Multiple Access. Difference between Multiplexing and Multiple Access

Transcription

1 Multiple Access (MA) Satellite transponders are wide bandwidth devices with bandwidths standard bandwidth of around 35 MHz to 7 MHz. A satellite transponder is rarely used fully by a single user (for example a single TV channel, a single stream of data,.). So, often several users have to access the same satellite transponder and have to avoid causing interference to each other. It is important to note that a signal cannot be split between different satellite transponders since there is a guard band between the band of different transponders. If the band of a signal is placed such that it occupies parts of the band of two consecutive transponders, part of the band of the signal that falls on the guard band between the two transponder bands will be lost. An Earth station can access a satellite in different formats. Some satellite applications require an Earth station to access a satellite in what is know as a fixed access while some other applications access the satellite in a demand based access: Fixed Access (FA): in FA, each user has a constant amount of bandwidth or data rate on the satellite over which a fixed amount of data can be transmitted the satellite regardless of the utilization of the satellite by other users, Demand Access (DA): in DA, a user will be assigned a varying bandwidth (or variable data rate) in which he can transmit varying amounts of data based on his demand. Difference between Multiplexing and Multiple Access In the context of satellites the difference between multiplexing and multiple access can be summarized as follows: Multiplexing: happens when several signals to be uplinked to a single transponder of a satellite are combined together at a single Earth station before being uplinked and then transmitted to the satellite as a single signal. Multiple Access: happens when different signals are uplinked to the same transponder of a satellite from different Earth stations separately. The following table summarized basic differences and types of multiplexing and multiple access: Difference Multiplexing Signals are combined at a single Earth station and then transmitted as one signal There is no possibility that a transmitted channel will damage other channels because they are Multiple Access Signals are transmitted from different Earth stations separately There is always the possibility of one of the transmitted channels interfering and causing damage to other transmitted signals

2 Types transmitted from the same Earth station. Time Division Multiplexing (TDM) Frequency Division Multiplexing (FDM) Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) Code Division Multiple Access (CDMA) o Direct Sequence (DS) o Frequency Hopping (FH) Space Division Multiple Access (SDMA) There are several methods by which multiple users can access a satellite transponder and have their transmission not interfere with each other. The following discussion illustrated the main type of multiple access (MA) used with satellite:. Frequency Division Multiple Access (FDMA) In this multiple access method, different users are assigned different frequency bands within the band of a single transponder that do not overlap in frequency. Basically, user will be assigned frequency band, user 2 will be assigned frequency band 2, and so on until the whole band of transponder is occupied. All users transmit at all time and none of them uses a code to distinguish his transmission from other users transmission (all users use the same code, which you will understand later). This can best be illustrated in the following form:

3 Note that the bands assigned to different users need not be of the same width as some of them may be wider than others. Often, a satellite TV broadcasting station that rents a band on a specific satellite owns several stations, in which all of the stations of that station are multiplexed together at the uplink site and are uplinked all as a single signal called Multiple Channels per Carrier (MCPC). Such transmission would usually have a bandwidth that is several times the width of a single TV channel (have you noticed that when setting the channels on a satellite receiver, the receiver may lock to a single carrier and show you a list of several channels associated with that single carrier frequency?). If a satellite TV broadcasting station is renting bandwidth on a satellite transponder for a single channel, that station will uplink its single channel signal called (Single Channel per Carrier (SCPC). The band of signals of different users appears as follows: A guard band is kept empty between the different bands assigned to different users to allow the use of real filters to extract the different channels. The width of the guard band is always selected such that practical real filters of reasonable complexity are used to extract the different channels. Main disadvantage of FDMA is that it may cause saturation of trnsponder High Power Amplifier (this will be seen in the example at the bottom). Since several users are sharing the same transponder and their signals get added at each time instant, when one ore more of the input signals to a specific transponder exceeds the power at which it is supposed to be transmitted (possibly at specific time instants), the addition of all input signals to the transponder may exceed the maximum allowed input, which may result in high power amplifer saturation. Satruation of the high power amplifier cause intermodulation, which reduces the C/N ratio of the transmitted signals. 2. Time Division Multiple Access (TDMA)

4 In TDMA, each group of users are assigned a single frequency band so they transmit at the same frequency and all of them use the same code for transmission, and yet, their signals do not interfere with each others because different users transmit at different time slots. This is illustrated in the following figure: In TDMA, the time is divided into small time slots that are assigned to the different users such that all users are using the same frequency band for transmission. For example, if N users are to share a single frequency band for transmission, the time is sliced into N time slots, one slot for each user. Once the last Nth user finishes transmission in the assigned time slot, the process is repeated again and again. The period in which all users are assigned one time transmission time slot is called a frame, and it contains N time slots. Usually, a small guard time is left between the different time slots used by the different users to prevent different transmissions from different users from mixing with each other, especially if the different users are transmitting from locations with different distances to the receiver (in our case that would be different Earth stations that are located at different geographical regions transmitting to the same satellite). This is seen clearly in satellite TDMA transmission where the difference in distance that different signals have to travel over until reaching the satellite may be several thousands of kilometers, resulting in delays of. seconds or possibly more. Therefore, this guard time helps in avoiding the overlap of the different transmissions of different users. An example of a satellite system that uses TDMA transmission algorithm is called Very Small Aperture Antenna (VSAT) system where several users share a single channel with a specific frequency band. This is illustrated in the following figure:

5 Although it appears that the different slots at which different transmitters transmit need not be separated (i.e., no need for guard times between time slots of different users), but the reality is that these time slots are very important to avoid the different transmissions of the different users from overlapping with each other in time. Consider two users (User and User 2) who are located at different distances from a common receiver. This is the situation that occurs in a cellular phone system when one user (User ) is located very far away from the cellular tower at the edge of a cell (which could be km or more away from that tower) while the second user (User 2) is located very close to the cellular tower. If User is assigned time Slot in the frame while User 2 is assigned Slot 2 in the frame, it is clear that due to the long travel of the signal of User, the signal of that user takes a duration of D/c (where D is the distance to the tower and c is the speed of light), while the other signal will arrive at the tower almost instantaneously. If the two time slots are adjacent to each other with no guard times separating them, the signals of the two users would overlap partially in time at the receiving tower. This concept can easily be used to determine the required guard time in a TDMA system for terrestrial systems by observing the furthest distance that a transmitter can communicated with the system. The guard time is set to be slightly higher than that to insure that there will be no overlap between the transmission of different users. In the case of satellite, the guard band cannot be se the same way because the difference in distance to the receiver may be several thousands of kilometers, which means that the guard time if set similarly to a terrestrial system would be too long resulting in a huge waste of transmission time. This concept is studied when discussion about VSAT system is given in the following lectures. In satellite TDMA communications, because of the long path between the satellite and Earth stations, note that several transmission of different users will be traveling from Earth stations to the satellite and from the satellite to the destination Earth stations. This places a significant amount difficulty in synchronizing the transmissions of the different Earth stations to avoid collisions at the satellite. One way of doing this is to have each Earth station monitor the transmitted signal from the satellite and check to see where its specific transmission falls within the frame. If the Earth station find that its transmission is a little behind, it would bring its transmission a little earlier. If it finds that its transmission is a little early, it would delay its transmission a little bit. Clearly this is not applicable if the transmitted signal from the satellite is

6 directed to a different region in which Earth stations cannot access. This issue even becomes a bigger problem in case the satellite is not a GEO satellite, which means that its position changes with respect to the Earth stations continuously. In this case, the Earth stations must be able to track the location of the satellite to be able to adjust the timing of their transmission continuously. It is important to note that because we are using TDMA, it does not mean that we can gain by squeezing as many users as we want through giving them different time slots without a cost. Since each user out of the N users sharing the same frequency channel transmits all the data that that user has generated (from voice for example) in a single time slot, the effective bit rate of transmission of a single user would be N times the bit rate if that user was transmitting his data over the complete time period (as in FDMA). Therefore, the bandwidth shared by all the N users

7 is N times the bandwidth that would be used by a single FDMA user. Therefore, the bandwidth efficiency of TDMA and FDMA are about the same. Also, note that since different transmits are transmitting at different time instants, their transmissions do not get added at the input of the transponder. This solves the problem of transponder high power amplifier saturation, or at least if a signal saturates the amplifier, this signal would be the only affected one. The main disadvantage of this method is that since each user is collecting his data and transmitting it in the form of bursts in a short period of time, high power must be transmitted during these bursts, resulting in needing good high power amplifiers at the Earth stations. Another disadvantage, is the need for good synchronization mechanism to avoid overlapping of the different transmissions of different users. 3. Code Division Multiple Access (CDMA) In the above two multiple access methods, we have seen that users can share the time of transmission but have different frequency bands of transmission or share the same frequency band but transmit at different time slots. It is still possible for multiple users to transmit at the same frequency band and transmit at the same time, yet their signals do not interfere with each other at the receiver and can be separated from each other. This is done using CDMA. This is illustrated as follows: The concept of CDMA is to spread multiple digital sequences using orthogonal spreading sequences. Each spreading sequence divides a bit of data into L chips (so the L chips all carry one bit of information) and the different spreading sequences are orthogonal over the period of one bit. Multiple

8 signals g(), t g(), t, gn () t are said to be orthogonal over one bit period T B (or any other time period) if they satisfy T B g () t g () t dt j k Positive Constant, = j j = k k The following signals which are constant over quarters of a bit period T B are all orthogonal (you can easily verify this by multiplying each of them by itself first and integrating them over T B and then multiplying each of them by all other signals and integrating over T B ):

9 The process of spreading the spectrum of signals in CDMA using spreading codes (or sequences similar to any of the above signals shown above) involves multiplying each bit of the signal with that spreading code. Different users are assigned different spreading codes (for example, the 4 signals shown above would be assigned to 4 different users). Once each user spreads his signal with the assigned code, all signals are transmitted over the same frequency band at the same time, which means that they are received as a combined signal at the input to each receiver. The combined signal now contains all the spread signals added to each other. At the receiving end, all that the different receivers have to do to extract a particular signal is to multiply the overall combined signal by the corresponding spreading code of the user for which it would like to receive from and then integrate over different bit periods. The integration over a bit period would give either a positive value corresponding to Logic or a negative value corresponding to Logic for each bit.

10 Again, we are not gaining anything here by using CDMA for free. Consider for example binary data that requires MHz of bandwidth to be transmitted, multiplying this signal by the spreading code generally results in dividing each bit into L smaller pieces (called chips), which results in the new signal having an apparent bit rate equal to L times the bit rate of the original signal, which requires L MHz of bandwidth to be transmitted. But this L MHz signal is used by L users. Since the different spreading sequences are all orthogonal, it can easily be shown that multiplying the combined signal containing all transmitted signals by one of the spreading sequences and integrating over the period of each bit produces the original bits of the signal that was spread using that spreading sequence and bits of the other signals are eliminated. Because the spreading sequence effectively increases the transitions in the message signal by the number of chips L in the spreading sequence, the bandwidth of the transmitted signal is L times the bandwidth of the original message signal. A practical example of this process is shown in the following figure. The input signals (each having 5 bits) are shown in Column. These signals are spread using the three orthogonal spreading sequences (Column 2). Note that the duration of each spreading sequence is the duration of one bit. The first spreading sequence happens to be constant for the whole period of the bit. The spread signals are shown afterwards (Column 3) where generally they will have wider bandwidth than the original signals. The spread signals are then combined as if they were transmitted and received by an antenna (Column 4). Upon multiplying each bit of the combined received signal by each of the spreading sequences and integrating over the bit period, which is the de-spreading process, we get the original bit sequences (Column 5). (VERIFY THAT THE SYSTEM WORKS BY TRACKING THE DIFFERENT SIGNALS IN THIS EXAMPLE YOURSELF) Original Bits Spreading Sequences

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12 4. Space Division Multiple Access (SDMA) In satellite systems, a forth multiple access method can be used. Space Division Multiple Access is based on the fact that different users (Earth stations) are located at different locations on Earth, and therefore on a satellite you may have several directional antennas that receive from/transmit to particular Earth stations ) at the same time, 2) over the same band of frequency, and 3) using the same code. This feature is capable of increasing the bandwidth of a satellite by several folds. Example: Three (3) Earth stations that can each transmit a maximum amount of power of 5 W are accessing the same satellite transponder in an FDMA form. The satellite transponder has a bandwidth of 36 MHz and a maximum output power of 4 W but must be operated at a back-off loss of 3 db to insure linearity in the power amplifier. In the linearity region, the gain of the transponder is 5 db. The three Earth stations are allocated bandwidth as follows BW for ES = 5 MHz BW for ES2 = MHz BW for ES3 = 5 MHz a) Find the power levels at the input and output of the transponder due to each of the 3 Earth stations assuming that the transponder is operated at maximum output power in the linear region. b) Find transmission power of each Earth station assuming that an Earth station that transmits an amount of power of 5 W causes the transponder to produce 4 W. Solution: To have maximum transmitted power in the linear region of the transponder power amplifier, the transmitted power of the transponder = log 4 W 3 db = 6 3 = 3 db = 2 W (remember that 3 db loss is equivalent to a reduction by a factor of 2). Clearly, this 2 W will be shared by all users of the transponder, according to their bandwidths The total bandwidth used by all Earth stations = = 3 MHz. At Output of Transponder

13 Pout (due to ES) = 5 MHz / 3 MHz *2 W = W P2out (due to ES2) = MHz / 3 MHz *2 W = W P3out (due to ES3) = 5 MHz / 3 MHz *2 W = W Using the gain of the transponder to find the input powers, we get the following powers at input of transponder Pin (due to ES) = log W 5 db = 95 db P2in (due to ES2) = log W 5 db = 96.8 db P3in (due to ES3) = log W 5 db = 99.8 db Now, we know that 5 W at the output of the ES results in 4 W at the output of the transponder. So, to get 2 W at output of transponder, an ES needs to transmit 25 W. Pout (at output of ES) = W /2 W * 25 = 25 W P2out (at output of ES2) = W /2 W * 25 = W P3out (at output of ES3) = W /2 W * 25 = 4.66 W Clearly, in here if one of the ES starts to transmit higher power, this will cause the transponder high power amplifier to saturate which damages all signals processed by this transponder. So, one way to jam a signal that is FDMA in a satellite is not to transmit a signal at the same frequency, but transmit a very high power signal in the same band of its transponder trying to saturate the HPA of that transponder to reduce the C/N ratio of the signal to be jammed.