WO1997011545A1

WO1997011545A1 - Method for increasing system capacity while maintaining channel bandwidth in a radio communication system

Info

Publication number: WO1997011545A1
Application number: PCT/US1996/010768
Authority: WO
Inventors: James Michael Keba; Clinton C. Ii Powell; Susan Chang
Original assignee: Motorola Inc.
Priority date: 1995-09-18
Filing date: 1996-06-24
Publication date: 1997-03-27

Abstract

A method achieves an increase of fifty percent in transmission capacity in a radio communication system (102, 104) utilizing a modulation process comprising four-level frequency shift-keyed (FSK) modulation having a maximum bit rate of 6400 bits per second (bps) and a minimum symbol frequency spacing of 3200 Hz prior to the increase. The method also maintains a channel bandwidth after the increase substantially equal to the channel bandwidth prior to the increase. The method comprises the steps of changing the modulation process (412, 414) to include a signal utilizing eight-level FSK modulation comprising four upper symbol frequency deviations and four lower symbol frequency deviations, increasing the maximum transmitted bit rate to 9600 bps, and changing the minimum symbol frequency spacing to 1600 Hz, thereby maintaining symbol orthogonality for a complex representation of the signal.

Description

METHOD FOR INCREASING SYSTEM CAPACITY WHILE

MAINTAINING CHANNEL BANDWIDTH IN A RADIO

COMMUNICATION SYSTEM

Field of the Invention

This invention relates in general to radio communication systems, and more specifically to a method for increasing system capacity while maintaining channel bandwidth in a radio communication system.

Background of the Invention

Motorola's FLEX™ paging protocol is well known in the art. The existing FLEX™ protocol supports bit rates of 1600 bits per second (bps), 3200 bps, and 6400 bps. While these rates are sufficient for a wide variety of applications, new wireless communication services such as electronic mail and faxes can benefit from a throughput beyond the 6400 bps offered by current FLEX™ paging systems.

Higher throughput must be accomplished, however, without exceeding the channel bandwidth assigned for messaging services by governmental regulating agencies. In addition, it is highly desirable to achieve the higher throughput economically.

Thus, what is needed is a method for increasing FLEX™ protocol transmission throughput while maintaining the currently assigned channel bandwidth, and without incurring high costs.

Summary of the Invention

An aspect of the present invention is a method for achieving an increase of fifty percent in transmission capacity in a radio communication system utilizing a modulation process comprising four- level frequency shift-keyed (FSK) modulation having a maximum bit rate of 6400 bits per second (bps) and a minimum symbol frequency spacing of 3200 Hz prior to the increase. The method also maintains a channel bandwidth after the increase substantially equal to the channel bandwidth prior to the increase. The method comprises the steps of changing the modulation process to include a signal utilizing eight-level FSK modulation comprising four upper symbol frequency deviations and four lower symbol frequency deviations, increasing the maximum transmitted bit rate to 9600 bps, and changing the minimum symbol frequency spacing to 1600 Hz, thereby maintaining symbol orthogonality for a complex representation of the signal.

A second aspect of the present invention is a method for achieving an increase of fifty percent in reception capacity in a radio communication system utilizing a modulation process comprising four- level frequency shift-keyed (FSK) modulation having a maximum bit rate of 6400 bits per second (bps) and a minimum symbol frequency spacing of 3200 Hz prior to the increase. The method also maintains a channel bandwidth after the increase substantially equal to the channel bandwidth prior to the increase. The method comprises the steps of changing the demodulation process to include a signal utilizing eight- level FSK demodulation comprising four upper symbol frequency deviations and four lower symbol frequency deviations, increasing the maximum received bit rate to 9600 bps, and changing the minimum symbol frequency spacing to 1600 Hz, thereby maintaining symbol orthogonality for a complex representation of the signal.

Brief Description of the Drawings

FIG. 1 is an electrical block diagram of a radio communication system in accordance with the preferred embodiment of the present invention.

FIG. 2 is an electrical block diagram of a controller utilized in the radio communication system of FIG. 1.

FIGs. 3 through 5 depict bandwidth occupied before and after the increase in channel capacity in accordance with the preferred embodiment of the present invention.

FIG. 6 is an electrical block diagram of a communication receiver utilized in the radio communication system of FIG. 1.

FIG. 7 is a flow chart depicting operation of the controller in accordance with the preferred embodiment of the present invention. FIG. 8 is a flow chart depicting operation of the communication receiver in accordance with the preferred embodiment of the present invention. Description of the Preferred Embodiment

Referring to FIG. 1, an electrical block diagram of a communication system in accordance with the preferred embodiment of the present invention comprises a fixed portion 102 and a portable portion 104. The fixed portion 102 includes a plurality of base stations 116, for communicating with the portable portion 104, utilizing conventional transmission techniques well known in the art, and coupled by communication links 114 to a controller 112 which controls the base stations 116. The hardware of the controller 112 is preferably a combination of the Wireless Messaging Gateway (WMG™) Administrator! paging terminal and the RF-Conductor!™ message distributor manufactured by Motorola, Inc. The hardware of the base stations 116 is preferably a Nucleus® Orchestra! transmitter manufactured by Motorola, Inc. It will be appreciated that other similar hardware can be utilized for the controller 112 and base stations 116.

Each of the base stations 116 transmits radio signals to the portable portion 104 comprising a plurality of communication receivers 122 via a transmitting antenna 120. The radio signals comprise selective call addresses and message transactions between the base stations 116 and the communication receivers 122. The controller 112 preferably is coupled by conventional telephone links 101 to a public switched telephone network (PSTN) 110 for receiving selective call messages therefrom. The selective call messages comprise voice and data messages received from the PSTN 110 using, for example, a conventional telephone 124 coupled to the PSTN 110 in a manner well known in the art.

Data and control transmissions between the base stations 116 and the communication receivers 122 preferably utilize an outbound protocol such as Motorola FLEX™ digital selective call signaling protocol described more fully in U.S. Patent No. 5,168,493 issued December 1, 1992 to Nelson et al., and assigned to the assignee of the present invention and which is hereby incorporated by reference. This protocol utilizes well-known error detection and error correction techniques and is therefore tolerant to bit errors occurring during transmission, provided that the bit errors are not too numerous in any one code word. Transmissions comprising data and control signals from the base stations 116 preferably utilize either two, four, or eight-level frequency shift keyed (FSK) modulation in accordance with the present invention, as will be described shortly below. Referring to FIG. 2, an electrical block diagram of elements of the fixed portion 102 in accordance with the preferred embodiment of the present invention comprises portions of the controller 112 and the base stations 116. The controller 112 comprises a processing system 210 for directing operation of the controller 112. The processing system 210 preferably is coupled through a transmitter controller 204 to a transmitter 202 via the communication links 114. The communication links 114 use conventional means well known in the art, such as a direct wire line (telephone) link, a data communication link, or any number of radio frequency links, such as a radio frequency (RF) transceiver link, a microwave transceiver link, or a satellite link, just to mention a few. The transmitter 202 can transmit either two, four, or eight-level FSK data messages to the communication receivers 122. The transmission of eight-level FSK can be any one of three embodiments. Table 1, shown below, compares two, and four-level FSK frequency deviations used in FLEX™ systems today to one embodiment of eight- level FSK frequency deviations.

Table 1. 8-Level FSK vs. 2 and 4-Level FSK (First Embodiment)

2-Level FSK 4-Level FSK 8-Level FSK (Gray Coded) (Gray Coded)

"100" +5600Hz

"1" +4800Hz "10" +4800Hz

"101" +4000Hz

"111" +2400Hz

"11" +1600Hz

"110" +800Hz

"010" -800Hz

"01" -1600Hz

"Oil" -2400Hz

"001" -4000Hz

"0" -4800Hz "00" -4800Hz

"000" -5600Hz

Several components of the modulation scheme proposed above should be noted. First, the frequency deviations are selected orthogonal with respect to each other. In the frequency spectrum, selecting orthogonal deviations results in a non-overlapping energy spectrum where the center of each deviation (where the energy is at a peak) occurs at a null in the frequency spectrum produced by each of the other frequency deviations. Orthogonality for a complex representation of the signal is accomplished by spacing the symbol frequencies at intervals of 1 /2T, where T is the symbol duration. A receiver that is designed for orthogonal reception advantageously can recover substantially all of the sensitivity that would otherwise be lost due to the higher 9600 bps bit rate utilized with the 8-Level FSK data messages. Second, none of the frequency deviations used by the added eight-level FSK correspond to existing two and four-level FSK frequency deviations utilized today in a FLEX™ radio communication systems. Third, simulations have shown that the added frequency deviations maintain a channel bandwidth substantially equal to the channel bandwidth of the two and four-level FSK systems. And fourth, the frequency deviations closest to the carrier frequency are ± 800Hz. A communication receiver 122 employing a frequency modulation (FM) discriminator (i.e., frequency to voltage converter) would demodulate the proposed eight-level FSK with these deviations, but at a somewhat reduced sensitivity.

However, a communication receiver 122 using a zero intermediate frequency (ZIF) receiver cannot easily demodulate the deviations depicted in Table 1 without great loss to sensitivity in reception. ZIF receivers inherently have a notch in the passband which has a variable bandwidth (BW) dependent on component specifications chosen in the design and implementation of the ZIF receiver. The above proposed frequency deviations centered at the carrier frequency (± 800Hz) are considered difficult for utilization in a ZIF receiver.

Table 2 shows a second embodiment which would better support the use of a ZIF receiver.

Table 2. 8-Level FSK vs. 2 and 4-Level FSK (Second Embodiment)

2-Level FSK 4-Level FSK 8-Level FSK (Gray Coded) (Gray Coded)

"100" +6400Hz

"1" +4800Hz "10" +4800Hz "101" +4800Hz

"111" +3200Hz

"11" +1600Hz "110" +1600Hz

"No Carrier" "No Carrier" "No Carrier"

"01" -1600Hz "010" -1600Hz

"011" -3200Hz

"0" -4800Hz "00" - 4800Hz "001" -4800Hz

"000" -6400Hz

This embodiment differs in two ways from the first embodiment. First, in this embodiment the proposed eight-level FSK frequency deviations share frequency deviations with the existing two and four- level FSK systems. And second, a ninth frequency deviation is reserved at the carrier frequency, but no carrier is transmitted, thereby spacing the symbol frequencies that are closest to the carrier frequency far enough (± 1600Hz) from the carrier frequency to better support a ZIF application. Table 3 shows a third embodiment.

Table 3. 8-Level FSK vs. 2 and 4-Level FSK (Third Embodiment)

2-Level FSK 4-Level FSK 8-Level FSK (Gray Coded) (Gray Coded)

"100" +6400Hz

"1" +4800Hz "10" +4800Hz "101" +4800Hz

"111" +3200Hz

"11" +1600Hz "110" +1600Hz

"Carrier" "Carrier" "Carrier"

"01" -1600Hz "010" -1600Hz

"011" -3200Hz

"0" -4800Hz "00" - 4800Hz "001" -4800Hz

"000" -6400Hz

In this embodiment a carrier frequency is transmitted in the ninth frequency deviation. The carrier signal can be used by a communication receiver 122 utilizing a coherent receiver design.

In accordance with the present invention, embodiments two and three are considered the preferred embodiments.

FIG. 3 depicts bandwidth occupied by the existing FLEX™ four- level FSK system. FIG. 4 depicts the first embodiment described above of a FLEX™ eight-level FSK system in accordance with the present invention. Fig. 5 depicts the second embodiment of a FLEX™ eight-level FSK system in accordance with the present invention without a carrier present at the ninth frequency deviation. Note the channel bandwidth depicted in FIGs. 4 and 5 is substantially the same as that of the existing FLEX™ protocol depicted in FIG. 3 (i.e., approximately ± 12KHz) and that the power is well inside the FCC out-of-band envelope 130. Simulations to determine bandwidth requirements of the third embodiment according to the present invention similarly show that the channel bandwidth thereof also is substantially the same as that of FIG. 3, and that the power remains well inside the FCC out-of-band envelope 130. Also note in FIG. 5 that when the ninth frequency deviation does not include a carrier, a frequency hole 123 is present in the center of the spectrum. The processing system 210 is also coupled to an input interface 218 for communicating with the PSTN 110 through the telephone links 101 for receiving selective call originations. In order to perform the functions (to be described below) necessary in controlling the elements of the controller 112, as well as the elements of the base stations 116, the processing system 210 preferably includes a conventional computer system 212, and a conventional mass storage medium 214. The conventional mass storage medium 214 also includes subscriber user information such as, for example, communication receiver 122 addressing, communication capability, programming options, etc.

The conventional computer system 212 is programmed by way of software included in the conventional mass storage medium 214. The conventional computer system 212 preferably comprises a plurality of processors such as VME Sparc processors manufactured by Sun Microsystems, Inc. These processors include memory such as dynamic random access memory (DRAM), which serves as a temporary memory storage device for scratch pad processing such as, for example, storing messages originated by callers using the PSTN 110, and for protocol processing of messages destined for the communication receivers 122, just to mention a few. The conventional mass storage medium 214 is preferably a conventional hard disk mass storage device.

It will be appreciated that other types of conventional computer systems 212 can be utilized, and that additional computer systems 212 and mass storage media 214 of the same or alternative type can be added as required to handle the processing requirements of the processing system 210.

FIG. 6 is an electrical block diagram of the communication receiver 122 in accordance with the preferred embodiment of the present invention. The communication receiver 122 comprises a receiver antenna 302 for intercepting RF signals from the base stations 116. The receiver antenna 302 is coupled to a receiver element 304 which includes a receiver utilizing conventional demodulation techniques for receiving the communication signal transmitted by the base station 116. The RF signals received from the base stations 116 use two, four, or eight-level FSK, as will be described shortly below. Radio signals received by the receiver element 304 produce demodulated information, which is coupled to a processing unit 310 for processing received messages. A conventional power switch 308, coupled to the processing unit 310, is used to control the supply of power to the receiver element 304, thereby providing a battery saving function.

To perform the necessary functions of the communication receiver 122, the processing unit 310 is coupled to a microprocessor 316, a random access memory (RAM) 312, a read-only memory (ROM) 314, and an electrically erasable programmable read-only memory (EEPROM) 318. Preferably, the processing unit 310 is similar to the M68HC08 micro¬ controller manufactured by Motorola, Inc. It will be appreciated that other similar processors can be utilized for the processing unit 310, and that additional processors of the same or alternative type can be added as required to handle the processing requirements of the processing unit 310. It will be also appreciated that other types of memory, e.g., EEPROM or FLASH, can be utilized for the ROM 314, as well as the RAM 312. It will be further appreciated that the RAM 312 and the ROM 314, singly or in combination, can be integrated as an integral portion of the microprocessor 316.

The processing unit 310 is programmed by way of the ROM 314 to process incoming messages on the outbound channel. During received message processing, the processing unit 310 decodes in a conventional manner an address in the demodulated data of the received message, compares the decoded address with one or more addresses stored in the EEPROM 318, and when a match is detected, the processing unit 310 proceeds to process the remaining portion of the message. Once the processing unit 310 has processed the message, it stores the message in the RAM 312, and a call alerting signal is generated to alert a user that a message has been received. The call alerting signal is directed to a conventional audible or tactile alerting device 322 for generating an audible or tactile call alerting signal. The message can be accessed by the user through user controls 320, which provide functions such as lock, unlock, delete, read, etc. More specifically, by the use of appropriate functions provided by the user controls 320, the message is recovered from the RAM 312, and then displayed on a display 324, e.g., a conventional liquid crystal display (LCD) and /or played out by the audio amplifier 326 and audio speaker 328 combination. FIG. 7 is a flow chart 400 depicting operation of the controller 112 in accordance with the preferred embodiment of the present invention. The flow chart 400 begins with step 402 where the controller 112 receives one or more messages from the PSTN 110 destined for one or more communication receivers 122. In step 404 the controller 112 processes the messages, and identifies the communication receivers 122 to which the messages are intended. In step 406 the controller 112 determines the communication capability of the selected communication receivers 122. In step 408, the controller 112 sends the messages to the transmitter 202 with transmission instructions. The transmission instructions comprise the transmission phases to be utilized by the transmitter when transmitting a message to a particular communication receiver 122.

Table 4 below shows the reception phase (used by the communication receivers 122) to the transmission phase (used by the base stations 116) mapping. The instructions given in step 406 determine the transmission phases to be used by the transmitter 202 when transmitting a message to a communication receiver 122 as defined by Table 4.

Table 4. Reception Phase vs. Transmission Phase Mapping at Different Bit Rates

Bit Rates and Corresponding Transmission Phases

Reception 9600 6400 3200 1600 Phase Value

0 a a a a

1 b a a a

2 c b a a

3 d c c a

4 e c c a

5 f d c a

As is well known by one of ordinary skill in the art, the least significant bits of a transmitted signal using M-ary FSK are subject to lower sensitivity for a communication receiver 122 employing an FM discriminator. For this reason, the transmission phases "a" and "c" are preferred over phases "b" and "d" when mapping a FLEX™ eight-level FSK to FLEX™ four-level FSK system. This is why the 6400 bps column in Table 4 has been chosen this way.

Assuming the second or third embodiments described above for eight-level FSK, in step 410 the transmitter 202 selects coherent or ZIF (noncoherent) transmission in accordance with the communication receiver 122 selected. Once the modulation mode selection is made the message is transmitted to the communication receiver 122.

FIG. 8 is a flow chart 500 depicting operation of the communication receiver 122 in accordance with the preferred embodiment of the present invention. The flow chart 500 begins with step 502 where the communication receiver 122 is prompted to receive a message in accordance with events in the FLEX™ protocol well known to one of ordinary skill in the art. In step 504, the communication receiver 122 determines the modulation format and multiplex factor from the FLEX™ protocol as indicated in SYNCl word (not shown) in the FLEX™ protocol. For a system in accordance with the present invention, the communication receiver 122 uses Table 4 to determine the reception phases in accordance with the programming of the communication receiver 122. In step 506 a determination is made from one of four operational quantities which define the multiplex factor utilized by the communication system 102, 104 for transmission and reception. When the operational quantity is "one," then the communication receiver 122 proceeds to step 508 and maps reception phases 0 through 5 to transmission phase "a" (i.e., the 1600 bps column of Table 4). When the operational quantity is "two," then the communication receiver 122 proceeds to step 510 and maps reception phases 0, 1, & 2 to transmission phase "a", and reception phases 3, 4, & 5 to transmission phase "b" (i.e., the 3200 bps column of Table 4). When the operational quantity is "four," then the communication receiver 122 proceeds to step 512 and maps reception phases 0 & 1 to transmission phase "a," reception phase 2 to transmission phase "b," reception phases 3 & 4 to transmission phase "c," and reception phase 5 to transmission phase "d" (i.e., the 6400 bps column of Table 4). When the operational quantity is "six," then the communication receiver 122 proceeds to step 514 and maps reception phase 0 to transmission phase "a," reception phase 1 to transmission phase "b," reception phase 2 to transmission phase "c," reception phase 3 to transmission phase "d," reception phase 4 to transmission phase "e," and reception phase 5 to transmission phase "f" (i.e., the 9600 bps column of Table 4).

Once the operational quantity has been determined, the communication receiver 122 proceeds to step 516 where it demodulates the received signal in accordance with its programming, and stores the message in the RAM 312. In step 518, the user is alerted to the pending message, and subsequently reads the message by using appropriate functions provided on the user controls 320. Thus, it should be apparent by now that the present invention provides a method and apparatus for increasing system capacity while maintaining channel bandwidth. In particular, the method and apparatus advantageously provides a novel method for increasing system capacity by fifty percent while maintaining a channel bandwidth after the increase substantially equal to the channel bandwidth prior to the increase. Three embodiments in accordance with the present invention have been described which provide for flexible receiver design.

Claims

1. A method for achieving an increase of fifty percent in transmission capacity in a radio communication system utilizing a modulation process comprising four-level frequency shift-keyed (FSK) modulation having a maximum bit rate of 6400 bits per second (bps) and a minimum symbol frequency spacing of 3200 Hz prior to the increase, the method maintaining a channel bandwidth after the increase substantially equal to the channel bandwidth prior to the increase, the method comprising the steps of: changing the modulation process to include a signal utilizing eight-level FSK modulation comprising four upper symbol frequency deviations and four lower symbol frequency deviations; increasing the maximum transmitted bit rate to 9600 bps; and changing the minimum symbol frequency spacing to 1600 Hz, thereby maintaining symbol orthogonality for a complex representation of the signal.

2. The method of claim 1, further comprising the step of inserting a ninth symbol frequency deviation, centered between the four upper symbol frequency deviations and the four lower symbol frequency deviations.

3. The method of claim 2, further comprising the step of minimizing transmission of energy at the ninth symbol frequency deviation for facilitating zero intermediate frequency (ZIF) demodulation.

4. The method of claim 2, further comprising the step of transmitting a carrier signal at the ninth symbol frequency deviation for facilitating coherent frequency demodulation.

5. The method of claim 1, further comprising the steps of: transmitting data by the radio communication system in an operational quantity of time-multiplexed data streams operating at 1600 bps each, the operational quantity selected from a group of operational quantities consisting of one, two, four, and six, the time-multiplexed data streams identified as a set of transmission phases "a", "b", "c" , "ά", "e" , and "f"; and assigning a communication receiver operating in the radio communication system at least one of six sample receive phases corresponding to the set of transmission phases "a" through "f", the six sample receive phases defined as "0", "1", "2", "3", "4", and "5".

6. The method of claim 5, further comprising the steps of: transmitting the data intended for receivers assigned to receive phases "0", "1", "2", "3", "4", and "5" in transmission phases "a", "b", "c", "d", "e", and "I" , respectively, in response to the operational quantity of time-multiplexed data streams being six; transmitting the data intended for receivers assigned to receive phases "0" and "1" in transmission phase "a", transmitting the data intended for receivers assigned to receive phase "2" in transmission phase "b", transmitting the data intended for receivers assigned to receive phases "3" and "4" in transmission phase "c", and transmitting the data intended for receivers assigned to receive phase "5" in transmission phase "d", in response to the operational quantity of time- multiplexed data streams being four; transmitting the data intended for receivers assigned to receive phases "0", "1", and "2" in transmission phase "a", and transmitting the data intended for receivers assigned to receive phases "3", "4", and "5" in transmission phase "c" , in response to the operational quantity of time-multiplexed data streams being two; and transmitting the data intended for receivers assigned to receive phases "0", "1", "2", "3", "4", and "5" in transmission phase "a", in response to the operational quantity of time-multiplexed data streams being one.

7. A method for achieving an increase of fifty percent in reception capacity in a radio communication system utilizing a modulation process comprising four-level frequency shift-keyed (FSK) modulation having a maximum bit rate of 6400 bits per second (bps) and a minimum symbol frequency spacing of 3200 Hz prior to the increase, the method maintaining a channel bandwidth after the increase substantially equal to the channel bandwidth prior to the increase, the method comprising the steps of: changing the demodulation process to include a signal utilizing eight-level FSK demodulation comprising four upper symbol frequency deviations and four lower symbol frequency deviations; increasing the maximum received bit rate to 9600 bps; and changing the minimum symbol frequency spacing to 1600

Hz, thereby maintaining symbol orthogonality for a complex representation of the signal.

8. The method of claim 7, further comprising the step of inserting a ninth symbol frequency deviation, centered between the four upper symbol frequency deviations and the four lower symbol frequency deviations.

9. The method of claim 8, further comprising the step of minimizing reception of energy at the ninth symbol frequency deviation for facilitating zero intermediate frequency (ZIF) demodulation.

10. The method of claim 8, further comprising the step of receiving a carrier signal at the ninth symbol frequency deviation for facilitating coherent frequency demodulation.

11. The method of claim 7, further comprising the steps of: receiving data from the radio communication system in an operational quantity of time-multiplexed data streams operating at 1600 bps each, the operational quantity selected from a group of operational quantities consisting of one, two, four, and six, the time-multiplexed data streams identified as a set of transmission phases "a", "b", "c", "d", "e", and "f"; and assigning a communication receiver operating in the radio communication system at least one of six sample receive phases corresponding to the set of transmission phases "a" through "f", the six sample receive phases defined as "0", "1", "2", "3", "4", and "5".

12. The method of claim 11, further comprising the steps of: receiving the data intended for receivers assigned to receive phases "0", "1", "2", "3", "4", and "5" from transmission phases "a", "b", "c", "d" , "e", and "i" , respectively, in response to the operational quantity of time-multiplexed data streams being six; receiving the data intended for receivers assigned to receive phases "0" and "1" from transmission phase "a", receiving the data intended for receivers assigned to receive phase "2" from transmission phase "b" , receiving the data intended for receivers assigned to receive phases "3" and "A" from transmission phase "c", and receiving the data intended for receivers assigned to receive phase "5" from transmission phase "ά" , in response to the operational quantity of time-multiplexed data streams being four; receiving the data intended for receivers assigned to receive phases "0", "1", and "2" from transmission phase "a", and receiving the data intended for receivers assigned to receive phases "3", "4", and "5" from transmission phase "c", in response to the operational quantity of time-multiplexed data streams being two; and receiving the data intended for receivers assigned to receive phases "0", "1", "2", "3", "4", and "5" from transmission phase "a", in response to the operational quantity of time-multiplexed data streams being one.