Reciprocity and differences between uplink and downlink
Two-way communication requires facilities for 'inbound', i.e., mobile-to-fixed, as well as
'outbound', i.e., fixed-to-mobile communication. In circuit-switched mobile communication,
such as cellular telephony, the inbound and outbound channel are also called the 'uplink' and
'downlink', respectively. The propagation aspects described on other pages are valid for inbound and
outbound channels. This is understood from the reciprocity theorem:
If, in a radio
communication link, the role of the receive and transmit antenna are functionally interchanged,
the instantaneous transfer characteristics of the radio channel remain unchanged.
In mobile multi-user networks with fading channels, the reciprocity theorem does not imply that
the inbound channel behaves identically as the outbound channel. Particular differences occur
for a number of link aspects:
- man-made noise levels
The antenna of the base station is usually mounted on an appropriate antenna mast, such that
it does not suffer from attenuation caused by obstacles in its vicinity. The mobile antenna, on
the other hand, is at most mounted a few metres above ground level. The man-made noise level,
particularly automotive ignition noise, is likely to be substantially higher at the mobile antenna
than at the base station antenna.
- effect of antenna diversity
Multipath scatters mostly occur in the immediate vicinity of the mobile antenna. The base
station receives more or less a transversal electromagnetic wave, whereas the mobile station
receives a superposition of a set of reflected waves from random angles. Two antennas at the
mobile terminal are likely to receive uncorrelated signal powers if their separation is more than
a wave length. At the base station site, however, all reflections arrive from almost
identical directions. Therefore, diversity at the base station requires much larger separation of
the antennae to ensure uncorrelated received signal powers at the two antennas. For the
same reason, antenna directivity has different effects at the mobile and the base station.
- correlation of shadow fading of wanted signal and interfering signals
In a cellular network, shadow fading of the wanted signal received by the mobile station is likely
to be correlated with the shadow fading of the interference caused by other base stations, or,
in a spread-spectrum network, with the shadowing of simultaneously transmitted signals
from the same base station. In contrast to this, at the base station, shadow fading of the wanted
signal presumably is mostly statistically independent from shadow fading of the interference.
However, experimental results for correlation of shadow attenuation are scarce.
- full-duplex channels
In full-duplex operation, multipath fading of inbound and outbound channel, which operate at
widely different frequencies, may be uncorrelated. This will particularly be the case if the delay
spread is large.
- multiplexing and multiple access
In a practical multi-user system with intermittent transmissions, inbound messages are sent
via a multiple-access channel, whereas in outbound channel, signals destined for different users
can be multiplexed. In the latter case, the receiver in a mobile station can maintain carrier and
bit synchronisation to the continuous incoming bit stream from the base station, whereas the
receiver in the base station has to acquire synchronisation for each user slot. Moreover, in
packet-switched data networks, the inbound channel has to accept randomly occurring
transmissions by the terminals in the service area. Random-access protocols are required to
organise the data traffic flow in the inbound channel, and access conflicts ('contention') may
occur.
In cellular networks with large traffic loads per base station, spread-spectrum modulation can
be exploited in the downlink to combat multipath fading, whereas in the uplink, the signal
powers from the various mobile subscribers may differ too much to effectively apply spread-
spectrum multiple access unless sophisticated adaptive power control techniques are employed.
- industrial design
From a practical point of view, the downlink and the uplink will be designed under entirely
different (cost) constraints, such as power consumption, size, weight and other ergonomic
aspects, energy radiated into the human body, and consumer cost aspects.
- data traffic patterns
In packet data networks applied for traffic and transportation, the characteristics of the data
traffic flows are known to differ for the uplink and the downlink. For instance, (outbound)
messages from a fleet management centre to the vehicles are likely to be of a more routine type, of
a more uniform length and occur in a more regular pattern than messages in the opposite
(inbound) direction.
