Link Budget

The establishment of a link budget is one of the first tasks that the RF engineer
needs to perform when beginning the design process. The establishment
of the link budget can only be done after a decision has been made as
to which technology platform(s) to use. When introducing, say, a 2.5G platform
into a 2G system, it will be necessary to have a link budget established
for each of the individual technology platforms involved. In addition, with
the introduction of packet data, the higher data rates have a direct influence
on the range of the site and/or its capacity.
What exactly is a link budget? The link budget is a power budget that is
one of the fundamental elements of a radio system design. The link budget
is the part of the RF system design where all the issues associated with
propagation are included. Simply put, the link budget can either be forward-
or reverse-oriented; it must account for all the gains and losses that
the radio wave will experience as it goes from the transmitter to the
receiver.
The link budget, as it is commonly called, is the primary method that an
RF engineer must first determine in order to ascertain if a valid communication
link can and does exist between the sender and the recipient of the
information content. The link budget, however, incorporates many elements
of the communication path. Unless the actual path loss is measured empirically,
the RF engineer has to estimate or rather predict just how well the RF
path itself will perform. The many elements involved in the communication
path incorporate assumptions made regarding various path impairments.
Figure 9-3 shows which part of the radio communication path the link
budget tries to account for. The link budget has two paths: up-link and
down-link. The up-link path is the path from the subscriber unit to the base
station. The down-link path is the path from the base station to the subscriber unit. Both the up-link path and the down-link path are reciprocal,
provided they are close enough in frequency. However, the actual paths
should be the same, with the exception of a few key elements that are
hardware-related. The actual path loss associated with the path the radio
wave transverses from antenna to antenna is the same whether it is uplink-
or down-link-directed.
The maximum path loss, or limiting path, for any communication system
used determines the effective range of the system.Table 9-1 involves a simplistic
calculation of a link budget associated with a 1G system and is used
for determining which path is the limiting case to design from. In this
example, the receiver sensitivity value has the thermal noise, bandwidth,
and noise figures factored into the final value presented.
The uplink path, defined as mobile to base, is the limiting path case. As
shown in Table 9-1, the talk-back path is 6 dB less than the talk-out path.
The limiting path loss is then used to determine the range for the site using
the propagation model for the network.
However, with the introduction of a 2.5G and/or 3G platform into the
wireless system, the issue of the components with a link budget becomes
more complicated. The complications arise due to differing modulation techniques,
bandwidth as well as process gain, and finally the Eb/No or Carrier
to noise ratio (C/N) values required for a proper Bit error rate (BER) or
Frame error rate (FER) rate. The link budget for both UMTS and
CDMA2000 is included in their respective chapters, Chapter 12, “UMTS
System Design,” and Chapter 13, “CDMA2000 System Design.” Therefore,
the individual link budgets for each technology platform will not be referenced
here; instead, because many issues are associated with either CDMA2000 or UMTS and their associated legacy systems, the fundamental
issues for a link budget will be discussed.
When putting together a link budget for a system, it will be common to
have more than one link budget based on the morphology and, of course, the
technology platform used. However, the morphology variation in the actual
link budget is included in the propagation analysis for the particular site
and is a varying value depending the local particulars. The link budget
itself is an establishment of the maximum path loss; either in the uplink or
downlink, that the signal can attenuate while still meeting the system
design requirements for a quality signal.
When calculating the actual link budget, the items in Tables 9-2 and 9-3
are recommended the be included in the calculation. The items listed
should have more items included in them than what may be utilized in the
physical system being installed. However, the inclusion or exclusion of any
of the items that can impact the link budget is included for reference. It is
also highly possible that other devices can be added in the path to either
enhance or potentially degrade the performance of the network.
Tables 9-2 and 9-3 help define the forward and reverse radio path components
that comprise the forward and reverse link budgets. One final note
on the path is that certain wireless access technologies utilize different
modulation formats on both the uplink and downlink paths. If this is the
case, then some of the reciprocity may not be applicable.
Because the link budget is such an integral part of the RF design process,
the link budget used for the system design needs to be documented and
made available for the design community to utilize.