XSD - define element and attribute with restrictions

I need to define XSD which has sequence of element with optional attribute. I also need to restrict string length of both element and attribute. Using following code I cannot seem to define 'restriction':

          <xs:element name="PeopleMentioned" minOccurs="0">
            <xs:complexType>
              <xs:sequence>
                <xs:element name="Person" minOccurs="1" maxOccurs="unbounded">
                  <xs:complexType>
                    <xs:simpleContent>
                      <xs:extension base="xs:string">
                        <xs:attribute name="PersonTopic" type="xs:string" />
                      </xs:extension>
                    </xs:simpleContent>                        
                  </xs:complexType>
                </xs:element>
              </xs:sequence>
            </xs:complexType>
          </xs:element>

I need to restrict length of 'Person' and 'PersonTopic', like would normally do for simple type:

                  <xs:simpleType>
                    <xs:restriction base="xs:string">
                      <xs:maxLength value="18" />
                    </xs:restriction>
                  </xs:simpleType>