RF signal

transport on the fiber plant

Digital RF transport offers improved flexibility in design for

indoor applications, too “Home run” fiber cable runs are

often required in analog RF transport systems, significantly

increasing installation costs and inhibiting expansion

capabilities Digital RF transport systems split and add

signals digitally, so the architecture of fiber cable runs can

be optimized for each application, minimizing installation

costs and time

Conclusion Digital transport of RF signals offers wireless service providers an edge that adds flexibility, improves QoS, and reduces capex/opex costs Flexibility in network deployment means networks can evolve as needs change, adding and distributing new radio capacity or moving excess capacity where needed Improved QoS from digital

RF transport reduces churn And most importantly, digital

RF transport significantly reduces capex and opex when deploying networks with centralized radio capacity through base station hotels Digital RF transport gives an edge to a wireless service provider to compete today and tomorrow

1 The Nyquist theorem states that the maximum bandwidth that can be accurately represented is less than one-half of the sampling rate So, to achieve a full 35 MHz bandwidth, sampling must be at least twice as fast—70 MHz, or more.

2 Blocking Dynamic Range (BDR) is the difference between the noise floor and the 1 dB compression point It is typically used to describe receiver (not transmitter) performance, and describes the effect an off channel strong signal will have on a weak signal When a strong signal reaches the 1 dB compression point, it will “block” or “desensitize” the weak signal by 1 dB For this spec to have meaning, the channel bandwidth must be defined Example: if a system has a 10 dB noise figure (a 30 kHz noise floor of -119 dBm) and a level limiting threshold 0f -40 dBm (which compresses gain by 1 dB at -39 dBm), then the blocking dynamic range would be 119-39 = 80 dB This definition can be interpreted as limited

to noise, not spur limited signals.

Dynamic Range of RF Transport

Systems

Digital RF transport systems are immune to the

noise and loss that degrades the performance of

analog transport systems In the above example,

the shaded area shows that acceptable system

operation requires dynamic range between 60 dB

and 80 dB The analog RF transport system starts

off strong with a dynamic range of 80 dB

However, as the analog RF signal traverses

various cables, modules, patch cords, and splices, noise and loss quickly degrade dynamic range to less than 40 dB On the other hand, digital RF transport maintains dynamic range of 70 dB all the way from remote antenna to the base transceiver station

ADC Telecommunications, Inc., P.O Box 1101, Minneapolis, Minnesota USA 55440-1101 Specifications published here are current as of the date of publication of this document Because we are continuously improving our products, ADC reserves the right to change specifications without prior notice At any time, you may verify product specifications by contacting our headquarters office in Minneapolis ADC Telecommunications, Inc views its patent portfolio as an important corporate asset and vigorously enforces its patents Products or features contained herein may be covered by one or more U.S or foreign patents An Equal Opportunity Employer

1239772 8/04 Revision © 2002, 2004 ADC Telecommunications, Inc All Rights Reserved

Web Site: www.adc.com

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Appendix A – Comparing Dynamic Range of RF Transport Systems

Patch cord (0.5 dB opt)

1 dB

Splice (1 dB opt)

2 dB

RF cable

RF cable loss

& NF

7 dB

Remote

Host Unit

antenna

CWDM Module (3 dB opt)

6 dB

80

70

60

50

40

D

N

A

M

I

C

R

N

G

E

(dB)

Analog System

CWDM Module (3 dB opt)

6 dB

Patch cord (0.5 dB opt)

1 dB

Cable (2 dB opt)

4 dB

Cable (2 dB opt)

4 dB

Cable (5 dB opt)

10 dB

Digital System

Acceptable Operation

_ Analog RF transport