Part II: Intermod60

A High Speed Intermodulation & Interference Analysis Tool

April 2003

Table of Contents

As telecommunications 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, cellular, PCS and WLAN (Wireless LAN) service providers need a comprehensive interference solution that enables them to quickly and reliably identify and resolve interference.

Voice quality and reliable high data transmission rates are dependent on Carrier/Interference (C/I) ratio and Received Signal Strength Indicator (RSSI) [4, 7, and 8]. 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. The C/I ratio has to be at least 17 dB for quality voice signal [1, 7, 9, 10, 11].

It is a competitive necessity that wireless 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 [2].

While the former is expensive and not feasible for many service providers, the latter is a possible solution for many service providers. In fact, numerous carriers are embracing the latter approach [3, 4]. Specifically, T-Mobile Wireless is overlaying GPRS and eventually EDGE in areas where they will offer high data rate solutions. To transmit at higher modulation or coding scheme, the C/I must be strong (stronger than required for either GPRS or GSM) [5]. 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 [2].

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), TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access) systems Consequently, the reduction of interference level merges as one of the most viable solutions for the service provider networks [12, 13, 14, 15, 16].

As new wireless network technologies emerge and the number of transmitters increases there is a corresponding increase in interference. WiFi and WLAN 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, due to significant interference with existing carrier signals. WLAN, especially within the 802.11b spectrum (~ 2.5 GHz) faces serious interference challenges [17, 21]. Another area where interference is highly sighted as a potential safety and quality problem is the medical RF environment [18, 19, 20].

RF interference 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. One of the major sources of interference is intermodulation [6]. 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 comprehensive interference (intermodulation) studies on a site-by-site basis [2]. As the number of collocated sites increases there is a growing need to understand and analyze all the issues in a unified manner. The main obstacle for this approach is the complexity of the analysis, which grows exponentially with the growth of the signals that could cause interference to occur. Current methods and tools, which are used to conduct interference detection and analysis lack the necessary performance required in the various RF fields [2].

Example:

In order to appreciate the computational complexity involved in the detection and analysis of intermodulation product interference, consider the following example. Assume a tower with four different carriers. The transmission spectrum of each carrier is 10 MHz with 30 KHz channel bandwidth. This configuration results in ~ 1300 transmission signals. Assume further that each carrier has one spectrum range for received signals. If we only consider 3^rd order intermodulation, then there are 5*[(1300*1299*1298)/6] = 1,826,610,500 different intermodulation products of the type (f1± f2 ±f3) that could interfere with 4 receiving frequency ranges. The time and space required to perform the search for all products that cause interference has been prohibitive. The current tools that exist in the market, such as Comsite Pro®[1] [http://www.rcc.com/] and TAP/SoftWright®[2] [http://www.softwright.com/], take more than 24 hours to complete such a task. The space requirement to solve such a problem is in excess of 8 GB of hard disk space (only to store the different product combinations).

The time and space complexity, until now, have been the main cause for not pursuing interference resolutions vigorously by service providers as well as by governmental agencies. Drop calls due to interference continues to be a hard to pursue issue, mainly because of the cost associated with detection and analysis [22]. Note that in order to bypass complex computations, service providers need to spend expensive engineering hours for troubleshooting interference issues.

It is estimated that the annual loss of revenue and engineering cost due to unresolved interference is in excess of $400,000 per site [2] ($300,000 lost revenue due to dropped calls and $100,000 is the average engineering cost for troubleshooting) [2]. Assuming that only 4% of the total number of sites (there are more than 120,000 sites in the US alone [2]) experience problems due o unresolved interference, the service providers’ loss is close to $2 billion. This figure is expected to grow much larger as more sites tend to neighbor one another, more collocation occurs on the same site, and more cellular penetration is achieved.

The need to address interference analysis in a timely manner is motivated by several factors:

The number of service providers continues to grow, due to the increased spectrum licenses auctioned world wide
Tower owners attempt to collocate more carriers on the same tower, for increased revenue. Some towers are already hosting 12 carriers. This phenomenon leads to more interference problems.
Tower farms are becoming more common[3], especially when service providers strive for more coverage along major highways as well as in highly populated areas.
Interference due to intermodulation can cause serious security issues, especially at the time of crisis. Any interference with government related signals during crisis can cause tremendous impact on security
Medical institutions, by and large, continue to be worried that radio interference may jeopardize the safety of people. Lack of comprehensive interference analysis justifies this type of fear.
The growing utilization and use of wireless LAN (WLAN) brings interference problems to the small offices and homes
The high cost associated with unresolved interference problems will contribute to the cost of cellular service for the end users.

With the current technology for intermodulation interference analysis, which is characterized by low performance, it is almost impossible to address the current market needs and requirements. Consequently, new methods and tools for interference detection, analysis, and resolution are necessary for the purpose of cost reduction, fast and reliable networks, and safe and secure environment.

This paper introduces a new intermodulation product interference detection and analysis tool, Intermod60®[4]. The main power of this tool is the speed at which it can analyze and detect all possible intermodulation products that could cause interference. While the complexity of intermodulation grows in a non-linear fashion, the performance of Intermod60® exhibits linear behavior.

Section 2 provides overview of the Intermod60 and its structure. Section 3 provides case studies that illustrate the capability of Intermod60. Performance evaluation is presented in section 4. Concluding remarks are given in section 5.

2. Intermod60®

2.1 Development Background

The idea of developing Intermod60 was motivated by RF engineers working in the area of network optimization for more than 10 years. An RF engineer would spend many hours and quite often days in order to diagnose an external interference related problem. In some cases, the desktop computer would actually crash, due to resource exhaustion (CPU, RAM and disk space). The obvious conclusion was that the industry lacks a high

performance interference analysis tool. The inspection of currently existing tools in the market revealed that the implementation of the SW tools reflects the exact nature of intermodulation interference complexity. Unfortunately, the complexity of intermodulation product analysis has a non-linear (exponential) growth pattern [27]. As such, the SW tools inherited the non-linear behavior of the intermodulation product analysis complexity.

The idea was to look for a different way of representing the intermodulation paradigm. This is a real application software architecture problem. The SW architect, who is specialized in high performance and availability systems [24, 25, 26], investigated the architecture underlying the intermodulation problem. The two main objectives of the architecture of Intermod60 were to reduce the time complexity, and to reduce the memory requirements of the SW system. The result of the investigation was the introduction of new algorithms for the detection of intermodulation products[5].

The algorithms explored some of the mathematical properties of intermodulation products formation. Consequently, the search algorithms were greatly simplified. The complexity of the 2^nd order intermodulation product detection was reduced from O(N²) to O(N.Log(N). The complexity of the 3^rd order intermodulation product detection was

reduced from O(N³) to O(N².Log(N) (Figure 1). The memory requirement of the algorithms during the analysis phase was reduced to O(N) for either 2^nd or 3^rd order. The core engine of Intermod60 is primarily based on these algorithms. Performance evaluation of Intermod60 will be presented in section (4).

2.2 Intermod60 Structure:

The structure of Intermod60 is shown in Figure 2. The core engine of Intermod60 implements the intermodulation algorithms. The engine is designed to be independent of he input or output format, for maximum flexibility. The user interface is designed in a modular manner to allow for all types of interference sources to be analyzed. Currently, the SW tool implements four types of interference sources, namely single tower, multiple towers, rooftop[6], and measured field data.

The single tower, multiple towers, and rooftop cases are used to carry exhaustive search for intermodulation products, used mostly for pre-planning and collocation analysis. The measured field data input allows for data gathered by frequency analyzers to be analyzed for intermodulation hits. Note that in the case of multiple towers and rooftops, the number of collocated carriers can be very large. Consequently, the number of all possible transmitting frequency channels can be very large. Currently, the only tool in the market capable of processing all four input variations is Intermod60. The main reason for this is the time complexity involved in executing rooftop and multiple tower data.

The structure of Intermod60 allows for multiple input sources to be analyzed without having to change or modify the core engine.

Similarly, the output format is independent of either the input or the core engine. Currently, Intermod60 generates output in PDF, HTML, graphics and/or text formats.

The Intermod engine carries all the necessary computation and generates a generic output to the output subsystem, which in turns generates the output reports.

Intermod60 engine analyzes both transmitter and receiver generated IM products. Specifically, it 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.

The modular design of Intermod60 provides the user with flexibility and ease of use. It also provides the software engineers flexibility in the modification, maintenance and upgrade of the system. It is worthwhile noting that Intermod60 is web-enabled tool. In other words, interference analysis can be conducted off the website (provided by AIMWS). This feature is made possible because of the high speed of the tool and the low memory requirements. Multiple studies can be conducted simultaneously on the same server.

In the next section, we will provide real examples, which demonstrate the functionality and the performance of Intermod60

3. Case studies Using Intermod60

3.1 Single Tower Case

Intermod60 interface allows the user to select between single and multiple towers. Most of the required information for conducting the study is available online from several web servers, e.g., the FCC web server, and web servers of various antenna makers. Once the user enters the location of the tower/site in terms of longitude and latitude, Intermod60 will pull most of the required information, such as the carriers’ names, the antenna and filter configurations. The user can modify or add any other relevant information. This process simplifies the task of the user a great deal. In addition, the SW interface verifies the validity of the user-supplied information. The interface automatically generates an input text file to the core engine. Table 1 shows an example of an input file. The data in Table 1 is necessary to calculate the intermodulation hits. The frequencies transmitted by a carrier antenna are determined by the minimum (TX Min), the maximum (TX Max) and the channel increment bandwidth (the last column). The RX_Min and RX_max give the receiving frequency range. For example, carrier 1 transmits at frequencies 850, 850.025, 850.05, 850.075, 850.1, … 865 (a total of 600 channels). Carrier 1 receives signals in the range 806-821 MHz. The intermodulation hits are further evaluated based on the power of the interference generated by the intermodulation products. The data required for this part of the calculation is shown in Table 2. The equations used in the power calculation are found in [2].

Table 1: User Provided Data for the Intermodulation Calculation Phase

Carrier Name[7]	TX Min (MHz)	TX Max (MHz)	RX Min (MHz)	RX Max (MHz)	TXChannel Bandwidith (KhZ)
C- 1	850	865	806	821	25
C- 2	870	880	824	835	30
C- 3	880	890	835	845	30
C- 4	890	891.5	845	846.5	30
C- 5	891.5	894	846.5	849	30
C- 6	902	928	902	928	1200

Table 2: User Provided Data for the Power Calculation Phase

Carrier Name	Height ft	Power Out(W)	Total Coax Loss	Equipment Receive Senseitivy (dBm)	Antenna Length (ft)
C- 1	170	30	-6	-100	6
C-2	105	30	-6	-105	4
C- 3	150	30	-6	-105	6
C-4	105	30	-6	-105	4
C- 5	150	30	-6	-105	6
C-6	125	2.5	-6	-110	8

Intermod60 calculates all the 2^nd and 3^rd order intermodulation products. Fourth and higher order intermodulation products were found to have too little power to cause harmful interference (C/I ratio is not significantly reduced). Consequently, Intermod60 reports only 2^nd and 3rd order intermodulation products with sufficient power to cause a harmful interference. The power level of interference is a user defined optimization parameter. Typical intermodulation products results are shown in Table 3.

Table 3: Sample Intermodulation Output Results

Victim Carrier		1^st Interfering Signal			2^nd Interfering Signal			3^rd Interfering Signal			Interference Power
Name	Frequency	OP	Name	Frequency	Op	Name	Frequency	OP	Name	Frequency	Interference Power
C-1	821	2	C-1	851	-1	C-2	881	0	-	0	-73.937
C-1	820.9	2	C-1	851	-1	C-2	881.1	0	-	0	-73.939
C-3	812.43	1	C-1	851	1	C-1	853.8	-1	C-2	892.37	-75.133
C-3	812.53	1	C-1	851	1	C-1	853.9	-1	C-2	892.37	-75.135

The results in Table 3 show the channel impacted by the intermodulation product (victim carrier). The OP field indicates the intermodulation operation, which results in the IM product (1 means add, -1 means subtract, and 2 means two adds for the same frequency- used for the third order harmonics). For example, the IM product 851+851-881 interferes with 821, which belongs to C-1 (first row). The last column presents the interference power generated by the intermodulation product (-73.937 dB for the first row).

The number of intermodulation products can be extremely large, depending on the number of collocated carriers, and the spectrum utilized by each carrier. Intermod60 provides a summary report, indicating the name of carriers participating in a harmful interference. An example of a summary report is shown in Table 4. The first column shows which carrier is being impacted by interference. The next three columns show the carriers that generate intermodulation hit against the victim carrier. The last column shows the amount of power loss (X) needed in order to avoid interference; X would be reduced to zero once interference is resolved.

Table 4: Example of a Summary Report

Victim Carrier	First Tx Component	Second Tx Component	Third Tx Component	Additional Loss Needed
C-v	C-i	C-j	C-k	X dB

Table 5 shows the final result of all the interference tests performed by Intermod60. Tables (1-5) are included in a detailed (PDF) and (HTML) reports for each interference analysis study.

Table 5: Summary Interference Results

Type of Interference	Status	Noise Above Threshold (dB)
Intermodulation	Failed	14.463
Transmission Noise	Failed	82.57
Receiver Desensitization	Failed	81.16
Transmitter Harmonics	Failed	16.415
Transmitter Spurious	Pass	0

3.2 Multiple Tower Case

Due to increased demand for service coverage in certain areas, the number of towers located in close proximity to one another has increased. It is typical in the industry to refer to towers located in close proximity as a “tower farm”. Each tower hosts one or several service provider carriers. Because the distance between the towers is relatively small, it is possible for intermodulation products to form among carriers on different towers. The intermodulation calculation process is similar to the single tower case, except that the scale of the calculation complexity is far greater than that for single towers. However, the relative distance between the towers has a direct impact on the power level of the intermodulation product. The mathematics for

Part II: Intermod60 A High Speed Intermodulation & Interference Analysis Tool April 2003 Table of Contents

Part II: Intermod60

Intermod60

A High Speed Intermodulation & Interference Analysis Tool