OFDMA New generation Broadband wireless access and its development prospect

1 Multimedia service drives the development of OFDM technology

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier digital modulation technology whose research history dates back to the mid 1960s, although the concept of OFDM has existed for a long time, but until recently, with the development of multimedia business, It is recognized that it is a good way of high-speed two-way wireless data communication. The technology, adopted by the European Digital Television standard DVB-T and the digital Audio Broadcasting (DAB) standard, is now the core technology for WLANs (ETSI HIPERLAN/2 and ieee802.11a) and broadband wireless access (IEEE 802.16). With the development of DSP chip technology, the mature technologies such as Fourier transform/reverse transform, 64/128/256QAM technology used in high speed modem, Shanger coding technology, soft decision technology, channel Adaptive technology, inserting protection period and reducing balance computation amount are gradually introduced into the field of mobile communication. People began to focus more and more on the development of OFDM in the field of mobile communications applications, the fourth generation of mobile communication is expected to be the mainstream technology will also be OFDM technology.

Since August 31, 2001, the National Ministry of Information Industry Radio Authority approved China Netcom to carry out OFDMA fixed wireless access system (this article referred to as Celerflex) test, Netcom has done a lot of work, the current test system has been opened, and conducted the necessary testing and business demonstrations, The experiment proves that the combination of fixed wireless access and mobile wireless access is feasible in Netcom company.

2 OFDMA Technology and products-provides integrated service access platform

The test System Center station for the Zhongguancun building, equipment placed on the 20 floor, through the optical cable connected to the 3-storey China network of Tongcheng Domain Edge node, through this node can connect with Internet network, at the same time through IP gateway connected to the PSTN network. The center base station uses a directional antenna with a half power angle of 33 degrees. Terminal station We have selected the National Library (4 km, completely do not see), Friendship Hotel (1.2 km, both visual and Garden Wing (4 km, visual, Multipath), CLP Mansion (1.2 km, towards the base station and back to the base station two cases, all do not see), such as the opening of the Internet browsing, e-mail, dynamic IP, file transfer, VOIP, video on demand, conference TV and other services, and conducted a preliminary mobility test.

Because of the performance of OFDMA adaptive modulation of the technology, the product has a great advantage over the existing technologies such as fixed wireless access and mobile wireless access.

(1) Improved spectrum utilization-increased system capacity

Because the OFDMA technique is to divide a wide bandwidth resource into several carriers, it is not limited by the frequency resources it occupies, For example: Wi-lan company based on 3.5Mhz frequency resources, Broadstorm company based on 5Mhz frequency resources, WI-DAX company based on 6Mhz and 8Mhz frequency resources, while Malibu company develops products based on 20Mhz frequency resources. This test product improves frequency utilization from several aspects of frequency domain, time domain and modulation:

* A product based on this technology divides a wide bandwidth resource into several sub carriers, users choose to use a good wireless carrier, when the multi-user access, the location of different terminals wireless transmission characteristics vary greatly, each terminal can complement the use of subcarrier, to achieve all the subcarrier are effectively applied.

* For low rate signals, such as voice services, even if the use of the list carrier communication can not make full use of frequency resources, so the use of time division multiplexing, a number of low-speed signals in different slots common to the same subcarrier.

* This test product realizes the 64QAM modulation, improves the spectrum utilization from the modulation way, thus enhances the system ability.

(2) Cost reduction

* This test system uses the whole IP technology, so the system complexity, system cost and so on greatly reduced.

* Compared with CDMA technology, because the product adopts the OFDMA technology, the user can send signals on the channel of good communication quality, avoid the complex signal processing such as channel equalization and rake receiver, and thus reduce the cost.

(3) Expand the coverage

* Because of the OFDMA technology for multipath signal processing capacity is strong, so the central base station and the client station does not require complete vision, thus expanding the coverage, improve coverage quality.

* Products based on OFDMA technology, the client station chooses the wireless performance of the sub channel centralized transmission power communication, and no longer like other access mode in the entire band, thus expanding the coverage.