The GSM frequencies

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GSM900 uses the radio frequency range 890-915 MHz for receive and 935-960 MHz for transmit. RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for use.

Extended GSM (EGSM)

The BSS software is capable of supporting an extra 10 MHz bandwidth in the uplink and downlink, known as Extended GSM900 (EGSM900). The extended frequency range is 880-890, trasmit and 925-935, receive. Provided the frequency type is specified as GSM900 in the equip site command, all cells at that site can be configured as GSM900 with the option to use EGSM900 as well. The EGSM frequencies can only be used if the correct radio hardware is available. The BCCH frequency and the channels configured as SDCCH must always be set within the GSM900 band to ensure MS compatibility. The database parameter to enable EGSM is chg_element freq_band_egsm900.

An idle MS is notified of the frequencies active in the cell via the cell channel description element in the system information messages. This element contains 16 available octets in which to encode the carrier frequencies in use. The number of frequencies that can be encoded depends upon the following criteria:

  • If EGSM frequencies are active, 49 carriers may be specified.
  • If channel 0 is also active, only 17 frequencies may be encoded.

When the CRM allocates a TCH for an EGSMMS it searches between extended then primary band frequencies starting from best to worst interference bands until a channel is found. To avoid blocking of primary MSs, an extended MS using a primary resource can be forced to handover to an idle extended band resource. This feature is controlled by use of the database parameter chg_element egsm_handover_threshold. If hopping is active within the cell, the hopping frequencies cannot be mixed; an MS is required to hop over primary frequencies or EGSM frequencies but not over a mixture of both within the same call.

DCS1800 frequencies

DCS1800 uses the radio frequency range 1710-1785 MHz for receive and 1805-1880 MHz for transmit. RF carriers are spaced every 200 kHz, allowing a total of 373 carriers for use (one used as a guard band).

High power DCS1800 with increased sensitivity receiver matrix

The receiver sensitivity increases restriction of a single cell in a single cabinet. The high powered DCS1800 radio has a calibrated output power of 32 watts measured at the antenna connector with no combining. The new radio operates in all DCS1800 TCU configurations. The new 32 watt radio does not use a 16 watt (16 watts measured at the antenna) TCU/SCU replacement radio and therefore is not used with other low powered radios within the same cell.

System impact

The high powered DCS1800 radio requires changes to the acceptable bay level offset range. This is due to the high gain, lower noise figure for the dual port preselectors (-108.5 dBm sensitivity).

The high power DCS1800 radio will also not support Rx daisy chain with the new dual path preselector. This feature has an impact on the power control algorithm, fault management, configuration management, the man machine interface, and the alarms processes.

The customer can order (or utilize) the high power feature independently from ordering the increased sensitivity receiver matrix.

A BTS cabinet can contain a high power DCS1800 radio and an existing DCS1800 radio, but will use only one radio type in a cell; no sharing is allowed. In addition, it would not be practical to install a pre-amplifier with the configuration.

Air interface properties

Overview

This section describes the technical characteristics of the GSM900, Extended GSM (EGSM900), DCS1800 and PCS1900 digital cellular Air Interfaces.

The GSM Air Interface is a noise-robust transmission medium. The speed of a radio channel used in GSM is 270.833 kbit/s. The modulation is 0.3 BT Gaussian Minimum Shift Keying (GMSK).

Frequency band characteristics

BTS radio channels (RF carriers) are full-duplex (transmit and receive) with the characteristics listed in Table 1-4.

Multipath fading

GSM provides features to overcome multipath fading, where the signal between the transmitter and the receiver travels by multiple paths. These paths are produced by signal reflections caused by the terrain in which the equipment is being used and the prevailing atmospheric conditions. The effect is that the received signal is the sum of a number of different copies of the transmitted signal, each copy reaching its destination at a different time.

The difference between the multiple times of reception of a given bit symbol (the time dispersion) is often longer than the 3.70 µs duration of the bit symbol itself when it was originally transmitted. A GSM system must be able to distinguish one bit from the next despite these problems, so sophisticated equalization techniques are employed to eliminate the effects of inter-symbol interference.

Optimized power control

Flexible power steps

The flexibility in defining power steps is added by allowing uplink and downlink power step sizes to be controlled separately. This is done by creating two increment and decrement step size elements, one pair each for uplink and downlink directions.

Extended range for increment step size

The range of the power control steps for increments is extended, to allow for a range of 2, 4, 6, 8, 10, 12, and 14 dB in both uplink and downlink directions. This allows the power to be brought back into the power box (the acceptable power range) quickly if it should suddenly fall below the acceptable range.

Dynamic step adjust procedures

Power increment and reduction step sizes are allowed to change dynamically (if the algorithm is enabled), based on the current power level and proximity to lower and upper power level and quality thresholds. This allows the power to be brought up or down at a faster rate when it has strayed out of the power box or quality box. Downlink oscillation control

Power oscillation occurs when a decision to reduce power due to good quality is immediately followed by a decision to increase power due to a low power level. To prevent unnecessary power pp(25) chapter#1 changes, oscillation must be detected and controlled. Oscillation prevention exists for uplink power control, but the Optimized Power Control feature extends this to downlink power control using a different algorithm (from the uplink oscillation control algorithm).

  • The total number of ARFCNs is 124 in PGSM & 374 in DCS.

The basic principles of calling a mobile phone

The fundamental difference between mobile phones and fixed telephones is that mobile phones transmit and receive voice and data calls using radio connections specifically designed to allow the user to move around whereas fixed telephones use connections (either wired or wireless) which are fixed in location. In a mobile network, the radio connections are only between the handset and the nearest base station,4 in the same way that a fixed telephone is connected to the local exchange (or concentrator unit).

The remainder of a mobile network is then similar to a fixed network. A series of switches and their associated processors support the radio coverage provided by the cells and supply the intelligence for the network. The processors decide the location to which the call should be switched, whether this is just to the next switch in the network or to another fixed or mobile network. The switches direct the calls across the network until they reach their intended destination or a point of interconnection. 3.12. In order for a mobile phone to be able to make or receive a call, it must be within radio cover-age of a base station and registered with the network. The area (or areas in the case where the coverage of a base-station is split into a number of sectors) of radio coverage provided by base stations are known as 'cells', so named because the pattern of coverage formed from a number of base stations is cellular (similar to a honeycomb).

Differences between GSM 900 and GSM 1800 3.32. Generally speaking, the higher the radio frequency employed, the shorter the transmission range that will be achieved for equivalent parameters (for example, transmitter power, antenna size and height, terrain, etc). Thus GSM cells operating at 900 MHz can cover greater areas than those operating at 1800 MHz. It follows that when initially rolling out a network to provide a particular level of cover-age, more cell sites will be required at 1800 MHz than at 900 MHz. However, these additional cells will give the 1800 MHz network additional initial capacity. 3.33. In addition, penetration inside buildings is generally regarded as more difficult at 1800 MHz than at 900 MHz2 and hence in order to provide equivalent levels of deep, dense urban coverage, more cells are required at 1800 MHz than at 900 MHz.3 3.34. However, as illustrated in Table 3.1, the amount of spectrum available to O2 and Vodafone is less than that available to Orange and T-Mobile. In principle, the more spectrum that is available to an operator, the easier it is to provide additional capacity without recourse to new cell sites; the less spec-trum available, the more difficult it is. Thus, once the required level of coverage is achieved, it is easier for Orange and T-Mobile to provide additional capacity without the need to develop new cell sites than it is for O2 and Vodafone.1 Table 3.3 shows the number of cell sites employed by each of the operators as of September 2001. September 2001 has been used as a reference point as this is the date to which the output of Oftel's modelling refers.

Terminology for the TSCs differs between operators with some calling TSCs Gateway MSCs (GMSCs). However, the fundamental network designs are based on the GSM standard and topologies are largely the same. 2A study conducted by the Institut für Nachrichtentechnik und Hochfrequenztechnik of the Technical University Vienna showed that: 'The penetration loss in small cells showed a much stronger dependence on elevation within the building than previously found. Losses were larger at 1800 MHz than at 900 MHz'. http://www.nt.tuwien.ac.at/nthft/dipl_diss_veroeff/diss94.html. 3Vodafone told us that because 900 MHz penetrates better through walls, whilst 1800 MHz penetrates better through openings (for example, windows) the two effects cancel each other out and concludes that penetration inside buildings is similar for both frequencies. 10

In the early years of the UK networks' operations, most phones sold for use operated only in single band, that is to say either 900 MHz or 1800 MHz, as the cost of providing dual-band handsets, ie capable of operating on both bands, was high.2 Over time the cost differential in producing single and dual-band phones has diminished, so most handsets being sold today are capable of operating on both 900 MHz and 1800 MHz. Thus as new handsets are purchased or old ones replaced, the majority3 of sub-scribers on all networks have become equipped with dual-band handsets.

This is important for a number of reasons. First, there are fewer 1800 MHz operators world-wide than 900 MHz operators. Thus the opportunities for a customer of a GSM 1800 network with only a single-band 1800 MHz phone to use their phone overseas, and the opportunities for GSM 1800 operators to generate revenue from these activities, is more limited than for customers and operators of a GSM 900 network. Using a dual-band handset overcomes these difficulties and gives equal capability to any user. Second, Vodafone and O2 have both 900 and 1800 MHz frequencies (as do many operators in other countries) and thus a dual-band phone needs to be used in order to allow the customers to use the net-work to the full. Dual-band handsets usually switch between the two frequencies automatically, so the caller does not know which frequency is being used.

Other differences between 900 MHz and 1800 MHz networks are:

  • Cost of infrastructure: 1800 MHz technology was developed more recently than 900 MHz tech-nology. In addition, whilst 900 MHz technology is used almost universally across the world, there are a more limited number of countries which have licensed operators at 1800 MHz. Orange and T-Mobile were among the first operators in the world to construct 1800 MHz net-works. The cost of the BTSs has historically been greater for 1800 MHz than for 900 MHz, but these differences are now largely insignificant. The rest of the infrastructure (BSC, MSC, TSC etc) is identical for 1800 and 900 MHz networks.
  • Cost of handsets: As for BTSs, the cost of 1800 MHz handsets has, in the past, been greater than for 900 MHz handsets. These differences are now minimal and, in any event, most handsets now being produced are dual-band.
  • Operation at high speed: The specifications for GSM 900 and GSM 1800 include an upper 'speed limit' for the handset, above which the radio connection is not guaranteed. For GSM 900 this is 250km/h, and for GSM 1800, 125km/h. This constraint is only likely to affect users on high-speed trains or motorists driving faster than the UK road speed limit.
  • Range of cell coverage: The maximum range achievable in ideal conditions for a GSM 1800 cell is half that of a GSM 900 cell, though coverage is usually limited by terrain and interference rather than the theoretical limits of the system.
  • Transmitter power of handsets: GSM 1800 handsets have a maximum transmitter power that is half that of GSM 900 handsets, reducing further the level of coverage that can be achieved.

However, as stated in paragraph 3.32, the initial capacity of Orange and T-Mobile's networks will have been higher than that of O or Vodafone at the launch of their respective networks. 2 2In some parts of the world, notably North America, GSM networks operate at frequencies around 1900 MHz (referred to as GSM 1900). Some handsets (sold in the UK) are capable of operating on 900, 1800 and 1900 MHz and are known as 'tri-band' phones. 3[] per cent for Vodafone UK now.

The situation with regards to the spectrum allocated to the four UK GSM operators is not, however, as clear-cut as simply 900 MHz versus 1800 MHz. Vodafone and O2 whilst being largely 900 MHz operators have, in reality, spectrum in three separate bands, E-GSM, GSM 900 and GSM 1800, whereas Orange and T-Mobile only have GSM 1800 spectrum.

  • E-GSM: Extended-GSM (E-GSM) spectrum is at 900 MHz (880-890 MHz paired with 925-935 MHz) and was used in the UK by Vodafone and O2 for TACS until 2001. Thus it has not been fully available for the provision of GSM services until recently. It is also spectrum which sits outside the normal GSM 900 spectrum allocation. As such, until it started to become avail-able for GSM services (in the UK and elsewhere in Europe), manufacturers did not produce E-GSM compatible handsets. Thus, even when the operators began operating service in the E-GSM band, most subscribers could not access it. This situation is now changing and many of handsets manufactured are E-GSM capable. Both O2 and Vodafone have 5 MHz of E-GSM spectrum.
  • GSM 900: The GSM 900 spectrum is at 900 MHz (890-915 MHz paired with 935-960 MHz) and has been used by O2 and Vodafone since the inception of their GSM services. Both O2 and Vodafone have 12.6 (or 12.8â€"see Table 3.1) MHz of GSM 900 spectrum each, in two non-contiguous blocks.
  • GSM 1800: The GSM 1800 spectrum is at 1800 MHz (1710-1785 MHz paired with 1805-1880 MHz) and is the only spectrum available to Orange and T-Mobile who each have a single, contiguous block of 30 MHz. O2 and Vodafone each have a single contiguous block of 5.7 MHz at 1800 MHz. As with E-GSM capable handsets, it has only been in the past few years that dual-band 900/1800 MHz phones, and the techniques for operating a dual-band networks, have become available. Until that time, it was not possible for O2 and Vodafone to use their GSM-1800 spectrum for additional capacity as handsets would not have been able to change seamlessly between the 900 and 1800 MHz services.

Until recently, therefore, Vodafone and O2 had only 12.6/8 MHz each of GSM 900 spectrum that was of use whereas Orange and T-Mobile had 30 MHz of GSM 1800 spectrum. Now, however, O2 and Vodafone have a total of 23.3/5MHz of spectrum, although this is in three non-contiguous blocks.

Call routing and charges

A proprietary technical means of implementing MNP, which results in calls and SMS text messages to numbers which have moved to other networks being routed via both the original and recipient operators' networks, has been developed for use in the UK. This system relies on the original network (ie the network on which the mobile user's number was originally registered) acting as an intermediary and routing the call onto the recipient network (ie the network on which the mobile user is now registered).

In terms of the current UK system, the charges paid by subscribers calling a ported mobile sub-scriber are as follows:

  • Calls to a subscriber mobile network A are charged according to the tariffs for calling network A.
  • This subscriber now moves to network B but keeps the same number by porting it across to net-work B. People calling the subscriber from mobile network A or B are charged according to the tariffs for calling network B. Callers from all other networks are charged according to the tariffs for calling network A as only networks A and B are aware of the change.
  • If the subscriber now moves to mobile network C, but again keeps the same number, the result-ing charges are as if the subscriber had moved directly from network A to network C.

In terms of the current UK system, the fees received by the relevant MNOs for calls made to a ported mobile subscriber are as follows:

  • For calls to a subscriber of mobile network A, network A receives its standard termination charge for the call or its charge for on-net calls for calls made by subscribers of network A.
  • This subscriber now moves to network B but keeps the same number by porting it across to net-work B. Network B receives its standard termination charge for calls originated on network A or its charge for on-net calls for calls made by subscribers of network B. For calls from all other networks, network B receives the termination charge for calls to network A minus a small transit fee which is retained by network A.
  • If the subscriber now moves to mobile network C, but again keeps the same number, the result-ing flow of payments is as if the subscriber had moved directly from network A to network C.

However, a ported subscriber is sometimes given a temporary new number on the recipient network, which can also be called by any party. This does not affect callers from the original or recipient network whose call charges already reflect the subscriber being on the recipient network, but for all other callers, it offers the opportunity to select one of two networks on which to call the user and thus two different tariffs. There is therefore some scope, for a limited period, for incoming callers to ported mobile subscribers to select one of two tariffs in order to minimize the cost of their call, albeit at the cost to the caller of having to remember two numbers.

Billing systems

Another key component in the ability of mobile (and fixed) operators to deliver differentiated services and tariffs is the billing system. For each call made on any given network, a Call Detail Record (CDR) is generated by both the network from which the calls was made and the network which the call was terminated. The CDR contains information on the number called, the number from which the call was made, the duration of the call, the time the call was initiated, and can store other information such as if certain network features (for example, call-back) were used. This raw information is then processed by the billing system which applies the appropriate tariff for each call before producing the retail or whole-sale bill.

At present, tariffs are applied based on the prefix of the number called (for example, 07xxx). However, should a database with the appropriate tariff for each individual number be made available (for example, together with the central database required to implement an IN solution), it would be possible to apply a different tariff for each number called (for example, 07xxx xxxxxx). If it were possible to contain, within the CDR, information on the call itself, such as whether the recipient or sender keyed in any additional codes, it may also be possible to bill for each individual call on a one-by-one basis.

A combination of new billing systems and IN platforms offer the opportunity for new services and technologies to be employed to deliver calls to mobile handsets. We sought the views of interested industry parties as to what such solutions may be feasible. The results of our consultation are sum-marized in the table in Appendix 3.1.

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