Communication Systems/Cellular Systems

Cellular Voice Networks

Mobile radio service was introduced in St. Louis in 1946. This radio dispatching system had an operator who patched the caller to the PSTN. Later, IMTS allowed customers to dial their calls without an operator. From this humble beginning came the present cellular phone system.

The cellular network is viewed by the PSTN as just another end-office where calls originate and terminate.

The STP (signal transfer point) handles the network routing by establishing the route to the HLR (home location register) for a specific mobile user. This simplifies network management, because only the routing tables in the STP need to be updated as the system grows. Each MSC (mobile switching center) does not have to maintain full routing tables to all other MSCs.

Cellular Voice Reference Model

AC — Authentication Center. This manages the authentication of the end user or equipment on behalf of the MS. It may serve many HLRs or in fact be an HLR itself.

BS — Base Station. The base station manages one or more cell sites and consists of a controller and one or more radio transceivers.

EIR — Equipment Identity Register. This is not presently well defined, but is used to identify end user equipment and reduce the incidents of fraud.

HLR — Home Location Register. This identifies the particular user and their service profile. It also records their current location and authorization period. The HLR may be distributed over more than one entity.

MS — Mobile Station. This is the actual radio based terminal that provides customer access to the network.

MSC — Mobile Switching Center. The telecommunications switch which routes calls between mobile users and the PSTN.

VLR — Visitor Location Register. This allows visitors to roam on other systems.

This reference model distinguishes between various tasks and does not necessarily reflect the actual physical equipment.

Unfortunately, there is a wide range of incompatible technologies being used at the Um air interface. Consequently, although it is possible to communicate with people all over the world, it is not possible to take your phone with you everywhere. This makes it difficult to adapt the present system to support global PCS.

The cellular infrastructure market is dominated by four major corporations: Lucent Technologies, Nortel, Ericsson, and Motorola.

The current cellular system is experiencing unprecedented growth in subscribers, services, and technological innovation. Some of the offered services include:

• Paging, vehicle location

• Text, data, facsimile transmission

• Traffic, weather information

• Emergency aid dialing

• Electronic funds transfer for fare payment

There are however, some areas of concern:

• Computerized call hand-offs between cell sites

• Caller identification

• Remote diagnostics

• Reliability

• Technology mix [digital, analog, UHF, audio, computer]

• Billing

• Long distance paging

Each subscriber is assigned transmit and receive frequencies for the duration of a call. The frequency pair is sometimes referred to as the forward and reverse channels, or the up and down link. Under control from the cell site, the cellular phone must be able to tune to any of the hundreds of frequency channels in the system.

Some of the adjacent cells will monitor the signal strength from the portable unit in order to help the central exchange to determine which cell site should handle the call. If at some time, this signal becomes weaker because the customer has moved into another cell service area, a call hand–off will be necessary. This requires that the cellular phone switch to another set of frequencies, and the calling path be rerouted. All of this must appear transparent to the user.

AMPS

AMPS is an analog system based on FDMA and used extensively in North America. It has been slightly modified since its inception and some of its characteristics are:

• Channel width: 30 KHz

• Duplex spacing: 45 MHz

• Data transmission: BFSK ±8 KHz deviation

• Output power: 0.6 to 4 watts

The present system divides the 832 channels into two blocks of 416 channels. Within each block, 21 channels are reserved for signaling.

Some of the weaknesses associated with this system include:

• Call blocking during busy hours in urban areas

• Misconnects and disconnects due to rapidly fading signals

• Lack of privacy and security

• Limited data transmission [1200 bps]

One technique, which has been put forward to resolve some of the congestion problem now found in the major urban areas, is to narrow the channel broadcast bandwidth. NAMPS(narrow AMPS) increases system capacity by splitting each 30 kHz AMPS channel into three 10 kHz channels. This is seen as an interim solution until a better scheme is agreed upon.

Cell Channel Allocation

It is not possible to assign all channels to each cell since adjacent cells using the same frequencies would interfere with each other. The channel distribution in the 832-channel system is as follows:

• A cell group of 7 adjacent cells share 416 full duplex channels

• No cell contains any adjacent frequency channels

• 4 cells are assigned 56 channels

• 3 cells are assigned 57 channels

• 21 channels are reserved for control

Frequency utilization can be improved by cell splitting and sectoring. These approaches effectively reduce the size of the customer service area and allow frequencies reuse. This increases the number of hand-offs and other demands on the MTX.

Cell splitting involves the creation of a new smaller cell from two larger ones, while sectoring is the breakup of a single cell into smaller ones. Typically, cell sites are split 3 or 4 to 1.

To minimize spill over into nearby cells, the cell antennas are given a slight downward tilt, and the output power is limited to 100 Werp.

Another way to increase utilization is by channel borrowing. A few channels are allowed to violate the normal frequency assignments and move between cells. This allows the system to dynamically vary the number of customers that can be served in a given cell. Careful consideration must be given to potential co-channel interference

This principle can be further extended to provide dynamic channel assignment, where the assigned cell frequencies are continually changing to meet the shifting demand patterns.

GSM

GSM is a third generation wireless telephone technology that allows roaming in 17 European countries. It started off as a pan-European standard but quickly gained world wide acceptance for its comprehensive and thoughtful implementation.

GSM uses a TDMA access format and has a call hand-off capability, thus increasing customer mobility and allowing inbound calls. Base stations can handle 124 frequency bands. Channel 0 is performs a dual role of providing a signaling channel and monitoring signal strength. All other channels can be assigned to subscribers.

• Up Link Frequency 890 - 915 MHz

• Down Link Frequency 935 - 960 MHz

• Carrier Spacing 200 KHz

• Can support 1000 speech or data channels

• Uses GMSK† modulation

UMTS