Wireless Application Protocol

Multiple Access Protocols

Multiple Access Protocols

Several strategies have been proposed to solve the shared medium access problem. These strategies attempt, by various mechanisms, to strike a balance between achieving the highest-quality resource allocation decision and the overhead necessary to reach this decision. These strategies can be classified in three major categories:

Fixed assignment(Channelization Protocol) Demand assignment (Controlled Access Protocols) Random assignment.

Multiple Access Protocols

Multiple schemes are used to allow many mobile users to share simultaneously a finite amount of radio spectrum.

The sharing of spectrum is required to achieve high capacity by simultaneously allocating the available bandwidth (the available amount of channels) in multiple users.

Multiple Access Protocols

Fixed-Assignment Protocols

Channelization Protocol

In fixed-assignment strategies, each node is allocated a predetermined fixed amount of the channel resources. Each node uses its allocated resources exclusively without competing with other nodes. Typical protocols that belong in this category include

Frequency-division multiple access(FDMA) Time-division multiple access (TDMA) Code-division multiple access(CDMA)

Demand Assignment Protocols Controlled Access Protocols The main objective of demand assignment protocols is to

improve channel utilization by allocating the capacity of the channel to contending nodes in an optimum or near-optimum fashion. Unlike fixed-assignment schemes, where channel capacity is assigned exclusively to the network nodes in a predetermined fashion regardless of their current communication needs, demand assignment protocols ignore idle nodes and consider only nodes that are ready to transmit.

Polling Reservation Token Passing

Random Assignment Protocols

In fixed-assignment schemes, each communicating node is assigned a frequency band in FDMA systems or a time slot in TDMA systems. This assignment is static, however, regardless of whether or not the node has data to transmit. These schemes may therefore be inefficient if the traffic sourceis bursty. In the absence of data to be transmitted, the node remains idle, thereby resulting in the allocated bandwidth to be wasted. Random assignment strategies attempt to address this shortcoming by eliminating preallocation of bandwidth to communicating nodes.

Random Assignment Protocols

Random assignment strategies do not exercise any control to determine which communicating node can access the medium next. Furthermore, these strategies do not assign any predictable or scheduled time for any node to transmit. To deal with collisions,the protocol must include a mechanism to detect collisions and a scheme to schedule colliding packets for subsequent retransmissions.

ALOHA CSMA CSMA/CD

Frequency Division Multiple Access (FDMA)

The FDMA scheme is used by radio systems to share the radio spectrum.Based on this scheme, the available bandwidth is divided into subchannels. Multiple channel access is then achieved by allocating communicating nodes with different carrier frequencies of the radio spectrum. The bandwidth of each node’s carrier is constrained within certain limits such that no interference, or overlap, occurs between different nodes. The scheme requires frequency synchronization among communicating nodes.

FDMA

f1

FDMA was the initial multiple-access technique for cellular systems

Separates large band into smaller channels.

Each channel has the ability to support user.

Guard bands are used to separate channel preventing co-channel interference

Narrow bandwidth (30 khz).

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FDMA

FDMA

FDMA is a continuous transmission scheme as compare to TDMA because fewer bits are needed for synchronization and framing.

In FDMA, as unique channels are assigned to each user, so FDMA systems have higher cell site system cost as compared to TDMA system.

In FDMA, both the transmitter and receiver operates at the same time so FDMA Mobile units require duplexers. These also increase the cost of FDMA subscriber units and base station.

FDMA

The number of channels in FDMA system that can be simultaneously supported in FDMA System is given by

Where B is total spectrum allocation,Bguard is guard bandwidh,Bc is channel bandwidth

FDMA

Advantages– Simple to implement in terms of hardware.– Fairly efficient with a small base population and with

constant traffic. Disadvantages

– Network and spectrum planning are intensive and time consuming.

– Channels are dedicated for a single user, idle channels add spectrum inefficiency.

Example

(i)Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.

(ii)Five channels, each with a l00-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10kHz between the channels to prevent interference?

Example

Example First-generation analog systems were allocated a total bandwidth of B =25 MHz for

uplink channels and another B =25 MHz for downlink channels. This bandwidth allocation was split between two operators in every region, so each operator had 12.5 MHz for both their uplink and downlink channels. Each user was assigned Bc =30 KHz of spectrum for its analog voice signal, corresponding to 24 KHz for the FM modulated signal and 3 KHz guardbands on each side. The total uplink and downlink bandwidths also requred guard bands of Bg =10 KHz on each side to mitigate interference to and from adjacent systems. Find the total number of analog voice users that could be supported in the total 25 MHz of bandwidth allocated to the uplink and the downlink.Also consider a more efficient digital system with high-level modulation so that only 10 KHz channels are required for a digital voice signal with tighter filtering such that only 5 KHz guard bands are required on the band edges.How many users can be supported in the same 25 MHz of spectrum for this more efficient digital system?

Solution:-

For either the uplink or the downlink, with guard bands on each side of the voice channel, each user requires a total bandwidth of Bc +2Bg. Thus, the total number of users that can be supported in the total uplink or downlink bandwidth B =25 Khz is

or 416 users per operator. Indeed, first-generation analog systems could support 832 users in each cell. The digital system has

users that can be supported in each cell, almost a three-fold increase over the analog system. The increase is primarily due to the bandwidth savings of the high-level digital modulation, which can accommodate a voice signal in one third the bandwidth of the analog voice signal

Time Division Multiple Access (TDMA

In TDMA, a radio spectrum is divided into time slots. These time slots are allocated for each user to transmit and receive information. The number of time slots is called a frame. Information is transferred and received in form of frame. A frame is consists a preamble, an information message and trial bits.

Time Division Multiple Access (TDMA

In TDMA, data transmission is not continuous and subscriber transmitter can be turned off which result in low battery consumption.

In TDMA, handoff process is much simpler for a subscriber because of discontinuous transmission.

In TDMA, duplexers are not required because different timeslots are used for transmission and reception.

In TDMA, the rate of transmission is very high as compare to FDMA.

TDMA

Entire bandwidth is available to the user for finite period of time.

Users are allotted time slots for a channel allowing sharing of a single channel.

Requires time synchronization. Each of the user takes turn in

transmitting and receiving data in a round robin fashion.

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TDMA

TDMA Frame Structure

TDMA Frame Structure

Each sequence of 8 time slots is known as a TDMA frame

TDMA

Preamble bits:- contains address and synchronization information that both the base station and subscriber use to identify each other.

Guard Bits:- bits are used to allow synchronization of the receivers between different slots and frames.

Example

Efficiency of TDMA

Efficiency of TDMA

Efficiency of TDMA

Efficiency of TDMA

Example

Example

TDMA Advantages & Disadvantages

Advantages– Extended battery life and talk time – More efficient use of spectrum, compared to FDMA – Will accommodate more users in the same spectrum space than an

FDMA system Disadvantages

– Network and spectrum planning are intensive – Multipath interference affects call quality – Dropped calls are possible when users switch in and out of different

cells.– Too few users result in idle channels (rural versus urban

environment) – Higher costs due to greater equipment

Examples

Examples

Examples

Code Division Multiple Access (CDMA)  In CDMA, all users transmit information simultaneously by

using the same carrier frequency. Each user has its own code word, which is orthogonal to other users. To detect the message, the receiver should know the codeword used by the transmitter.

Code Division Multiple Access (CDMA)

CDMA includes the following features: In CDMA system many users share the same frequency. In CDMA unlike FDMA and TDMA the number of users is

not limited. It has a soft capacity. But due to large number of users its performance degrades.

In CDMA, each user operates independently with no knowledge of the other users.

CDMA

CDMA is a spread spectrum technique used to increase spectrum efficiency.

SS has been used in military applications due to anti-jamming and security.

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CDMA

CDMA

Code-Division Multiple Access (CDMA)

We start with a data signal with rate D, Which we call the bit data rate. We break each bit into k chips according to a fixed pattern that is specific to each user, called the user's code.The new channel has a chip data rate of kD chips per second.

Basic Principles of CDMA– D = rate of data signal– Break each bit into k chips

Chips are a user-specific fixed pattern – Chip data rate of new channel = kD

CDMA Example

If k=6 and code is a sequence of 1s and -1s– For a ‘1’ bit, A sends code as chip pattern

<c1, c2, c3, c4, c5, c6>– For a ‘0’ bit, A sends complement of code

<-c1, -c2, -c3, -c4, -c5, -c6> Receiver knows sender’s code and performs electronic

decode function <d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s code

665544332211 cdcdcdcdcdcddSu

CDMA Example

User A code = <1, –1, –1, 1, –1, 1>– To send a 1 bit = <1, –1, –1, 1, –1, 1>– To send a 0 bit = <–1, 1, 1, –1, 1, –1>

User B code = <1, 1, –1, – 1, 1, 1>– To send a 1 bit = <1, 1, –1, –1, 1, 1>

Receiver receiving with A’s code– (A’s code) x (received chip pattern)

User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 ->6 unwanted signal ignored

CDMA Advantages

– Greatest spectrum efficiency: – CDMA improves call quality by filtering out background noise, cross-talk, and

interference – Simplified frequency planning - all users on a CDMA system use the same

radio frequency spectrum. – Random Walsh codes enhance user privacy; a spread-spectrum advantage – Precise power control increases talk time and battery size for mobile phones

Disadvantages – Backwards compatibility techniques are costly – Currently, base station equipment is expensive – Low traffic areas lead to inefficient use of spectrum and equipment resources