In packet radio (PR) access techniques, many subscribers attempt to access a single channel in an uncoordinated (or minimally coordinated) manner
- Packet radio multiple accesses is very easy to implement but has low spectralefficiency and may induce delays.
- Transmission is done by using bursts of data.
- Collisions from the simultaneous transmissions of multiple transmitters are detected at the base station receiver, in which case an ACK or NACK signal is broadcast by the base station to alert the desired user (and all other users) of received transmission.
- ACK signal indicates an acknowledgment of a received burst from a particular user by the base station
- NACK (negative acknowledgment) indicates that the previous burst was not received correctly by the base station.
- By using ACK and NACK signals, a PR system employs perfect feedback, even though traffic delay due to collisions may be high.
Packet Radio Protocols:
- To study packet radio protocols, it is assumed that all packets sent by all users have a constant packet length and fixed, channel data rate, and all other users may generate new packets at random time intervals.
- It is also assumed that packet transmissions occur with a Poisson distribution having a mean arrival rate of X packets per second.
- If t is the packet duration in seconds, then the traffic occupancy or throughput R of a packet radio network is given by
- R is the normalized channel traffic (measured inErlangs) due to arriving and buffered packets, and is a relative measure of thechannel utilization.
- If R> 1, then the packets generated by the user’s exceed the maximum transmission rate of the channel. Thus, to obtain a reasonable throughput, the rate at which new packets are generated must lie within 0<R<1.
- The normalized throughput is always less than or equal to unity and may be thought of as the fraction of time a channel is utilized
- The normalized throughput is given as the total offered load times the probability of successful transmission, i.e.
Pr [no collision] is the probability of a user making a successful packet transmission.
- The probability that n packets are generated by the user population during a given packet duration interval is assumed to be Poisson distributed and is given as
- Packet is assumed successfully transmitted if there are no other packets transmitted during the given packet time interval.
- The probability that zero packets are generated (i.e., no collision) during this interval is given by