Basic Packet Multiplexed Streams/Scheduling

INTRODUCTION TO WIRELESS NETWORKS

To provide wireless communications within a particular geographic region (a city, for example), an integrated network of base stations must be deployed to provide sufficient radio coverage to all mobile users.

The base stations, in turn, must be connected to a central hub called the mobile switching centre (MSC).
The MSC provides connectivity between the public switched telephone network (PSTN) and the numerous base stations, and ultimately between all of the wireless subscribers in a system.
The PSTN forms the global telecommunications grid which connects conventional (landline) telephone switching centres (called central offices) with MSCs throughout the world.
Radio links are established using a carefully defined communication protocol called common air interface also known as the handshake communication protocol.

Differences Between Wireless and Fixed Telephone Networks

Transfer of information in the public switched telephone network (PSTN) takes place over landline trunked lines (called trunks) comprised of fiber optic cables, copper cables, microwave links, and satellite links.
The network configurations in the PSTN are virtually static, since the network
connections may only be changed when a subscriber changes residence and requires reprogramming at the local central office (CO) of the subscriber.
Fixed networks are difficult to change
The available channel bandwidth for fixed networks can be increased by installing high capacity cables (fiber optic or coaxial cable).
Transfer of information is by wireless radio links.
Wireless networks are highly dynamic, with the network configuration being rearranged every time a subscriber moves into the coverage region of a different base station or a new market.
Wireless networks must reconfigure themselves for users within small intervals of time (on the order of seconds) to provide roaming and handoffs between calls as a mobile moves about.
Wireless networks are constrained by the limited RF cellular bandwidth provided for each user.

Comparison of Circuit switching and Packet switching

Circuit Switching

First generation cellular systems provide connection-oriented services for each voice user by a technique called circuit switching.
A physical radio channel is switched in to use for two-way traffic between the mobile user and the MSC, and the PSTN dedicates a voice circuit between the MSC and the end-user.
Circuit switching establishes a dedicated connection between the base and mobile, and a dedicated frill duplex phone line between the MSC and the PSTN for the entire duration of a call.
Wireless data networks are not well supported by circuit switching, due to their short, bursty transmissions which are often followed by periods of inactivity.
Circuit switching is best suited for dedicated voice-only traffic, or for instances where data is continuously sent over long periods of time.

Packet Switching

Packet switching (also called virtual switching) is the most common technique used to implement connectionless services
It allows a large number of data users to remain virtually connected to the same physical channel in the network.
Call set-up procedures are not needed to dedicate specific circuits when a particular user needs to send data.
Packet switching breaks each message into smaller units for transmission and recovery and adds a certain amount of control information to each packet (fig below).
It provides excellent channel efficiency for bursty data transmissions of short length.
The channel is utilized only when sending or receiving bursts of information which is useful in cases of limited bandwidth.
Packet switching supports intelligent protocols for data flow control and retransmission.

INTRODUCTION TO MULTIPLE ACCESS

Cellular systems divide a geographic region into cells where a mobile phone in each cell communicates with a base station. The main objective of a cellular system design is to handle as many calls as possible within the given bandwidth with maximum reliability.
The techniques that have been developed to allow many users to simultaneously share the finite amount of resources or radio spectrum in a most efficient way are known as multiple access techniques. As a result, high capacity is achieved by simultaneously allocating the available bandwidth to multiple users.
The possible multiple access methods are
Frequency Division Multi `Access (FDMA)
Time Division Multiple 'Access (TDMA)
Code Division Multiple Access (CDMA)
Space Division Multiple Access (SDMA)

Frequency Division Multiple Access (FDMA)

Frequency division multiple access (FDMA) assigns individual channels to individual users. Each user is allocated a unique frequency band or channel. These channels are assigned on demand to users who request service. During the period of the call, no other user can share the same frequency band. In FDD systems, the users are assigned a channel as a pair of frequencies; one frequency is used for the forward channel, while the other frequency is used for the reverse channel.
The first U.S. analog cellular system, the Advanced Mobile Phone System (AMPS), is based on FDMA/FDD.

Features of FDMA

FDMA channel carries only one phone circuit at a time.
If an FDMA channel is not in use, then it sits idle and cannot be used by other users
To increase or share capacity.
After the assignment of a voice channel, the base station and the mobile transmit simultaneously and continuously.
FDMA is a narrowband system.
The symbol time is large as compared to the average delay spread which means that the amount of intersymbol interference is low.
Less complex when compared to TDMA.
Fewer overhead bits are needed as compared to TDMA.
FDMA systems have higher cell site system costs as compared to TDMA because of the single channel per carrier design.
More expensive due to the use of duplexer in the mobile unit.
FDMA requires tight RF filtering to minimize adjacent channel interference

Time Division Multiple Access (TDMA)

Time Division Multiple Access (TDMA) systems divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive. Each user occupies a cyclically repeating time slot, so a channel may be thought of as particular time slot that reoccurs every frame, where N time slots comprise a frame. TDMA systems transmit data in a buffer-and-burst method, thus the transmission for any user is noncontinuous. This implies that digital data and digital modulation must be used with TDMA.
The transmission from various users is interlaced into a repeating frame structure.
A frame consists of a number of slots. Each frame is made up of a preamble, an information message, and trail bits.

Features of TDMA

TDMA shares a single carrier frequency with several users, where44th user makes use of nonoverlapping time slots.
Data transmission is in discrete bursts which gives extended battery life.
Handoff process is much simpler for a subscriber unit, since it is able to listen for other base stations during idle time slots.
TDMA uses different time slots for transmission and reception, thus duplexers are not required.
Adaptive equalization is usually necessary in TDMA systems.
The transmission rates are generally very high-as compared to FDMA channels.
In TDMA, the guard time should be minimized.
High synchronization overhead is required in TDMA systems because of burst transmissions.
It is possible to allocate different numbers of time slots per frame to different users.
More efficient use of spectrum.

Code division multiple access (CDMA).

Code division multiple access (CDMA) is one type of Spread Spectrum Multiple Access Technique. It uses the spread spectrum techniques to increase spectrum efficiency over current 1DMA and FDMA systems.
Spread spectrum multiple access (SSMA) uses seals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth.
There are two main types of spread spectrum multiple access techniques;
Frequency hopped multiple access (FH)and
Direct sequence multiple access (DS)
Direct sequence multiple access is called code division multiple access (CDMA).
CDMA uses unique spreading c61es to spread the baseband data before transmission.
CDMA assigns a unique code sequence to each user that is used to code data before transmission. If the receiver knows the code sequence related to a user, it is able to decode the received data. The codes are called Pseudo-noise (PN) sequences.
The narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal which is the pseudo-noise code sequence.

Features of CDMA

All users in a CDMA system, use the same carrier frequency and may transmit simultaneously. Each user has its own pseudorandom code word which is approximately orthogonal to all other code words.
The receiver performs a time correlation operation to detect only the specific desired code word. All other code words appear as noise due to decorrelation. For detection of the message signal, the receiver needs to know the code word used by the transmitter. Each user operates independently with no knowledge of the other users.
CDMA has a soft capacity limit. increasing the number of users in a CDMA system raises the noise added to the system.
Multipath fading may be substantially reduced because the signal is spread over a large spectrum.
Channel data rates are very high in CDMA systems.
Self-jamming is a problem in CDMA system.

Frequency Hopped Multiple Access (FHMA)

Frequency Hopped Multiple Access also known as Frequency Hopped Spread Spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudo random sequence known to both transmitter and receiver. The carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel.
The digital data is broken into uniform sized bursts which are transmitted on different carrier frequencies. The instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth.
In the FH receiver, a locally generated PN code is used to synchronize the receivers instantaneous frequency with that of the transmitter.
At any given point in time, a frequency hopped Signal only occupies a single, relatively narrow channel.
A frequency hopped system provides a level of security, especially when a large number of channels are used, since an unintended (or an intercepting) receiver that does not know the pseudorandom sequence of frequency slots must retune rapidly to search for the signal it wishes to intercept.
The FH signal is somewhat immune to fading, since error control coding and interleaving can be used to protect the frequency hopped signal against deep fades.
Bluetooth uses FHSS technique.

Space Division Multiple Access (SDMA)

Space division multiple access (SDMA) uses the spatial dimension for multiplexing of different data streams by transmitting the data streams over different non-overlapping transmission channels. SDMA enables users to share simultaneously the same bandwidth in different geographical locations.
Space division multiple access (SDMA) controls the radiated energy for each user in space. SDMA serves different users by using spot beam antennas. These different areas covered by the antenna beam may be served by a TDMA or CDMA system at the same frequency or by different frequencies in an FDMA system.
Sectorized antennas are an application of SDMA. Adaptive antennas can be used to simultaneously steer energy in the direction of many users at once and appear to be best suited for TDMA and CDMA base station architectures.
By using adaptive antenna arrays also called smart antennas in mobile radio systems, signals can be received and sent only from and into a limited angular range, following the directional nature of multipath This improves coverage in noise-limited situations and enhances capacity in interference-limited situations.

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