Abstract:
A method reduces interference adaptively in a CDMA receiver. A baseband signal is demodulated to determine target symbols. The baseband signal is also demodulated to estimate interfering symbols. The interfering symbols are modulated to generate a cancellation signal, and the cancellation signal is subtracted from the base band signal to reduce interference in the target symbols.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to code division multiple access (CDMA) communication networks, and more particularly, to interference reduction and cancellation in mobile CDMA networks. 
     BACKGROUND OF THE INVENTION 
     A third generation (3G) code division multiple access (CDMA) can concurrently supports multiple services and multiple symbol rates on multiple channels. For example, services can include data services at relatively high symbol rates, and voice services at relatively low symbol rates. It is very difficult to find a simple solution that supports variable symbol rates for the various services, in real time, particularly, when the channels are confined to a fixed frequency and fixed bandwidth, i.e., a fixed “chip” rate. 
     In a CDMA network, the symbols representing data and voice signals can be orthogonally modulated using quadrature variable length Walsh codes to distinguish the respective channels and minimize cross-correlation. Error correction is typically carried out by convolution coding using variable encoding rates for the respective channels. Combining the CDMA with a spread-spectrum technique can increase network capacity. This makes each orthogonally coded control and traffic channel of the CDMA network practical for broadcast and mobile digital cellular networks (DCN). 
     A variable length Walsh code can be denoted as W N   M , where N is the Nth row of the Walsh metrics, and M is the length of a specific Walsh code W. For a channel with a given bandwidth, e.g., 1.25 MHz, and a fixed spreading/modulation method, e.g., quadrature phase shift keying (QPSK), higher symbol rates are modulated with shorter length Walsh codes and lower symbol rates are modulated with longer Walsh codes. This code assignment ensures that the symbols on the various channels remain orthogonal. 
     For a desired frame error rate (FER), the signal power must vary with the symbol rate. Usually, high symbol rate data services require a lower FER than low symbol rate voice services. Consequently, on a channel providing data services, the symbols are transmitted at a relatively higher signal power than on channels providing voice services. As a result, high symbol rate channels can interfere with low symbol rate channels, especially in a mobile communications network, such as a cellular phone network where a base station concurrently transmits symbols for data and voice services on multiple channels. Depending on the relative locations of the mobile receivers, the interference can become so serious that low power signals cannot be recovered. 
     Therefore, it is desired to reduce interference in a wireless communications network that simultaneously supports asymmetric services such as Internet data access and voice communications in real time, particularly when the receivers in the network are mobile. 
     SUMMARY OF THE INVENTION 
     The invention provides reliable communication channels in a CDMA network. The invention is particularly suited for mobile networks where symbols are concurrently transmitted over multiple fixed frequency and fixed bandwidth channels at variable symbol rates and signal powers. More particularly, the invention provides reliable low signal power voice services in the presence of high signal power data services that otherwise would interfere to meet associated application needs. More specifically, the invention can provide a mobile user good voice communications while experiencing strong interference from high data rate users. 
     The invention method and apparatus reduces interference adaptively in a CDMA receiver. A baseband signal is demodulated to determine target symbols. The baseband signal is also demodulated to estimate interfering symbols. The interfering symbols are modulated to generate a cancellation signal, and the cancellation signal is subtracted from the base band signal to reduce interference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an apparatus to reduce interference according to the invention; and 
     FIG. 2 is a block diagram of an interference estimator of the apparatus of claim  1 ; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention provides means that adaptive reduce interference in a CDMA network to improve performance on low symbol rate-low signal power voice channels in the presence of high symbol rate, high signal power data channels. 
     FIG. 1 shows an apparatus  100  for reducing interference in symbols received via a CDMA channel. The apparatus can be used in a mobile receiver or the receiver of a fixed base station. 
     The apparatus includes a down-converter  110  connected to an antenna  101  to receive a spread-spectrum signal. The down-converter produces a baseband signal  111  from the received RF signal. De-spreading of user-specific pseudo random noise (PN) sequences on the baseband signal takes place in the PN de-spreader  120  The de-spreader can include a complex multiplier and an integrator. Spread-spectrum signals provide multiple-access. The de-spreaded signal is fed to a delay controller  130  and a Walsh decoder  140  before the target symbols  141  are fully decoded. 
     As stated above, the received signal can be mixed with a high power interference signal intended for an other high symbol rate user. The interfering signal is spread with a short-length orthogonal Walsh code. In general, due to the power control and targeted service requirements, e.g., a desired SNR, higher symbol rates have higher power and shorted code lengths. 
     Therefore, before Walsh demodulation and soft combining, the receiver according to the invention performs interference reduction, or cancellation, to improve performance. 
     An interference estimator  200  is used to estimate the high power interfering symbols. From the interfering symbols, the estimator generates a cancellation signal  201  that approximates the interference. The cancellation signal  201  is then subtracted  135  from the delayed PN-de-spreaded signal  131  to form the input to the Walsh demodulation block  140 . A control system  150  measures a signal-to-noise ratio (SNR) of the partially decoded target symbols  141 , and compares the measured SNR with a pre-determined threshold. If the SNR is greater than the predetermined threshold, the interference estimator  200  is activated. If the SNR is below the threshold the estimator  200  is not required. 
     FIG. 2 shows the interference estimator  200  in greater detail. The estimator includes a Walsh searcher  210 , a delay  220 , a Walsh demodulator  230 , an adder  240 , and a gain controller  250 . The estimator also receives the de-spreaded baseband signal  121 . The function of the estimator  200  is to generate a cancellation signal  201 . The estimated cancellation signal is then subtracted from the delayed de-spreaded signal  131  to reduce or cancel interference. 
     The searcher  210  attempts to find the Walsh code of the high symbol rate user signal so that the interfering signal can be re-generate. Because the Walsh code of the high power interfering signal is short, the Walsh code is one of a small set of candidate codes. Thus, in a low complex implementation, a trial and error method can be used to estimate the Walsh code of the interfering signal. 
     In general, the searcher  210  can perform a partial correlation between Walsh code of the input signal  121  and all possible candidate codes, and find the Walsh code with the maximum likelihood. After the partial Walsh code has been found, the interfering signal is demodulated in block  230  to produce estimated interfering symbols  131 . Block  240  is used to generate the cancellation signal  201  from the estimated interfering symbols, as transmitted for the high symbol rate user. The gain controller  250  matches the power of the cancellation signal  210  with the power of the de-spreaded signal  121 . The cancellation signal  201  is than subtracted  135  from the delayed de-spreaded signal  121  to reduce or cancel interference. The delay controller  130  delays the de-spreaded signal  121  by a time interval that is equal to the latency of the Walsh estimator  200 . 
     Although the invention has been described with respect to a mobile receiver in a wireless CDMA network, it should be understood that the invention can also be deployed in a receiver of a fixed base station, or any other type of CDMA receiver, for example, a satellite receiver. 
     As an advantage, the base station has access to the Walsh codes that are used by the mobile receivers. Therefore, the base station actually re-generate the interfering signal directly to effect interference cancellation in the physical media independent layer (PHY). 
     Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.