6.1 Time division multiplexing | unit 6 multiple access techniques

Multiple Access Techniques

6.1 Time division multiplexing

In TDM the complete channel bandwidth is allotted to one user for fixed time slot. For instance, if there are ten users then every user can be given the time slot of one second. Thus, complete channel can be used by each user for one second time in every ten seconds.

Fig 1 Time Division Multiplexing

This technique is suitable for digital signals because the digital signals are transmitted intermittently and the time spacing between two successive digital codewords can be utilised by other signals. The TDM is classified in two types

Synchronous TDM

In synchronous time division multiplexing, each transmitter is allotted with a fixed time slot, regardless of the fact that the transmitter has any data to transmit or not. The device has to transmit data within this time slot. If the transmitter does not have any data to send then its time slot remains empty.

Fig 2 Synchronous TDM

The above figure shows four dedicated time slots A, B, C and D. The transmitter A data is sent at time slot A, transmitter B data is sent at time slot B, transmitter C data is sent at time slot C and transmitter D data is sent at time slot D.

In the time frame 2, the transmitter B and C does not have any data to send so the time slot B and C remains empty.

The main drawback of synchronous time division multiplexing is that the channel capacity is not fully utilized. Hence, the bandwidth goes wasted.

Asynchronous TDM

In this case the time slots are not fixed. The number of transmitters are not equal to the number of time slots. In asynchronous TDM the number of slots are always less than the transmitters.

Fig 3 Asynchronous TDM

As seen from above figure the system has four transmitters and 3 time slots. Frame 1 is completely filled with data from devices A, B, C and D. Frame 1 has only 3 time slots. The data from D is filled in frame 2 in timeslot 1. The data from A and D are filled in timeslot 2 and 3 in time frame 2.

In asynchronous time division multiplexing, the multiplexer scans all the transmitters and accepts input only from the devices that have actual data to send and fills all the frames, and then sends it to the receiver. If there is not enough data to fill all the slots in a frame, then the partially filled frames are transmitted. In most of the cases, all the time slots in frames are completely filled.

Key takeaway

6.2 Frequency division multiplexing

This technique allows to allot a fixed frequency band to every user in the complete channel bandwidth. Such frequency slot is allotted to each frequency user. The figure shows FDM below. The transmitter end contains multiple receivers and transmitters. The transmitter end sends a signal of different frequency. The transmtter1 sends signal of 30kHz, transmitter 2 sends signal of 40kHz and so on as shown below. These signals of different frequencies are multiplexed and transmitted.

Fig 4 Frequency Division Multiplexing

At the receiver end, the multiplexed signals are separated by using a device called demultiplexer. It then sends the separated signals to the respective receivers. In the above figure, the receiver 1 receives signal of 30 kHz, receiver 2 receives signal of 40 kHz, and receiver 3 receives signal of 50 kHz.

Key takeaway

This technique allows to allot a fixed frequency band to every user in the complete channel bandwidth. Such frequency slot is allotted to each frequency user.

6.3 Code division multiplexing

Code division multiplexing (CDM) is a networking technique in which multiple data signals are combined for simultaneous transmission over a common frequency band.

When CDM is used to allow multiple users to share a single communications channel, the technology is called code division multiple access (CDMA).

CDMA stands for Code Division Multiple Access. CDMA is a technique for spread spectrum multiple access.

Data signals are XOReD with pseudorandom code and then transmitted over a channel.

Different codes are used to modulate their signals, selection of code to modulate is important aspect as it relates with performance of system.

In CDMA, single channel uses entire bandwidth and it does not share time as all stations transmit data simultaneously. Due to this CDMA differ from FDMA and TDMA.

Let us assume we have 4 different stations A, B, C and D. All 4 stations connected to same channel. Data from station A is d1, B is d2, C is d3 and D is d4 similarly code assigned to A is C1, B is C2, C is C3 and D is C4.

To transmit data, all 4 stations have multiplication strategy i.e. either code multiply by itself. It results in 4 stations or code multiply results in 4 stations or code multiply results in 4 station of code multiply by another, it results in zero station.

Fig. 5 Communication of station with code

If station C and station D are talking to each other, D wants to listen whatever C is saying. It multiplies data on channel by C3 of station C.

As C3  C3 is 4 and rest of combination i.e. C1  C3, C2  C3 and C4  C3 are all zero.

Every station has some code, which is sequence of numbers known as chips.

Sequence should be selected in proper manner to get appropriate codes. Orthogonal sequence has some properties :

Multiplication of sequence by a scalar.

Inner product of two equal sequences.

Inner product of two different sequences.

Sequence generation uses Walsh table to generate chip sequence.

Key takeaway

6.4 Introduction to upcoming techniques of transmission

The basic techniques used for transmissions are listed and explained briefly below.

6.4.1 FDMA

It sub-divide frequency into a several frequency band with non-overlapping.

Fig.6 FDMA

FDMA allows user to transmit signals simultaneously to satellite transponder with the help of assigning a specific frequency to every user among a channel.

Each transaction of signal has own unique radio channel. Channel are probably 30 KHz or less used to transmit or receive channels.

Frequency allocation made by national policies. Uplink (i.e. from mobile station to base station). Downlink (i.e. from base station to mobile station) uses frequencies bands like. Uplink and downlink with frequencies mentioned below :

Uplink	890.2 MHz, 915 MHz
Downlink	935.2 MHz to 960 MHz

Uplink (UL) and downlink (DL) can be described with the help of one relation among them as follows:

fd = fu + 45 MHz

For Example :

If UL frequency is :

fu = 890 MHz + n∙ 0. 2 MHz

Then

fd = 935 MHz + n ∙ 0. 2 MHz

Fixed assigned frequency makes scheme very inflexible limits to no. of sender turn into disadvantage of FDMA.

6.4.2 TDMA

It is based on time slots. TDMA more flexible than FDMA scheme.

Dynamic allocation of channel or by some fixed pattern channels get allotted for time slots to takes places synchronized communication.

Fig. 6 TDMA

Fixed channel are allocated for communication as best practice solution for wireless phone system in Medium Access Control (MAC) reserved time slot access is for crucial.

6.4.3 CDMA

CDMA stands for Code Division Multiple Access. CDMA is a technique for spread spectrum multiple access.

Data signals are XOReD with pseudorandom code and then transmitted over a channel.

Different codes are used to modulate their signals, selection of code to modulate is important aspect as it relates with performance of system.

In CDMA, single channel uses entire bandwidth and it does not share time as all stations transmit data simultaneously. Due to this CDMA differ from FDMA and TDMA.

Fig.7: Communication of station with code

If station C and station D are talking to each other, D wants to listen whatever C is saying. It multiplies data on channel by C3 of station C.

As C3  C3 is 4 and rest of combination i.e. C1  C3, C2  C3 and C4  C3 are all zero.

Every station has some code, which is sequence of numbers known as chips.

Sequence should be selected in proper manner to get appropriate codes. Orthogonal sequence has some properties :

Multiplication of sequence by a scalar.

Inner product of two equal sequences.

Inner product of two different sequences.

Sequence generation uses Walsh table to generate chip sequence.

6.4.4 Mobile Radio

6.4.4.1 Cellular Infrastructure

The cellular system is replaced by large number of base stations (BS) which are hexagonal cells. They cover certain range of areas.

Fig 8 Cellular Structure

For having the communication in cellular structure, we need Mobile station (MS). In these types of structures, the MS communicates with the BS of cell. This is the location of MS in a BS cell. This BS acts as a gateway. For establishing connections, the MS needs to be in the range of the system.

6.4.4.2 Cell Splitting

Cell splitting is the process of subdividing a congested cell in to smaller cells, each with its own base station and corresponding reduction in antenna height and transmitter power.

Cell splitting increases the capacity of a system since it increases number of times that channels are reused.

In cell splitting original cell is split in to smaller cells. New cell radius is half of the original radius.

In this the cell boundaries need to be revised so that the local area which was earlier considered as a single cell can now contain number of smaller cell ,these new cells are called microcells

Dynamic cell splitting: This technique is based on utilizing the allocated spectrum efficiency in real time. In this of splitting techniques cells are not splitted permanently depending on requirement of traffic the splitting of the cells are carried out.

The algorithm for dynamically splitting cell sites is a tedious job since we cannot afford to have single cell unused during cell splitting at heavy traffic hours. Proof:

When the cell radius is reduced by a factor, it is also desirable to reduce the transmitted power. The transmit power of the new cells with radius half that of the old cells can be found by examining the received power PR at the new and old cell boundaries and setting them equal.

This is necessary to maintain the same frequency re-use plan in the new cell layout as well. Assume that PT1 and PT2 are the transmit powers of the larger and smaller base stations respectively. Then, assuming a path loss index n=4, we have power received at old cell boundary = PT1/R4 and the power received at new cell boundary = PT2/(R/2)4. On equating the two received powers, we get PT2 = PT1 / 16. In other words, the transmit power must be reduced by 12 dB in order to maintain the same S/I with the new system lay-out.

At the beginning of this channel splitting process, there would be fewer channels in the smaller power groups.

As the demand increases, more and more channels need to be accommodated and hence the splitting process continues until all the larger cells have been replaced by the smaller cells, at which point splitting is complete within the region and the entire system is rescaled to have a smaller radius per cell.

If a cellular layout is replaced entirety by a new layout with a smaller cell radius, the signal-to-interference ratio will not change, provided the cluster size does not change. Some special care must be taken, however, to avoid co-channel interference when both large and small cell radii coexist.

It turns out that the only way to avoid interference between the large-cell and small-cell systems is to assign entirely different sets of channels to the two systems.

So, when two sizes of cells co-exist in a system, channels in the old cell must be broken down into two groups, one that corresponds to larger cell reuse requirements and the other which corresponds to the smaller cell reuse requirements.

The larger cell is usually dedicated to high speed users as in the umbrella cell approach so as to minimize the number of hand-offs.

Fig 9 Cell Splitting

6.4.4.3 Frequency reuse

It is process of combining analog and digital signal to send over shared medium.

It divides the capacity of communication channel into multiple channels

Multiplexing is divided into space division, frequency division and time division multiplexing.

Space Division Multiplexing

In wired medium, separate point to point conductors are used to each channel.

In wireless medium multiple elements of antennas are used such that it forms phased array antenna.

Multiple output, multiple input and single input multiple output are the examples of this.

Frequency Division Multiplexing

In these multiple signals are send in distinct frequency in single medium.

The signals are electrical.

Radio, television broadcasting are the examples of FDM.

The service provides can send several channels or signals continuously to all subscribers even the customer has single cable connection.

Time Division Multiplexing

In this for separation of data streams the time is used instead of frequency and space.

It consists of group of bits in sequence one after another.

Each sequence is associated with each receiver.

Carriers sense multiple access and multidrug are the eaxples of time division multiplexing.

Key takeaway

It is process of combining analog and digital signal to send over shared medium.

It divides the capacity of communication channel into multiple channels

Multiplexing is divided into space division, frequency division and time division multiplexing.

6.4.4.4 Fading and Multipath: Small scale multipath propagation

Multi path causes large and rapid fluctuations in a signal

These fluctuations are not the same as the propagation path loss.

Multipath causes three major things in wireless communication

Rapid changes in signal strength over a short distance or time.

Random frequency modulation due to Doppler Shifts on different multipath signals.

Time dispersion caused by multipath delays

These are called “fading effects

Multipath propagation results in small-scale fading.

6.4.4.5 Synchronous optical network (SONET)

It defines the rate and format for the optical transmission of digital data. The data rates of 51.84Mb/s to 9.953Gb/s are specified by SONET. This system has four layers.

Photonic Layer: This specifies the type of optical fiber, the minimum required laser powers, sensitivity of the receiver and dispersion characteristics of lasers.

Section Layer: This layer generates SONET frames and converts the electronic signal to photonic signals.

Line Layer: It synchronizes and multiplexes the data into SONET frames.

Path Layer: It performs end to end transport of data at the power rate.

Key takeaway

Approach	SDMA	TDMA	FDMA	CDMA
Idea	Segment space into cells/sectors	Segment sending time into disjoint time-slots, demand driven or fixed patters	Segment the frequency band into disjoint sub-bands	Spread the spectrum using orthogonal codes
Terminals	Only one terminal can be active in one cell/one sector	All terminals are active for short periods of time on the same frequency	Every terminal has its own frequency. uninterrupted	All terminals can be active at the same place at the same moment uninterrupted.
Signal separation	Cell structure, directed antennas	Synchronization in the time domain	Filtering in the frequency domain	Code plus special receivers
Advantages	Very simple, increases capacity per km2	Established, fully digital, flexible	Simple, established robust	Flexible, less frequency planning needed, soft handover
Disadvantages	Inflexible, antennas typically fixed	Guard space needed (multipath propagation), synchronization difficult	Inflexible, frequencies are a scarce resource	Complex receivers, needs more complicated power control for senders
Comment	Only in combination with TDMA, FDMA or CDMA useful	Standard in fixed networks, together with FDMA/SDMA used in many mobile networks	Typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)	Still faces some problems, higher complexity, lowered expectations : will be integrated with TDMA/FDMA

References

1. “Communication Systems”, Simon Haykin, Wiley publication, 4th Edition, 2004

2. “Digital Communication Fundamentals and Applications”, Bernard Sklar, Pearson

Education India, 2nd Edition, 2009

3. “Modern Electronic Communication”, Miller Gary M, Prentice-Hall, 6th Edition, 1999

4. “Digital Communications”, John Proakis, Tata Mc Graw Hill, 5th Edition, 2007

5. “Electronic Communication Systems, Fundamentals Through Advanced”, Wayne Tomsi, Pearson Education, 4th Edition, 2001

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