Co-channel and adjacent-channel interference evaluation of an outdoor telecommunications cabinet’s (BS) wireless reporting systems

FACULTY OF ENGINEERING DEPARTMENTS OF ELECTRONICS ENGINEERING AND AUTOMATION ENGINEERING

TECHNOLOGICAL EDUCATIONAL INSTITUTE OF PIRAEUS

Module: CI7150: (Wireless Communications and Networks) Module Coordinator: Dr. Stylianos Savvaidis & Dr. Myridakis Nikolaos

MSc IN NETWORKING AND DATA COMMUNICATIONS

COURSEWORK

MODULE:

CI7150: (Wireless Communications and Networks)

Module Coordinator:

Dr. Stylianos Savvaidis & Dr. Myridakis Nikolaos

Date of Module:

03/01/2015

Name of Student:

Stamatakis Konstantinos

Kingston University London

FACULTY OF ENGINEERING DEPARTMENTS OF ELECTRONICS ENGINEERING AND AUTOMATION ENGINEERING

TECHNOLOGICAL EDUCATIONAL INSTITUTE OF PIRAEUS


Subject: Co-channel and adjacent-channel interference evaluation of an outdoor telecommunications cabinet’s (BS) wireless reporting systems Submission Date: 03/01/2015

Kingston University London

Stamatakis Konstantinos, MSc student in Networking & Data, TEI of Piraeus - Kingston University


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Abstract— The objective of the present paper is to evaluate the

impact of adjacent and co-channel interference on the performance of some standard report systems that exists in an outdoor telecommunications cabinet - base station (BS) (also known as KV, from the acronym of the German word Kabelverzweiger) and devices that technicians frequently use. Specifically, the interference analyzed is between devices, such as Bluetooth handset, laptop and measurement tools, that are usually used during inspections and repairs by technicians and wireless reporting systems installed inside BS that provide information about BS’s condition and real time connection with the help desk back office

Index Terms— Adjacent and Co-channel Interference, Wireless Reporting Systems, Coexistence, telecommunications cabinet base station

I. INTRODUCTION In the past decade wireless technologies have had an amazing growth as they managed to overcome numerous limitations in the fields of speed and quality of service (QoS), providing users the ability of enjoying a variety of wireless technologies required by different applications used in our every day life. This growth made it almost impossible for various sectors of industry to consider their operational status without the existence of such wireless systems and inevitable, many of their services and departments have been entirely replaced or depend, to wireless communication systems. As a result, more applications have been brought up, to enhance numerous sectors of industry, by providing workers and employees with a wide range of tools that help them to perform more efficient at their tasks, reducing the amount of unwanted or unnecessary actions that appear in the absence of wireless communication access. For instance, technologies such as Bluetooth, WiFi, WiMAX, ZigBee, or any other cordless devices, usually operating at the 2.4 GHz ISM band, have been considered to be mandatory in various appliances of

Stamatakis Konstantinos is a student of “MSc in Networking & Data Communications” of TEI of Piraeus and Kingston University (email: [email protected])

industry because of their flexibility, mobility and fast adaptability, in any event which takes place, in order to perform at higher standard, with constant support and real time feedback, increasing not only performance but also contribute to other important factors such as security and employers’ safety. This wireless technology adoption by ISM industries in fields where real time reporting or feedback support from back-office helpdesk is required, performs at the unlicensed 2.4 GHz ISM band of radio frequency, characterized by ITU [1]. With millions of mobile and fixed wireless devices, the major issue that must be dealt with is the interference because of coexistence between all these radio devices operating simultaneous, with minimum possible failure of operational status. Specifically as ITU answers the way 2.4 GHz band is used, “Radio communication services operating within these bands must accept harmful interference, which may be caused by these applications. ISM equipment operating in these bands is subject to the provisions of RR No. 15.13” [1]. In brief, most used protocols for 2.4GHz applications, are WiFi, described by IEEE 802.11 family of protocols, Bluetooth, an industrial protocol with huge commercial success, described by IEEE 802.15.1, ZigBee, described by IEEE 802.15.4-2003 and WiMAX, described accordingly by IEEE 802.16 protocol, with 2.4GHz band included in, after the 802.16-2004 revision update. Interference problems, due to the simultaneous operation of different protocols in 2.4GHz, must be examined and confronted, as the majority of devices operating at 2.4GHz demand connection reliability, adaptability and high data rate. This interference between systems using the 2.4 GHz band is located in three main categories, Intersymbol Interference (ISI), Co-Channel Interference (CCI) and Adjacent Interference (ACI). Concisely, ISI is created due to multipath propagation, while CCI and ACI are detected around the spectrum of frequency over the dimension of time and both are an enormous research field of communication science, thoroughly examined and handled with different techniques such as optimization algorithms, fixed or dynamic channel

Co-channel and adjacent-channel interference evaluation of an outdoor telecommunications cabinet’s (BS) wireless reporting systems




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assignments, coding techniques and optimum engineering improvement, to both physical and upper wireless network layers. Despite all research, there is no final solution for interference problems but all these approaches, combined together, reduce major effect, aiming to evolve to a step further than the traditional best effort approach and correspond to criteria of higher QoS. In our case study, we will examine a real-type scenario of simultaneous use between different wireless technologies, at 2.4 GHz, used by technicians during inspections and repairs inside an outdoor BS, part of the core network of a telecommunication company’s wired backbone MAN that supports the local area, focusing on CCI and ACI.

II. CO-CHANNEL AND ADJACENT INTERFERENCE In general CCI arises when a frequency band is multi-shared among different radios, synchronously through time, due to excessive use of 2.4GHz, false network’s design plan, extreme weather conditions, and finally re-uses of same frequency in cellular networks. Specifically in cellular networks, appears when neighboring cells start using the same frequency at the same time and therefore this re-use decreases one of key elements of the cellular network, the frequency reuse factor (FRF). Along with re-use distance, both determine frequency’s re-use ability, in order to enlarge capacity and space coverage [2][4]. According to Dr. Wajih A. Abu-Al-Saud (2010) and Ajal A. Jose (2013), we realize that in order to increase network’s capacity, FRF must decrease, which inevitable leads to higher CCI as the re-use distance between cells decreases similarly [2][5]. But in this paper’s model scenario, where is examined the behavior at 2.4 GHz, CCI can also be treated with other approaches, such as spread spectrum techniques, optimization algorithms, fixed and/or dynamic channel assignment and power level transmission control, as CCI is more vulnerable to interference from nearby wireless sources that operate in the same or nearby frequency bands [3][4]. To evaluate a system with CCI, all above approaches should be examined, through simulations or real time adjustments to observe which are more successful. Spread spectrum techniques, used in IEEE 802.11 systems and applied with frequency hopping at the PHY, confronts the collision of packets from other networks that operate at 2.4GHz. Furthermore, use of adaptive filtering or spatio-temporal interference mitigation with MIMO antennas and diversity combing techniques also encounter CCI successfully [4][6][7]. Also, MIMO allows exploitation of Beamforming and Transmit Diversity, both being considered very efficient, as Beamforming through dynamic or fixed incorporations of radiation patterns, minimize CCI, whereas Transmit Diversity applies space time encoding with antenna diversity that leads to co-channel reduction [4][7][8].

In addition, the Multiuser Detection technique combined with MIMO for ISM bands, mitigates CCI effects and provides gain, while Spatial Diversity and Space Time Spreading takes advantage of independent fading from the positioning of antennas in the longest possible distance between them, whereas Space Division Multiple Access (SDMA) distinguish users by detecting the unique spatial signature of each one, allowing simultaneous support of multiple users within the same frequency and time [4][9]. Last but not least, as CISCO proposes, a proper positioning of APs avoids CCI, and particular for WLANs at 2.4 GHz, CISCO agrees with the popular recommendation of three-channel setup approach and recommends it, as the most appropriate for 802.11b/g protocols [10]. However, CISCO notes that a four channel installation might turn to a tricky and rather troublesome situation, because as more users are introduced inside the wireless system, the signal of each separate device turn to noise, for one another, because more sideband energy is created, which inevitable leads to poor performance and lower throughput [10]. Messing up APs create disputes between them as they constantly to control frequency, which hurts wireless network’s total performance, as each AP must wait until others stop transmit, or crank up its power output in order to drown interference from neighboring APs and that is the reason for this energy leakage to nearby channels that create interference noise, as any neighboring transmitter is vulnerable at his tolerance band around its channel [10][11]. The phenomenon where interference caused by neighboring or adjacent transmitters’ energy leaks to another channel, on distinct frequency channels, is defined as Adjacent Interference (ACI) [10][11]. When ACI is present, SINR is reduced, therefore number of errors in the receiver is highly increased and when used in 802.11 among non-overlapping channels, ACI becomes a problem. Additionally, due to space constraints, it is even higher in devices equipped with multiple 802.11 radios, as distance between antennas is minimum but as mentioned before, a careful engineering design increases network’s capacity, using overlapping channels, but this can be succeeded only by optimal APs positioning, power transmission check and channel assignment, that is estimated according to spatial spacing among radios installed, PHY modulation, RF band selection, and traffic pattern’s examination. Proper engineering design, also avoids matters like Hidden and Exposed Terminal, who correlate to additional ACI and waste of resources [11][12]. All above conclusions, were based on research usually performed in 802.11b/g protocols, however late 802.11n, brought up lots of improvements in signal filtering, which minimizes leakage of energy to adjacent channels, wider channels use and also less guard carriers which have a positive effect on channel’s orthogonality [12].



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In advance, as Andrzej Zankiewicz determines in “Susceptibility of IEEE 802.11n networks to adjacent-channel interference in the 2.4GHz ISM band” [13], ACI among 802.11 protocols varies and notice that “IEEE 802.11n standard are much more susceptible to a foreign transmission than links which use the older 802.11g or 802.11b standard.” [13]. As regards to the rest 802.11 protocols and experimental comparison between them, Zankiewicz’s results showed that 802.11g remained rather unaffected by an adjacent 802.11n transmission, except when both protocols operated inside the same channel, where 802.11g throughput got poorer by 75%, while was unaffected in cases of direct adjacent channel operation between them [13]. In contrast, Zankiewicz’s observations in relation to 802.11b/n compare, showed that 802.11b throughput got poorer by 50% in every overlapping channel use of an adjacent 802.11n link. Additional, similarly to previous the 802.11g/n comparison, 802.11b was unaffected only when introduced a complete separation into the channel deployment between operational links of two protocols, therefore is advised to use a clear separation and far distance deployment between channels, in both cases, in order to avoid ACI, due to the fact that even for 802.11n protocol, the vulnerability to ACI, when compared to older protocols, is even higher in respect to the importance of signal that is carried through [13].

III. CASE STUDY PRESENTATION This scenario analyzes coexistence of wireless systems

operate inside a KV, as part of a telecommunications company’s core network, during repairs or maintenance by technicians’ crew, through simulation. As any other cabinet of this model company, this BS (KV) provides connection services to a perimeter that covers several city blocks, through copper and fiber cable systems, between last mile connections from all nearby apartment blocks, and buildings and company’s core network, in order to provide services such as phone, xDSL, VoIP, leased lines and other.

Since majority of the last mile connection is copper wired

and still considered as the main transmission medium, it is vital to have accurate report measurements about important factors that influence the network’s quality such as humidity and temperature, in order to step in rapidly, when a problem occurs that may lead to QoS loss. Furthermore, real time report is also essential for security factors, such as BS’s status or clarification of access to different divisions of BS according to technicians’ access rights, as ensures further security and reliability in sectors that require exceptional care, like governmental, military or medical.

IV. CASE STUDY ANALYSIS In the model scenario, BS is using two separate wireless

systems in order to safeguard its QoS and security criteria. First wireless system installed is an 802.11g/n AP that

allows technicians to establish a backup wireless data connection with back-office Helpdesk in cases of emergency, such as mobile signal loss, device’s failure, battery’s discharge, technicians’ vehicle theft or any other worst case scenario that would demand data transmission, back and forth to Helpdesk, in order to send and receive all necessary information about incidents took place and continue any operations left, smoothly. No special criteria required, apart from decent bandwidth and data rate that allow email, VoIP or any other similar data exchange between technicians and Helpdesk, in order to maintain operational job status active until further assistance is provided. Concerns about range or bandwidth, considered limited but it is suggested to expect a future increase in QoS or data rate, therefore selection of equipment must be made with criteria higher than those required.

As device example, Terrawave High-Density 2.4/5 GHz

14dBi Patch Antenna is used, designed for indoor and outdoor applications.

Frequency Range 2400-2500

MHz 5150-5850 MHz

Bandwidth 100 MHz 700 MHz Gain 14 dBi Azimuth Beamwidth 35° Elevation Beamwidth 35° VSWR ≤2 Nominal Impedance 50 Ohm Isolation ≥28 dB Maximum Power 50 W

Tab. 1 TerraWave High-Density Antenna Specifications

As indicated by specifications, this antenna minimizes channel-to-channel interference and features higher gain, without sacrificing network performance, due to its 35-degree beamwidth (Fig. 1) at both frequency bands [14].

Fig. 1 TerraWave High-Density 2.4/5 GHz Antenna

Patterns [14]



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Future increase in QoS or need for frequency change, is considered to be adequate for short-future periods, as it operates in greater, than needed, distance for both bands but also because it is compatible with major manufacturers like CISCO, Motorola and others [14]. More sophisticated solutions, such as LTE Multiband antenna or MIMO, are rejected, as we do not wish to invest further to a secondary backup line due to cost restrictions.

BS’s second wireless system is the most important one, as

monitors crucial factors mentioned before, such as humidity, temperature and light presence inside the cabin, that being considered top priority as BSs serve thousands of users with different standards, from minimum best effort to highest QoS with zero tolerance in network failure. Therefore real time monitoring is vital for this model company, as allows rapid response to any situation occurs by creating an alternative standby task force, that exploits technicians’ constant mobility throughout the area, providing them this real time monitoring of all these factors, as they drive with their vehicles by various BSs that are scattered through any domestic area. Examples that require rapid response are extreme weather condition such as periods of heavy rainfall which influence sensitive to humidity copper wired networks, breach of security which is detected by the presence of light inside a BS without prior update through VPN call or PIN input, possible failure or loss of quality in leased lines service-level agreement (SLA) that require instant repair and many other occasions where security and QoS is at risk.

MeshNet Environmental Temperature, Humidity, Light and Pressure Sensor is used as device example, MN-ENV-THPLR EDS-1068 (Fig. 2), which provide highly accurate humidity and temperatures readings, pulse counter, and a discrete input with user-defined parameters to provide the appropriate responsiveness & battery life. [15][16]

Fig. 2 MN-ENV-THPLR EDS-1068 [15][16]

Frequency Range 2.405 MHz 2.475 MHz Bandwidth 100 MHz 700 MHz Gain 0 dBm Transmission Range Up to 120m Humidity Range 0 – 90 %RH Temperature Range -40 - 85 °C Light Measurement Range 0 – 65535 Lux Pressure Range 300 – 1100 hPa /Millibar Enclosure Dimensions 80x80x21 mm

Tab. 2 MN-ENV-THPLR EDS-1068 Specifications The EDS-1068 automatically transmits whenever any alarm

occurs and its transmission range (under good conditions) at 120 meter, is considered to be more than adequate, as BSs in urban environments are located approximately every two or three domestic blocks. Technicians scattered in the area of interest, combining Star and Mesh network topology, among themselves, sensors and DHCP server, enlarge even further the wireless network with additional transmission routes for sensor data so every BS is monitored.

Fig. 3 MN-ENV-THPLR EDS-1068 DHCP server [15][16] In advance, using internal buffer memory, an average sum

of inside-limit rates about humidity, pressure and temperature, could be stored and transmitted in custom selected periods, giving technicians and Helpdesk the ability to be informed on the average daily status through the DCHP server (Fig. 3), reducing this way the amount of energy used, as less data will be transmitted [15][16].

Third wireless system is the common Bluetooth, used by

technicians as they communicate with clients or Helpdesk and Sony’s Smart Bluetooth® Handset SBH52 is used as example device, which supports Bluetooth 3.0, assuming the average maximum power output estimated approximately at 1.5mW (1.76dB) for EDR and HS [17].



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Fig. 4 Simulink Block Diagram of our BS wireless informative systems interference

V. SIMULATION MODEL ANALYSIS In this section, a simulation of the case study is presented,

using Simulink, a block diagram environment integrated with MATLAB® [18]. As base, Adjacent and Co-Channel Interference model was chosen, presented in examples of MathWorks libraries, which simulates the effects of ACI and CCI on a generic PSK modulated signal [19].

Figure 4 exhibits Simulink block diagram, modified to model’s needs, with necessary changes and updates in order to adapt to case study.

EDS-1068 wireless sensor, is set as the main wireless

transmitter, creating a 16-PSK modulated signal, applied into a square root raised cosine filter, in order to produce the original signal that will be transmitted and interfere with three transmissions, the antenna of our AP, the Bluetooth Headset during a call and AWGN noise, all added by the sum block, in order to produce the final signal that reaches the receiver, where it would be filtered, down sampled, and demodulated.

Bluetooth’s transmitter’s creates an 8DPSK modulated signal, as proposed in Bluetooth v2.0+EDR specifications for 3 Mb/s EDR packets, because 8DPSK allow a symbol rate of three bits per symbol, resulting 3 times as great in the data rate. It must be pointed that this increase has its tradeoff, as the 8-DPSK is considered to be more sensitive to noise, when compared to older modulations [20].

Bluetooth transmission is considered as Interferer 1.

AP’s antenna, uses a 16QAM modulation signal, applied similarly into a square root raised cosine filter that eventually will interference with all transmissions in order to observe the impact to the final signal at the receiver.

AP’s antenna transmission is considered as Interferer 2. Both interference signals have a modifiable frequency offset

and power gain and can be deactivated by choice, as both are active by default.



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VI. PERFORMANCE EVALUATION Performing simulation, examine all transmitted signals

separate and its’ interference with AWGN in the receiver. Initially, using Discrete Time Eye Diagram block, the impact of coexistence, for each transmission separate, is presented in the following diagrams.

Fig. 5 - MN-ENV-TH EDS-1068 Sensor Eye Diagram

Fig. 6 – Access Point’s Eye Diagram

Fig. 7 – Bluetooth’s Eye Diagram

Fig. 8 – Received Signal’s Eye Diagram

From Figures 5 to 8, Distortion and Signal-to-Noise Ratio

(SNR) is significantly increased, as well as the impact in the receiver’s signal. The effect of interference is clearly visible in the eye diagrams generated, as the final received signal suffers from multiple reflections and distortion in time, from both AP’s and Bluetooth’s simultaneous operation.

In order to value further performance of model, additional

presentation of signals’ interference to the spectrum of frequency follows, using signal diagrams and constellations.

Fig. 9 – Access Point’s transmitted signal

Fig. 10 – Bluetooth transmitted signal

Fig. 11 – Sensor’s final transmitted signal with AWGN

Fig. 12 – Received transmitted signal

Fig. 13 – Received Signal Constellation



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Figures 9 to 13 represent how interfering signals alter the initial sensor’s signal, compared to final transmitted signal with AWGN, with the received one, especially at frequencies where AP’s and Bluetooth have its greatest power output.

From the received signal constellation, received data are noticed to be close to ideal constellation locations, therefore concluding that system is adequate for simultaneous coexistence of all three wireless systems.

Decreasing frequency offset of both interfering signals,

spectrum analyzers (Figures 14 and 15) show interfering signals of Bluetooth and AP slowly moving from the adjacent channel into the frequency band of the original sensor signal and eventually start causing CCI, introducing some significant concerns, but not that important to consider our system non functional.

Fig. 14 – Received transmitted signal with decreased

frequency offset

Fig. 15 – Received transmitted signal with decreased

frequency offset

Fig. 16 – Received Signal Constellation with decreased

frequency offset

For additional performance evaluation, examination of BER applied, using Error Rate Display block and Bit Error Rate Analysis Tool (BERTool) [21] in which noticing ACI’s and CCI’s big impact upon.

Fig. 17 – BER comparison (theoretical & simulated)

BER initially is estimated at 0.1827 and compared to three

theoretical BER plots from AP, Bluetooth and Sensor (Figure 17), it’s obvious that corresponds at the beginning with theoretical BER values, while afterwards is slightly reduced but remains constant, as no encoding technique is applied, at a high rate of ≈ 18%.

VII. CONCLUSION In the performance evaluation of the case study scenario,

some important ACI and CCI problems are noticed, due to simultaneous operation of three systems in the same band but despite the appearance of high BER, from examination of signal constellation and diagrams and acknowledging the simultaneous operation of all three wireless systems, as a worst case scenario, the operational status of the system is considered to remain active and functional.

Further improvements that will mitigate ACI and CCI, are

channel assignments, a selection of equipment with higher QoS standards such as coding techniques, lower but more resistant modulation schemes, supplementary hardware or software methods and finally an additional power control through an average estimation of values from sensor’s data that will be transmitted more periodically, allowing BS’s wireless reporting systems to be more stable, as less transmissions are required but also more energy efficient, having less power consumption.



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Engineering

Co-channel and adjacent-channel interference evaluation of an outdoor telecommunications cabinet’s (BS) wireless reporting systems