WO1994021072A1

WO1994021072A1 - Method for increasing communication range in a two-way communication system

Info

Publication number: WO1994021072A1
Application number: PCT/US1994/000531
Authority: WO
Inventors: Charles J. Malek; James Macdonald; David A. Hume
Original assignee: Motorola, Inc.
Priority date: 1993-03-08
Filing date: 1994-01-14
Publication date: 1994-09-15
Also published as: CN1098579A; AU5994994A

Abstract

A portable communication unit (120) which operates in a wireless communication system (100) comprising at least one fixed communication unit (102). The fixed communication units (102) are capable of communicating with the portable communication unit (120) at a first bit rate. There exists a method for decreasing group delay distortion and for increasing the communication range between the plurality of fixed communication units (102) and the portable communication unit (120) comprising the steps of: (a) establishing communication between the fixed communication units (120) and the portable communication unit (120) at the first bit rate, the established communication having a communication range associated with the first bit rate; (b) dividing the first bit rate into integer multiples for obtaining a second bit rate lower than the first bit rate; and (c) establishing communication at the second bit rate wherein the communication at the second bit rate decreases the group delay distortion thereby increasing the communication range between the fixed communication units (102) and the portable communication unit (120).

Description

METHOD FOR INCREASING COMMUNICATION RANGE IN A TWO-WAY COMMUNICATION SYSTEM

Field of the Invention

This invention relates in general to two-way communication systems, and more specifically, to a method for increasing communication range in a two-way communication system.

Background of the Invention

Radio communications involves either one-way (e.g. selective call signaling or paging systems) or two-way communications (cellular, cordless telephone, or digital personal communication systems) over radio waves. Two-way communication takes place on channels, comprising time division multiplexed (TDM) timeslots, code division multiplexed (CDM) codewords, or frequency division multiplexed (FDM) frequencies, or a combination thereof.

For various radio communications, a fixed radio frequency spectrum is assigned thereto. For example, in the United States, the

Federal Communications Commission (FCC) reserves various portions of the radio frequency spectrum to designated communication services. The radio frequency use is therefore limited to and for the assigned services. The radio frequency use is more overtaxed within certain areas where the number of frequencies allocable for communicating between radio transceivers in a system are severely limited. Therefore, as the existing available spectrum is "used-up" by previously assigned communication systems, the later developed technologies, for example the Digital European Cordless Telephone (DECT), must be designed for other portions of the spectrum that are not already "used-up".

Generally, the later developed communication systems are therefore designed to operate at higher frequency bands. However, while the higher frequency systems are necessary because the present allocated frequency bands are over crowded, the higher frequencies have numerous designs and operating drawbacks. For Example, at the higher bit rate, group delay-dispersion significantly reduce the range of the personal cordless telephone systems, and at high bit rates, the sensitivity is also reduced. Under certain circumstances, this reduction in communication range and sensitivity are intolerable.

Thus, what is need is a method for reducing group delay-dispersion for increasing the range of the communication system and reducing bit rate for improving sensitivity.

Summary of the Invention

A portable communication unit which operates in a wireless communication system comprises a plurality of fixed communication units The fixed communication units are capable of communicating with the portable communication unit at a first bit rate. A method for decreasing group delay distortion and increasing communication range between the plurality of fixed communication units and the portable communication unit, comprising the steps of:

(a) establishing communication between the fixed communication units and the portable communication unit at the first bit rate, the established communication having a communication range associated with the first bit rate; (b) dividing the first bit rate into an integer multiple for obtaining a second bit rate lower than the first bit rate; and

(c) establishing communication at the second bit rate wherein the communication at the second bit rate decreases the group delay distortion thereby increasing the communication range between the fixed communication units and the portable communication unit.

Brief Description of the Drawings

FIGs. 1-2 are electrical block diagrams of two wireless communication systems in accordance with a preferred embodiment of the present invention.

FIGs. 3-6 are timing diagrams showing the contents of a time division multiple access and time division duplex (TDMA/TDD) frame constructed according to the Digital European Cordless Telecommunications (DECT) standard as used in accordance with the preferred embodiment of the present invention. FIG. 7 is an electrical block diagram of a fixed communication unit (FCU) in accordance with the preferred embodiment of the present invention.

FIG. 8 is an electrical block diagram of a portable communication unit (PCU) in accordance with the preferred embodiment of the present invention.

FIG. 9 is an electrical block diagram of a TMDA/TDD module in accordance with the preferred embodiment of the present invention.

FIG. 10 is a constellation diagram illustrating the phase relationship for π/4 QPSK and 0.5 GMSK modulation schemes.

FIG. 11 is a flow diagram illustrating the operation of the PCU shown in FIG. 9 according to the preferred embodiment of the present invention.

Description of a Preferred Embodiment

Referring to FIG. 1, a wireless communication system 100 is shown in accordance with the preferred embodiment of the present invention. The wireless communication system 100 comprises a plurality of fixed communication units (FCUs) 102 that provide radio coverage in a plurality of radio coverage areas 108, 110, 112. The FCUs 102 are coupled to the Public Switched Telephone Network (PSTN) 114 by a plurality of telephone lines 116. Those of ordinary skill in the art will recognize that the wireless communication system 100 according to the present invention can also be used with telephone systems other than the PSTN, e.g., a private branch exchange (PBX). The wireless communication system 100 further comprises at least one portable communication unit (PCU) 120 having hand-off capability and transmitting and receiving in a time division multiple access and time division duplex (TDMA/TDD) digital format. The wireless communication system is coupled to at least one wired telephone set 124 for sending and receiving calls to and from a PCU 120. A PCU 120 may also communicate with another PCU 120 through one or more of the FCUs 102 as depicted. The PCUs 120 also include a user selectable switch, which will be illustrated in details below, to select an option to communicate at a first (or normal operating) bit rate and at a second lower bit rate, for example one-third normal operating bit rate.

Further enabling the wireless communication system according to the present invention is the use of a standard protocol for communication between the plurality of FCUs 102 and the at least one PCU 120. The standard protocol defines messages and procedures for requesting and establishing wireless communication links, for transmitting and receiving signaling data, for transmitting and receiving user communications, and for defining the TDMA/TDD format used therefor. An example of such a standard protocol is the Digital European Cordless Telecommunications (DECT) standard.

FIG. 2 shows a block diagram of a second embodiment of a wireless communication system according to the present invention. The second embodiment comprises a telepoint communication system 200 having a switch control point (SCP) 202 which is coupled a switch service point (SSP) 204. The SSP 204 is coupled to at least one FCU 120 which is RF linked to the PCU 120 for communicating therebetween. The telepoint system 200 (and sparse residential system) is an outdoor system and generally requires a fewer number of channels because of a fewer number of callers. Also, being an outdoor system, it is desirable to have an increased range between the FCU 102 and the PCU 120. Accordingly, the telepoint system 200 performs adequately with a reduced DECT system protocol bit rate. This reduced bit rate also reduces the number of time slots from twenty-four to eight, and the reduction in group delay dispersion associated with the given bit rate allows an increase in the communication range and sensitivity therebetween, in a fading multi-path environment.

FIG. 3 shows a TDMA/TDD frame 300 constructed according to the DECT standard protocol as used in accordance with the preferred embodiment of the present invention. The TDMA/TDD frame 300 comprises twelve time slots 302 for FCU transmission and twelve time slots 304 for PCU transmission illustrated by numerals 0-9 and letters A-B. The time slots 302, 304 are paired on a positional basis for transmitting and receiving information. For example, an FCU 102 (FIG. 1) transmitting in the time slot 302 labeled "0" would receive in the time slot 304 also labeled "0."

FIG. 4 illustrates a more detailed timing diagram of the time slots as shown in FIG 3. The time slots 0-B of the FCU and PCU transmit sides 302, 304 of the TDMA/TDD frame comprise 480 bits. The 480 bits of each time slots 0-B are divided into a synchronization marker 306 having thirty two bits for synchronizing a linked PCU 120 (FIG. 1) to the FCU 102, a data portion 308 comprising 388 bits, and a Z-field having 4 bits for detecting unsynchronized interference sliding by monitoring bit errors in the synchronization marker 306.

FIG. 5 is a timing diagram which further illustrates the data portion 308 which comprises a control field 310 for passing control information, e.g., frame and slot identification and other control messages between the linked PCU 120 and the FCU 102, and a user data portion 312 for carrying user data, e.g., speech. The synchronization portion 306 (FIG. 4) and the control portion 310 are used to synchronize the information carried in the user data part 312 as well as any user signaling that is carried in the control portion 310.

FIG. 6 is a timing diagram illustrating the modified DECT protocol of FIG. 3 for the one-third bit rate reduction of the DECT standard protocol. According to the DECT standard, the normal operating bit rate is 1.152 Mbits/second and a frame having twenty-four time slots. As shown, the one-third reduction in the DECT bit rate results in a bit rate of 384

Kbits/second and a modified DECT frame having eight time slots 602, 604. Each time slot of the reduced bit rate frame comprises three time slots of the standard DECT frame. The time slots 602, 604 are similarly paired on a positional basis for transmitting and receiving information. For example, an FCU 102 (FIG. 1) transmitting in the time slot 602 labeled "2" would receive in the time slot 604 also labeled "2."

Referring to FIG. 7, a block diagram of a preferred embodiment of the FCU 102 (FIG. 1) in accordance with the present invention is shown. The FCU 102 comprises an antenna 600 coupled a radio frequency (RF) transceiver 602 for transmitting and receiving radio signals comprising digital information transmitted and received in a TDMA/TDD format well known to one of ordinary skill in the art. The RF transceiver 602 is coupled to a microprocessor 604 for controlling the transceiver 602 by a bus 612. The microprocessor 604 is coupled by the bus 612 to a system frame synchronization circuit 608 for maintaining frame synchronization among all the FCUs in the system. The frame synchronization circuit 608 receives a master system synchronization signal at a terminal 606. If the interface with the PSTN 114 (FIG. 1) is digital, the master synchronization signal can, for example, be derived from synchronization markers contained therein, after adjustments are made for differential delays between the PSTN 114 and the plurality of FCUs 102. The RF transceiver 602 is also coupled to a TDMA/TDD circuit 610 for interfacing the RF transceiver 602 to a plurality of CODECs 614 for performing analog-to-digital and digital-to-analog conversions of signals transmitted and received, respectively, by the FCU 102. The plurality of CODECs 614 are coupled to a plurality of telephone interfaces 616 for coupling a plurality of telephone lines 116 to the plurality of CODECs 614. A clock generator (FIG. 9) operating at the serial bit rate of the TDMA/TDD circuit 610 (e.g., 1.152 Mbits/sec. for the standard DECT protocol or 384 Kbits/sec. for the modified DECT protocol) and the CODECs 310 and synchronized by the TDMA/TDD circuit 610 to the master system synchronization signal for passing information between the CODECs 614 and the TDMA/TDD circuit 610 in accordance with the present invention.

The TDMA/TDD circuit 610, the CODECs 614, and the telephone interfaces 616 are also all coupled to the bus 612 for receiving control signal from the microprocessor 604. A memory 618 is also coupled to the microprocessor 604 for storing program control software and for storing programming values. A plurality of memory locations 620, 622 are reserved for hand-off completion times, each of the two corresponding plurality of memory locations 620, 622 being associated with a corresponding plurality of paired receive and transmit TDMA/TDD time slots 202, 203 (FIG. 3) used by the FCU 102.

FIG. 8 shows a block diagram of the PCU 120 of the DECT system 100 (FIG. 1) according to the preferred embodiment of the present invention. The PCU 120 comprises an antenna 170 coupled to a transmitter circuit 172 and a receiver circuit 174. A microprocessor controller 178 receives a signal from the receiver circuit 174 indicating the received signal strength (the RSSI signal). The transmitter 172 and the receiver 174 are coupled to a TDMA/TDD 176 module which controls the signal provided to the transmitter 172 and received from the receiver 174 to facilitate two-way communications by synchronizing communications to the pair of timeslots 302, 304 allocated for communication. The operation of the TDMA/TDD 176 is controlled by a signal from the microprocessor controller 178. The microprocessor controller 178 provides a signal to a frequency synthesizer 180 for controlling the operation thereof. The frequency synthesizer 180 supplies the operating frequency information to the transmitter 172 and the receiver 174 for receiving and transmitting the communication signal. The microprocessor controller 178 is also coupled to a memory 179 for accessing and updating stored information.

The signal received by the receiver 174 or transmitted by the transmitter circuit 172 is a digitally encoded signal which is processed by the TDMA/TDD module 176, the operation of the TDMA/TDD 176 will be discussed in detail below. The signal from the TDMA/TDD 176 passes to a codec 184 for digital-to-analog or analog-to-digital conversion. For example, the signal received via the receiver circuit 174 and converted by the codec 184 is supplied as an analog signal to audio circuitry 186 and thence to a speaker 188. Likewise, an analog signal received from a microphone 190 passes through the audio circuitry 186 and is converted to a digital signal by the codec 184 before being provided to the transmitter circuit 172 via the TDMA/TDD module 176. In addition, control signals, such as call initiation requests, call hand-off, and call disconnect requests, can be provided from the microprocessor controller 178 to the transmitter 172 for transmission therefrom. Control signals received by the receiver 174 are likewise provided to the microprocessor controller 178.

For other operations, such as dialing up a telephone number, user controls 183 provide appropriate signals to the microprocessor controller 178. In addition, the microprocessor controller 178 supplies a signal to a display driver 192 for generation of a visual message for presentation to the user on a display 194.

FIG. 9 is a block diagram of the TDMA/TDD module 176 in accordance with the preferred embodiment of the present invention. The signal from the receiver 174 (FIG. 8) is passed to the TDMA/TDD module 176 which demodulates the signal by a π/4 Quadrature Phase Shift Keying (π/4 QPSK) demodulator 706 or a 0.5 Gaussian Minimum Shift Keying (0.5 GMSK) demodulator 708. A user of the PCU 176 or by software control operates a switch 700 to select between the 0.5 GMSK 708 and π/4 QPSK demodulators 706. It will be appreciated that the DECT standard protocol uses the 0.5 GMSK modulation scheme.

Referring to FIG. 10, a constellation diagram is shown which illustrates In-phase (I) and Quadrature (Q) phase relationships for decoding the π/4 QPSK. There are eight symbol (or baud) levels 0-7 associated with demodulation of the π/4 QPSK modulation scheme, these symbol levels are represented as 2³ numbers of levels. All combinations of the eight symbol levels are therefore represented as a three bit symbol. However, for 0.5 GMSK there are only two symbol levels (0, 4 or 2, 6) which are represented by 2¹ number of levels. Therefore, all combinations of the two levels of 0.5 GMSK are represented as a one bit symbol. It will be appreciated by one of ordinary skill in the art that, with 0.5 GMSK modulation technique, two bits will cause up-to two transitions while with π/4 QPSK modulation techniques, one di-bit pair will always cause a one symbol transition. This can be easily seen by referring to the I-axis which illustrates that each baud rate is capable of four transitions (00, 01, 10, and 11). For example, if di-bit pair 00 is received at baud 0, the transition is to baud 1, likewise, when di-bit pair 11 is received is received at baud 0, the transition is to baud 5. Also when di-bit pair 10 is received at baud 5, for example, the transition is to baud 4, etc. As a result, the π/4 QPSK modulation technique has one-half the symbol rate of 0.5 GMSK modulation techniques. Therefore, by switching modulation scheme from 0.5 GMSK to π/4 QPSK, a two-to-one reduction in symbol rate is obtained.

Referring back to FIG. 9, the switch 700 facilitates the switching between the 0.5 GMSK and π/4 QPSK modulation schemes which results in a reduction in the symbol rate by one-half. Also, when the switch 700 is operated, switches 702, 704 simultaneously operate as indicated by control line 750 such that both the received and transmitted signals have compatible modulation schemes. Operationally, the signal when received will be modulated as 0.5 GMSK and with the switch 700 set accordingly, a 0.5 GMSK demodulator 708 demodulates the received signal which is later processed by the oversample correlation detector 710 which aligns the demodulated bits. A receiver processor 712 processes the signal in a manner well known to one of ordinary skill in the art, and passes the signal to a memory pointers computer interface 714 for receiving control signals from the microprocessor controller.

A crystal 740 and an amplifier 742 are coupled together to form a crystal oscillator which is coupled to the TDMA/TDD module 176 to generate clock signals for operating the TDMA/TDD module 176 at 1.152 Mbits/sec. A switch 746 facilitates the switching (by user control or by software) of the 1.152 Mbits/sec. bit rate to second bit rate of an integer multiple of the 1.152 Mbits/sec. bit rate. A bit rate divider 7-44 divides the 1.152 Mbits/sec. bit rate. According to the preferred embodiment, the bit rate divider 744 divides the 1.152 Mbits/sec by three for operating the TDMA/TDD at 384 Kbits/sec. It will be appreciated by one of ordinary skill in the art that other bit rates of lesser integer divisors of 1.152 Mbits/seconds could be used as well. The output of switch 746 is applied to all blocks within the TDMA/TDD module 176. One of ordinary skill in the art will appreciate that the invention applies to either switching from the high data rate to the low data rate or switching from the low data rate to the high data rate, and vice versa.

The memory pointers computer interface 714 receives and transmits data between an audio I/O port 732 and the micro-computer. A transmitter processor 716 processes the data to be transmitted in a manner well known to one of ordinary skill in the art. The transmitter processor 716 is coupled to a filter 720 which filters the 0.5 GMSK signal which passes to the transmitter via the switch 702 and the In-phase Modulator Block (IMOD) 726 to an I-Q summer 730 for summing the In-phase and Quadrature products from IMOD and QMOD blocks 726, 728. As is well known to one of ordinary skill in the art, the 0.5 GMSK modulation produces a Quadrature phase of zero, resulting in no contribution in the QMOD block 728. When the π/4 QPSK modulation scheme is switched 702, 704, the output of the transmitter processor 716 is processed by the π/4 QPSK modulator 718 which generates the in-phase and quadrature phase products. The in-phase product is filtered by a filter 722 and passes to the I- Q summer 730 via the switch 702 and the IMOD block 726. Similarly, the quadrature phase product is filtered by a filter 724 and passes to the I-Q summer 730 via the switch 704 and the QMOD block 728. The IMOD 726, QMOD 728 and the I-Q summer 730 operate in a manner well known to one of ordinary skill in the art for producing the π/4 QPSK modulated signal to be transmitted.

Therefore, since at higher bit rates, for example 1.152 Mbits/second, the group delay dispersion significantly reduces the range and sensitivity of the PCU 120. By providing a switch and a bit rate divider for dividing the DECT standard bit rate to a lower bit rate, the group delay dispersion is decreased and the sensitivity are increased. This decrease in group delay and increase in sensitivity cause an increase in the range between the PCU 120 and the FCU 102. Furthermore, according to the preferred embodiment, the bit rate is divided by three which increases the DECT system protocol time slot by three times, thereby ensuring that both bit rates are compatible to the DECT system protocol. In this way, the PCU may be manually switched to the required bit rate of operation, or the PCU can be operated to automatically scan when entering a new system coverage area to determine the bit rate being transmitted in the new system coverage area. Therefore, the lower bit rate is chosen to ameliorate the effects of delay dispersion at the higher symbol rates when channel capacity can be sacrificed while maintaining the DECT system specification. Also, it will be appreciated that the modulation technique can either be coupled to the bit rate of transmission, or alternately independently selected irrespective of the bit rate of transmission or selected by scanning the received transmissions. For example, the switching of the bit rate can be determined by errors in the synchronization code word, or in the A-field and the B-field in the cyclic redundant check (CRC). As is well known, the A-field is the embedded signal channel, and the B-field includes other user data, such as speech data.

FIG. 11 is a flow diagram illustrating the operation of the PCU 120 shown in FIG. 9 according to the preferred embodiment of the present invention. Operationally, in step 1100, the PCU 120 powers-up and initializes all variables. The receiver turns on, step 1102, and at step 1104, the PCU set its default bit rate and modulation technique, preferable, the DECT standard. The receiver begins receiving transmission from the FCUs to determine the bit rate and modulation scheme of the signal being received, step 1106. Accordingly, the PCU can scan the information received for determining the bit rate of the information being transmitted, and when necessary, toggles the bit rate switch when the PCU begins to receive information from an FCU that is transmitting information at the alternative bit rate. The PCU then checks if the signal is being transmitted in the DECT standard bit rate of 1.152 Megabits-per-seconds, step 1108. If so, the PCU checks to see if the low bit rate switch has been selected (by the user or by software), step 1110. When the low bit rate switch is not selected, the PCU determines if the received information is modulated in 0.5 GMSK, step 1116. If yes, the modulation switch is checked to determine if the π/4 QPSK modulator /demodulator switch is set, step 1118, and if no, the received information is processed accordingly, step 1124. However, when the modulator/demodulator switch is set, the PCU toggles the modulator switch, step 1120, to select the corresponding demodulation scheme, and to begin processing the received information, step 1124.

Returning the step 1108, when the received information is being transmitted at the low-data rate, the PCU checks to determine if the high data rate is selected, step 1114, and if so, the bit rate switch is toggled, step 1112, to select the low bit rate for receiving the information determined to be transmitted at the low bit rate. Alternately, if the high data rate was not selected, the PCU proceeds to step 1116. Also at step 1110, when the low bit rate has been selected when the PCU has determined that the information is being received at the high bit rate, the bit rate switch is toggled, step 1112, to select the corresponding bit rate. At step 1116, after the PCU has determined that the information is modulated in the π/4 QPSK (not the 0.5 GMSK) scheme (or format), the PCU checks if the modulation switch is set to 0.5 GMSK, step 1122. If so, the modulation switch is toggled to the π/4 QPSK modulation scheme, step 1120, and processing the received information begins, step 1124. Alternately, when the modulation is set to π/4 QPSK (or not set to 0.5 GMSK), the PCU similarly begins to process the received information, step 1124.

In this way, the PCU can operate on at least two bit rates, the DECT standard bit rate, and the second lower bit rate which increases the range of communication between the FCU and the PCU when a modified DECT system is transmitting information at the reduced bit rate of transmission. Accordingly, while still conforming to the general DECT specification, a private communication systems, when capacity is not at issue, can operate at a lower bit rate which decreases the number of available in the time slots of the DECT protocol. However, for the decrease in capacity, an increase in range is achieved because at the lower bit rates results in a reduction in delay dispersion. Also, it will be appreciated that the choice of modulation techniques can either be coupled to the bit rate of transmission, or alternately independently selected irrespective of the bit rate of transmission or selected by scanning the received transmissions. For example, the switching of the bit rate can be determined by errors in the synchronization code word, or in the A-field and the B-field in the cyclic redundant check (CRC). As is well known, the A-field is the embedded signal channel, and the B-field includes other user data, such as speech data. In summary, a portable communication unit which operates in a wireless communication system comprising at least one fixed communication unit. The fixed communication units being capable of communicating with the portable communication unit at a first bit rate. The exists a method for decreasing group delay distortion and increasing communication range between the at least one fixed communication unit and the portable communication unit, comprising the steps of: (a) establishing communication between the fixed communication units and the portable commumcation unit at the first bit rate, the established communication having a communication range associated with the first bit rate; (b) dividing the first bit rate into integer multiples for obtaining a second bit rate lower than the first bit rate;

(c) establishing communication at the second bit rate wherein the commumcation at the second bit rate decreases the group delay distortion thereby increasing the communication range between the fixed communication units and the portable communication unit;

(d) selecting one of a first modulating format having a first symbol rate and second modulation format having a second symbol rate for communicating at the first and second bit rates; and

(e) scanning communication signals received from the fixed communication units for determining the bit rate and the modulation scheme of communication signals. What is claimed is:

Claims

1. A portable communication unit which operates in a wireless communication system comprising at least one fixed communication unit, the fixed communication units being capable of communicating with the portable commumcation unit at a first bit rate, a method for decreasing group delay distortion and increasing communication range between the at least one fixed commumcation unit and the portable communication unit, comprising the steps of: (a) establishing communication between the fixed communication units and the portable communication unit at the first bit rate, the established communication having a communication range associated with the first bit rate;

(b) dividing the first bit rate into integer multiples for obtaining a second bit rate lower than the first bit rate; and

2. The method according to claim 1 further comprising a step of increasing a sensitivity of the communication signal being received by the portable communication unit in response to the step of dividing the first bit rate.

3. The method according to claim 1 further comprising the step of selecting between a first modulating format having a first symbol rate and second modulation format having a second symbol rate for communicating at the first and second bit rates.

4. The method according to claim 3 wherein the first modulation format comprises 0.5 Gaussian minimum shift key modulation scheme.

5. The method according to claim 3 wherein the second modulation format comprises π/4 quadrature phase shift keying modulation scheme.

6. The method according to claim 1 further comprising the step of maintaining frame synchronization between transmissions at the first and second bit rates.

7. The method according to claim 1 further comprising the step of scanning communication signals received for determining the bit rate of the information being received from the fixed communication units.

8. The method according to claim 3 further comprising the step of scanning communication signals received for determining the modulation scheme of the information being received from the fixed communication units.

9. A portable communication unit which operates in a wireless communication system comprising at least one fixed communication unit, the fixed communication units being capable of communicating with the portable communication unit at a first bit rate, a method for decreasing group delay distortion and increasing communication range between the at least one fixed commumcation unit and the portable communication unit, comprising the steps of:

(a) establishing commumcation between the fixed communication units and the portable communication unit at the first bit rate, said established communication having a communication range associated with the first bit rate; (b) dividing the first bit rate into integer multiples for obtaining a second bit rate lower than the first bit rate;

(c) establishing communication at the second bit rate wherein the communication at the second bit rate decreases the group delay distortion thereby increasing the communication range between the fixed commumcation units and the portable communication unit; and

(d) selecting one of a first modulating format having a first symbol rate and a second modulation format having a second symbol rate for communicating at the first and second bit rates.

10. A portable communication unit which operates in a wireless communication system comprising at least one fixed communication unit, the fixed communication units being capable of communicating with the portable commumcation unit at a first bit rate, a method for decreasing group delay distortion and increasing commumcation range between the at least one fixed commumcation unit and the portable communication unit, comprising the steps of:

(a) establishing communication between the fixed communication units and the portable communication unit at the first bit rate, said established communication having a communication range associated with the first bit rate;

(b) dividing the first bit rate into integer multiples for obtaining a second bit rate lower than the first bit rate; (c) establishing communication at the second bit rate wherein the communication at the second bit rate decreases the group delay distortion thereby increasing the communication range between the fixed communication units and the portable communication unit;

(e) scanning communication signals received from the fixed communication units for determining the bit rate and the modulation scheme of communication signals.