Base Station Equipment and Radio-Frequency Signal Flow

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Base Station Equipment and Radio-Frequency Signal Flow

Tower Mounting of Omnidirectional Transmit and Receive Antennas

When looking at a base station tower a casual observer would notice two stick-shaped

antennas pointing downward, and one antenna pointing upward. If so, this means that

this is an omnidirectional cell base station.

Receive Antennas

The two antennas that are pointing downward on the base station tower are receiving

antennas (RX 0 and RX 1). These antennas receive RF signals at the base station from

the mobile telephone. They receive the uplink signal-the mobile-to-base signal. The

diversity receive antenna (the second antenna) compensates for Rayleigh fading in the

uplink to the base station. Diversity is a tool used to optimize the signal received by a

base station antenna. It counteracts the negative effects of Rayleigh fading.

The two receive antennas provide for space diversity as follows:

When a wireless customer presses the send button on the phone to place a call, the

signal enters both of the receive antennas, and travels down the coax cable on the tower

into the base station.

At that point, a device known as a comparator in the base station transceiver examines

both of the received signals, and selects the best signal of the two received. The

comparator continues to dynamically select the best of the two receive signals for the

duration of the wireless call. The call can change from being carried by one receive

antenna to being carried by the other receive antenna dozens or hundreds of times

during an average wireless call.

Note: Space diversity can provide anywhere from a 0.5- to 12-dB difference in received

signal strength between one receive antenna (RX 0) and the other receive antenna

(RX 1).

Figure: Tower mounting of omnidirectional antennas (side view).


Transmit Antennas

The antenna that is pointing upward at an omni cell base station tower is the transmit

antenna. This antenna is used to transmit from the cell base station to the mobile phone.

These antennas transmit the downlink signal-the base4omobile signal.

In omnidirectional cells, the reason that the receive antennas point downward and the

transmit antenna points upward is to reduce or eliminate intermodulation interference.

Even though the cellular transmit and receive bands are separated by 45 MH4 having

the omni transmit and receive antennas mounted so they point in different directions

helps to reduce the potential for interference.

Cell Site Configuration

Figure: Typical base station transceiver.

Transceivers (Base Station Radios)

Transceivers, or base station radios, have two receive ports (diverse receive signals) one

transmit port, an audio-in channel, an audio-out" channel, and a data line. The data line

runs the functions of the transceiver as the transceiver communicates to the base station

and the MSC. See figure or a depiction of a typical base station transceiver.

RF Signal Flow through a Cell Site: The Downlink

For illustration purposes, the scenario of a mobile-terminated call will be used here. A

wireless customer is receiving a call. From the PSTN trunk that was assigned by the

MSC, the incoming call is routed to the (land) audio-in" input into a transceiver.


Note: The MSC has already assigned a paired channel and frequencies to this call. The

assigned channel and frequencies will remain the same for the duration of the call, or

until a call handoff is executed.

A power amplifier is used in the downlink signal path to boost the radio signal up to 45

W maximum output. Amplifier signal levels ate measured in decibels. There is one

power amplifier assigned to each channel pail, or transceiver.

If a given cell site has a 16-channel radio bay then there will be four 4-channel

combiners at that cell site. The purpose of the combiner is to allow for the use of one

antenna for multiple radio channels, rather than having one separate antenna dedicated

to each channel at the cell. In other words, without a combiner there would theoretically

be 16 antennas at the cell. Combiners eliminate the cost and poor aesthetics of having to

install a separate antenna and run coax cable down the tower for each and every radio

channel. A by-product of the combiner's call processing is that it reduces the signal level

by half.

Note: A decrease in a power level by one-half means that the overall power has been cut

by 3 dB. Therefore, the signal power at the star connector is now half what it was when

it entered the combiner: It is now 22'/2 W.

Each combiner port must be tuned to the frequency of the radio channels assigned to the

cell by a wireless technician.

A star connector links together all four of the 4-channel combiners.

The RF signal is now routed into a duplexer. The function of the duplexer is to enable

both transmit and receive signals to be routed through the same antenna. The call is then

transmitted out from the cell base station to the mobile telephone at a base station

transmit frequency Duplexers are not always used by all carriers, but they avoid the cost

of having to use two receive antennas instead of one.

Note: The entire process described above goes hand in hand with the actual processing

of a wireless call.

Figure gives a graphical depiction of signal flow over the downlink.


Figure: Downlink signal processing in a typical Nortel AMPS cell site configuration.

RF Signal Flow through a Cell Site: The Uplink

Once an uplink signal is received into the RX 0 and RX 1 antennas at the cell site, it is

transmitted down into the base station into a special band-pass filter The function of this

equipment is to filter out all frequencies except the receive frequency for the specific

channel processing a given wireless call.

The signal is then routed to a low-noise amplifier, or preamp." The purpose of the

preamp is to boost the received signal to a level that's strong enough to be split into 16

outputs. This is necessary because of loss from impedance as the signal flows through

the coax cable. The low-noise amplifier contributes very little noise to the received

signal.

The call is then routed to a device called a multicoupler. The function of the

multicoupler is to split the two received signals independently into 16 coaxial receive

outputs, which are then routed into 16 separate transceivers.

Note: Each transceiver has two receivers: one connected to the RX 0 diversity antenna

and one connected to the RX 1 diversity antenna.


The multicoupler's function is somewhat analogous to the combiner's function in that

both pieces of equipment allow for the consolidation and interleaving of signals.

Both received signals now enter the transceiver The transceiver constantly selects the

best of the two received diversity signals via the comparator, and continues to process

the call Once the transceiver has selected the best receive signal, it then routes the call

to the audio-out output, an& then to the MSC. Once the call reaches the MSC, the

switch completes processing. Least cost routing is taking place at this point.

See Figure for a graphical depiction of signal flow over the uplink.

Figure: Uplink signal processing in a typical Nortel AMPS cell site configuration.

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Base Station Equipment and Radio-Frequency Signal Flow