XSD: required nested tags if optional parent exists

I have tags which are optional in my XML, if the optional tag is present I then need the child tags of that element to be required, how can I achieve this using XSD (or another solution if this is not possible in XSD)?

An example part of my XML:

                <xs:element name="deviceLocation" minOccurs="0" maxOccurs="unbounded" nillable="true">
                  <xs:complexType>
                    <xs:sequence>
                      <xs:element type="xs:string" name="xPos"/>
                      <xs:element type="xs:string" name="yPos"/>
                      <xs:element type="xs:string" name="zPos"/>
                    </xs:sequence>
                  </xs:complexType>
                </xs:element>

As you can see the deviceLocation tag is optional - if this tags exists I then want the child tags xPos, yPos and zPos to be required.