soap request报错

一个Soap Request的xsd文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="unqualified" targetNamespace="http://schemas.xmlsoap.org/soap/envelope/" 
  xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <xs:element name="Envelope">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="soap:Header"/>
        <xs:element ref="soap:Body"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="Header">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="AuthenticationHeader">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Username" type="xs:string"/>
              <xs:element name="Password" type="xs:string"/>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="Body">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="availabilityRequest">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="arrivalDate" type="xs:date"/>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

以这个xsd为参照，我写了一个request的xml，如下：

<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <soap:Header>
    <AuthenticationHeader>
      <Username>Username</Username>
      <Password>Password</Password>
    </AuthenticationHeader>
  </soap:Header>
  <soap:Body>
    <availabilityRequest>
      <arrivalDate>2001-01-01</arrivalDate>
    </availabilityRequest>
  </soap:Body>
</soap:Envelope>

结果validate的时候，"<AuthenticationHeader>"总是报错：

Description    Resource    Path    Location    Type
cvc-complex-type.2.4.a: Invalid content was found starting with element 'AuthenticationHeader'. One of '{WC[##other:"http://schemas.xmlsoap.org/soap/envelope/"]}' is expected.    DemoRequest.xml    /TrivagoTask/src/com/trivago/task1    line 6    XML Problem

请教各位，到底错在哪里呢，如何修改？多谢先！