Continuing advances in digital signal processing and analog-to-digital converters are putting RTSA within reach of more users

Interference
is becoming more common and more severe in today’s increasingly crowded
wireless spectrum. Given the complex and dynamic nature of many wireless signals,
equally dynamic measurement tools are needed to effectively troubleshoot and
maintain deployed systems.

One such tool is real-time spectrum analyzer
(RTSA) capability, which provides high-speed, gap-free measurements and a
variety of informative display modes. When these functions are added to a
handheld spectrum or combination analyzer, field personnel can use a single
instrument to detect, locate, and resolve issues such as co-channel
interference and uplink interference (Fig.
1
). Continuing advances in digital signal processing (DSP) and
analog-to-digital converters (ADCs) are putting RTSA within reach of more
users.

 

RTSA_1

Fig. 1. With frequency coverage to 50 GHz and
RTSA functionality, advanced handheld analyzers enable efficient troubleshooting
of interference in the field.

Reviewing common interference issues

 

Any
type of interference can have a profound effect on performance metrics such as
quality of service and quality of experience. Fortunately, troubleshooters can
focus on a few types of interference that commonly cause problems in wireless
systems. Relative to signal interaction, the interference may be co-channel
(CCI), adjacent channel (ACI), or intermodulation distortion (IMD). From the
network-operation perspective, there may be interference in the downlink or
uplink between a base station and a mobile unit.

 

Within
a wireless system, interference is often caused by closely situated cell sites and
base stations that are always transmitting. Noise affects downlink connections
to mobile users, leading to increased uplink noise at the base station antenna
and, ultimately, reducing cell-site capacity. The impaired signal-to-interference
ratio shows up in the channel quality indicator (CQI) reports generated by
mobile devices and sent to the base station. Low CQI results in greater
retransmission of data and a decrease in network speed.

 

In
commercial digital wireless networks, external interference is often the result
of the slim frequency guard bands that exist between operators. Also, any
illegal use of the frequency spectrum will exacerbate these problems.

 

Assessing traditional interference
analysis

 

If
interference exists within a network, typical performance-monitoring tools may
report a variety of issues: connection failures, high signal-to-noise ratio
(SNR), and a rising uplink noise floor when traffic is low.

 

The
next step is detecting and locating the source of the problem. The traditional
approach has been to use a directional antenna connected to a spectrum
analyzer, and this works well when the culprit is sending a relatively constant
signal.

 

Detecting
non-constant signals requires use of the “maximum hold” display feature of most
swept-tuned and FFT-based spectrum analyzers. However, because these analyzers often
have large dead times during sweep reset or FFT processing time, max hold can be
time consuming as it gradually accumulates the details in “bursty,” or random-like,
signals. It may also miss exceedingly brief signals.

 

Utilizing gap-free measurements

 

An
RTSA is an FFT analyzer operating in a mode that continuously processes
incoming data, and this solves the problems of analyzer dead time and
random-like signals. The analyzer is set to a start or center frequency of interest
and a frequency span less than or equal to its maximum real-time analysis
bandwidth. With its large buffer memory synchronized with the DSP and display
engines, the analyzer can process and empty the memory fast enough to capture all
incoming data (Fig. 2). The stream of
gap-free measurements enables detection of dynamic transients and narrow pulses
that are within the analyzer’s real-time bandwidth.

RTSA performance is a function of the underlying
ADC and DSP hardware. In current-generation analyzers, maximum real-time bandwidth
ranges from 10 MHz in handheld analyzers to 1 GHz in benchtop signal analyzers.
In field troubleshooting, 10 MHz is often sufficient for most of today’s
over-the-air applications.

 

RTSA_2

Fig. 2. Gap-free RTSA processing uses weighting,
or “window,” functions and overlapping FFT calculations to enhance the
measurement and display of dynamic signals.

Two
more crucial and related characteristics are “100% probability of intercept”
(POI) and minimum detectable signal. POI defines the shortest signal duration that
the analyzer can detect with 100% probability and full amplitude accuracy; the minimum
detectable signal is shorter and is a function of raw ADC and DSP performance.
In handheld analyzers, typical values are 12 µs for POI and 22 ns for minimum
detectable signal.

 

When
chasing interference in the field, the other key specification is spurious-free
dynamic range (SFDR). The better the analyzer’s front-end performance, the more
easily RTSA can discern low-level signals of interest from internal spurious
signals.

 

Mitigating interference problems

 

CCI
refers to any interfering signals that are either at the same frequency as the
serving carrier or are within its channel bandwidth. In digital wireless
systems, the interfering signals have an effect only when synchronized with the
baseband frames.

 

Typically,
CCI has a greater effect on the downlink because the system has no direct
feedback regarding this type of interference. For example, if an interferer
blasts RF energy into the middle of an LTE downlink channel, the mobile unit detects
poor SNR and, in response, transmits more power on the uplink.

 

Detecting
and troubleshooting CCI can be difficult because interferers are often hidden
beneath the carrier signal. When using a traditional spectrum analyzer, the
only reliable way to detect such signals is to shut off the carrier transmitter
— and this is too disruptive to be practical in the field.

With RTSA, the density display is a spectrum
measurement that adds a third dimension — frequency of occurrence — to
frequency and power. The display is color-coded to show the rate of occurrence,
ranging from red (often) to blue (less often). Because interferers differ from
carriers in terms of signal-level distribution, the density display makes it
easier to detect multiple signals in the same channel (Fig. 3).

 

RTSA_3

Fig. 3. Compared to a conventional
spectrum analyzer (left), RTSA and the density display (right) reveal the
presence of a two-way FM signal in the center of a W-CDMA signal.

 

In
broadband systems such as LTE, noise in the uplink limits network capacity and
performance. Because cell sites and mobile devices within a system operate on
the same frequency, controlling all noise — internal and external — to the
system is crucial.

 

With
RTSA, the ability to perform a gap-free capture and then apply the density
display enables the user to find the time signature of a specific signal and see
the statistical distribution of power in the signal. This makes it easier to
separate various signal types, even within the same network.

 

Checking for hardware problems

 

More
than half of interference issues are caused by malfunctioning RF subsystems or
components within a network, and the most common culprits are antennas and
cables.

 

In
an antenna, the key performance parameter is return loss, or voltage standing
wave ratio (VSWR). If a transmit antenna has a decrease in return loss, less
energy will be transmitted to the coverage area. Mobile units respond by increasing
transmit power, and this causes noise to increase at the base-station receivers.
Field personnel may misinterpret this as external interference, sending them on
a wild goose chase. Whenever external interference is suspected, a good first
step is to test (i.e., sweep) the antenna using a suitably equipped handheld
analyzer.

 

Cabling
is exposed to a wide range of environmental forces and, over time, connectors may
corrode and cables can bend. These types of degradations often lead to an
increase in cable loss, reducing the received power level near the cell edge
and causing declining SNR. Performing routine cable-loss measurements relative
to the link budget is a proactive way to avoid interference issues within the
network.

Interference is a symptom of many underlying
problems. Handheld analyzers equipped with RTSA can help engineers and
technicians quickly detect interference issues, locate the cause and, after
implementing the fix, verify system performance.