Part I:  A Comprehensive Analysis of External Interference

 

 

September 2002

 

AIM Wireless


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Tel(+1) 703 760 7898
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Email: info@aimws.com
Http://www.aimws.com

 

 
 

 

 


 



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 Shannon’s 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

 
 

 

 


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. 

 

Text Box: Figure 2: C/I Verses Throughput

 

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 device’s transfer characteristic (eqn 1).

 

Figure 3: Intermodulation Mechanism

 

Equation 1

                                                                                 [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 device’s 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

 

                                                                                                                                            [5]

                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 other’s 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 hardware’s 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 receiver’s in-band can severely affect the receiver’s 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