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IETE TECHNICAL REVIEW, Vol 23, No 5, 2006

 

Difficulty in wide area coverage using higher frequencies

  • 1.8/1.9 GHz bands were added to GSM (900MHz) as DCS1800/ 1900; 1.9GHz band was to AMPS family; and 1.5GHz band was to PDC. None of them are nationwide system because of narrower coverage, only used to compensate for shortage in 800/900MHz bands or to allow competitive service by new operator.

  • Seeing rapid increase of PHS (1.9GHz) subscribers at its early stage, any PDC operators were afraid;“the future might be PHS”. But the result was different in spite of every improvement made in PHS: high speed mobility, control channel power up at BTS (Base Transmitter Station), etc. due to issue in ‘anywhere’ caused not only by low power but by high frequency.

  • Problem of PHS WLL by coconuts tree, due to intense radio absorption.

  • 3G in 2GHz needs 4-6 times number of cell sites compared with 800/ 900MHz, causing the system cost increase.

  • High cost of 3G: full coverage will not be available for several years, first 3G services: in markets with a high concentration of potential users such as large cities and their immediate surroundings.

    SDR Base station transceiver (BTS)

  • SDR BTS can support multiple means to enhance spectrum efficiency and reuse.

  • SDR technology may be used to implement: improved codes, adaptive interference management, and smart antennas.

  • To fully realize the benefits of dynamic spectrum allocation/ adaptive non-interfering basis operation, the functions of identifying, allocating, and assigning spectrum resources within the cell are best done in well-managed wireless networks by BTS.

  • Ideally, handset and BTS should feature: reprogrammable radio frequency, modulation, and interface characteristics, but the spectrum management functions should remain the domain of BTS.

  • Spectrum efficiency is improved by using a multiplicity of means. Some of these may be jointly applied in synergitic combinations. Examples of principal methods listed below:
    1. Coding: Advanced coding/compression methods allow fewer bits to transmit any given amount of information.
  
    1. Modulation: Higher-order modulation methods improve spectral efficiency from; 1 bps/Hz (BPSK) up to: 3 bps/Hz by 8-ary PSK, or 5 bps/Hz by 64QAM.

    2. Adaptive Interference Management

    3. Smart Antennas

    4. Spectrum Management


Multi-band, multi-mode SDR user terminals and base stations may monitor the spectrum in selected bands:

- identify unused spectrum,
- If needed, negotiate use of that spectrum
  for the mobile services.


ADAPTIVE INTERFERENCE MANAGEMENT:
ADAPTIVE EQUALIZER

The basic problem of receiver design in communication system is the presence channel distortion, which is not known at a priori. The channel distortion results in inter symbol interference (ISI), which, if left uncompensated, causes high error rates. The solution to the ISI problem is to design a receiver that employs a means for compensating or reducing the ISI in the received signal. The compensator for the ISI is called an equalizer (Fig 1).

Fig 1 Basic communication system with equalizer

From the definition of the equalizer we have:

In most of the communication system that employ equalizers, the channel characteristics are unknown a priori and, in many cases, the channel response is
time-variant. In such a case the equalizers are designed to be adjustable to the channel response and, for time –variant channels, to be adaptive to the time variations in the channel response.

The equalizers based on algorithms for automatically adjusting the equalizer coefficients to optimize a specified performance index and to adaptively compensate for time variations in the channel characteristics are called adaptive equalizers.