Satellite communication

1. Multiple
Access
Techniques

2. Multiple access means access to a given facility or a resource by multiple users. In the context of satellite
communication, the facility is the transponder and the multiple users are various terrestrial terminals under the
footprint of the satellite. The transponder provides the communication channel(s) that receives the signals
beamed at it via the uplink and then retransmits the same back to Earth for intended users via the downlink.
Multiple users are geographically dispersed and certain specific techniques, to be discussed in this chapter, are
used to allow them a simultaneous access to the satellite’s transponder. The text matter is suitably illustrated
with the help of a large number of problems.
What is Multiple Access?
► Multiple access is a technique whereby a variable number of users can access a common resource for the
purpose of communications.
► Multiple access is employed in most wireless systems, particularly in satellite systems and cellular systems.
► Objective is to achieve higher capacity through sharing of the limited radio spectrum while optimizing the
system performance.
► Similar to multiplexing in a wire-line system.
Multiple Access

3. Introduction to Multiple Access Techniques
Commonly used multiple access techniques include the following:
1. Frequency division multiple access (FDMA)
2. Time division multiple access (TDMA)
3. Code division multiple access (CDMA)
4. Space domain multiple access (SDMA)
Three primary methods:
► Use a unique frequency (FDMA).
► Use a unique time slot (TDMA).
► Use a unique code (CDMA).
Two other variants:
► Orthogonal frequency division multiplexing (OFDM).
► Space division multiple access (SDMA)

4. Frequency Division Multiple Access (FDMA)
► In the case of frequency division multiple access (FDMA), different Earth stations are able to access the total
available bandwidth in the satellite transponder(s) by virtue of their different carrier frequencies, thus
avoiding interference amongst multiple signals.
► The term should not be confused with frequency division multiplexing (FDM), which is the process of grouping
multiple base band signals into a single signal so that it could be transmitted over a single communication
channel without the multiple base band signals interfering with each other.
► Here, multiple base band signals modulate different carrier frequencies called subcarrier frequencies and the
multiplexed signal then modulates a common relatively higher frequency carrier, which then becomes the
signal to be transmitted from the Earth station.
► Similarly, other stations may also have similar frequency division multiplexed signals with a different final
carrier frequency.
► These multiplexed signals, by virtue of their different final carrier frequencies, are able to access the satellite
simultaneously.
► In this type of multiple access, we assign each signal a different type of frequency band (range). So, any two
signals should not have same type of frequency range. Hence, there won’t be any interference between them,
even if we send those signals in one channel.
► One perfect example of this type of access is our radio channels. We can see that each station has been
given a different frequency band in order to operate.

5. Demand Assigned FDMA
► In a demand assigned FDMA system, the transponder frequency is subdivided into a number of channels and the Earth station
is assigned a channel depending upon its request to the control station.
► Demand assignment may be carried out either by using the polling method or by using the random access method.
► In the polling method, the master Earth station continuously polls all of the Earth stations in sequence and if the request is
encountered, frequency slots are assigned to that Earth station which had made the request.
► The polling method introduces delays more so when the number of Earth stations is large.
► In the random access method, the problem of delays does not exist.
► The random access method can be of two types namely the centrally controlled random access method and the distributed
control random access method.
► In the case of centrally controlled random access, the Earth stations make requests through the master Earth station as the
need arises.
► In the case of distributed control random access, the control is exercised at each Earth station.
Pre-assigned FDMA
► In a preassigned FDMA system, the frequency slots are pre-assigned to the Earth stations.
► The slot allocations are pre-determined and do not offer flexibility.
► Hence, some slots may be facing the problem of over-traffic, while other slots are sitting idle.

6. Time Division Multiple Access (TDMA)
► In the case of time division multiple access (TDMA), different Earth stations in the satellite’s footprint make use of the transponder by
using a single carrier on a time division basis. Again it should not be confused with time division multiplexing (TDM), which is the
technique used at a given Earth station to simultaneously transmit digitized versions of multiple base band signals over a common
communication channel by virtue of their separation on the timescale. The composite time multiplexed signal modulates a high
frequency carrier using any of the digital carrier modulation techniques. Multiple time multiplexed signals from other stations having the
same carrier frequency are then able to access the satellite by allowing each station to transmit during its allotted time slot.
► As the name suggests, TDMA is a time based access. Here, we give certain time frame to each channel. Within that time frame, the
channel can access the entire spectrum bandwidth.
► Each station got a fixed length or slot. The slots, which are unused will remain in idle stage.
► Suppose, we want to send five packets of data to a particular channel in TDMA technique. So, we should assign them certain time slots
or time frame within which it can access the entire bandwidth.
► In above figure, packets 1, 3 and 4 are active, which transmits data. Whereas, packets 2 and 5 are idle because of their non-
participation. This format gets repeated every time we assign bandwidth to that particular channel.
► Although, we have assigned certain time slots to a particular channel but it can also be changed depending upon the load bearing
capacity. That means, if a channel is transmitting heavier loads, then it can be assigned a bigger time slot than the channel which is
transmitting lighter loads. This is the biggest advantage of TDMA over FDMA. Another advantage of TDMA is that the power
consumption will be very low.

7. ► As outlined earlier, time division multiple access (TDMA) is a technique in which
different Earth stations in the satellite footprint having a common satellite
transponder use a single carrier on a time division basis.
► Different Earth stations transmit traffic bursts in a period time-frame called the
TDMA frame.
► Over the length of a burst, each Earth station has the entire transponder
bandwidth at its disposal.
► The traffic bursts from different Earth stations are synchronized so that all
bursts arriving at the transponder are closely spaced but do not overlap.
► The transponder works on a single burst at a time and retransmits back to Earth
a sequence of bursts.
► All Earth stations can receive the entire sequence and extract the signal of their
interest. Figure 6.5 illustrates the basic concept of TDMA.
► The disadvantages of TDMA include a requirement for complex and expensive
Earth station equipment and stringent timing and synchronization requirements.
► TDMA is suitable for digital transmission only.
► In preassigned TDMA systems, every Earth station is allotted a specific time slot.
► In a demand assigned TDMA system, the time slots are allotted to the Earth
stations on request from the control station.
► In satellite switched TDMA systems, several antenna spot beams are utilized to
provide services to different regions on the Earths surface.
► Limited preassigned TDMA is a technique that allows the traffic to be handled
during busy hours by demand.

8. Reference Burst
► The reference burst is usually a combination of two reference bursts (RB-1 and RB-2).
► The primary reference burst, which can be either RB-1 or RB-2, is transmitted by one of the stations, called the primary
reference station, in the network.
► The secondary reference burst, which is RB-1 if the primary reference burst is RB-2 and RB-2 if the primary reference burst is
RB-1, is transmitted by another station, called the secondary reference station, in the network.
► The reference burst automatically switches over to the secondary reference burst in the event of primary reference station’s
failure to provide reference burst to the TDMA network.
► The reference burst does not carry any traffic information and is used to provide timing references to various stations
accessing the TDMA transponder.
Traffic Burst
► Different stations accessing the satellite transponder may transmit one or more traffic bursts per TDMA frame and position
them anywhere in the frame according to a burst time plan that coordinates traffic between various stations.
► The timing reference for the location of the traffic burst is taken from the time of occurrence of the primary reference burst.
► With this reference, a station can locate and then extract the traffic burst or portions of traffic bursts intended for it.
► The reference burst also provides timing references to the stations for transmitting their traffic bursts so as to ensure that they
arrive at the satellite transponder within their designated positions in the TDMA frame.
Guard Time
► Different bursts are separated from each other by a short guard time, which ensures that the bursts from different stations
accessing the satellite transponder do not overlap.
► This guard time should be long enough to allow for differences in transmit timing inaccuracies and also for differences in range
rate variations of the satellite.

9. Code Division Multiple Access (CDMA)
► In the case of code division multiple access (CDMA), the entire bandwidth of the transponder is used
simultaneously by multiple Earth stations at all times. Each transmitter spreads its signal over the entire
bandwidth, which is much wider than that required by the signal otherwise. One of the ways of doing this is by
multiplying the information signal by a pseudorandom bit sequence. Interference is avoided as each transmitter
uses a unique code sequence. Receiving stations recover the desired information by using a matched decoder
that works on the same unique code sequence used during transmission.
► In CDMA technique, a unique code has been assigned to each channel to distinguish from each other. A
perfect example of this type of multiple access is our cellular system. We can see that no two persons’ mobile
number match with each other although they are same X or Y mobile service providing company’s customers
using the same bandwidth.
► In CDMA process, we do the decoding of inner product of the encoded signal and chipping sequence. Therefore,
mathematically it can be written as
► The basic advantage of this type of multiple access is that it allows all users to coexist and use the entire
bandwidth at the same time. Since each user has different code, there won’t be any interference.
► In this technique, a number of stations can have number of channels unlike FDMA and TDMA. The best part of
this technique is that each station can use the entire spectrum at all time.

10. Space Division Multiple Access (SDMA)
► Space domain multiple access (SDMA) uses spatial separation where different
antenna beam polarizations can be used to avoid interference between multiple
transmissions. Beams with horizontal and vertical or right-hand circular and left-
hand circular polarizations may be used for the purpose. Use of the SDMA
technique on board a single satellite platform to cover the same Earth surface
area with multiple beams having different polarizations allows for frequency re-
use. In the overall satellite link, SDMA is usually achieved in conjunction with
other types of multiple access techniques such as FDMA, TDMA and CDMA.
► Omni-directional communication space is divided into spatially separable sectors.
► BS uses smart antennas, allowing multiple MSs to use the same channel
simultaneously.
► The communication characterized by either time slot, carrier frequency or
spreading code can be used.
► Use of smart antenna maximizes the antenna gain in the desired direction and
directing antenna gain in particular direction leads to range extension, which
reduces the number of cells required to cover a given area.
► Focused transmission (spot beams) reduces the interference from undesired
directions by placing minimum radiation patterns in the direction of interferers.
► Spot beams enhance the quality of the communication link significantly and
increase overall system capacity.

11. Comparison of Multiple Access Methods