This paper presents experimental results of a coverage analysis in a WCDMA network using the 850 MHz band; an urban electromagnetic propagation environment is considered. Obtained coverage estimation maps are based on Krige statistical method of interpolation. Key performance parameters used in this work are the CPICH and Ec. The results could be used to identify problems and help to optimize cell coverage and capacity in a given service area.
WCDMA (Wideband Code Division Multiple Access) is a technology of third generation 3G that increases data transmission rates by broadening the signal at the air interface through the Multiple Access Code Division Multiplexing instead of Time Division used by GSM systems. WCDMA supports multimedia services with very high speed and full-motion video, Internet access and videoconferencing. It also easily handles applications that make use intensively of bandwidth such as data and image transmission via Internet. In a communication system based on multiple access, an amount of users want to access the channel simultaneously. To implement a multiple access communication there are several techniques that allow users to share the channel. In the technique of multiple access code division (CDMA), users can access the channel simultaneously and share the same bandwidth. WCDMA is a spread spectrum technology [
Currently, WCDMA is the wireless access technology broadest deployed worldwide. WCDMA is a wide band system where the transmitted signal with a rate R is spread by combining it with a wideband spreading signal, creating a spread signal with a bandwidth W. The effective bandwidth for a WCDMA air interface is 3.84 MHz and with guard bands, as a result the required bandwidth is 5 MHz [
The Base Station (BS), also known as Node B, is part of the Network of Terrestrial Radio Access of UMTS (UTRAN). The Node B takes in charge several tasks such as: transmission and reception of radio signals, signal filtering, amplification, modulation and demodulation of the signal and be an interface to the controller of Radio Network (RNC) [
Common Pilot Channel (CPICH) transmits a carrier used to estimate the channel parameters. It is the physical reference for other channels. It is used for power control, transmission and coherent detection, channel estimation, measurement of adjacent cells and obtaining the scrambling code (Scrambling Code SC) [
The RSCP is the power value after the process of correlation/separation (dBm) and should be measured for each code. Denotes the power measured by a receiver on a physical channel communications in particular. It is used as an indicator of signal strength as a criterion for delivery in the downlink control energy and for calculating path loss. And can only be measured once the receiver has found the dominant pilot.
Interference is typically measured by the energy per chip to total received power (Ec/Io) of the CPICH, in other words, how clear is the signal received. Eb/Io is the ratio of energy per bit to interference after despreading. This value is the one that has direct effects on bit error rate. Ec/Io is only defined by the pilot channel, is measured before the despreading; all channels contribute to the total output of Io. Eb/Io is the energy per bit to noise spectral density, is the signal to (noise + interference) after despreading, from some channels except the pilot. There is a one to one relationship between Ec/Io of the pilot and Eb/No traffic channel. At one point in particular, the value of Ec/Io is almost constant, but the value of Eb/No is constantly changing due to forward link power control. The value of Ec/Io should be –9 dB or higher in 95% of the coverage area, as shown in
Measurements were done, along the test area, (
analyzer with advanced Software profiles to analyze WCDMA/HSDPA signals [5,6]. The measurement capabilities of this unit include Return Loss, Cable Loss, Distance to Fault, two port gain/loss, one port and two ports magnitude and phase, interference analysis, spectrogram, and power meter. The BTS Master can measure Node B performance Over the Air by connecting an antenna or by connecting Node B equipment directly to the BTS Master MT8222A. Measuring a WCDMA signal over the air, the antenna must be connected appropriately to the frequency band to be measured. Among all the parameters measured by the BTS Master MT8222A [5-7], the CPICH (Common Pilot Indicator Channel) was the main focused.
In the UMTS cellular communication system denotes the power measured by a receiver in a specific communication physical channel. It is used as an indicator of signal strength as a criterion for delivery in the downlink control energy and the calculation of loss per path. In WCDMA systems a physical channel corresponds to a particular spreading code, hence the name.
Measurements were done specifically in Channel 1087 of WCDMA Band V of additional channels, using the Spectrum Master MS2721B, GPS options and an omnidirectional antenna in the range of 870-960 MHz [
While measurements were obtained, it was necessary to have geo-referenced information at the point where measurements were made. GPS functionality was activated in the menu “System” of the spectrum analyzer and a GPS antenna was connected [
In the analyzed area (see
This paper shows the measurements for 226, 181 and 246 Scrambling codes. The next section exhibits the results of the measurements described before.
The following maps (Figures 2 and 3) belong to Node B
with SC 226. The Node B is located in the right bottom corner and its SC is 224, 225 and 226. The Antennas height for this Node B is about 18 meters.
Node B is located in the right bottom corner of the Figures 2 and 3; we can see that the strength level of the WCDMA signal decreases when the measurement point is located further from the Node B. The SC 226 is the strongest signal in the studied area.
The higher power levels are identified by the first band of color scale on the bottom of the image-white color. Then the signal strength starts to decrease, and the next level there is a new increase in power levels in a region northwest of the area where the stadium and a wooded area are located, so this increase in power level must be caused by phenomena such as multipath or reflections in the stadium structure which cause this increase in power WCDMA signal. Coverage maps for the Scrambling Code 181 for values of CPICH and Ec associated with the Node B are shown in Figures 4 and 5, respectively. The Node B is located on Wilfrido Massieu Avenue (bottom left of the figures). In this case the attenuation of the signal is observed to be from left to right, going from the lighter shades to darker; Red color identifies the higher attenuation (12 dB at distances no greater than 200 meters)
The Node B, identified as Ticoman, is a node that is located North-East of the area selected, (top-right of the figures). Based on the sector 246, (Scrambling code 246) we were able to get the coverage maps for the CPICH and Ec values shown in Figures 6 and 7, respectively. The Node B is near to the selected area, approximately 500 meters and the observed power levels are defined by the first four colors of power. At the top left of the
Based in the previous sections, we can point out several concerns related to the Ec level, expressed on the Guideline Section [
cept for the white area in the top of
The cell size can be modified depending on the CPICH power level. If the power of the cell is lower, it could be possible that some areas do not have coverage; in an area without coverage, a mobile station is not able to receive and detect a CPICH dominant (fallen calls, bad voice quality and a lower throughput can be the result of this effect). In the other hand, a very high CPICH power level, can provoke that multiple SC can be detected in the Soft Handover area; fallen calls, blocked calls can be experienced for the mobile stations. This type of experimental analysis is important in order to adequately plan the coverage and capacity of a WCDMA network. It should continue to analyze the presence of other SC in the area and
determine the crosstalk between themselves. It should also make an analysis of coverage taking into account the three sectors of base stations nearest to the studied area.