Reconfiguring ERICSSON RBS 884 Microcells for 3G

Implementation

Darrell FergusonLead RF Field EngineerAT&T WirelessSan Francisco Bay Area

Preface

This document is intended as a guide to assist in the reconfiguration of existing ERICSSON 884 Microcells needed for the continued deployment of 3G in the San Francisco Bay Area.

Identifying the need for reconfiguration:

When implementing 3G base station equipment, many times the existing diversity receive antenna and coaxial cable are removed from the 850 MHz Microcell. This depends on the ability to run new coaxial cable for use with 1900 MHz equipment, what the current

site configuration is, and how long the existing coax runs are between the radio equipment and the antennas.

The majority of Microcells requiring reconfiguration are equipped with two omni-directional antennas. One of these antennas are used for both transmit and receive signals. The other antenna is used to provide diversity receive capabilities to the radio equipment. This diversity receive line in many cases will be used exclusively for 3G equipment. For these sites, the existing transmit/receive antenna is upgraded to ensure continued radio performance, but consideration must be given to both the transmit and receive signal paths.

For the transmit path, typically little reconfiguration is required. The receive path however must be modified if the site is to continue to perform as designed. For ERICSSON CMS 8800 radio equipment, a receive signal must be detected at both the “A” and “B” receive ports of each radio. In the case of a reconfigured Microcell, the signal from the diversity receive antenna that has been disconnected needs to be supplied from the primary antenna. In order for this to take place, one of the expansion ports on the “A” side of the Multicoupler is used to supply the receive signal to the input on the “B” side. When this takes place, the receive path must be balanced in order for the radio to perform well. Since the “Exp” port gain exceeds the losses in the Power Splitter in the back plane of the radio cabinet, an attenuator must be used to balance the signal. In this case, an INMET 2AH-1.5dB attenuator is added to the receive path to cancel the gain created by the Multicoupler. Moreover, special attention must be paid when using only one expansion port on the Multicoupler to terminate the unused output. If this does not take place, reflected signals from the un-terminated port can interfere with receive signals on the expansion port in use.

Finally, the receive path for the Microcell must be routed through the combiner filter in such a way that the transmit signals will not introduce unwanted interference in the receive band. While newer combiner filter designs provide internal duplexing, older models do not provide sufficient isolation between paths and an external duplexer must be used. Although these sites typically have duplexers for the existing antenna configuration, a reference is provided for these “COMBFILT” units to ensure the proper configuration is implemented.

Once the necessary changes have been made and verified by testing the radio receiver performance, the secondary antenna can be disconnected from the “B” side receive antenna input for use with the 1900 MHz GSM Microcell Base Station.

Steps for reconfiguration

1. Identify whether or not the site needs to be reconfigured.2. Check the ERICSSON part # on the Combiner Filter to verify the model of

COMBFILT in use

3. If the COMBFILT does not provide sufficient isolation, an external duplexer must be used. These sites should already have a duplexer in use, if not, an external duplexer must be installed between the radio cabinet and the primary antenna feed line.

4. After verifying that the transmit signals are pathed through the “A” antenna, disconnect the receive B antenna feed line from the COMBFILT Unit. Be sure to leave this antenna disconnected for easy identification during 3G implementation.

5. Redistribute the receive path on the primary Multicoupler to feed the B side Multicoupler input from the A side output (either “Exp 1A” or “Exp 2A”). Depending on the configuration of the existing receive path and the # of cabinets in use, the Multicoupler in the second and/or third radio cabinet may also need to be reconfigured.

6. One the receive path has been reconfigured, verify the receive path is balanced in all three cabinets. Using WINFIOL, select a radio in each cabinet for testing. One the radio and it’s associated devices have been blocked, the command MBRMI:CEQ=MBCEQ-1875; will test the radio receive performance for both receive branches. The signal level for the MBRMI test should be between –73.0 and -75.0 dBm +1 dB. If the signal level is not balanced, verify that attenuators are installed on each “Exp” port in use. If only one Exp port is used on a Multicoupler, a 50 ohm termination should be installed on the other Exp port. If the receive signal does not cascade to additional radio cabinets (no Exp port is used), the installation of 50 ohm terminations are not necessary. The MBRMI test must be performed on at least one radio in each cabinet.

7. Once the receive path has been verified and one of the receive paths has been freed, the site is ready for 3G equipment installation.

When reconfiguring the receive path, the B path input to the Multicoupler is supplied from an open A path expansion port. In the picture above, Exp 1A and Exp 1B cascade to the 2nd Microcell cabinet. In this case, the signal from Exp 2A will supply input B.

The A side Expansion port is cabled to the Input B port. A 1.5 dB attenuator is added in line to cancel the Multicoupler gain on the expansion outputs. Once this reconfiguration is complete, the MBRMI command should be used to verify receive path performance in all radio cabinets. Although the site no longer has antenna diversity, by feeding the B path receive branch with the A path signal, overall radio performance is improved.

The Type 0 and Type 1 Combiner Filters (COMBFILT) are nearly identical. Type 0 units are designed for low power applications where the transmit output (ANT A) runs to a multi-carrier power amplifier before it continues to the antenna. In a Type 0 configuration, the “OUT A” signal connects directly to “CAB 1”. Type 1 Combiner Filters are designed to handle higher RF throughput levels than the Type 0 design. Type 1 COMBFILT units should be configured for the amplified power signal from the MCPA to enter the combiner filter on the “CAB 1” port where the output power can be measured by the RFTL before continuing through the “ANT A” port to the antenna. In both the Type 0 and Type 1 configurations, the A side receive signal is supplied via the ANT C connection. For both of these designs, a duplexer must be used to separate the transmit and receive signals since there is inadequate isolation. Notice the diversity antenna receive port (ANT B) does not have “CAB 2”, “FWD B” or “REF B” connections.

The Type 2 COMBFILT design is equipped with internal duplexing. In Type 2 units, the “ANT C” port is eliminated since the “ANT A” port handles both transmit and receive signals. Early Type 2 versions used the same chassis as the Type 0 and Type 1 designs. Later versions use an improved chassis design moving the “ANT A” and “ANT B” ports side by side.

The picture above shows the newer Type 2 design where both the A and B sides have the ability to carry both transmit and receive signals. Notice that the CAB 1 and CAB 2 inputs have N type connections.

ERICSSON 884 Microcell Combiner Filter Reference Chart

The diagram above outlines the transmit path (red) and receive path (blue) using 1 antenna with a Type 0 Combiner Filter. These combiner filters must use an external duplexer. Notice that the A path receive signal is supplied from the ANT C connection.