Part
I: A Comprehensive Analysis
of External Interference
September
2002
AIM
Wireless
8000
Towers Crescent Drive
Suite 1350
Vienna
VA 22182
Tel(+1) 703
760 7898
Fax(+1) 703 760 7899
Email: info@aimws.com
Http://www.aimws.com
|
Part
I: A Comprehensive Analysis
of External Interference September
2002 |
|
AIM
Wireless
|
Table
of Contents
Overview
.
.
3
Background
.
4
Data
Transmission
.
.
.
4
Interference
is on the Rise
5
Types
of Interference
..
..
.
6
Intermodulation
6
Transmitter
Noise
.
10
Receiver
Noise
11
Harmonics
.11
Spurious
Noise
..
.12
RF
Calculation, Antenna Isolation and
Modeling
12
The
Solution: AIM Wireless Interference
Software
..
15
Conclusion
.
16
References
.
17
Overview
As
service providers begin transmitting voice and data over the network to meet the
demands of enterprise customers for dispersed information, creating an
interference-free network has become imperative. To meet customer requirements,
service providers need a comprehensive interference solution that enables them
to quickly and reliably identify and resolve interference. The AIM Interference
Management System (IMS) and AIM Intermod60 meet those
needs through web-enabled solutions that provide full analysis of various types
of interference in as little as 20 minutes. IMS and Intermod60 represent a
dramatic shift in the way interference issues are identified and resolved,
providing a comprehensive family of interference products backed by in-depth
engineering knowledge and services. With IMS and Intermod60, AIM Wireless is
helping service providers rapidly identify and resolve all types of interference
and better achieve market success.
This paper outlines the
various types of interference and provides detailed calculations, assessing the
negative impact caused by each type of interference. It is the first in a two
part series of white papers, where Part Two provides information on how the
AIM Interference Management System and AIM Intermod60 help you to identify and
resolve even the most challenging types of interference.
Background
Voice quality and reliable,
high data transmission rates are dependent on Carrier/Interference (C/I) ratio
and Received Signal Strength Indicator (RSSI). In CDMA and similar systems, C/I
can be translated into Ec/Io (Energy per chip over
Interference), or Eb/No (Energy per bit over Noise). For
instance, adequate voice quality can be sustained for a PCS CDMA system with
Ec/Io = -10 dB and RSSI = -90 dBm. However, it is common
for an adequate signal to be present and still have poor voice quality and low
data transmission rates. When this occurs, it is often due to the presence of
external interference, resulting in reduced C/I. According to Shannons
telecommunication theory, spectrum utilization efficiency is less than 75% as a
consequence.
Figure 1 illustrates the
ideal noise floor and the real noise floor that result from internal and
external interference.
A
I M
Wireless Enabling a
Wireless World
Figure 1: Rise in Noise
Floor
|
Figure 1: Rise in Noise Floor |
Data Transmission
It is a competitive necessity that service providers deliver reliable, high-speed data if they are to make a successful foray into the enterprise market fast, reliable access to information and communication is mission critical. Delivery of higher data transmission rates requires the use of either a wider spectrum or a different modulation and coding scheme and packet data techniques. While the former is expensive and not feasible for many service providers, the latter is a possible solution for many carriers. In fact, numerous carriers are embracing this approach. Specifically, AT&T Wireless is overlaying GPRS and eventually EDGE in areas where they will offer high data rate solutions. GPRS and EDGE employ capacity on demand whereby more than 1 GSM time-slot may be combined and different modulation schemes may be used concurrently in the system. GPRS is dependent on the ratio of C/I and received signal level, which contributes directly to the receiver sensitivity threshold. EDGE utilizes the 8PSK modulation scheme. The major difference between EDGE and GPRS is the number of bits per transmission or symbol rate. 8PSK is capable of transmitting at 3 times the data rates of GPRS or GSM. To transmit at higher modulation or coding scheme, the C/I must be strong (stronger than required for either GPRS or GSM). Given the fact that most wireless networks are up-link limited, the C/I ratio can be increased by building more sites, reducing the interference level or both.
The buildout of new sites is expensive and often presents new challenges. Specifically, additional sites result in more signals contributing to a rise in the spectrum noise floor and limiting frequency re-use in FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) systems.
Figure 2 below shows the relationship in typical GPRS/EDGE systems between the C/I and the throughput rate per time slot for different convolution code rates (R). Note that as throughput increases so does the C/I requirement.
New methods and tools that contribute to improved levels of spectrum optimization through reduced interference are highly effective solutions for a healthy network.
Interference is on the Rise
As new wireless network technologies emerge and the number of transmitters increase there is a corresponding increase in interference. WiFi is a technology with a growing presence in the high-data rate space. WiFi not only poses a threat to service providers market share, but it also threatens the very quality of voice and data throughput on providers networks. AIM Wireless has conducted intermodulation analysis using AIM Intermod60 and discovered that WiFi contributes transmitting components generating 3rd order intermodulation products on both the Cellular and PCS frequencies. Obviously each product must be analyzed separately to determine whether the product is harmful. Nonetheless, their presence serves to add additional network optimization challenges. Also, of particular concern to wireless carriers is the fact that WiFi is being deployed rapidly in many parts of the country and antennas are being located on towers and rooftops that host wireless voice and data equipment.
Types of Interference
RF interface can occur in any radio system. Interference is dependent on interactions within the systems that are in turn dependent on the system equipment, system design parameters and site antenna configurations.
When collocation
occurs this further complicates interference analysis and control. For a single
system installation the interference analysis and control is comparatively
simple since site equipment and antenna configurations can be restricted to
tried and tested set-ups. For
multiple collocated systems the scenarios are too complex to predict and other
than providing broad guidelines for collocation there is no option but to
perform the interference studies on a site-by-site basis. As the number of
collocated sites increase there is a growing need to understand and analyze all
the issues in a unified manner. The AIM Intermod60 solution
helps you effectively identify
and resolve intermodulation in single site and multiple collocated systems in
as little as 20 minutes.
Intermodulation
Intermodulation distortion (IMD) is the term given to the phenomenon by which signals present in a non-linear device combine to create new signals. To illustrate the mechanism, consider the non-linear device in Figure 3 below. The device is fed with two sinusoidal input signals (eqn.2) and the output is given by the devices transfer characteristic (eqn 1).
Figure 3: Intermodulation
Mechanism
Equation 1
where
vo =
output signal (what units?)
vi =
input signal
a0,
a1, a2, a3 =
constants
Equation 2
[2]
The first two terms of
the transfer characteristic represent the linear response while the third and
higher terms represent the nonlinear response of the device. These terms contain
the distortion products and are referred to as the nth-order products. For
example, the second-order products of the square-law term,
a2vi2, consist of a dc term, second harmonics
of the input frequencies and intermodulation products:
[3]
Similarly, the cube-law term gives rise to the third-order products including third-order intermodulation (IM) products. Table 1 summarizes the second and third order products.
Table 1: Intermodulation
Product
|
IM Products |
Frequency |
Amplitude |
|
2nd-Order |
f1 + f2 |
a2 A1 A2 |
|
|
f1 - f2 |
a2
A1 A2 |
|
3rd-Order |
2f1 + f2 |
(0.75)a3 A12 A2 |
|
|
2f1 - f2 |
(0.75)a3 A12 A2 |
|
|
2f2 + f1 |
(0.75)a3 A1 A22 |
|
|
2f2 - f1 |
(0.75)a3 A1 A22 |
The third-order IM products are the most severe in terms of interference and therefore the most analyzed. The relationship of the fundamental and third order IM products can be seen in Figure 4 below. Amplitude of the IM products is a function of the amplitudes of the fundamental (or component) signal levels. Assuming that the fundamental signal magnitudes are equal it can be shown that for each dB change in the fundamental signal level the third-order IM level will change by 3 dB.
Figure 4: Fundamental Frequencies and
Intermodulation Products
This is best seen using an
intercept diagram and the concept of the third-order intercept point (IP3) as
shown in Figure 5. 
The third-order intercept diagram consists of
plots of the fundamental and the third-order IM signal levels plotted against
the output or the input levels of the fundamental signal. Please note that all
signal levels are specified using the log
scale.
Figure 5: IP3 Intercept
Point
The slope of the 3rd-order IM is 3 times that of the fundamental signal.
Intermodulation Rejection (IMR) is the difference between the fundamental and IM signal levels. The intercept point, IP3 (obtained by extrapolation of both lines) is the point at which the IM level is equal to the fundamental level. It is a measure of the devices non-linearity.
Actual device operation does not
extend to the intercept point but is limited to the small-signal region below
the 1 dB compression point and well below the intercept point. Therefore, given
IP3 and the fundamental signal level it is possible to determine the
corresponding IM level:
[4]
where
IP3o =
3rd-order output intercept point, dBm
IP3i =
3rd-order input intercept point, dBm
G = gain,
dB
where
IMR3 = 3rd-order
intermodulation rejection, dB
IP3o =
3rd-order output intercept point, dBm
Po =
fundamental signal output level, dBm
[6]
=
3Po - 2IP3o
where
IM3 = 3rd-order
intermodulation signal level, dBm
Po =
fundamental signal output level, dBm
IMR3 = 3rd-order
intermodulation rejection, dB
IP3o =
3rd-order output intercept point, dBm
For the purposes of this paper, it suffices that IM signals are generated due to mixing of two or more signals in a non-linear operation. The multiple transmit signals at a site can leak into each others paths and mix in the base station hardware. The frequencies and power levels of the IM products are a function of the mixing signals and the hardwares transfer characteristics. IM generation can occur at any of several points along the transmit and receive signal paths, but keeping in mind the scope of this study, the primary sources of IM are the transmit amplifiers, receiver front-ends and antennas. IM frequencies that fall within a collocated receivers in-band can severely affect the receivers sensitivity. Reducing IM interference requires controlling the level of the mixing signals and their frequencies.
AIM Intermod60 analyzes both transmitter and receiver generated IM products. Specifically, Intermod60 analyzes worse case scenarios by calculating the IM products at the receiver antenna, examining each channel licensed to the carriers in the area. In addition to IM, AIM Intermod60 analyzes transmitter noise, receiver desensitization noise, spurious noise and transmitter harmonics noise.
Transmitter Noise
The power output from a transmitter should ideally lie completely within the assigned frequency band without any splatter into the adjoining frequencies. In reality however, outp