Derek Chauvin, who was earlier this year jailed for the murder of George Floyd, has appealed against his conviction, BBC News reports.
The former Minneapolis police officer was sentenced in June to 22 and a half years after kneeling on Floyd's neck.
Cops arrested Floyd after the proprietor accused him of using a counterfeit banknote to pay for his purchases.
Floyd was then handcuffed lying face down in the street, while Chauvin pressed his knee on the back of his neck for a total of nine minutes - fatally asphyxiating him.
Cell phone footage recorded by teenage bystander Darnella Frazier showed that Floyd repeatedly asked for help and pled for leniency from ground level, even stating audibly "I can't breathe" more than 20 times.
Floyd's death sparked mass global protests against racism and police brutality in the US.
And now, after having 90 days to lodge an appeal from the date of sentencing, Chauvin has filed court documents on Thursday, September 23.
Per Chauvin alleges that the trial judge abused his discretion at several key points of the case, including denying a request to postpone or move the hearing from Minneapolis due to pre-trial publicity.
The former officer said he had no legal representative for the appeal process as the Minnesota police department's "obligation to pay for my representation terminated upon my conviction and sentencing", the Associated Press news agency reports.
He has asked the Supreme Court to review an earlier decision to deny him a publicly-financed lawyer.
Chauvin was given 90 days from the date of his sentencing on 25 June to appeal against his conviction.
He was found guilty of second-degree murder and other charges, was barred from owning firearms for life, and was also told to register as a predatory offender.
Chauvin is the first white police officer in Minnesota to be convicted of killing a black man.
Per Sky News, the maximum sentence for second-degree unintentional murder is 40 years in prison under Minnesotan law.
However, Chauvin's lack of previous convictions meant that his sentence was expected to have been lighter.