Multiple Access Schemes for wireless communication – A simple overview

In the post on the GSM Standard, we learned that data and other signals are transmitted through allocated bands of frequencies called channels. In this post, we shall have a look at different multiple access schemes that are employed on these channels to facilitate wireless communication.

What are Multiple Access Schemes?

• Multiple Access Schemes enable multiple users to gain access to the wireless network and use it simultaneously. As simple as that.
• This is generally done by alloting a part of the available bandwidth to each user, in such a way that mutual interference between two parts is brought to a minimum, and the entire available spectrum can be utilized efficiently.
• These Multiple Access Schemes can be categorized into different types based on how the available spectrum is split, which we shall see in detail in the coming sections.

First, we shall have a look at the three main multiple access schemes, namely:-

1. Time Division Multiple Access (TDMA)
2. Frequency Division Multiple Access (FDMA), and
3. Code Division Multiple Access (CDMA).

Time Division Multiple Access

• Time Division Multiple Access or TDMA, as the name suggests, enables multiple users to gain access to a network by dividing the available spectrum into parts based on time slots, and allocating each time slot to a user.
• Each user transmits in rapid succession, during their corresponding time slot, as bursts.

• Note that the time difference between two bursts belonging to the same time slot is quite small and the sampling of the speech data cannot be noticed by the human ear, hence it appears continuous.

The figure shows the functioning of TDMA, with the spectrum having been split into eight time slots. The Mobile Station (MS) at terminal A has been allocated time slot 1 to communicate, while MS at B uses the spectrum at time slot 6.

Advantages

• The data rate can be varied from as low as 64 kbps to as high as 120 Mbps by varying the length of the time slot.
• Mutual interference is quite small as different conversations are carried out at different time slots.
• No frequency guard band is required.
• The same single carrier frequency can be shared with multiple users.
• Makes efficient use of the spectrum, and hence more users can be accommodated within the same frequency bands.

Disadvantages

• There are more chances of calls being dropped during handoffs as the time slot allocated to the MS at the initial Base Station (BS) might already be in use at the other BS. The reallocation results in at least a delay at the receiver’s end if the call does not get dropped.
• Is highly susceptible to multipath distortion, where the waves carrying the same information emitted at the same time, arrive at the destination at different instances, distorting the information to be passed.
• Network and Spectrum planning is quite intensive.

Use of TDMA

• TDMA was first used in satellite communication systems by Western Union in its Westar 3 communications satellite in 1979. It is now used extensively in satellite communication.
• TDMA also finds its application in 2G cellular systems such as the Global System for Mobile Communication (GSM), which we had seen in an earlier post in this course on Wireless and Mobile Communication.

Code Division Multiple Access

• Code Division Multiple Access or CDMA, enables multiple users to gain access to the network by employing spread spectrum technology and allocating different codes for different transmitters.
• This means that each MS is given a code that is used to encode/modulate the signal being sent, and the same code is shared with the receiver, ensuring that only the entities who possess the code can decode the message.

The figure shows how in CDMA, the channels are split on the basis of code, ensuring that the entire spectrum and time are available for communication.

Advantages

• CDMA is a spread spectrum technology and hence a wider bandwidth is available for transmission.
• This also results in the transmitted signals being immune to interference to a greater extent.
• CDMA systems are robust and are resistant to fading.
• In order to receive and be able to decode the data, the receiver must have a knowledge of the spreading code (used to encode the signal), which gives a measure of security to the transmitted waves.
• Using CDMA, unlike in the case of TDMA, it is possible for a terminal to communicate with two base stations at once. As a result, the old link only needs to be broken when the new one is firmly established. This provides significant improvements in terms of the reliability of handover (handoff) from one base station to another.
• CDMA has the ability to handle a larger load than the other Multiple Access Schemes (almost six times than what TDMA can handle).
• CDMA is quite energy-efficient. A CDMA cell phone employs a dynamic power control system to make sure that the signal transmitted has enough power to reach the receiver at ease. But not in excess as it causes interference, and drains the battery in the device.

Disadvantages

• CDMA uses soft handoff, which we had seen in the post about handoffs. Moreover, the handoff procedure is quite complex.
• An increase in the number of users will decrease the overall Quality of Service (QoS).
• Chances of jamming can be high if the codes allocated to the subscribers are not orthogonal.

Used of CDMA

• As CDMA is proprietary, it is not found around the world like TDMA (GSM) is.
• CDMA is mainly found in the United States, South Korea, and Russia.
• CDMA was also employed in Global Positioning Systems (GPS) and in transport logistics.

Frequency Division Multiple Access

• Frequency Division Multiple Access or FDMA, enables multiple users to gain access to the network by splitting the available bandwidth into non-overlapping channels that are assigned to each user to communicate.
• To ensure that channels operating at adjacent frequency intervals do not interfere with each other (which could result in loss of information), a small range of frequencies in between the two, called the guard band, is left empty.

The figure above shows how different frequency bands are allocated for different channels.

Advantages

• The complexity of designing FDMA systems is lower when compared to TDMA and CDMA systems.
• A capacity increase in the network can be managed by reducing the information bit rate and using efficient digital codes, providing better load distribution.
• Since the transmission in FDMA is continuous, fewer bits are needed for overhead purposes (synchronization majorly) as compared to TDMA.
• The guard bands present substantially reduces intersymbol interference (ISI).

Disadvantages

• FDMA does not optimally use the bandwidth available. If an FDMA channel is not in use, then it sits idle and cannot be used by other users to increase or share capacity, wasting it.
• FDMA requires tight filtering mechanisms to minimize adjacent channel interference.
• The presence of the guard bands also decreases network utilization.
• The maximum bit rate per channel is fixed and small, inhibiting the flexibility in bit-rate capability that may be a requirement for 3G applications in the future.

Use of FDMA

• A common example of FDMA would be FM broadcasting, where each radio station is given a particular frequency range at which they’re supposed to transmit.
• Apart from this, FDMA was used in early age telephony, which has evolved into more of a TDMA and FDMA hybrid presently.
• Walkie Talkies and such close-range wireless communication devices generally employ FDMA.

We have seen the three major multiple access schemes, now we move on to other multiple access schemes being used in communication.

Space Division Multiple Access

• Also known as Spatial Division Multiple Access, Space Division Multiple Access (SDMA) is a multiple access scheme that optimizes the use of the available spectrum and minimizes power consumption by taking advantage of the directional properties of dish antennas.
• In traditional networks, the base station has no information on the position of the mobile units within the cell and radiates the signal in all directions within the cell in order to provide radio coverage.
• This method results in wasting power on transmissions when there are no mobile units to reach, in addition to causing interference amongst nearby cells operating at the same frequency.
• Hence, a MIMO (multiple inputs multiple outputs) system is employed where directional signals beams are sent directly to the receiver whose spatial location can be found out by smart antenna technologies, which uses the direction of arrival (DOA) of signals to determine the spatial location of the mobile station.

Advantages

• The usage of power is optimized and wastage of energy in the transmission is minimized.
• Multiple receivers operating on the same frequency can receive signals from the same transmitter without having to divide the frequency range (assuming that their spatial location is different).
• Completely free from interference.
• Enhanced Performance.
• The radiated energy for each receiver can be controlled and regulated.

Disadvantages

• If two receivers operating in the same frequency have the same spatial location, the frequency bands have to be split into channels to accommodate both the mobile stations.
• Network design can get a little tedious.

Use of SDMA

• In GSM, though the Base Transceiver Station (BTS) is not aware of the direction of the MS, it gets to know of the distance to the MS with the help of a parameter called “Timing Advance” (TA). With the help of the TA, the BTS can then power up or power down the signal to be sent depending upon how far away the MS actually is, conserving energy and reducing its wastage.
• Similarly, in the 5th Generation (5G) networks focuses on directing the whole energy required only to the “zone” containing the mobile station, enabling power savings for the MS, reducing the amount of radiation and power wastage, and increasing spectral efficiency.

Power Division Multiple Access

• The idea of Power Division Multiple Access (PDMA) came up in the early 2000s, where the key idea was to have different received power for each user in order to permit multiple users to access the network at the same time.
• The power transmitted is divided into multiple regular power segments (PSs) to simultaneously transmit multiple independent information/data streams in peer to peer communications.

Advantages

• The complexity of the network is kept low.
• No requirement of assigning a code signature to all the users in the network.

Disadvantages

• Has not been implemented on a large scale and is relatively new.
• The chances of interference occurring is fairly high, and the design of the infrastructure required to reduce it can be quite complex.

Beam Division Multiple Access

• In Beam Division Multiple Access (BDMA), the base station allocates a separate antenna beam for each mobile station.
• This is done after evaluating the position of the mobile stations as well as their moving speeds, then deciding the widths and directions of the beams to be transmitted such that it uses the right amount of power that the information reaches the mobile device and excess power is not used.
• Thus multiple users can be accommodated by making sure they’re at different angles with respect to the base station.
• If there is more than one user within the proximity of the same beam, the beam itself has to be divided by employing another Multiple Access Scheme like TDMA or FDMA.

Advantages

• Better utilization of bandwidth and resources.
• Eliminates signal deterioration at the cell edge.
• Reduces the overhead of channel estimation.
• Reduces the processing complexity at both the transmitter and receiver.

Disadvantages

• Requires the implementation of an additional Multiple Access Scheme for it to work.
• Has not been implemented on a large scale and is fairly new.

Use of BDMA

• BDMA was proposed to be the backbone of 5G technology because of all the advantages it offers.

5G Multiple Access Schemes

• With 5G technology being developed at an astonishingly fast pace, there is also a lot of debate going on about which Multiple Access Scheme would be the best contender that can provide what 5G promises, which includes but is not limited to improved spectral efficiency, lower latency, better data rates and better Quality of Service (QoS) and enhanced security.
• Apart from Beam Division Multiple Access, which we already had a look at, there are a couple of other Multiple Access Schemes that are strong candidates here, which we shall have a look at in this section.

Non Orthogonal Multiple Access (NOMA)

• The key idea of Non Orthogonal Multiple Access (NOMA) is to use the power domain for multiple access (similar to PDMA), unlike its orthogonal counterparts that split the spectrum based on time, frequency, or code.
• The main issue with this orthogonal multiple access (OMA) technique is that its spectral efficiency is low.
• Employing NOMA on the other hand ensures very high spectral efficiency, very low latency, and massive device connectivity, as the criteria of each channel which is assigned to each user needing to be orthogonal is not required.

Sparse Code Multiple Access (SCMA)

• Sparse Code Multiple Access (SCMA) enables non-orthogonal transmissions of multiple users’ signals among both code and power domain, which could greatly improve the spectral efficiency.
• Here, each channel is assigned specific codewords such that each channel can support multiple users and transmit and receive data from those multiple users.
• This technique of assigning codewords to each channel has been developed to use what are termed sparse code and in this way, significant numbers of users can be added while maintaining the spectral efficiency levels.
• Although the complexity does get a tad higher, it is compensated for by the larger amount of users that can be accommodated and the expected lower latency.

Orthogonal Frequency Division Multiple Access (OFDMA)

• Orthogonal Frequency Division Multiple Access (OFDMA) has been widely used and very successful for 4G, along with LTE,  and could be a potential contender at being the 5G multiple access scheme.
• In OFDMA, the channels are further divided into subsets which are then assigned to individual users, similar to how multiple access was achieved in the case of SCMA.
• Each subset is modulated independently and simultaneously, forming OFDM symbols separated in time by guard intervals.
• The receiver is able to track all the subsets simultaneously and demodulate the symbols independently.

Advantages of Orthogonal Frequency Division Multiple Access (OFDMA)

• Allows simultaneous low-data-rate transmission from several users.
• Shorter and constant delay.
• Offers frequency diversity by spreading the carriers all over the used spectrum.
• Can accommodate more users in the network.

Disdvantages of Orthogonal Frequency Division Multiple Access (OFDMA)

• Higher sensitivity to frequency offsets and phase noise.
• Dealing with co-channel interference from nearby cells is more complex in OFDMA than in other multiple access schemes.

There are several multiple access schemes that could be employed for 5G. The one used (if not a hybrid of multiple schemes) will be chosen as a result of the standardization process for the technology.

In this post, we have seen the different multiple access schemes that have been employed in the wireless communication industry, along with its various advantages, disadvantages, and uses. We hope the explanations were clear for you. If you have any queries, please let us know in the comments section below!