Indoor Distributed Antenna System for the University of Baghdad Building

1,2 Laith Awda Kadhim, 3Salih Mohammed Salih,

Senior Member, IEEE 1 Al-Khwarizmi College of Engineering, Mechatronics Engineering Department,

University of Baghdad, Iraq, laithawda@yahoo.com 2 Photonic Technology Lab (PTLab), Centre for Research in Photonics,

University of Ottawa, Canada, lkadhim@uottawa.ca 3 Renewable Energy Research Center, University of Anbar, Iraq, dr_salih_moh@yahoo.com

Abstract

In-building solutions as investigated and implemented in this paper is a way to enable efficient usage of wireless mobile applications within different kinds of structures. This requires that sufficient coverage and capacity with good radio quality to be available inside the buildings. Although the mobile operators will cover most buildings from outdoor sites in their macro network, there is a need to provide many buildings with extended radio coverage and capacity. In-building solutions are well-proven methods for an operator to capture new traffic and new revenue streams. There are several different ways to implement in-building solutions. Dedicated radio base stations (RBSs) that are connected to distributed antenna systems (DASs) are commonly implemented solutions. These solutions provide additional capacity as well as cover all area inside different kinds of buildings. A number of different types of both RBSs and DASs are available and the solutions can be customized for different buildings and needs. In this paper, a new model for improving the performance of indoor distributed antenna systems is proposed and analyzed using passive DAS for the university of Baghdad building. In order to reduce the overall cost, the proposed designed Passive system use ½” RF cable, Omni antenna, panel antenna, splitters and Couplers. The proposed model assumed a “worst case scenario” for the location of the equipment room (Balcony of the 7th floor) which is also used for maintenance.

Keywords: Radio base stations, distributed antenna systems, In-building solutions, received signal strength indicator, tower-mounted amplifier.

1. Introduction

Indoor sites are built to cater capacity and coverage issues in indoor compounds where outdoor macro

site can’t be a good solution [1]. In dense urban clutter where buildings structures and indoor environment losses are quite large for macro site which makes it an inappropriate solution. Generally floors underground (basements and lower ground) have poor received signal strength indicator (RSSI).

Most of the reflections take place from the ground and this portion underground has poor signal coverage. On the other hand, floors above third have quality and dropped-call rate (DCR) issues. Due to fewer obstacles in the LOS path, path losses are less compared to ground floors. So there is a multi-servers environment due to fewer path losses and cells overshooting which leads to ping pong handovers and interference issues inside the compound. In urban areas, there are buildings that generate high traffic loads like commercial buildings, offices; shopping malls may need indoor systems to take care of the traffic demands. For such areas indoor is the efficient solution regarding cost, coverage and capacity. In indoors downlink is the critical link in the air interface. There is no need to use the uplink diversity in an indoor system or use amplifiers like a tower-mounted amplifier (TMA) for improving the uplink signal. Multi-antenna indoor system is providing diversity as uplink signals received by several antennas [2] [3].

In-building solutions distributed antenna system-indoor building system (DAS-IBS) technology [4-10] is one of the fastest changes in mobile network rollouts. It has been estimated that 70-90% of all mobile calls are made inside the buildings; therefore to improve the quality of service (QOS), operators today have started concentrating more on this aspect of network rollouts. The most efficient way to achieve optimal quality, coverage and capacity result inside the building is to use Microcell [11] with DAS-IBS Solution; specifically the solutions of radio network design are needed to enhance QOS and

Indoor Distributed Antenna System for the University of Baghdad Building Laith Awda Kadhim, Salih Mohammed Salih

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capacity of the network. Most of the calls are generated from inside of buildings so it does require special attention for enhancing the network performance.

2. Distributed Antenna System 2.1. System Description

This paper presents a new approach for indoor building solution at the University of Baghdad. The

approach aims to increase the quality of indoor signal at different public and business locations. There are various solutions that can be implemented for a particular site. For a design approach, the most cost-effective solution to meet the performance criteria will be selected. The useful application of antenna’s in indoor systems is the idea of distributed antennas. The philosophy behind this approach is to split the transmitted power among several antenna elements, separated in space so as to provide coverage over the same area as a single antenna, where reduced total power and improved reliability. The advantage of this approach is that the network is simple and requires minimal maintenance. The smaller coverage footprint of each antenna element provides for controlled coverage and reduces excessive interference and spillage effects. A distributed antenna system can be implemented in several ways, and passive network design with optical fiber solution will be used.

The university of Baghdad building consists of 19 floors, ground floor for electrical transfer and basement for mechanical and pump water. One GSM 900 MHz Base station is configured for one sector equipped with two Remote RF Unit (2 RRU) each of two transceiver connected via 30m multimode fiber (MMF) to 7th floor and 70m MMF to 15th floor) for the capacity requirements for the building . Coaxial cable is used from floor 7 to floors 1, 2, 3, 4, 5, 6, 8, 9 and 10 respectively, also from floor 15 to floors 11, 12, 13, 14, 15, 16, 17, 18 and 19 respectively. The proposed model uses one sector configuration for meeting the stipulated coverage and capacity requirements. In order to fulfill the – 60 dBm signal strength for the coverage area, the base station power shall be set at 43 dBm per carrier after all TRXs have been combined. This takes into account current radio environment (interference), future use of dedicated IBS broadcast control channels (BCCH’s) [12] and future possible technology insertion. Due to the fact the we had strong clashes frequencies penetrates the building hence results in bad quality services, the target is set to get the RX signal around -60dBm.

Most antennas used in IBS design are Omni directional and flat panel directional antennas [13] [14]. The selection of antenna types is based on the availability, feasibility; retain ability, compatibility and performance with selected solution. The usage of different type of antennas varies for different physical atmosphere. The antennas are connected with coax feeders inside the building. Omni directional antennas are used to transmit a signal in all direction, it contains low gain horizontal direction pattern all over the place, but vertical direction concentrated. The individual antennas in a distributed system will have different effective isotropic radiated power (EIRP) due to different feeder lengths and losses in power splitters as well as different gain in the antennas. There is an increased risk for disturbances (uplink interference, receiver blocking, intermodulation) if the EIRP differs too much.

The radiated power from the base station antenna (EIRP) equal to [15]:

EIRP = BTS output power at antenna connector − feeder loss − Coupler loss − Splitter loss − Attenuator loss + antenna gain

In addition to the said antennas, distributed antennas connected with couplers using passive coaxial

cable, power splitters, jumpers and feeder cable, link budget [16] calculations based on how many couplers and splitters are used and losses of coupler, splitters and feeder cable length in design.

The total losses and splitter losses as below:

Total loss = distance (m) × attenuation per meter Splitter loss = 10 log (no of ports) + insertion loss

Splitters are used to split antenna feeder network power equally over the output ports. Two-way,

three-way and four-way splitters are generally used. Couplers are used to split antenna feeder power unequally among output ports.

(1)

(2) (3)

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Table 1. System and installation information

Description Details and Reference

Proposed solution Passive Distributed Antenna System Equipment room location In Shelter at 7th floor

RBS Equipment Type distributed base stations (DBS3900) BTS configuration 4 TRX

Type of internal partitioning Wall, Glass, Concrete, Concrete Wall and Ceiling Frequency Band GSM 900

CELL ID 3398-1 and 3398-2 BCCH 121and 111

RBS Configuration 3 Total Number of TRX 6

Number of Floors Building 19 floors +Ground Number of Omni Antenna 70

2.2. Typical Characteristics

The design of the proposed antenna location for the University of Baghdad Building based on the

primary measurements taken inside the university and shown in Figure 1. for ground floor and Figure 2. for other typical floors. The service area for ground floor is (L×W) 10×3=30 m2 and for typical floors are (L×W) 10×10=100 m2, the antenna distributed (2 for ground floor and 4 for typical floors) to cover whole area and to provide good coverage with less number of antennas. The organizing of nodes, are to minimizing the co-channel interference in the network, so by this antenna distribution overall network interference will be minimized.

Figure 1. Ground floor for university tower 30m2. Figure 2. Typical floor for university tower 100m2

After the path loss and link budget calculations the RF plan is made floor by floor on the layout of the

building, antenna, cable lengths and passive elements are drawn accurately according to the plan. Figure 3. shows antenna tree diagram with antenna and cable layout for the university tower that made to have a quick overview of the IBS design. This design use less number of optical fiber and coaxial cables length then less number of feeders, jumpers, connectors, splitters and tappers as shown in Figure 3.

Table 2. shows link budget calculation for the university tower that consider in calculation the losses for feeders, jumpers, connectors, splitters, tappers, EIRP, service area, wall losses, BS power and antenna gain, the calculation as below:

Indoor Distributed Antenna System for the University of Baghdad Building Laith Awda Kadhim, Salih Mohammed Salih

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Total losses (dB) = feeder (1/2") length × feeder losses + feeder (7/8") × feeder losses + (3 x No. of 2-way splitter + 4.8 × No. of 3-way splitter + 6 × No. of 4-way splitter) + No. of connector ×

connector losses + No. of jumper × jumper losses + No. of tapper. Minimum Rx = EIRP − (50 + 0.8 × service area) − margin for wall losses. (5)

Figure 3. Antenna Tree diagram for university tower

(4)

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Table 2. Link budget calculation

Con

figu

rati

on o

f an

ten

na

Fee

der

s 1/

2" (

m)

Jum

per

Con

nect

or

HC

an

d 2

-way

Sp

litt

er

4-w

ay S

pli

tter

Tap

per

EIR

P (

dB

i) 9

00

Serv

ice

Are

a (m

)

Min

imu

m R

x 90

0

F1A1 53 1 14 2 0 5 26.99 10 -54.0

F1A2 69 1 14 2 0 5 25.87 10 -55.1

F2A1 25 2 12 1 1 11 19.75 10 -61.3

F2A2 30 2 12 1 1 11 19.40 10 -61.6

F2A3 35 2 12 1 1 11 19.05 10 -62.0

F2A4 30 2 12 1 1 11 19.40 10 -61.6

F3A1 21 2 10 1 1 10 21.23 10 -59.8

F3A2 26 2 10 1 1 10 20.88 10 -60.1

F3A3 31 2 10 1 1 10 20.53 10 -60.5

F3A4 26 2 10 1 1 10 20.88 10 -60.1

Once the indoor site is implemented the result of coverage and quality measurements based on the system walk test (at busy hour) performance and subscriber perceived quality for IBS sites is needed. The evaluation includes the performance of radio access network with a focus on radio related items such as coverage, radio quality; call accessibility and call retain ability.

The methodology used in this measurement is as follows: One TEMS mobile [17] [18] shall be used to record continuous dedicated mode GSM Voice

Call measurement and idle mode GSM measurement for under antenna measurements. Rx Level Under antenna, Hand Over and Spillage measurements (spill of indoor signal

outside the indoor location). Measurement tools use for measurement is TEMS Light running TEMS Investigation

software. Planed drive tests performed on a regular basis, follow normal traffic routes, cover low signal areas

and high population areas.

3. Simulation Aspects

GSM Rx-Lev and Rx-Qual are used in GSM is a part of the network measurement reports (NMR) [19]. The radio signal strength (Rx-Lev) is measured as Rx-Level and is reported in a range from 0 to 64. The Rx-Level is the signal level over -110 dBm. Rx-Level 30 is: -110 + 30 dB = -80 dBm. The mobile will also measure the radio quality and signal strength of the serving cell, as well as the other adjacent cells (specified by the neighbor list received from the base station controller, BSC). The mobile measures and decodes the base station identity code (BSIC) and reports the strongest cells back to the BSC for handover evaluation/execution.

The radio quality (Rx-Qual or bit error rate) of both the uplink and downlink connection is divided into eight levels and defined as ‘Rx-Qual’ 0–7, 0 being the best quality. Rx-Qual 7 will start a network timer that will terminate the call if the quality does not improve within a preset time. If the Rx-Qual rises above 4 it will degrade the voice quality of a speech call to an extent that is noticeable to the user. A Rx-Qual better than 3 must be striven for when planning indoor solutions; preferably, a Rx-Qual of 0 should

Indoor Distributed Antenna System for the University of Baghdad Building Laith Awda Kadhim, Salih Mohammed Salih

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be insured throughout the building. The main cause of degradation of the Rx-Qual is interference from cells using the same radio frequency, or by too low a reception level of the servicing signal as shown in Table 3.

Table 3. Antenna Rx-Level and Rx-Quality parameters

RX level Strength Color on Screen

-120<x<-95 Poor Red Colour

-95<x<-83 Good Yellow Color

-83<x<-70 Very good Green Light Color

-70<x<-10 Excellent Dark Green Color

RX quality Strength Color on Screen

0<x<4 Excellent Dark Green Color

4<x<5 Good Yellow Color

5<x<7 Poor Red Color

Table 4. Shows statistical and coverage information from walk test for floors (Ground, 2, 18) and ground, the result shows better performance for RX-level and RX-Qual for the 95% of coverage area also Handover and CSSR rate within acceptable range:

Table 4. Statistical and Coverage Information from Walk Test

Items Key performance Indicators (KPI)

RESULT

GSM Ground Floor

RX Level -80 dBm or better

@ 95% Coverage Area 99%

RX Qual 4 or better

@ 95% Committed Coverage Area 98%

Handover Success Rate >=98% 96% Drop Call Rate =< 2% 0%

CSSR Rate >=98% 100% GSM 2nd Floor

RX Level -80 dBm or better

@ 95% Coverage Area 98%

RX Qual 4 or better

@ 95% Committed Coverage Area 99%

Handover Success Rate >=98% 98% Drop Call Rate =< 2% 0%

CSSR Rate >=98% 100% GSM 18th Floor

RX Level -80 dBm or better

@ 95% Coverage Area 100%

RX Qual 4 or better

@ 95% Committed Coverage Area 95%

Handover Success Rate >=98% 98% Drop Call Rate =< 2% 0%

CSSR Rate >=98% 100%

Figure 4. and Figure 5. show the Rx-Lev and Rx-Qual for ground and fourth floor respectively (coverage information from walk test), the result shows better performance for RX-level and RX-Qual (Dark green color) and less performance (light Green color).

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Figure 4. Antenna Rx-Level and Rx-Quality for ground floor

   

Figure 5. Antenna Rx-Level and Rx-Quality for the fourth floor

Figure 6. and Figure 7. showed the Rx-Lev and ERIP for all floors respectively except floor 7 as the equipment room is located there. The values in these figures are different due to a difference in feeder length, losses, and a number of tapper, number of splitters, number of jumpers and number of connectors. The calculation which was given in Table II for a link budget is based on equation (1) and (5) and is shown in Figure 8.

Figure 6. Antenna Rx-Level for all floors

‐65.0

‐60.0

‐55.0

‐50.0

‐45.0

Floor 1

Floor 3

Floor 5

Floor 6

Floor 8

Floor 10

Floor 12

Floor 13

Floor 15

Floor 17

Floor 19

Rx Level

Rx Level

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Figure 7. EIRP for all floors

Figure 8. Total losses for each floor

Figure 9. and Figure 10. show the handover test for the main entrance from indoor to outdoor (Rx-Level=-58 dBm) and from outdoor to indoor (Rx-Level=-38 dBm) respectively that work successfully between cells (antenna cell ID inside the tower is 33981 and 33982) and the calls proceed uninterrupted when a mobile moves from one cell service area to a new one.

As shown in Figure 9. and Figure 10. the Rx-levels inside the entrance floor is acceptable (between -10 dBm and -70 dBm) and at the main entrance room is -58 dBm (because of long distance from indoor antenna and main entrance) and it is within acceptable range also the Rx-level inside the entrance room is -38 dBm (small distance) and it is within acceptable range also.

Figure 9. Handover test for the main entrance from indoor to outdoor

0.00

20.00

40.00

Floor 1

Floor 3

Floor 4

Floor 6

Floor 7

Floor 9

Floor…

Floor…

Floor…

Floor…

Floor…

Floor…

Floor…

ERIP

ERIP

‐80.00

‐60.00

‐40.00

‐20.00

0.00

20.00

40.00

Floor 1

Floor 3

Floor 5

Floor 7

Floor 9

Floor 11

Floor 13

Floor 15

Floor 17

Floor 19 Total Losses

EIRP

Rx Level

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Figure 10. Handover test for the main entrance from outdoor to indoor

Figure 11. and Figure 12. show the spillage test for the main entrance from indoor to outdoor and from outdoor to indoor respectively and as shown in these figures there is no spillage from indoor (cell ID 3398) to outdoor and vice versa, so unnecessary reselections and handovers are avoided.

Figure 11. Spillage test for the main entrance from indoor to outdoor

Figure 12. Spillage test for the main entrance from outdoor to indoor

4. Simulation Results

The performance of the DAS is obtained in terms of carried traffic channel (TCH), call setup success rate (CSSR), TCH call drop rate, TCH congestion and handover success rate (HOSR). Figure 13. shows

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the TCH for channel that contains all the user data for speech and data for two cells ID also showed high traffic in busy hour (09-10 am) and drop in peak amplitude for other hours.

Figure 13. Carried Traffic Channel (TCH)

The call setup success rate (CSSR) in Figure 14. is the fraction of the attempts to make a call that result in a connection to the dialed number (due to various reasons not all call attempts end with a connection to the dialed number). This fraction is, usually, measured as a percentage of all call attempts made. The CSSR can be obtained through traffic measurement and drive tests. The ideal value of CSSR is 1 i.e. the network should be capable of accepting 100 % of the calls made. The formula used for calculating CSSR is:

CSSR = Successful Assignments/Call-related Requests × 100%.

Figure 14. shows accepted CSSR and there is some drop in CSSR Due to low traffic at that time.

Figure 14. Call Setup Success Rate (CSSR)

The formula used for calculating TCH drop rate (that shown in Figure 15.) is:

TCH Call Drop Rate= (TCH call drop/ Successful TCH seizures).

This figure shows some acceptable drop in one day as high traffic at that time and normal for other days.

Figure 15. TCH Call Drop Rate

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The formula used for calculating TCH Congestion (that shown in Figure 16.) is as follows:

TCH Congestion= (Attempted TCH seizures meeting TCH overflow/ Attempted TCH seizures (all)) ×100%.

Figure 16. Shows some congestion in one hour and it is normal for other hours so it meet the target during the whole period under test.

Figure 16. TCH Congestion

The formula for calculating HOSR (that show in Figure 17. and it is a normal result) is:

Handover Success Rate (HOSR) = (Successful intercell handovers +Successful intracell handovers)/(Attempted intercell handovers+ Attempted intracell handovers ) ×100%.

Figure 17. shows handover in some busy hour and it is normal and meet the target for the whole period.

Figure 17. Handover Success Rate (HOSR)

5. Conclusions

Indoor-distributed antenna systems have been proposed for providing coverage and increasing the capacity of indoor wireless communication systems by using multiple antenna elements (access points). The proposed scheme enhances existing system capacity significantly making it cost-effective and at the same time increasing the quality of indoor signal at different public locations. The advantage of this proposal is that the network is simple and requires minimal maintenance. The Received Signal Level (Rx-Lev) for all 19 floors is within accepted range also radio quality (Rx-Qual or bit error rate) for all 19 floors is acceptable also other Key Performance Indicator parameter meet the target and within acceptable levels.

6. Acknowledgment

The authors would like to thank The Islamic Development Bank (IDB), Dr. Trevor Hall and Dr. Sawsan

Abdul –Majid at The Photonic Technology Lab, Centre for Research in Photonics at the University of Ottawa, for their support in completing this work.

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