Receiver DC-offset reduction method
Original Publication Date: 2001-Dec-01
Included in the Prior Art Database: 2003-Jul-23
For the existing Global System of Mobile Communi- cation (GSM) a new radio interface technology called EDGE (Enhanced Data Rates for GSM evolution), also known as EGPRS, will enable large scale mobile videos and internet applications. In order to achieve this, EDGE allows for an increased data throughput up to around 384 kbits/s per carrier using 8PSK modulation scheme instead of binary GMSK (Gaus- sian Minimum Shift Keying) modulation. In general mobile cellular phone receivers introduce impairments due to the receiving process e.g. direct conversion receivers have DC-offsets that shifts the received base band signal from the origin. Since 8PSK modulation has constellation points with low distance to each other compared to GMSK, this modulation type is more sensitive against DC-shifts. This means that DC-offset compensation algorithms usually used for GMSK are not sufficient for EDGE receivers in order to achieve adequate receiver per- formance.
Information / Kommunikation
Receiver DC-offset reductionmethod
Idee: Dr. Hans Dieterich, Ulm
For the existing Global System of Mobile Communi-cation (GSM) a new radio interface technology calledEDGE (Enhanced Data� Rates� for� GSM� evolution),also known as EGPRS, will enable large scale mobilevideos and internet applications. In order to achievethis, EDGE allows for an increased data throughputup� to� around� 384� kbits/s� per� carrier� using� 8PSKmodulation scheme instead of binary GMSK (Gaus-sian Minimum Shift Keying) modulation.
In general mobile cellular phone receivers introduceimpairments due to the receiving process e.g. directconversion receivers have DC-offsets that shifts thereceived base� band� signal� from� the� origin.� Since8PSK modulation has constellation points with lowdistance to each� other� compared� to� GMSK,� thismodulation type is more sensitive against DC-shifts.This means that DC-offset compensation algorithmsusually used for GMSK are not sufficient for EDGEreceivers in order to achieve adequate receiver per-formance.
An object of the present� invention� is� to� provide� amethod for the reduction of the DC offset from thereceived digital base band signal for EGPRS / EDGEreceivers� in� an� efficient� way� so� that� the� specifiedperformance� in� the� GSM 05.05 specification forEDGE can be fulfilled using normal 8PSK equalisers.
The idea behind this invention is to estimate jointlythe channel� impulse� response� (CIR)� and� the� DC-offset during the received training sequence symbolsof an EDGE burst. The schematic of the invention isshown in the figure 1 below.
At� first� an� initial� DC-offset� reduction as usuallyknown for GMSK is applied. After the de-rotation ofthe DC-offset reduced samples an initial CIR estima-tion is performed. In general two kinds of approachescan be used for the CIR estimation:
a) In the conventional approach a part of the trainingsequence� is� correlated� with� the� received� sequencethat corresponds to the training sequence. This ap-proach is often used but it has the disadvantage that at
the� start� and� the� end� of� the� correlation� user dataswhich are transmitted� before� and� after� the� trainingsequence may affect the CIR estimation. This maylead to so called phantom taps which do not exi...