XGP | Technology | Feature and Outline

	Features and Outlines

	Feature of XGP

OFDMA/TDMA/TDD

XGP adopts OFDMA/TDMA as multi dimension access method and TDD as duplex method. Besides FDMA/TDD method, SC-FDMA/TDD method can also be adopted for UL (uplink) .

The co-existence of OFDMA/TDD and SC-FDMA/TDD method is possible during uplink. The structure of TDMA slot is symmetrical with 625 us x 4 slots for uplink and 625 us x 4 slots for downlink. This structure is the same with the existing PHS system, which made its co-existence with XGP possible.

The symmetrical structure of TDMA slot guarantees its good affinity with AAS and SDMA. XGP adopted AAS and SDMA technology to improve the efficiency in using radio frequency resource.

The technical reference for XGP are displayed in the following charts: General parameter for XGP

Duplex method	TDD
Downlink access method	OFDMA/TDMA
Uplink access method	OFDMA, SC-FDMA/TDMA
TDMA slot period	625 us
TDMA frame period	5 ms
Number of slots in one frame	8 slots, 4 slots for transmission and 4 slots for reception symmetrically

System bandwidth [MHz]	1.25	2,5	5	10		20
Effective channel bandwidth [MHz]	0.9	1.8	3.6	8.1	9	16.2	17.1	18
Guard bandwidth [MHz]	0.35	0.7	1.4	1.9	1	3.8	2.9	2
Number of subchannels	1	2	4	9	10	18	19	20
Total number of PRU	4	8	16	36	40	72	76	80

	Individual parameters for each system bandwidth

The following chart displays TDMA/TDD method adopted in existing PHS system and
OFDMA/TDMA/TDD method adopted in XGP.
One communication channel is assigned to one timeslot in existing PHS.
Taking advantage of the OFDM feature of XGP, multiplexing plural units (PRU) in one timeslot is realized.
The amount of multipliable PRUs can increase or decrease according to the system bandwidth.

Individual parameters for each system bandwidth

	Dynamic Channel Assignment by DCA

	The PRU to be used in XGP adopts DCA method to execute “carrier sensing” function to detect if a source has already been used by other users, hence, to avoid interference with other users while assigning adaptive channel. That is, BS carries out carrier sensing after receiving a wireless link establishment demand from BS. Unused resource of PRU is assigned to the demanding user according to the result of carrier sensing. By adopting DCA method, the centralized control device in wireless base station is saved and Autonomous Decentralized Control is realized. As one base station can share PRU with its adjacent base stations, installation design on adding new base stations becomes unnecessary

	Micro Cell


	Mobile system such as GSM and W-CDMA adopts macro cells with each covering an area of several kilometer to several ten kilometers in radius. On the other hand, XGP adopts micro cells with each covering an area of no more than several hundred meters in radius. The reasons to choose micro cell system are stated below: 1, Per unit efficient on frequency usage improves and system capacity increases. This is how flat rate service becomes possible . 2, As service at reasonably affordable rate begins, more cell stations shall be installed to cover more areas and more users. 3, Autonomous Distributed Control function is activated in heavy traffic areas with plural cell stations. Hence, per user throughput is maximized. 4, Comparing the declining rate of throughput by number of increased users in certain area, micro cell system suffers a lower one than macro cell system, which covers the whole area with one cell. Since reduction of electric output to the terminal is realized, size of the terminal can be minimized and battery becomes more durable. Meanwhile, base station can be minimized for the same reason. 2, Hence, cell station can be installed in narrow and closed space like inner building or underground street. 3, Layout of cell stations can be carried out in accord with traffic condition, such as increasing cell density in area with concentrated access. 4, Besides, when on cell station suffers unexpected breakdown, other cell stations around the area can take instant action as complement. 5, Therefore, we consider micro cell system a better communication system at time of disaster.

	Two MAC Methods


	The Mac methods adopted by PHS are QS-Mode and FM-Mode. QS-Mode is a fixed assignment method that assigns PRUs as CSCH to all users. As wireless band is occupied by specified user during the assignment, the mode is adaptive to services such as voice communication with high demand of real time and high possibility to capture random data. FM-Mode is a fixed assignment method that assigns one PRU as ANCH to all users. PRU for communication (EXCH) is dynamically assigned from each TDMA frame by MAP in the ANCH. That is, EXCH is shared by plural users instead of being monopolized by one user. FM-Mode is adaptive to services such as high speed data communication as it can respond to the burst increase and decrease of data instantly. QS-Mode is always applied to control users who occupy the bandwidth in voice communication and FM-Mode is always applied to control users in data communication. QS-Mode can also be used to control low-speed packet communication together with voice communication, in case when bandwidth for a system Is narrow. Besides, QCS-ID, which is adopted in XGP, can realize the function to assign multiple services to one user.

	Overcoming the Weak Points of Micro Cell System


	XGP takes into consideration of the adoption of macro cells as supplyment to micro cells during system deployment. By doing this, times for handover reduce as macro cells are adopted to cover area that correspond to connect requests during high speed movement. Therefore, communciation quality is expected to improve during handover. In addition, it is neccessary to adopt plural base stations to cover a big area with wireless network that's constructed of micro cell only. Hence, it is most practical to expand service area by building micro cell system in areas with dense polution and adopting macro cell system in areas with sparse population.

	Terminal Valuation and Development Cost


	As long as a terminal keeps one PRU for transmitting and receiving data, it can be used for communication in XGP system. That is, the system doesn’t request all terminals to hold ability to transmitting and receiving data with full bandwidth. One can choose OFDMA or SC-FDMA as UL access method according to the function of a terminal. If SC-FDMA is chosen as the UL access method, the development of terminal can be easier as the disposal process can be better simplified than OFDMA. For example, high-end terminal for high-speed communication uses OFDMA for both UL and DL communication and is equipped with wireless device that can transmit and receive data with full bandwidth. Low-end terminal used basically for voice communication uses SC-FDMA for UL communication and is equipped with only one PRU to transmit and receive data, in order to reduce cost. This feature allows development of terminals with different functions and usages by choosing access method for UL and bandwidth for data transmission. That is, developing different types of terminals in correspondence with different users’ needs becomes possible.

	Top of page