Evolution Of Cellular Systems • The advent of Cellular communications began in 1940s known as MTS(Mobile Telephone Systems) • There has been a gradual advancement in the capacity and signal strength. • The development can be broadly divided into 3 categories namely first, second and third generations.
hidden and exposed terminals • Hidden terminals • • • •
A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C
• Exposed terminals
• B sends to A, C wants to send to another terminal (not A or B) • C has to wait, CS signals a medium in use • but A is outside the radio range of C, therefore waiting is not Necessary • C is “exposed” to B
near and far terminals • Terminals A and B send, C receives • signal strength decreases proportional to the square of the distance • the signal of terminal B therefore drowns out A’s signal • C cannot receive A
MULTIPLE ACCESS METHODS • There are predominantly three types of multiple access methods. 1.SDMA(Space Division Multiple Access) 2.FDMA(Frequency Division Multiple Access) 3.TDMA(Time Division Multiple Access) 4.CDMA(Code Division Multiple Access) 5.DAMA(Demand Assigned Multiple Access) 6.PRMA(Packet Reservation Multiple Access)
SDMA(Space Division Multiple Access) • SDMA is used for allocating a separated space to user in wireless networks. • SDMA never used in isolation but always in combination with one or more other schemes.
• A comparative study between the FDMA,TDMA and CDMA technologies with respect to time and frequency is as shown below.
FDMA(Frequency Division Multiple Access)— • With FDMA, frequencies are separated in the frequency domain, each user is allotted a different set of frequencies to operate upon. • Subscribers are assigned a pair of voice channels (forward and reverse) for the duration of the call.
FDD/FDMA - general scheme, example GSM
TDMA(Time Division Multiple Access) • assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time. • each user is allocated a different time slot. Forward link frequency and reverse link frequency is the same. A synchronous switch is responsible for the time switching.
TDD/TDMA - general scheme
Aloha/slotted aloha • Mechanism • random, distributed (no central arbiter), timemultiplex • Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries
DAMA - Demand Assigned Multiple Access(Reservation-ALOHA) • Channel efficiency only 18% for Aloha, 36% for Slotted Aloha • Reservation can increase efficiency to 80% • Reservation period is followed by transmission period. • a sender reserves a future time-slot • sending within this reserved time-slot is possible without collision • reservation also causes higher delays under light load. • typical scheme for satellite links
Access method DAMA: Explicit Reservation • Explicit Reservation (Reservation Aloha): • two modes: • ALOHA mode for reservation: – competition for small reservation slots, collisions possible
• reserved mode for data transmission within successful reserved slots (no collisions possible)
• it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time.
Access method DAMA: PRMA • Implicit reservation (PRMA - Packet Reservation MA): • a certain number of slots form a frame, frames are repeated • stations compete for empty slots according to the slotted aloha principle • once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send • competition for this slots starts again as soon as the slot was empty in the last frame
MACA - collision avoidance • MACA (Multiple Access with Collision Avoidance) uses short signaling • packets for collision avoidance • RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet • CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive • Signaling packets contain • sender address • receiver address • packet size
• MACA avoids the problem of hidden terminals • A and C want to send to B • A sends RTS first • C waits after receiving CTS from B
• MACA avoids the problem of exposed terminals • B wants to send to A, C to another terminal • now C does not have to wait for it cannot receive CTS from A
3.CDMA(Code Division Multiple Access)— There is no restriction on time and frequency in this scheme. All the users can transmit at all times and at all frequencies. Because users are isolated by code, they can share the same carrier frequency, eliminating the frequency reuse problem encountered in other technologies.
CDMA SPECIFICATIONS Frequency of operation: 824-849Mhz & 869-894 Mhz Duplexing Method : Frequency Division Duplexing Access Channel per carrier: Maximum 61 Channels RF Spacing: 1.25 Mhz verage: - 5 Km with hand held telephones and approx. 20 Km with fixed units. • CDMA channel in the trans and receive direction is a FDD (Frequency Division Duplexing) channel. • Frequency-division duplexing means that the transmitter and receiver operates at different carrier frequencies.