Which mobile phone?

Which mobile phone? A question that everyone, who is interested in buying a mobile phone, is asking. As mobile phones are getting more widely used, new service providers are emerging and intense competition is building up among the existing ones. These involve lowering the call price and giving customers free goods and call time after being connected.
This article is divided into two parts : the first covers and compares different standards in the UK, while the second introduces the idea of CDMA (code division multiple access) as compared to the multiple access system used by GSM.

PCN phones
PCN stands for Personal Communication Network, a digital mobile communications system using the DSC1800. The 1800 refers to the operating frequency which is 1800MHz (unlike the GSM which uses 900MHz).

Mercury One2One
Mercury was formed in 1992 and launched its PCN service in September 1993. The One2One is a joint-venture company owned equally by telecommunications giants Cable & Wireless and US West. The initial coverage area was within the M25 (motor-way), but this was improved considerably and now covers over 60% of the population - which is still a long way behind rival networks.
Mercury One2One offers a very good service within the London area, but its higher radio frequencies do not penetrate buildings as well as analogue frequencies do - this is compensated by its large number of cells in the covered areas.

The newest and fastest-growing digital network. Orange was launched in April 1994 as a Hutchison Telecom's PCN network. Since it was the newest of the networks, Orange had a massive challenge to make a mark as a major mobile phone company, which it did with its superb advertising campaign.
As stated earlier, Orange uses PCN. Before it was launched, it was tested in a centre (built by Orange) called Testbed. This testing allowed new aspects to be developed: the digital system opens up the possibility of using the phone with a computer to send and receive data.
When it was launched, Orange had the most rapid expansion plans of any mobile operator in the UK - from 50% in its year of launch to 70% by 1994 and then to 90% by 1995. This is mostly due to the improved inside buildings coverage and the fact that Orange charges calls by the second (now recently joined by Mercury) unlike other operators who charge by the nearest minute or half minute.

So which one is the better choice? Well, the answer to that really depends on the buyer. If you absolutely need good UK coverage today, then One2One simply isn't for you. However, if you travel predominantly in its regions of coverage, the One2One could be the right choice particularly if you take advantage of the free weekend calls that it is currently offering (although no one knows how long this offer will last!). One the other hand, Orange might seem the better choice if you need fax, data, SMS and Caller ID today.
Caller ID (or calling line identity) displays the number of the caller, provided they are calling from a BT phone.
SMS (or short messages service) allows text messages of up to 160 characters to be sent between phones. These two are currently only possible with Orange.

GSM phones
Having a GSM phone has an obvious advantage over the PCN system. As discussed in the previous article, GSM is the standard used in most European countries. Therefore having a GSM phone means that the same phone could be used in all those countries outside the UK (roaming). The GSM phones in the UK are Vodafone and Cellnet.

Vodafone was the first company to launch a digital network in the UK, back in 1991. It uses GSM digital phones which are secure. The global coverage of Vodafone is constantly improving.
Vodafone has announced the UK's first roaming agreement with a US company, American Personal Communication (APC) of Washington. This will allow UK customers to use their Vodafone SIM in Washington. The US does not use GSM and hence the GSM handsets will not work there. However, the roaming agreement allows Vodafone users to insert their SIM into a PCS phone (Personal Communication System - used in the US). In the long run, Vodafone expects that it will be introducing a duel-mode phone for roaming in the UK and US.

Cellnet launched its digital network in 1994. Over 100 European networks use the same technology and hence Cellnet Digital could be used in most major European cities as well as many other places including Australia, South Africa and Hong Kong.
The problem is that when a Cellnet Digital phone is used from abroad, the customer will pay a great deal more for using the foreign network. But the advantage is that even if you are abroad, the phone number does not change; a caller will dial exactly the same number whether the receiver is in the UK or anywhere else.
Since Cellnet supports transmitting data, it could be used to receive and transmit emails via a computer.
Cellnet and Vodafone digital phones have about the same features, and hence the choice really depends on the billing and charging criteria.


Multiple Access Systems
All the mobile networks discussed above use multiple access systems to allow multiple users over the same channel. Traditionally each multiple access systems user is given certain resources, such as frequency (FDMA) or time slots (TDMA used in GSM), which are disjoint from those of any other user. Each channel's capacity is limited only by the bandwidth or time alloted to it, the degradation caused by background noise and propagation anomalies - which produce multipath fading and shadowing effects. However these multiple access systems suffer from many limitations:
The first is that it assumes that all users are transmitting continuously for the same length of time. This may be nearly be true for packet switching data transmission, but usually not true for circuit-switched voice transmission; It is a known fact that during a two-person conversation, each speaker is active less than half the time.

The second is the geographical reallocation of spectrum. In the first article of my colleague ( Existing Technology of Cellular Networks-Mobile Phones), the frequency allocation of the cells in a cellular network was explained: the same frequency or time slots were never reused in contiguous cells, but maybe be reused in noncontiguous cells. Two cells that are allocated the same frequency or time slot are separated by more than one cell diameter, hence preventing interference. The consequence of this, however, is that the number of channels per cell is reduced by the reuse factor.

The third source of degradation particularly in terrestrial environments, is multipath. Phase cancellation between different propagation paths can cause severe fading; this makes the interference problem worse by lowering the signal power.


The CDMA Cellular Standard
With CDMA (Code Division Multiple Access), unique digital codes, rather than separate RF (radio frequencies) frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called pseudo-Random Code Sequences. All users share the same range of radio spectrum.
For cellular telephony, CDMA is a digital multiple access technique specified by the Telecommunications Industry Association (TIA) as "IS-95." In March 1992, the TIA established the TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gave its approval of the CDMA IS-95 standard.

IS-95 systems divide the radio spectrum into carriers which are 1,250 kHz (1.25 MHz) wide. One of the unique aspects of CDMA is that while there are certainly limits to the number of phone calls that can be handled by a carrier, this is not a fixed number. Rather, the capacity of the system will be dependent on a number of different factors. This will be discussed in later sections.

Spread Spectrum
CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.
A CDMA call starts with a standard rate of 9600 bits per second (9.6 kilobits per second). This is then spread to a transmitted rate of about 1.23 Megabits per second. Spreading means that digital codes are applied to the data bits associated with users in a cell. These data bits are transmitted along with the signals of all the other users in that cell. When the signal is received, the codes are removed from the desired signal, separating the users and returning the call to a rate of 9600 bps.

Traditional uses of spread spectrum are in military operations. Because of the wide bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere with, and difficult to identify. This is in contrast to technologies using a narrower bandwidth of frequencies. Since a wideband spread spectrum signal is very hard to detect, it appears as nothing more than a slight rise in the "noise floor" or interference level. With other technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect.

Increased privacy is inherent in CDMA technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an analog conversation, a simple radio receiver will not be able to pick individual digital conversations out of the overall RF radiation in a frequency band.

In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions of the same pseudo-random code. In order to assure that the time offsets used remain unique from each other, CDMA stations must remain synchronised to a common time reference.
The Global Positioning System (GPS) provides this precise common time reference. GPS is a satellite based, radio navigation system capable of providing a practical and affordable means of determining continuous position, velocity, and time to an unlimited number of users.

CDMA Benefits
When implemented in a cellular telephone system, CDMA technology offers numerous benefits to the cellular operators and their subscribers. The following is an overview of the benefits of CDMA.
Capacity increases of 8 to 10 times that of an AMPS analogue system and 4 to 5 times that of a GSM system
Improved call quality, with better and more consistent sound as compared to AMPS systems
Simplified system planning through the use of the same frequency in every sector of every cell
Enhanced privacy
Improved coverage characteristics, allowing for the possibility of fewer cell sites
Bandwidth on demand

In conclusion to this article, I would like to emphasize that the choice of cellular phone in the UK or anywhere else depends on where it will be used, how often and what sort of servises are required from the mobile.
CDMA is the newest of the multiple access systems. Although not many networks are using it at this moment, but I suspect that will change in the near future. The world's first CDMA network was launched last September - in Hong Kong operated by Hutchinson Telecom.

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