Multiple Access Techniques is a very interesting and important part of networking which needs a very in-depth knowledge as many questions in both academics and interviews are from this very portion. Hence to help the students of mainly electronics and computer science, we have brought together some important questions on multiple access techniques with answers .
Question-1 Give the introduction of Multiple access techniques ?
1. It shares many users at same time
2. share a finite amount of radio spectrum
3. high performance
4. duplexing generally required
5. It works in frequency domain
6. It also works in time domain
Question-2 What do you mean by narrow band and wide band system?
Answer- NARROW BAND SYSTEM
1. Channel system: generally total spectrum is divided into a number of relatively narrow radio channels (e.g. FDMA). If all the channels are being used, call blocking occurs. Unused bandwidth in each channel cannot be used by other users.
2. Transmission experiences non selective fading. This means that when fading occurs, whole of the information (i.e. the whole channel) is affected.
WIDE BAND SYSTEM
1. The main feature of wide band systems is that either complete spectrum is available (e.g. CDMA, TDMA) or a considerable portion of it is used by each user (e.g. TDMA+FDMA).
2. The advantage of wideband systems is that the transmission bandwidth always exceeds the coherence bandwidth for which the signal experiences only selective fading. That is, only a small fraction of the frequencies composing the signal is affected by fading.
3. Signal can be distorted and therefore equalization is needed but unlikely a total signal fade occurs.
Question-3 Write the types of Multiple Access Techniques?
Answer- Types of multiple access techniques are:
1. Frequency division multiple access (FDMA)
2. Time division multiple access (TDMA)
3. Code division multiple access (CDMA)
4. Space division multiple access (SDMA)
They all are grouped as narrow band and wide band system.
Question-4 What is Random Access i.e Aloha system?
1. Aloha is a packet-switching system. The time interval required to transmit one packet is called a slot.
2. When transmissions from two or more users overlap, they destroy each other, whether it is complete overlap or partial overlap – collision takes place.
3. The maximum interval over which two packets can overlap and destroy each other is called the vulnerable period.
4. The mode of random access in which users can transmit at anytime is called pure Aloha. In this case, the vulnerable period is two slot times.
5. A version in which users are restricted to transmit only from the instant corresponding to the slot boundary is referred to as slotted Aloha. The alignment of transmission to coincide with the slot boundary means that packets can only experience complete overlap. So, the vulnerable period is only one slot time.
Question-5 What is the difference between Multiple Access & Multiplexing?
Answer- Both are very different terms.
1. Multiplexing is the process of transmitting several messages simultaneously on the same circuit or channel.
On the other hand Multiple Access are techniques that have been developed in the satellite industry which allow satellite spectrum and power to be shared efficiently among multiple users.
2. In multiple access, more than one simple signal can thus be transmitted as part of a single complex signal and separated out at the receiving end. This is not possible in multiplexing.
Question-6 What is Near Far Problem?
Answer- At the receiver, the signals may come from various multiple sources.
1. The strongest signal usually captures the modulator. The other signals are considered as noise.
2. Each source may have different distances to the base station
Question-7 What is Duplexing?
Answer- For voice or data communications, must assure two way communication (duplexing, it is possible to talk and listen simultaneously). Duplexing may be done using frequency or time domain techniques.
1. Forward (downlink) band provides traffic from the BS to the mobile
2. Reverse (uplink) band provides traffic from the mobile to the BS.
Question-8 Write the features of TDMA?
Answer- TDMA is Time Division Multiple Access technique, its features are:
1. In TDMA a single carrier frequency with a wide bandwidth is shared among multiple users. Each user is assigned non-overlapping time slot.
2. Number of time slots per frame depends on (1) available bandwidth, (2) modulation techniques etc.
3. Transmission for TDMA users is not continuous, but occurs in bursts, resulting in low battery consumption. The subscriber transmitter may be turned off during non-transmission periods.
4. Hand off process is simpler for a subscriber, since it can listen to other base stations during non-transmit times.
5. An enhanced Link control such as provided by MAHO can be implemented by a subscribe by listening in an idle time slot in the TDMA frame
6.TDMA uses different time slots for TX and reception, thus duplexers are not required.
Even in TDMA/FDD a switch rather than a duplexer is required in the mobile unit to switch between the TX and RCRVR
Question-9 What is CDMA?
Answer- CDMA is Code Division Multiple Access Technique.
1. For code-division multiple access, each transmitter is assigned a different pseudo-noise (PN) sequence.
2. If possible, orthogonal sequences should be used.
3. The PN sequence for the transmitter is only given to the receiver that is to operate with the transmitter.
4. The receiver will then only receive the correct signals and ignore all others.
Question- 10 What is frequency hopping?
1. It is a digital multiple access technique
2. A wide band radio channel is used.
a. Same wide band spectrum is used
3. The carrier frequency of users are varied in a pseudo-random fashion.
a. Each user is using a narrow band channel (spectrum) at a specific instance of time.
b. The random change in frequency makes the change in using the same narrow band channel very low.
4. The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized.
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