Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-06-22
2004-06-08
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C370S337000, C455S069000, C455S093000
Reexamination Certificate
active
06748021
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a cellular radio communication system. In particular the present invention relates to a broadband wireless access system suitable for the delivery of multi-media services.
BACKGROUND OF INVENTION
There is a growing demand for broadband wireless access systems which can deliver the high data rates required for the provision of multi-media services. Such wireless access systems operate within licensed frequency bands. Accordingly, these systems are continually developing to carry more data across the limited frequency band allocated to them. Pressure for this development is two fold. Firstly, there is increased demand for multi-media services from subscribers to the system. Secondly, revenue for the network operator will increase as billing is calculated on a per byte of information delivered basis as opposed to on a per second basis.
The performance of wireless access communication systems is prone to dynamic degradation, ie. time variant degradation, due to changing environmental conditions. Wireless transmissions in the frequency range from 10 to 50 GHz are particularly prone to dynamic degradation resulting from rain and from the growth and movement of foliage located in the transmission path. For example, radio wave propagation through rain causes absorption and scattering of the radio energy. These effects cause signal attenuation and must be considered in the design of radio communications systems, particularly at frequencies above several GHz, as the attenuation effects increase with frequency.
FIG. 1
shows the attenuation of a 30 GHz signal in dB per kilometer due to rainfall against the percentage of time that such rainfall occurs within climate zone ‘F’ which zone covers the UK.
This type of dynamic degradation has been taken account of in existing wireless access systems by designing the systems for operation in worst case environmental conditions. This has been achieved by the use of robust modulation schemes such as QPSK (Quadrature Phase Shift Keying), also known as 4-QAM (Quadrature Amplitude Modulation) which deliver low BERs (bit error rates) of the order of 10
−9
, ie. one incorrect bit per 10
9
bits transmitted, in poor environmental conditions. However, designing such systems for worst case environmental conditions in this way results in low rates of data transmission.
Amplitude dependent modulation schemes, such as 64-QAM, 16-QAM and QPSK in which the amplitude of the modulation envelope is varied according to the symbol to be transmitted (referred to hereafter as amplitude dependent modulation schemes) are considered to be the most spectrally efficient modulation schemes. These modulation techniques have therefore been preferred in radio communication systems such as mobile, satellite and fixed networks where the available bandwidth is limited. When using amplitude dependent modulation schemes, linear amplification is required in order to keep third order intermodulation products and spectral regrowth to within acceptable limits. Thus, some of the potential gain available from amplification has to be sacrificed.
As can be seen from
FIG. 1
, for the majority of time transmission conditions are good. Adaptive modulation techniques have been proposed which enable higher data rates to be achieved by the use of 16-QAM or 64-QAM modulation schemes when the transmission conditions across a wireless link are improved or where the distance over which the link extends is relatively short. In this way the rate of data transmission within a limited frequency band can be improved. However, for very poor transmission conditions or for transmissions over longer distances a break in communication can occur. This is because for amplitude dependent modulation schemes, the modulated signal has to undergo substantially linear amplification before transmission, as discussed above. This limits the output power of the amplifier, thus limiting the power of the signal that can be transmitted over the transmission link.
In known cellular wireless access system a frequency plan is implemented over a geographical area. The frequency plan allocates channels within the frequency band to localised cells and due to attenuation of a radio signal across the cells, the same channel can be re-used within other cells in the frequency plan. The aim is to maximise frequency re-use without causing interference between parts of the frequency plan which use the same channels. Generally, a base station is associated with a cell to transmit radio frequency signals to all end user terminals or CPEs (Customer Premise Equipments) located within the geographical area covered by the cell. The uplink from the CPEs in the area to the base station may be a common medium access uplink, for example a FTDMA (Frequency or Time Division Multiple Access) uplink in which time and frequency carrier slots can in some way be allocated for use by the CPEs to send signals to the base station. The downlink from the base station to the CPEs may be a TDA (Time Division Access) downlink, with time slots over which the base station sends signals to the CPEs.
Another approach to optimising the use of bandwidth is automatic repeat request (ARQ). In this approach the receiving unit, be it a base station or a CPE, detects which signals sent across the transmission link have been received with errors in them and sends a feedback message to the transmitting unit requesting that the signals which have not been correctly received are sent again. This is an alternative way of increasing or decreasing the amount of information which is sent across the transmission link dependent on environmental conditions. In poor transmission conditions, more data will have to be re-sent and so data rates will be low. In good transmission conditions, less data will have to be re-sent and so data rates will be higher. However, this method has a degree of transmission delay inherent within it which may not be appropriate for all multimedia services. It is also inefficient in terms of the amount of uplink or downlink resource used for services requiring a low bit error rate.
It is also known to use forward error correction (FEC) in which a FEC code is added to the data payload of a packet sent across a transmission link. The FEC code is used by the receiving unit to detect and correct errors in the data payload received by the receiving unit.
OBJECT OF THE INVENTION
The present invention seeks to provide an improved cellular radio communication system which uses adaptive modulation and which can provide transmission links having optimised data rates with low bit error rates dependent on the prevailing transmission conditions, while maintaining the links in the poorest transmission conditions. Because links can be maintained in poor transmission conditions the present invention can enable operation over longer distance transmission links.
STATEMENT OF INVENTION
According to a first aspect of the present invention there is provided a cellular radio communication system for transmitting data over a plurality of transmission links comprising means for generating a modulated signal by applying a constant amplitude envelope modulation scheme to data to be transmitted across poor quality transmission links and amplifier means for non-linearly amplifying the modulated signal. By using a constant amplitude envelope modulation scheme higher gain non-linear amplification can be used without prejudicing the ability to recover data from the modulated signal. Thus, signals transmitted over the poorest quality transmission links can have a higher power which enables transmission links to be maintained in poor transmission conditions or over longer distances.
The system preferably additionally comprising means for generating a second modulated signal by applying an amplitude dependent modulation scheme to data to be transmitted across higher quality transmission links and amplifier means for linearly amplifying the second modulated signal. Thus, for higher quality links a higher spectral efficiency can be
Barnes & Thornburg LLP
Chin Stephen
Nortel Networks Limited
Pathak Sudhanshu C.
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