When the mobile unit is active (i.e. when a mobile phone is switched on), it registers^[5] with the appropriate BS , depending on its location, and its cell position is stored at the responsible MSC. When a call is set-up (when a user makes a call), the base station monitors the quality of the signal for the duration of the call, and reports that to the controlling MSC, which in turn makes decisions concerning the routing of the call.

When a cellular phone moves from one cell to the other, the BS will detect this from the signal power and inform the MSC of that. The MSC will then switch the control of the call to the BS of the new cell, where the phone is located. This is called handover . It normally takes up to 400ms, which is not noticeable for voice transmission.

A cellular phone user can only use his/her mobile within the covered area of the network. Roaming is the capacity of a cellular phone, registered on one system, to be able to enter and use other systems. Those other systems must be compatible to enable roaming (i.e. they must have the same type of networks). In Europe, the standard cellular network is called GSM (Global System for Mobile Communication). Incoming calls to GSM^[8] users are routed to them, irrespective of where they are, as long as they are within Europe.

From the above argument, it would seem that increasing the number of cells in the covered area (i.e. by decreasing the cell size) would increase the capacity. But by doing so, a number of difficulties arise:

• Interference : decreasing the cell size, especially with a low repeat factor increases the problems of interference between cells which are using the same frequency.
• Handovers: Decreasing the cell size increases the frequency of handovers, since a moving cellular phone would be changing cells more often. Since the MSC needs time to switch (for handovers), increasing the handovers will increase that time delay.

It has been pointed out that decreasing the cell size increases capacity but causes other problems such as increased interference and time to handle handovers. However having an intelligent cell, which is able to monitor where exactly the mobile unit is and find a way to deliver confined power to that mobile unit, will increase channel capacity without causing these problems.
In a microcellular system^[1], each cell is divided into a number of microcells; each microcell (or zone) has a zone site and the cell itself has one base station. It is necessary to note that all the microcells, within a cell, use the same frequency used by that cell; that is no handovers occur between microcells.

Base Station Signals
When a call is made to a cellular phone, the system already knows the cell location of that phone. The base station of that cell knows in which zone, within that cell, the cellular phone is located. Therefore when it receives the signal, the base station transmits it to the suitable zone site. The zone site receives the cellular signal from the base station and transmits that signal to the mobile phone after amplification. By confining the power transmitted to the mobile phone, co-channel interference is reduced between the zones and the capacity of system is increased.

The benefits of Microcellular systems

• Interference reduced (compared to decreasing the cell size)
• Handovers reduced (also compared to decreasing the cell size) since the microcells within the cell operate at the same frequency; no handover occurs when the mobile unit moves between the microcells
• Size of the zone apparatus. The zone site equipment are small, so the can be mounted on the side of a building of on poles.
• Increased system capacity. The microcell is an intelligent cell. The new microcell knows where to locate the mobile unit in a particular zone of the cell and deliver the power to that zone. Since the signal power is reduced, the microcells can be closer and therefore increase capacity

In TDMA, the bandwidth allocated for the channel is divided into time slots - the number of slots depends on the system. Each user is then allocated a slot, and hence multiple users share the same frequency but at different times.
In FDMA, the channel is divided into frequency bands, and each user is allocated a frequency band.

2. DATA TRANSMISSION OVER EXISTING CELLULAR NETWORKS

CMPS for Data Transmission Coping with Handover Buffering

Digital Data Cellular Network for Data Transmission Architecture Modifications Medium Access Protocol Buffer Administration Techniques Performance of the Digital Data Cellular Network

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The cellular network described in section 1 is a form of Cellular Mobile Phone Service (CMPS) network meant especially for mobile phone services. Therefore we have so far considered voice transmission on the cellular network. Now transmission that allows transactions such as sending an e-mail to a remote user or a portable computer talking to a central database is considered. This form of transaction requires data transmission and can be done using the existing cellular networks. Now, a discussion of data transmission on existing cellular networks is presented.

Here, two ways in which the cellular network can be used for data transmission are explained. They are

• Using the existing CMPS with no architectural modification. CMPS is a circuit-switched network which means once a connection has been made between two telephones, this connection would be dedicated to these telephones for the whole conversation.
  
• Using a Digital Data Cellular Network.
  This network unlike the CMPS would use packet switched technology and requires slight architectural modification to the CMPS. A conceptual model of the Digital Data Cellular Network is used to explain this method of data transmission.
  

More emphasis will be given on the second method, since it is more suitable than the first for data transmission (

2.1.2 Buffering

Once handover occurs, the MS and the BS stops transmitting data. However, both of them will still be receiving data from the data terminal and the PSTN network respectively. Therefore the buffer needs to be implemented in two places. One between the data terminal and the MS for upward transmission (transmission from MS to BS) and the other between the BSC and the BTC for downward transmission (opposite to upward).

The diagram below illustrates these queues.

2.2.1 Architecture Modifications

The MSC is now replaced by a packet switch exchange since packet switch technology is used. The diagram below shows the overall architecture of this network.

An example of a digital cellular network that is opened for public service is Paknet, a joint venture company between Racal Telecom and Mercury Communications.
Other than the change illustrated above, everything else about the architecture remains the same with that of CMPS. There is still a central computer accessible by all the base stations which continuously monitors the location of the MSs. Even now when an MS moves to another cell, handover occurs and control is handed to another BS.
The cell operation and spectrum utilisation methods remain the same as the architecture described in section 1.

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2.2.2 Medium Access Protocols

Protocols define a set of steps which allows meaningful conversation to take place. In cellular networks, a set of protocols, which describes the data that is to be transmitted and to be expected by both mobile station and base station, is defined . This allows these components within the cellular network to know what and when to listen, transmit and receive the relevant data. This data could be either control information or the information itself.

2.2.3 The Protocol

The protocol described here is a combination of TDMA and Polling. The right to access each channel is controlled by the BS. In the channels, data packets are transmitted at discrete time intervals known as time slots.

There are three kinds of channels. They are control, uplink data transfer and downlink data transfer channels.

The uplink data transfer channel is used to transmit packets from the MS to the BS while the downlink data transfer channel is used to transmit packets in the opposite direction.

The control channel transmits system packets from the BS to all the MSs in the cell. The most important contents of these packets are the Header and the Data Assignment Table (DAT). The Header contains information regarding number of channels available and its corresponding frequency. Meanwhile, the DAT acts to inform which MSs are allowed to transmit and receive packets in the next time slot.

In the next time slot, the MS informed to transmit uses the appropriate upward data transfer channel to transmit packets if it has message, otherwise a null reply is sent. This shows that the BS is effectively polling the MSs to transmit packets in the upward channel.
The MS informed to receive tunes to the selected downward transfer channel and receive the packets.

2.2.4 Buffer Administration Technique

The MS and the BS receive and transmit data packets. If the MS has only one receiver, it can only tune into one channel at a time and receive a single packet from this channel time slot. So, the packets that arrive at the base station from the packet switch exchange, meant for several MSs, have to be queued and forwarded one at a time to the appropriate transmitting channel (channel which the MS is waiting to receive the packets from). Transmitting more than a single packet to a mobile station would result in packet overlap and thus packet loss.

Buffer administration Techniques describes how the arriving packets are queued and selected to be transmitted from the BS to the appropriate MS.

There are three Buffer Administration Techniques :-

1. Cyclic Polling
2. Channel Splitting
3. Reservation

2.2.4.1 Cyclic Polling

In this technique there is an arbitrary number of queues of fixed capacity. Each MS has its own queue that is not shared with other MSs. There are several number of transmitters. This system would consist of a single server (channel controller) which would repeat the following steps :-

1. Check the queue for any packet.
   1. If there are, then send the packet to a free transmitter
   2. If there are no packets, then move to the next queue

repeat step (I) until

1. all the queues have been visited
2. there are no free transmitters.

And then point to the next queue to be visited in the next discrete time slot.

The diagram below illustrates this technique

2.2.4.2 Channel Split Technique

This is the same as the Cyclic Polling Technique but it has several channel controllers which operate in parallel. Each channel controller is dedicated to a group which consists of several queues. Each group of queues also has its own dedicated transmitter. Packets queued in a group can only be transmitted using this group's transmitter.

The diagram below illustrates this graphically.


The steps performed by the channel controller still remains the same as cyclic polling but now it only visits the queues in its group.

2.2.4.3 Reservation Technique

In this technique, similar to the Channel Split Technique, the transmitters are dedicated to groups. But now, in each group there is only a single queue. The incoming stream of packets which would have been put into different queues within a group in the Channel Splitting Technique would be put into this single queue. Therefore the queue capacity in this technique is normally multiples of the queue capacity in both Channel and Cyclic Polling Techniques.

The diagram below illustrates this technique

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WHY?

When the packet arrival rate is unequal in each queue (which is normally the case) , in both the Reservation and Channel Splitting Techniques, the transmitters allocated to the queues with lower arrival rate will be inefficiently used.
Meanwhile, the Cyclic Polling Technique would use all its transmitters efficiently since any one of these transmitters can be used to transmit the packets.

(b) Incoming Traffic Level Supported

The BS is said to support an incoming traffic level when it can operate without any queue overflow and the packets queued has acceptable mean waiting time in the queue.
The incoming traffic level considered here can be divided into moderate and high levels.
Mathematical analysis^[3] shows that the results obtained for both moderate and high incoming traffic levels are the same.

The Reservation Technique gives the best support for both traffic levels without any possibility of queue overflow. The Cyclic Polling Technique gives the second best support followed by the Channel Splitting Technique.

WHY?

Since the queue capacity in the Reservation Technique is the largest, it can accommodate larger number of packets at both incoming traffic levels. Since both Cyclic Polling and Channel Splitting Techniques have the same queue size, the inefficient use of the transmitters in the Channel Splitting Technique causes the Cyclic Polling Technique to have a better ability to prevent queue overflow.

However, the reservation channel causes the highest mean waiting time of the packets in the queue. The Cyclic Polling Technique gives the lowest mean waiting time followed by the Channel Splitting Technique.

WHY?

The queues in the Reservation Technique which has a higher capacity than the rest causes more packets to be queued. So, any packet in the queue has to wait longer before getting transmitted. In the Cyclic Polling Technique, the packets have lower mean waiting time than the Channel Split Technique as it uses the transmitters more efficiently.

Final point on buffer administration techniques

1. The Cyclic Polling Technique supports the highest maximum rate of packet arrival without any possibility of congestion.
2. The Reservation Technique gives support to the highest number of arriving packets without any possibility of congestion at the expense of an increased packet delay.

i. Dynamic changes in the queue size

The queue size is changed depending on the rate of packet arrival to the queue. In all three, techniques this would lead to reduction in queue overflow. The queue with a higher rate or number of packet arrival should be allocated a larger queue space. This could be obtained from the queues with lower rate or number of packet arrival respectively.

In order to do this extra intelligence to monitor the rates and number of packet arrival to all the queues is needed. The system should then be able to predict future arrivals based on the previous data obtained. This system should also be able to determine the optimal queue size needed as increasing the size too much would lead to an increase in the mean waiting time of packets.

ii. Dynamic Channel Assignment

One of the main reasons packet overflow occurs in both Reservation and Channel Splitting Techniques is because of the limited number of transmitter channels available in each group.
So an improvement would be to add intelligence that allocates channels to groups that has higher rate or number of packet arrival. Again the same intelligence, as discussed above, is needed.

Disadvantage of the Improvement Methods Suggested

Both dynamic channel and queue size assignment requires extra processing power. This could prove to be very costly to implement.

But on the other hand if these methods give an improvement that is more than proportionate to the cost increase, it would be prudent to implement a queuing system with these improved techniques.

2.3.1.2 Rate At Which BS Polls MS

The BS controls the rate at which a MS transmits data since the MS transmits data only if it had been polled to do so in the previous time slot (refer section 2.2.2). If the rate is too high the receive buffer for this MS in the BS would be congested.

Solution

Reduce the rate at which the BS polls the MS to transmit packets in the upward data transfer channel. Care must be taken not to reduce the polling rate too much as this can cause the transmit buffer in the MS to suffer queue overflow.

2.3.1.3 Number of Channels Per Cell

When the number of channels per cell is small (less transmitting channels) base station congestion is very likely to occur. Increasing the number of channels to solve this problem could be a problem since :-
the bandwidth allocated for uplink and downlink transmission is fixed. Therefore increasing the channel number per cell would cause the channel frequencies used in a cell to be re-used in a closer cell. This increases co-channel interference.

Solution

Use a microcellular network since it can increase the number of channels per cell without an increase in co-channel interference (see section 1.4.2).

2.3.2 Fading

This is the reduction of signal power. Fading is caused by many factors - the most important ones being multipath and shielding.

Shielding is the absence of field strength. Most common causes are tunnels, hills and inside certain buildings.

CDMA is a 'spread spectrum' technology; it spreads the information contained in a particular signal over the entire bandwidth allocated for the mobile communication. With CDMA, unique digital codes, rather than radio frequencies or channels, are used to differentiate the different signals. These codes are shared by both the transmitter and receiver; and hence the receiver (the mobile phone) receives all the signals but can only recognise the one with the same codes.

CDMA has many advantages over the existing cellular systems

• Increases capacity and improves quality of the signal.
• Simplified system planning through the use of the same frequency all over the covered area.
• Enhanced privacy