WO2000076109A1

WO2000076109A1 - System and method for adaptive modulation in a cellular telecommunications network

Info

Publication number: WO2000076109A1
Application number: PCT/EP2000/004906
Authority: WO
Inventors: Jorma Matero; Kari Kananen; Jouni HÄNNINEN
Original assignee: Nokia Corporation
Priority date: 1999-06-04
Filing date: 2000-05-26
Publication date: 2000-12-14
Also published as: AU5526600A; GB9913101D0

Abstract

The present invention relates to a transmitter and a method of transmission, particularly for use in a wireless communication network. Signals are transmitted from a first user to a second user, the transmitter being capable of selecting one of a plurality of different modes of transmission, 16-QAM for example, the selection of transmission mode taking into account the type of information service to be provided.

Description

SYSTEM AND METHOD FOR ADAPTIVE MODULATION IN A CELLULAR TELECOMMUNICATIONS NETWORK

The present invention relates to a transmitter and a method of transmitting for use, for example, in a wireless cellular telecommunications network.

In wireless cellular telecommunications networks, the network is divided into a plurality of cells each of which is served by a base station. Each base station is arranged to communicate with the mobile terminals in the cell associated with the base station. Networks may be arranged to deal with different information rates. For example, some voice information services can require a lower rate as compared to certain data information services. Other types of data service may require a lower rate than voice services such as certain types of packet data services .

The signals which are transmitted between the base station and the mobile station are modulated. The modulation method used is generally specified in the standard which is being used in the particular wireless telecommunications network. Different modulation methods have been specified in different standards. One standard which is currently being proposed requires the use of two different modulation methods. The modulation method is selected in accordance with the data rate.

However the inventors have appreciated that there is a fundamental flaw with this proposal . Different modulation methods will have different sensitivities to interference. Typically, modulation methods which support greater data rates are more sensitive to interference. If no steps are taken to deal with this problem, then the signal may not be correctly received. This is particularly disadvantageous with some types of data service. One possibility to deal with this problem would be to increase the power with which the signals are transmitted, if the interference is relatively large and an interference sensitive modulation method is to be used. However this would unnecessarily increase the interference suffered by other users in the system and may lead to a reduction in capacity and/or signal quality.

It is therefore an aim of embodiments of the present invention to address this problem.

According to a first aspect of the present invention, there is provided a transmitter for transmitting signals from a first user to a second user, said transmitter comprising means for selecting one of a plurality of different modes to be used for a signal to be transmitted, and selection means for selecting the mode, taking into account at least one characteristic of a channel defined between said first and second user.

By ensuring that the modes are selected in accordance with at least one characteristic of the channel, it can be ensured that the most appropriate mode is used for a given characteristic.

Preferably, the selection means takes into account the type of data to be transmitted when determining the said mode . For example, the data may be voice data, packet data or any other type of data.

The selection means may take into account the bit rate of data to be transmitted when determining the mode. Thus, the mode can take into account not only a characteristic of the channel but also the bit rate of the date to be transmitted.

Preferably at least two of the modes use different modulation methods . The different modulation methods comprise two or more of the following modulation methods : GMSK; π/4-DQPSK; 8-PSK; 16-QAM; PSK; and QAM.

In preferred embodiments of the present invention, the π/4-DQPSK and 8-PSK are the preferred modulation methods. In preferred embodiments of the present invention, two different modulation methods are provided. These different modulation methods may have different data rates associated therewith. This may be taken into account when selecting the modulation method as well as the characteristic of the channel.

In one embodiment of the present invention, at least two modes have different channel widths associated therewith for the transmission of signals. Wider channel widths may be associated with faster data rates. It should be appreciated that different modulation methods or the same modulation method may be associated with the different channel widths. In one embodiment of the present invention, two different modulation methods are provided, with at least one modulation method being associated with two different channel widths.

Preferably, in one mode, signals are transmitted at the same time signals are received and in another mode, signals are transmitted at different times to when signals are received. Again, these modes will take into account a characteristic of the channel.

The at least one characteristic of the channel may be the quality thereof . The characteristic of the channel may be determined based on the bit error rate of signals transmitted from between the first and second users.

The at least one characteristic of the channel may be determined based on the strength of the signal received from the second user by the first user. In a particularly preferred embodiment of the present invention, the quality of the channel and the strength of the signal received from the second user by the first user are both taken into account when selecting a particular mode.

The transmitter described hereinbefore may be incorporated in a mobile station or a base transceiver station.

According to another aspect of the present invention, there is provided a method for transmitting signals from a first user to a second user, said method comprising the steps of selecting one of a plurality of modes taking into account at least one characteristic of a channel defined between said first and second user; and transmitting a signal in accordance with the selected one of the plurality of different modes to a signal to be transmitted.

For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which:

Figure 1 shows a typical wireless cellular telecommunications network;

Figure 2a shows a constellation diagram for the GMSK modulation method;

Figure 2b shows a constellation diagram for the π/4-DQPSK modulation method;

Figure 2c shows a constellation diagram for the 16-QAM modulation method;

Figure 2d shows a constellation diagram for the 8-PSK modulation method;

Figure 3 shows a transceiver embodying the present invention;

Figure 4a shows a known slot structure for D-AMPS;

Figure 4b shows a first alternative slot structure; and

Figure 4c shows a second alternative slot structure.

Reference will now be made to Figure 1 which shows a wireless cellular network 2. The network comprises a plurality of cells 4, each of which is served by a respective base transceiver station 6. Each base transceiver station 6 is arranged to send radio signals to and receive radio signals from terminals 8 in the cell associated with the respective base station 6. It should be noted that, depending on the standard being used, terminals may be in communication with more than one terminal at a time and accordingly the cells of adjacent base stations may overlap. The terminals may be mobile or fixed terminals. For example the terminal may be a mobile telephone or a data communication device . Before describing the embodiment of the present invention, four different modulation methods will be described purely by way of example with reference to Figures 2a to 2d.

Referring first to Figure 2a, this shows the constellation diagram for the GMSK (Gaussian minimum shift keying) modulation method which is used in the GSM (Global system for mobile communications) standard. As can be seen there are only two constellation points which are separated by π which means that this modulation method is relatively resilient to errors. This because the chances of confusing the values is small. One symbol is required to transmit one bit . However the data rate supportable by this modulation method is relatively low. GMSK modulation is a constant envelope modulation. This means that no transitions through the origin of the constellation diagram are allowed. This is achieved with Gaussian filtering in the modulator. The benefit of the constant envelope modulation is that a nonlinear power amplifier can be used in the transmitter, which gives optimum efficiency (low current consumption, long talk time) . The GSM mode is very efficient.

For each symbol only one bit of data can be sent . The symbol rate thus equals the data bit rate. In GSM, the symbol rate is 280.8333 ksps (kilosymbols per second). The modulation is robust, since the distance between constellation points is large. The transitions in the constellation are on the circle of the constellation diagram. There is no amplitude variation because of the constant envelopes and this method can be used with nonlinear amplifiers .

Reference is now made to Figure 2b which shows the constellation diagram for the π/4-DQPSK (π/4 differential quadrature phase shift keying) modulation method which is used in the D-AMPS (digital advanced mobile phone) standard. This modulation method is typically used with a channel width of 30kHz. This constellation has eight constellation points which are separated by π/4 and accordingly this modulation method is less resilient to interference than that shown in Figure 2a. The dotted lines shown in this Figure represent the permitted transitions between the eight possible states. The permitted transitions are such that there is no large variation in power. This means that the linearity requirement of the amplifiers is relaxed and efficiency is improved. Non linear amplifiers are more efficient than linear amplifiers. With this modulation method, with one symbol it is possible to transfer two bits. For each symbol, two bits of data can be sent. The data bit rate is thus equal to two times the symbol rate. In D-AMPS, the symbol rate is 24.3 ksps . QPSK modulation actually has four constellation points, but due to π/4 phase shifting and differential encoding, II/4-DQPSK has 8 constellation points. This modulation method has amplitude (envelope) variation due to transitions between constellation points. This amplitude variation sets higher linearity requirements than with GMSK, but still provides reasonable performance .

In return for increased data rate, a better signal to noise (S/N) or signal to noise and interference (S/(N+I)) ratio is required than in GMSK.

Reference is now made to Figure 2c which shows the constellation diagram for a 16-QAM (quadrature amplitude modulation) which has even more constellation points than the method shown in Figure 2b and is even less resilient to interference. Four bits can be transferred by one symbol.

Reference is made to Figure 2d which shows the 8-PSK (8-phase shift keying) modulation method. Three bits can be transferred by each symbol. With this modulation method, all possible transitions from one state to another are permitted. It has been proposed to use this method with channels having a width of 200kHz. As this modulation method allows three bits to be transferred by each symbol as compared to the two bits per symbol of the II/4-DQPSK modulation method, a higher data rate can be achieved than with the GMSK and II/4-DQPSK modulation methods. However, the signal is less tolerant to noise and interference. This modulation scheme is adopted because three bits can be transferred by each modulation symbol. This gives a high data transfer rate for the given symbol rate. The symbol rate may be 270.833 ksps, which would give a data rate of 812.5 kbps . Transitions between all the constellation points are allowed in the proposed modulation scheme. This requires a greater linearity for the amplifiers, as compared to the π/4-DQPSK modulation. Likewise, a better signal to noise ratio S/N is required than for π/4-DPSK method since the symbol energy is now divided between three bits.

Thus, more complex modulation schemes have to be implemented in order to achieve higher datarates.

Occupied frequency bandwidth of the modulation depends on symbol rate, the modulation method and pulse shaping.

Reference will now be made to Figure 3 which shows an embodiment of the present invention. In particular Figure 3 shows a transceiver 20 which may be incorporated in a mobile station or a base station. For the following description, it is assumed that the transceiver is in a mobile station.

The transceiver 20 comprises an antenna 22 which is connected both to the receive part 24 of the circuit and the transmit part 26 of the circuit. The receive part 24 of the circuit will be described first. The receive part 24 of the circuit receives signals from the antenna 22 via the part of a duplex filter 28 tuned to the frequency band of the received signal. The output of the duplex filter 28 is input to a first amplifier 30 which amplifies the received signal. The output of the first amplifier 30 is input to a first bandpass filter 32 which filter out any

noise introduced by the first amplifier 30 which is outside the bandwidth of interest .

The output of the first bandpass filter 32 is input to a first mixer 34 which also receives an input from a first synthesizer 36. The first mixer 34 mixes the output of the first bandpass filter 32 with the output of the first synthesizer 36 to reduce the frequency of the received signal to an intermediate frequency which is less than the radio frequency of the received signal. The output of the first mixer 34 is output to one of the second or third bandpass filters 38 and 40 depending on the position of a switch 37. The second bandpass filter 38 may have a bandwidth of 200kHz whilst the third bandpass filter 40 will have a bandwidth of 30kHz. The second bandpass filter is selected for signals which use the 8-PSK modulation method whilst the third bandpass filter is selected for signals which use the π/4-DQPSK modulation method.

The output of the second and third bandpass filters 38 and 40 are connected to an automatic gain control amplifier 42 which is controlled by a digital signal processor 62. This automatic gain control amplifier (AGC) 42 amplifies the output of the second and third bandpass filters 38 and 40. The output of the automatic gain control amplifier 42 is connected to the inputs of second and third mixers 44 and 46. The second and third mixers 44 and 46 also receive an input from a second synthesizer 48. The signals received by the second and third mixers 44 and 46 from the second synthesizer are 90° out of phase with respect to each other so that the outputs of one of the second and third mixers 44 and 46 provides the in phase signal whilst the output of the other of the second and third mixers 44 and 46 provides the quadrature signal . It should be noted that the output of each of the second and third mixers 44 and 46 will be at the base band after the signal from the automatic gain control amplifier 42 has been mixed in the respective mixers with a signal at the intermediate frequency from the second synthesizer 48.

The output from each of the second and third mixers 44 and 46 are input to respective first and second low pass filters 50 and 52. The first and second low pass filters 50 and 52 filter out any undesired mixing products introduced by the respective mixers outside the bandwidth of interest. The output of the fourth and fifth low pass filters are input to respective second and third amplifiers 54 and 56 which amplify the signal. The output of each of the amplifiers 54 and 56 is connected to a respective analogue to digital converter 58 and 60.

The output of the analogue to digital converters 58 and 60 are input to the digital signal processor 62 for further processing. The information contained in the received signal is extracted. Additionally, based on the strength of the signals detected and the gain applied by the automatic gain control unit 42, a measure of the strength of the received signal is obtained. The strength of a received signal will depend on a number of factors such as the distance between the base station and the mobile station, the amount of interference (the signal to noise ratio) and the path followed by the signal from the base station to the mobile station. Typically, the signal will be strong if the mobile station is close to the base station and weak if the mobile station is far from the base station.

The digital signal processor 62 also makes a determination as to the quality of the radio channel between the base station and the mobile station. The signal transmitted by the base station includes a sequence of known bits. The digital signal processor identifies that sequence of known bits in the received signal and compares the received sequence with the sequence which should have been received. This allows the bit error rate to be determined. If there is a good match between the received sequence and the sequence which should have been received, then it can be determined that the quality of the radio channel is good. On the other hand, if the match is poor it can be determined that the quality of the radio channel is poor. A poor quality radio channel is one with a poor signal to noise ratio. Likewise a good quality radio channel is one with a good signal to noise ratio.

In an alternative embodiment, the base station will instead send information based on measurements made by the base station as to the quality of the channel between the base station and the mobile station. The digital signal processor 62 will then extract the information as to the radio channel quality from the received signal. The transmitter will use the technique outlined hereinbefore to determine the radio channel quality. As described hereinafter, the radio channel information is used to control a parameter of the signals to be transmitted to the base station from the mobile station.

This latter embodiment is advantageous in that the information on the channel from the mobile station to the base station is used to determine the channel quality and that is then used to control transmissions from the mobile station to the base station. The first embodiment assumes that the radio channel is the same in both directions. This will not necessarily be the case, particularly where different frequencies are used in the different directions. Nevertheless, the measurements obtained in the first embodiment may be useful in some cases. Embodiments of the invention may alternatively use both of these measurements to obtain the quality of the channel. It is preferred that the quality in both the downlink (base station to mobile) and uplink (mobile to base station) directions be known. In embodiments of the inventions different modulation schemes can be used in the uplink and downlink directions depending on the required data transfer rate and the signal to noise S/N or signal to noise and interference S/(N+I) conditions in the respective directions.

The transmit part 26 of the transceiver 20 will now be described. The signals to be transmitted are output in the baseband frequency and in digital form to first and second digital to analogue converters 64 and 66 from the digital signal processor 62. It should be appreciated that the digital signal processor 62 is arranged to select one of the 8-PSK modulation method and the π/4-DQPSK modulation method. The following criteria is used for selecting the modulation method. Firstly the bit rate of the data to be sent is determined. If the bit rate is below a certain threshold, then the π/4-DQPSK modulation method is selected. This method may be automatically selected for certain types of communications such as voice communication and certain types of data service.

If the bit rate is above the threshold, then the quality of the radio channel and the received signal strength will be taken into account. If the received signal strength is strong and the quality of the channel is good, then the 8-PSK modulation method may be selected. If the received signal strength is weak and/or the quality of the channel is poor, then the π/4-DQPSK modulation method is selected or the user is advised that a requested service is unavailable. Various threshold values may be defined for the received signal strength and the quality of the radio channel values. For example, the π/4-DQPSK modulation method is used if one or other or both of the received signal values are below first threshold values and above second threshold values. The user may be advised that the requested services is not available if one or other or both of the received signal strength and radio channel quality values are below the second threshold values .

The bitrate threshold depends on required service. For example if there is a voice call only, the I1/4-DQPSK modulation method is used, where bitrate is 48.6 kbps. The digital to analogue convertors would output I and Q components of the modulation symbol at the rate of 24.3 ksps. However, if high speed data is to be sent, bitrate of 812.5 kbps and the 8-PSK modulation method are selected. The symbol rate would be 270.833 ksps. The output of each of the digital to analogue converters 64 and 66 would be half this rate.

In case of 8-PSK modulation, if the base station receives the signal in poor conditions i.e. with a low signal to noise ratio, the bit error rate (BER) may be above an acceptable level. The base station will then send a command to the mobile station to increase its power or switch modulation to the 1/4-DQPSK method which is less sensitive to interference.

In some embodiments of the present invention, different services with which it is preferred to use the π/4-DQPSK modulation method will have different threshold values for the received signal strength and the radio channel quality. These threshold values may depend on the data rate of the service in question.

In one preferred embodiment of the present invention, the 8-PSK method is used for data services and the π/4-DQPSK method is used for voice services.

Typically, more complex modulation schemes require the output amplifier on the transmit side to be more linear. This means that the output of the amplifier will have less power. Likewise on the receive side, a higher signal to noise ratio is required to receive the signal. This means that the more complex modulation schemes require an environment where the received signal strength is relatively high and the radio channel quality is reasonably good.

If a service cannot be provided, the user may be advised of this by a message displayed on a display of the mobile station.

It should be noted that the width of the channel will depend on the modulation method used. The signal will be transmitted on a channel having a width of 30kHz if the π/4-DQPSK modulation method is used and on a channel having a width of 200kHz if the 8-PSK modulation method is used.

The form of the analogue data output by the digital signal processor 62 will depend on the modulation method selected. The in phase signal information is input to the first digital to analogue converter 64 and the quadrature signal information is input to the second digital to analogue converter 66. The signals are converted to analogue form and input to third and fourth low pass filters 68 and 70 respectively. The third and fourth low pass filters 68 and 70 are used to filter out any noise introduced by the first and second digital to analogue converters 64 and 66.

The outputs of the third and fourth low pass filters are input to respective fourth and fifth mixers 72 and 74. The fourth and fifth mixers 72 and 74 also receive a signal from a third synthesizer 76 at an intermediate frequency. The signal provided by the third synthesizer 76 for the fourth mixer 72 is 90° out of phase with that signal provided by the third synthesizer 76 to the fifth mixer 74. The outputs of the fourth and fifth mixers 72 and 74 are at an intermediate frequency and are input to a second automatic gain control amplifier 78. The second automatic gain control amplifier 78 amplifies the received signals in accordance with the power with which the signal is ultimately to be transmitted. The second automatic gain control amplifier 78 is controlled by the digital signal processor 62.

The output of the second automatic gain control amplifier 78 is output to a fourth bandpass filter 80 which filters out any spurious signals introduced by the second automatic gain control amplifier 78. The output of the fourth bandpass filter 80 is output to a sixth mixer 82 which mixes that signal with the output of a fourth synthesizer 84 to provide a signal at the correct radio frequency.

The output of the sixth mixer 82 is input to a fifth bandpass filter 86 which filters out the undesired mixing products introduced by the sixth mixer 82. The output of the fifth bandpass filter 86 is input to a fourth amplifier 88 for amplification. The output of the fourth amplifier 88 is input to a sixth bandpass filter 90 which filters out any spurious signals introduced by the fourth amplifier 88.

The output of the sixth bandpass filter 90 is input to a fifth amplifier 92 which amplifies the signal. The output of the fifth amplifier 92 is input to the transmit part of the duplex filter 28 so that the signal can be transmitted by the antenna 22.

Typically, the bandpass filters in the transmit part of the transceiver are wider than those in the receive part of the circuit and therefore different filters for the different channel widths are not required.

In one modification to the embodiment described hereinbefore, the 8-PSK modulation method or any other modulation method may be used with a 30kHz channel with lower bit rates or with a 200kHz channel for higher bit rates. The same principles outlined in respect of the selection of different modulation methods can be used in the selection of the channels of different lengths.

Reference is now made to Figure 4. Figure 4a shows the known slot structure of D-AMPS. The first slot 100 is used to receive signals, the second slot 102 is an idle slot and the third slot 104 is a transmit slot.

Figure 4b shows an alternative scenario which similar to that shown in Figure 4a, except the idle slot 108 is now used to transmit or receive signals as well as the first and third slots 106 and 110. Figure 4c shows a fully duplex operation where signals can be received and transmitted at the same time in the respective slots 112 -122. This embodiment of the invention is applicable to time division multiple access systems and hybrids thereof .

In one alternative embodiment of the invention, there are two alternative modes, both of which may but not necessarily use the same modulation method. The mode shown in Figure 4a will be used for lower data rates whilst the mode shown in Figure 4b or 4c will be used for higher data rates if the received signal strength and/or the quality of the radio channel are good enough. Otherwise the mode shown in Figure 4a will be used. The duplex filter is optimized for the TDMA mode (where transmission and reception do not occur at the same time) . In this mode the insertion loss of the duplex filter is small but its rejection of noise generated in transmitter is low. If this same filter is used in full duplex operation (where transmission and reception can occur at the same time) , then the transmitter will introduce noise into the receiver. Full duplex operation should be allowed only when received signal is relatively high.

Accordingly, the full duplex operation shown in Figure 4c can cause problems particularly where the transmit and receive frequencies are not separated by much. In particular, the duplex filter will suffer a high insertion loss. This means that the full duplex operation should only be allowed if the received signal strength is relatively high and/or the radio channel quality is relatively good. The modes shown in Figure 4a or 4b should be used where the signal strength is relatively poor and/or the quality of the radio channel is relatively poor.

Embodiments of the invention may incorporate two or more of the modes of Figure 4 as well as the different modulation methods as described hereinbefore.

The function of the AGC 42 is to regulate the received signal in the receiver so that the signal level is at an optimum at the input of the analogue to digital converters. In the DSP 62 there is an algorithm which monitors the signal level at the outputs of the analogue to digital converters. If the level is too low then the DSP 62 increases the gain control signal value applied AGC amplifier 42 in the receiver section and vice versa when the signal is too high. The DSP62 thus knows the gain setting of the receiver.

The DSP can calculate the received signal strength (signal level at the antenna) based on its own knowledge about signal level at the output of the analogue to digital converters and the receiver gain .

It should be appreciated that in embodiments of the present invention, different modulation methods and/or modes can be used during a single call even if the data rate is constant so as to take into account changes in the received signal strength and/or the quality of the radio channel.

Preferred embodiments of the present invention are designed so that the receiver and transmitter is optimized to one of the modulation methods or modes of operation. This will be the mode or modulation method which is used more often in normal use. For example the standby times and talk times are optimised for one of the possible modulation methods. It is for example possible to optimize a terminal for voice operations and not data operations or vice versa.

Embodiments of the present invention may have more than two modulation methods available. In addition any other suitable modulation methods can be used and not just the two used in the preferred embodiment of the present invention. In addition any combination of modulation methods can provided.

In alternative embodiments of the present invention, the different modulation methods may use channels of the same width.

Embodiments of the present invention may be used with any suitable access technique such as time division multiple access, space division multiple access, frequency division multiple access, spread spectrum or hybrids of any one or more of these techniques .

Claims

CLAIMS ;

1. A transmitter for transmitting signals for providing information services from a first user to a second user, said transmitter comprising means for selecting one of a plurality of different modes to be used for a signal to be transmitted, and selection means for selecting the mode, taking into account at least one characteristic of a channel defined between said first and second user wherein the selection means takes into account the type of information service to be provided when determining the said mode, said type being one of data services and voice services .

2. A transmitter as claimed in claim 1, wherein the selection means takes into account the bit rate of data to be transmitted when determining the mode .

3. A transmitter as claimed in any preceding claim, wherein at least two of said modes use different modulation methods.

4. A transmitter as claimed in claim 3, wherein said different modulation methods comprise two or more of the following modulation methods:

GMSK; π/4-DQPSK; 8-PSK; 16-QAM; PSK; and QAM.

5. A transmitter as claimed in claim 3 or 4 , wherein two different modulation methods are provided.

6. A transmitter as claimed in any one of the preceding claims, wherein at least two modes have different channel widths associated therewith for the transmission of signals.

7. A transmitter as claimed in any one of the preceding claims, wherein in one mode signals are transmitted at the same time signals are received and in another mode signals are transmitted at different times to when signals are received.

8. A transmitter as claimed in any one of the preceding claims, wherein said at least one characteristic of said channel is the quality thereof.

9. A transmitter as claimed in claim 8, wherein the quality of said channel is determined based on the bit error rate of signals transmitted between the first and second users.

10. A transmitter as claimed in any one of the preceding claims, wherein said characteristic of said channel is determined based on the strength of the signal received from the second user by the first user.

11. A transmitter as claimed in any preceding claim wherein when the first and second users are relatively close, a mode which supports a relatively high data rate is selectable and when the first and second users are relatively far apart, a mode which supports a lower data rate is selectable.

12. A mobile station incorporating a transmitter as claimed in any one of the preceding claims.

13. A base transceiver station incorporating a transmitter as claimed in any one of claims 1 to 11.

14. A method for transmitting signals providing information services from a first user to a second user, said method comprising the steps of: selecting one of a plurality of modes taking into account at least one characteristic of a channel defined between said first and second user; and transmitting a signal in accordance with the selected one of the plurality of different modes, wherein said selecting step takes into account the type of information services to be provided when determining the said mode, said type being one of data services and voice services.