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

 

 

 

 

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 E_c/I_o (Energy per chip over Interference), or E_b/N_o (Energy per bit over Noise). For instance, adequate voice quality can be sustained for a PCS CDMA system with E_c/I_o = -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. 

 

 

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 3^rd 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

v_o = output signal (what units?)

v_i = 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, a₂v_i², 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

 

 

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

IP3_o = 3rd-order output intercept point, dBm

IP3_i = 3rd-order input intercept point, dBm

G = gain, dB

 

                                                                                                                                            [5]

                where

IMR3 = 3rd-order intermodulation rejection, dB

IP3_o = 3rd-order output intercept point, dBm

P_o = fundamental signal output level, dBm

 

                                                                                                                                                  [6]

                        = 3P_o - 2IP3_o

                where

IM3 = 3rd-order intermodulation signal level, dBm

P_o = fundamental signal output level, dBm

IMR3 = 3rd-order intermodulation rejection, dB

IP3_o = 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, output power is spread over a much larger band than the assigned bandwidth (see Figure 6 below). This out-of-band power is variously referred to as broadband noise, transmitter noise or far-out noise. Filters placed at the output of the transmitter serve to attenuate broadband noise but cannot completely eliminate it. Vertical and horizontal separations are often used to minimize transmitter noise. The portion of residual broadband noise in the receiver’s in-band will enter the receiver and raise its noise floor effectively reducing the receiver’s sensitivity. This noise cannot be filtered at the receiver since it occupies the same frequency band as the receiver’s desired signals.

Figure 6:  Transmitter Broadband Noise

 

The analysis predicts each transmitter’s noise signal level present at the input of each receiver. It takes into account the transmitter’s noise characteristics, frequency separations, power output, transmission line losses, filters, duplexers, combiners, isolators, multi-couplers and other RF devices that are present in both systems. Additionally, the analysis considers the antenna separation space loss, horizontal and vertical gain components of the antennas as well as how they are mounted on the structure. The gain components are derived from the antenna pattern data published by each manufacturer.

Receiver Noise

Receiver Desensitization (also known as receiver overload or receiver overdrive) occurs when the receiver is subject to a strong out-of-band signal that raises the receiver noise floor and may even drive the receiver front-end into saturation. Receivers typically have a wideband frequency response and therefore any signal energy impinging on the receiver can desensitize it - even if it is outside the receiver desired frequency band. These signals are within the transmit band of their transmitters and are therefore not filtered at the transmitter. However, they can be filtered out at the receiver input. Filtering attenuates the signal but does not completely suppress it and the residual energy can still adversely affect the receiver sensitivity. 

 

This analysis predicts each transmitter signal level present at the input of each receiver. It takes into account the receiver filter characteristics, frequency separations, power output, transmission line losses, filters, duplexers, combiners, isolators, multi-couplers and characteristics of other RF devices that are present in both systems. Additionally, the analysis considers the antenna separation space loss, horizontal and vertical gain components of the antennas and how they are mounted on the structure.

 

Harmonics

Transmitter harmonic interference is due to non-linear characteristics in a transmitter. The harmonics are typically created due to frequency multipliers and the non-linear design of the final output stage of the transmitter. If the harmonic signal falls within the pass band of a nearby receiver and the signal level is of sufficient amplitude it can degrade the performance of the receiver. 

 

The analysis takes into account the transmitter harmonic characteristics, transmitter signal output level, transmission line losses, filters, duplexers, combiners, isolators, multi-couplers and other RF devices that are present in each system. Additionally, the analysis considers the antenna separation loss, horizontal and vertical gain components of the antennas as well as how they are mounted on the structure. The gain components are derived from the antenna pattern data published by each manufacturer.

 

Spurious Noise

Transmitter spurious output can be caused by many different elements within a transmitter, including its non-linear characteristics and physical components. If the frequency of the generated spurious signal falls within the pass band of a nearby receiver and the signal level is of sufficient amplitude it may degrade the performance of the receiver. The analysis takes into account the transmitter’s spurious output characteristics, transmitter’s signal output level, transmission line losses, filters, duplexers, combiners, isolators, multi-couplers and other RF devices that are present in each system. Additionally, the analysis considers the antenna separation loss, horizontal and vertical gain components of the antennas as well as how they are mounted on the structure. The gain components are derived from the antenna pattern data published by each manufacturer.

 

RF Calculation, Antenna Isolation & Modeling

Antenna isolation through spatial separation is possibly the most flexible and versatile collocation parameter especially if the collocation interference analysis is being performed at system buildout when equipment modification is a last resort option. This is because unlike other measures antenna isolation combats all types of interference mechanisms and is also independent of the base station equipment.

 

Antenna isolation is primarily a function of the spatial configuration and the mounting structure.   In the case of duplexed antennas, duplexed transmit and receive are isolated by a fixed amount through a duplexer provided in the base station hardware. Assuming omni dipoles, the free space antenna isolation values for a given configuration can be estimated using equations 7, 8 and 9 - approximations commonly used in the industry. For a more rigorous theoretical treatment refer to [i] and [ii].  Needless to say, the antenna types (omni vs panel), their characteristics (downtilt, pattern etc.) and relative configurations together with the mounting structures will influence the isolation values.

The exact value can only be determined through actual field measurements but the equations are useful tools in making preliminary assessments.

 

 

 

 

 

The antenna isolations are given by the following equations[iii]:

                                                                                                                         [7]

                                                                                                    [8]

                                                                                          [9]

where,

·         L_v = vertical isolation (Affanasievⁱⁱⁱ) in dB

·         L_h  = horizontal isolation (Friis transmission formula) in dB

·         L_e  = echelon isolation (linear interpolation between L_v and L_h as a function of angle) in dB

·         h    = vertical tip-to-tip separation - same units as d and l

·         d    = horizontal separation - same units as h and l

·         l   = wave-length - same units as h and d

·         tan^-1 (h/d)^c = arc tan in radians

·         G_t ,G_r   = effective antenna gains in dBi (see figure 7, rated gain is achieved only in the far-field).

 

Figure 7, is an example illustrating the variation of antenna gain with distance from the

 

 

 

 

 

 

 

 

 

	Figure 7: Maximum Effective Gain Verses Distance

 

 

antenna. The measurements were made with a Sinclair SRL-480 10dBd gain antenna at 850 MHz. The rated (constant) full gain of an antenna is only realized in the far-field or Fraunhofer zone. In the near-field or Fresnel zone the gain increases with distance. The boundary between the two regions is at a distance from the antenna given by eqn 10

                                                                                                                                                                                [10]

                where

·         R = distance from the antenna (near-field, far-field boundary)

·         L = length of the antenna

·         l = wavelength

The Solution: AIM Wireless Interference Software

AIM Wireless recognized early on that interference would continue to increase and eventually plague the industry. As such, AIM Wireless is considered a pioneer in the development of interference solutions, developing software since the PCS auctions. Intermod60 is our web-enabled intermodulation tool that helps you identify and eliminate interference in as little as 20 minutes. Service providers and tower owners alike rely on Intermod60 to optimize their network. Intermod60 not only addresses internal interference, but also external interference.

 

Intermod60 has enabled companies to run interference studies painlessly and effortlessly due to the ease of use, comprehensiveness, flexibility and unprecedented speed. A full interference study can be run in 20 minutes or less with as many as 2500 channels considered. As a result, external interference can be eliminated quickly and economically. Although, Intermod60 is designed to optimize the network, it is also to be applied toward cell site design by running what-if scenarios.

 

AIM Wireless has collected field data and found that there are many systems vulnerable to intermodulation interference. In most cases, this vulnerability is site and location dependent. The more transmitters in a certain location, the greater the probability of intermodulation and external interference.

 

Please note the histogram below. It represents the intermodulation resultant power on the PCS-C band at a Northern Virginia site. Spectrum usage was recorded at a typical site and analyzed using Intermod60. The analysis, including data entry took less than 20 minutes. The histogram data represents 3-minutes of continuous and simultaneous signals.

 

 

 

 

 

 

 

 

 

 

In many instances, service providers run into problematic sites where a number of visits have been made and the conventional methods of optimizations have been applied without a subsequent improvement in the performance of these sites. We have found that these sites are suffering from external interference problems rather than internal, which explains the inefficiency of conventional optimization methods. Intermodulation interference is the strongest offender, since it is harder to track and cannot be filtered out. The solutions normally are obtained after a series of analysis including Intermod60 and field visits. We have found that in many cases with a slight re-arrangement of channel use, the performance is greatly improved. This solution is normally the first to be considered since it is the least expensive.

Conclusion

The culprits of interference have changed dramatically in the past several years. Intermodulation, harmonics, transmitter noise, spurious noise and receiver desensitization are now the common causes of interference and cause more harm with the plethora of transmitters. Therefore, new tools and processes are necessary to effectively eliminate these types of interference.

 

 

 

References