The Advanced PHS Committee, a subordinate body to the Telecommunications Council (a Japanese government advisory body), compiled its key findings on PHS advancement measures as a report in June 2001.

By combining several proposed advancement measures, high-speed data transmission speeds up to some 1Mbps will be enabled. It is scheduled that the Association of Radio Industries and Businesses (ARIB) will start the revision of the RCR STD-28 based on the report.

In succession of our previous issue (PHS MoU News No. 35), we introduce the following two measures from the report:

• Speed up of symbol speed
• Slot linked communications

In order to realize 1Mbps/RF information transmission speed, on the assumption that all four slots are used for data transmission, an information transmission speed at 1Mbps/4-256kbps per slot is required. In consideration of coexistence with the current PHS, for realizing the information transmission speed of 256kbps per slot without changing frame lengths or slot lengths, since the frame length is 5ms, the length of data portion in one slot becomes 1,280 bits (-256 kbps × 5ms). If the 16QAM is used for the modulation method, it is equivalent to 320 symbols, because one symbol is four bits.

On the assumption that overhead portion in slot is 20%, the number of symbols of 1 slot is 400 symbols (320 + 0.8), and the symbol transmission speed is 640 ksymbol/s [400 symbols + 625μs (5ms + 8)].

From the above, the symbol transmission speed of 640 ksymbol/s is required to realize the information transmission speed of 1Mbps/RF.


1-2. Channel interval at high symbol transmission speed

The symbol transmission speed of 640 ksymbol/s acquired as the above is 3.33 times the existing PHS's 192 ksymbol/s. As a bandwidth of signals is in proportion to its symbol transmission speed, the channel interval will be about 1,000 kHz (300kHz × 3.33).

However, for the fully efficient channel allocation in consideration of coexistence with the existing PHS, the efficient channel interval is 900 kHz (300 kHz × 3), which is odd-number times of the current 300 kHz. In addition, the 900 kHz channel interval enables use of the existing PHS carrier numbers without changes. Therefore, in order to alter the 1,000 kHz channel interval to 900kHz, roll-off ratio α of route Nyquist characteristics.

1-3. Optimal roll-off ratio

2. Coexistence with current PHS

  As is stated in the previous chapter, the signal bandwidth expands to three times the current PHS by speeding-up the symbol speed. In order that 900 kHz bandwidth signals and 300 kHz bandwidth signals share the common frequency bandwidth, it is necessary to create systems with affinity.

In this chapter, we show methods for carrier sense, definition of adjacent channel power leakage, guard time and antenna power, which are especially taken into consideration in coexistence with the current PHS.

2-1. Carrier sense

As the channel interval becomes 900 kHz at the speeding up of symbol speed, when a cell station assigns a channel, the cell station is to conduct the carrier sensing in the same way as the current system, and is to assign a channel only when the cell station judges that all of three consecutive channels are vacant at the channel assignment.

Regarding carrier sensing at the mobile station side, the mobile station is to conduct carrier sensing in slot timing of assigned channel to a carrier number and the adjacent two carrier numbers thereof. These enables coexistence with current PHS.

2-2. Definition of adjacentchannel power leakage

Regarding the adjacent-channel power leakage, it is provided for the existing PHS that 600 kHz detuning and 900 kHz detuning be not over 800 nW and 250 nW, respectively.
  In consideration of coexistence with the existing PHS, at speeding-up of symbol speed, based on the definition of adjacentchannel power leakage in the current PHS, it is appropriate to stipulate that 900 kHz detuning and 1,200 kHz detuning be not over 800 nW and 250nW, respectively.
We explain the adequacy of theseprovisions using Fig. 1.

The transmission spectrum in speeding-up of symbol speed is shownin Fig. 1(a). In this case, since the channel interval is provided to be 900 kHz (300 kHz × 3), it is equivalent with signals with the current PHS bandwidth assigned in three consecutive channels.

In this equivalent spectrum, the detuning frequency defined as adjacent-channel power leakage of the current PHS is defined as -600 kHz and -900 kHz for signal B, being assigned in the lowest channel, and +600 kHz and +900 kHz for signal C, being assigned in the highest channel as shown in Fig. 1(b).

Based on the signal's central frequency at speedingup of symbol speed, these detuning frequencies will be those of ±900 kHz and ±1,200 kHz. Therefore, in the same way as the current PHS, by providing that the power leakage not over 800 nW and 250 nW, respectively, the high symbol speed is able to coexist with the current PHS without influencing signals thereof.

2-3. Guard time

In the current PHS, the guard time of 41.67μs (625μs 120 symbols × 8 symbols) is secured.
This is the time period with which burst transmission from a mobile station gives no interference to adjacent slots for some ten seconds after the onset of out of synchronism in the mobile station, in the case where the maximum distance between an origin of synchronization and a cell station is 2 km (transmission delay 6μs/km in a wire interval) and transmission clock deviation between a cell station and a mobile station is 3 ppm.

Therefore, at speeding- up of symbol speed, it is sufficient to secure the current PHS' guard time (including ramp-down time), which requires 27 symbols (41.67μs × 640 ksymbol/s = 26.7 symbols).

Fig.1

1. Slot linked method

In line with the explosive diffusion of the Internet use, web browsing and program downloading are used more frequently, expanding data volume from servers to users. In PHS, the downlink (from a cell station to a terminal) data volume is increasing.

Therefore, it is necessary to realize the speeding up of downlink data transmission. By linking physical slots for communications, downlink high-speed data communications is enabled. The method for realizing it is as follows:

PHS communications methods employ TDMA/TDD, and in each slot, bits for securing transient response time or establishing synchronization are inserted. It is clear that the effective transmission speed would be improved if these bits are not necessary.

By linking downlink slots inside a carrier and conducting continuous communications, high-speed data communications is realized.

2. Method for realizing linked slots

Here, the case of linking four slots is depicted. The structure of the current physical slot for communications is shown in Fig. 2. In order to link four slots, the following operation is made to frame formats of each slot:

1) The first slot
By deleting 16 bits each of CRC and G, and by inserting the information bit (I) instead, I becomes 192 bits. (Fig. 3)

2) The second and third slot
By deleting all 80 bit except I, and by inserting I instead, I becomes 240 bit. (Fig. 4)

3) The fourth slot
By deleting 48bit of R, SS, PR, UW, CI and SA, and by inserting I instead, I becomes 208bit. (Fig.5)

As the above, in four consecutive slots, R, SS, PR, UW, CI, SA, CRS and G are not needed in the second and third slots in the middle, and by inserting I bit instead, the speeding-up of transmission is enabled.The slot structure in four-slot link is shown in Fig. 6. The transmission speed would be 176 kbps, which is 1.4 times faster than the case where ordinary four slots (128 kbps) are used.

Fig. 6. Slot structure in four-slot link

3. Example of 16QAM

In the case where a modulation method is 16QAM, the linked slot is made in the same way as the previous section. Here, the structure example of communications physical slot is shown in Fig. 7, when four slots are linked in slots exemplified in Appendix 1. The location to insert a pilot symbol is 1 symbol each at the beginning and the end of a slot. By this, transmission speed becomes some 356 kbps.

It enables high-speed transmission, which is about 1.3 times in comparison with the case where four independent slots are used without linking 16QAM, and about 2.8 times of four independent slots (128 kbps) of current system (π/4 shift QPSK).

Fig. 7. Example of slot structure of 16QAM (four-slot link)

4. Number of slots linked

Since one slot is defined to be 625μsec (240 bits)by the Rules for Radio Equipment, it is clear that we can link slots in this unit. As a result, the maximum number of downlink slots linked is 4. The definition is for the case of π/4 shift QPSK modulation method. In 16QAM whose symbol rates double, it is appropriate to define in 480 bits.

5. Conditions for coexistence

In the current system, it is necessary to satisfy slot transmission requirements of carriers for communications, in order to link slots. That is, every slot conducting linked slots cannot be used if the level of interference waves is over the first level. In the current system, no assignment is made in consideration of the order of slots (slots are assigned from the first or fourth slot at the both ends of slots).

  For example, in three-slot link, if the level of interference waves of the second or third slot is more than the first level, the link cannot be used. Many such cases tend to occur, there may be concern about lowered frequency of usage. Therefore, upon introduction of this method, efficient use is enabled by setting dedicated frequency in assigning channels in consideration of linked slot.