the preferred embodiments of the present invention will now be explained with reference to the accompanying drawings . it should be understood however that the disclosed embodiments are merely exemplary of the invention , which may be embodied in various forms . the following description and drawings are not to be construed as limiting the invention and numerous specific details are described to provide a thorough understanding of the present invention , as the basis for the claims and as a basis for teaching one skilled in the art how to make and / or use the invention . however in certain instances , well - known or conventional details are not described in order not to unnecessarily obscure the present invention in detail . the present invention provides a system and method of providing ue assisted backward handover in heterogeneous wireless networks . the generalized ue assisted backward handover between the heterogeneous wireless networks is shown in fig2 . the functionality of the interworking function / unit ( iwf / iwu ) is to convert the ran and the cn containers / protocol / parameters of one access system to another . iwf interface with different network entities , with the entity specific interface protocol . the iwf can be co - located with any other network entity in the serving network or in the target network or alternatively can be a separate entity in the serving network or in the target network . the operation of the ue assisted backward handover is given in details below : ue sends periodic or event based measurements to the serving access network . if the serving access system finds that ue measurement is below the threshold or serving access system decides by any other mean that serving rat cannot be continued , then the serving access system can request the ue to start scanning other rats or alternatively by l2 or by some other means , the ue decides that the serving ran cannot be continued and starts scanning the other rats . ue decide the target system based on the measurement report compared with the rat specific handover criteria and ranking orders configured in the ue . the ue initiates the backward handover procedure by sending handover request message to the target network through the serving network . the serving network checks for the capabilities and authorizes the ue requested handover . then the serving network forwards the ho request to the interworking function ( iwf ). the serving network includes the security context of the ue along with this message . the iwf converts the serving access network specific parameters to the target specific parameters and forwards the request to the target network . the iwf lookup the target system parameters in the ho request message and forwards the ho request to the appropriate target access system entity . the target system then prepares for the ue handover using the ho request parameters . after successful ho preparation the target system sends ho response message to the iwf . the target system includes the target system related parameters like entity id and / or ip address and selected security configuration along with the ho response . the iwf then converts the parameters according to the serving system specific parameters and forwards the ho response to the serving network . then the ue initiate l2 connectivity to the target system . illustrated example for backward handover from the wimax to the lte / sae access system as shown in fig3 ue sends periodic or event based measurements to the bs of wimax network . if the bs finds that ue measurement is below the threshold or bs decides by any other mean that wimax ran cannot be continued , then the bs can request the ue to start scanning other rats or alternatively by l2 or by some other means , the ue decides that the serving ran cannot be continued and starts scanning the other rats . ue decided the target system as lte / sae based on the lte / sae measurement report compared with the rat specific handover criteria and ranking orders configured in the ue . the ue initiates the backward handover procedure by sending handover request message to the lte / sae network through the wimax network . the ue includes the ue security capabilities , tai , cell id , indiation of inter rat ho , serving and target rats , etc . the ue send the ho request to the bs , and then the bs forwards the ho request to the asn gw . the asn gw then contacts the aaa server for authorization . the aaa server checks in user &# 39 ; s profile if inter rat ho is allowed . if the user is capable of inter rat handover , in authorization answer , the aaa server includes the unused avs , latest ck and ik or alternatively the master key ( mk ). the asn gw forwards the ho request message along with the security keys given by aaa server to the iwf . the iwf function then forwards the ho request to the mme through the interface specific protocol . the iwf obtains the ip address of the mme by resolving tai or alternatively the iwf have the mapping of tai to the mme . the mme decides the target enb and derives the lte / sae specific keys and prepares for the ue ho . the mme derives the lte / sae specific key k asme using the ck and ik or the master keys of the wimax network . the derivation of k asme from ck and ik is by concatenation of ck and ik or alternatively using the pseudo random function ( prf ) and input to the prf are ck and ik or alternatively exclusive or &# 39 ; ing the keys ck and ik or alternatively some other means using ck and ik or using mk . then from k asme all other keys are derived as specified in the 3gpp tr 33 . 821 . the mme passes the security context to the target enb and setup the rab for the ue . the mme sends the preferred algorithm as the algorithm used in the wimax system by the ue to the target enb . after successful ho preparation the mme sends ho response message to the iwf . the mme includes the target system related parameters like enb id ( s ) and selected security configuration parameters along with the ho response . the iwf then converts the parameters according to the serving system specific parameters and forwards the ho response to the ue through aaa , asn gw and bs . then the ue initiate l2 connectivity to the enb . illustrated example for backward handover from the wimax to the lte / sae access system as shown in fig4 ue sends periodic or event based measurements to the bs of wimax network . if the bs finds that ue measurement is below the threshold or bs decides by any other mean that wimax ran cannot be continued , then the bs can request the ue to start scanning other rats or alternatively by l2 or by some other means , the ue decides that the serving ran cannot be continued and starts scanning the other rats . ue decided the target system as lte / sae based on the lte / sae measurement report compared with the rat specific handover criteria and ranking orders configured in the ue . the ue initiates the backward handover procedure by sending handover request message to the lte / sae network through the wimax network . the ue includes the ue security capabilities , tai , cell id , indication of inter rat ho , serving and target rats , etc . the ue send the ho request to the bs , and then the bs forwards the ho request to the asn gw . the asn gw forwards the ho request message to the iwf . the iwf then contacts the aaa server for authorization through aa request message . the aaa server checks in user &# 39 ; s profile if inter rat ho is allowed . if the user is capable of inter rat handover , in authorization answer message , the aaa server includes the unused avs , latest ck and ik or alternatively the master key ( mk ). the iwf forwards the ho request message along with the security keys given by aaa server to the mme . the iwf function then forwards the ho request to the mme through the interface specific protocol . the iwf obtains the ip address of the mme by resolving tai or alternatively the iwf have the mapping of tai to the mme . the mme decides the target enb and derives the lte / sae specific keys and prepares for the ue ho . the mme derives the lte / sae specific key k asme using the ck and ik or the master keys of the wimax network . the derivation of k asme from ck and ik is by concatenation of ck and ik or alternatively using the pseudo random function ( prf ) and input to the prf are ck and ik or alternatively using exclusive or function on the keys ck and ik or alternatively some other means using ck and ik or using mk . then from k asme all other keys are derived as specified in the 3gpp tr 33 . 821 . the mme passes the security context to the target enb and setup the rab for the ue . the mme sends the preferred algorithm as the algorithm used in the wimax system by the ue to the target enb . after successful ho preparation the mme sends ho response message to the iwf . the mme includes the target system related parameters like enb id ( s ) and selected security configuration parameters along with the ho response . the iwf then converts the parameters according to the serving system specific parameters and forwards the ho response to the ue through asn gw and bs . then the ue initiate l2 connectivity to the enb . illustrated example for backward handover from the lte / sae to the wimax access system as shown in fig5 ue sends periodic or event based measurements to the enb of lte access network . if the enb finds that ue measurement is below the threshold or enb decides by any other mean that wimax ran cannot be continued , then the enb can request the ue to start scanning other rats or alternatively by l2 or by some other means , the ue decides that the serving ran cannot be continued and starts scanning the other rats . ue decided the target system as wimax based on the lte / sae measurement report compared with the rat specific handover criteria and ranking orders configured in the ue . the ue initiates the backward handover procedure by sending handover request message to the wimax network through the lte / sae network . the ue includes the ue security capabilities , bs id , nai ( formaed based on imsi ), asn id , indiation of inter rat ho , serving and target rats , etc . the ue send the ho request to the enb , and then the enb forwards the ho request to the mme . the mme checks in user &# 39 ; s profile , if inter rat ho is allowed for the ue . if the user is capable of inter rat handover , then the mme forwards the ho request message to the iwf . the mme includes the unused avs , latest ck and ik along with the ho request message . the iwf then contacts the aaa server for authorization and key derivation through the interface specific protocol . the iwf obtains the ip address of the aaa server by resolving nai or alternatively the iwf have the mapping of nai to the aaa server . the iwf sends the aa request message to the aaa server for authentication and authorization . the aa request message includes ue security capabilities , bs id , nai ( formed based on imsi ), asn id , indication of inter rat ho , serving and target rats , unused avs , etc ., along with the security keys . the security key can be ck and ik derived from the k asme or alternatively the mme sends k asme as the security key . the aaa server maps the target asn gw based on the bs id or alternatively by asn id or alternatively through some other means and derives the wimax specific keys . the wimax specific key master key ( mk ) are derived from the ck and ik keys received from the lte / sae system as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )) or alternatively if k asme is received from the lte / sae system then the aaa using k asme as the mk . from master key the aaa server derives the following keys , transient eap keys ( teks ), master session key ( msk ) and extended master session key ( emsk ) as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )). the aaa server passes aa response message along with the msk key to the target asn gw . the asn gw derives wimax specific keys from the msk and passes the wimax specific keys and security context to the target bs . the asn gw optionally setup the rab for the ue . after successful key derivation the asn gw sends aa answer message to the aaa server . the asn gw includes the pmk sequence number along with the aa answer message . the iwf sends the ho request message to the asn gw . the iwf includes the ue id and the bs id along with the other parameters in the message . the asn gw sends the ho response message with the target bs id . the iwf then converts the parameters according to the lte / sae specific parameters and sends the ho command to the ue through the mme and the enb . the iwf includes the target bs id , pmk sequence number . the ue generates the wimax specific keys using the latest ck and ik keys agreed in the lte / sae system . the wimax specific key master key ( mk ) are derived from the ck and ik keys used in the lte / sae system as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )) or alternatively if k asme is used , then the ue using k asme as the mk . from master key the ue derives the following keys , transient eap keys ( teks ), master session key ( msk ) and extended master session key ( emsk ) as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )) and start handover procedure . the ue does the 802 . 16e link up procedure . the ue set the ho process optimization bit . or alternatively the ue can indicate inter ho optimization procedure by having a new bit . during the security capability exchange through the sbc_req message , the ue can send the preferred algorithm as the algorithm used in the lte / sae system . then the wimax bs begins the pkmv2 three way handshake and establish the sa for link layer protection . the bs may accept the ue preferred algorithm , if it supports . illustrated example for backward handover from the lte / sae to the wimax access system as shown in fig6 ue sends periodic or event based measurements to the enb of lte access network . if the enb finds that ue measurement is below the threshold or enb decides by any other mean that wimax ran cannot be continued , then the enb can request the ue to start scanning other rats or alternatively by l2 or by some other means , the ue decides that the serving ran cannot be continued and starts scanning the other rats . ue decided the target system as wimax based on the lte / sae measurement report compared with the rat specific handover criteria and ranking orders configured in the ue . the ue initiates the backward handover procedure by sending handover request message to the wimax network through the lte / sae network . the ue includes the ue security capabilities , bs id , nai ( formaed based on imsi ), asn id , indiation of inter rat ho , serving and target rats , etc . the ue send the ho request to the enb , and then the enb forwards the ho request to the mme . the mme checks in user &# 39 ; s profile , if inter rat ho is allowed for the ue . if the user is capable of inter rat handover , then the mme forwards the ho request message to the iwf . the mme includes the unused avs , latest ck and ik along with the ho request message . the iwf then contacts the asn gw using the bs id or through any other means . the iwf sends the ho request message to the asn gw through the interface specific protocol . the ho request message includes ue security capabilities , bs id , nai ( formaed based on imsi ), asn id , indication of inter rat ho , serving and target rats , unused avs , etc ., along with the security keys . the asn gw contacts the aaa server for authorization and key derivation . the asn gw sends the aa request message to the aaa server . the aa request message includes ue security capabilities , bs id , nai ( formaed based on imsi ), asn id , indication of inter rat ho , serving and target rats , unused avs , etc ., along with the security keys . the security key can be ck and ik derived from the k asme or alternatively the mme sends k asme as the security key . the aaa server checks for authorization and derives the wimax specific keys . the wimax specific key master key ( mk ) are derived from the ck and ik keys received from the lte / sae system as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )) or alternatively if k asme is received from the lte / sae system then the aaa using k asme as the mk . from master key the aaa server derives the following keys , transient eap keys ( teks ), master session key ( msk ) and extended master session key ( emsk ) as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )). the aaa server passes aa response message along with the msk key to the asn gw . the asn gw derives wimax specific keys from the msk key as specified in the wimax specification “ wimax forum network architecture — stage 3 — detailed protocols and procedures — release 1 . 0 . 0 ” and passes the wimax specific keys and security context to the target bs . the asn gw optionally setup the rab for the ue . after successful key derivation the asn gw sends ho response message to the aaa server . the asn gw includes the pmk sequence number and the target bs id along with the ho response message . the iwf then converts the parameters according to the lte / sae specific parameters and sends the ho command to the ue through the mme and the enb . the iwf includes the target bs id , pmk sequence number . the ue generates the wimax specific keys using the latest ck and ik keys agreed in the lte / sae system . the wimax specific key master key ( mk ) are derived from the ck and ik keys used in the lte / sae system as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )) or alternatively if k asme is used , then the ue using k asme as the mk . from master key the ue derives the following keys , transient eap keys ( teks ), master session key ( msk ) and extended master session key ( emsk ) as specified in the ietf rfc 4187 ( extensible authentication protocol method for 3rd generation authentication and key agreement ( eap - aka )) and start handover procedure . the ue does the 802 . 16e link up procedure . the ue set the ho process optimization bit . or alternatively the ue can indicate inter ho optimization procedure by having a new bit . during the security capability exchange through the sbc_req message , the ue can send the preferred algorithm as the algorithm used in the lte / sae system . then the wimax bs begins the pkmv2 three way handshake and establish the sa for link layer protection . the bs may accept the ue preferred algorithm , if it supports . it will also be obvious to those skilled in the art that other control methods and apparatuses can be derived from the combinations of the various methods and apparatuses of the present invention as taught by the description and the accompanying drawings and these shall also be considered within the scope of the present invention . further , description of such combinations and variations is therefore omitted above . it should also be noted that the host for storing the applications include but not limited to a microchip , microprocessor , handheld communication device , computer , rendering device or a multi function device . although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings , it is to be noted that various changes and modifications are possible and are apparent to those skilled in the art . such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart there from . backward handover : the source ran node initiates the handover , and resources are prepared in the target ran nodes . examples of backward handover concept are reported in tr 25 . 931 . forward handover : the ue changes to the target ran node without any preparation in the network . examples of this concept are reported in tr 25 . 931 geran : gsn edge radio access network consisting of the bts and bsc ha : home agent , a router on a mobile node &# 39 ; s home network that tunnels packets to the mobile node while it is away from home . inter as mm : inter access system mobility manager , an entity assisting in mobility across access systems iwf / iwu : interworking function or interworking unit . iwf and iwu are same and interchangeable . user terminal : the end user equipment e . g ., the mobile station ( ms ) or user equipment ( ue ). utran : umts terrestrial radio access network consisting of the node b and the rnc .