Abstract:
Apparatus for communication includes at least one RF receiver circuit, which is coupled to receive and downconvert first and second RF signals that are transmitted respectively over different, first and second wireless networks in accordance with different, first and second network protocols, so as to output first and second downconverted signals. A baseband processing circuit includes processing components that are coupled to receive and process the first and second downconverted signals so as to extract first and second data from the signals. The processing components have a first configuration for demodulating the first downconverted signals in accordance with the first network protocol and a second configuration for demodulating the second downconverted signals in accordance with the second network protocol.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 60/772,167, filed Feb. 9, 2006, which is incorporated herein by reference. This application is related to two other U.S. patent applications, filed on even date, entitled, “Simultaneous Operation of Wireless LAN and Long-Range Wireless Connections,” and “Scanning for Network Connections with Variable Scan Rate,” both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to wireless communications, and specifically to wireless terminals capable of operating in multiple different data networks. 
       BACKGROUND OF THE INVENTION 
       [0003]    Wireless local area networks (WLANs) have gained broad popularity. The original IEEE 802.11 WLAN standard was designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b, 802.11e, 802.11g, 802.11n and other extensions to the original standard, in order to enable higher data rates. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise. 
         [0004]    WiMAX (Worldwide Interoperability for Microwave Access) is a new technology for wireless packet data communications, which is similar in concept to IEEE 802.11, but has a number of enhancements designed to improve performance and range. The original WiMAX standard, IEEE 802.16, specified WiMAX in the 10-66 GHz range. More recently, IEEE 802.16a added support for the 2-11 GHz range, and IEEE 802.16e (approved as IEEE 802.16-2005) extended WiMAX to mobile applications, using an enhanced orthogonal frequency division multiple access (OFDMA) modulation scheme. In the context of the present patent application and in the claims, the term “802.16” is used to refer collectively to the original IEEE 802.16 standard and all its variants and extensions, unless specifically noted otherwise. 
         [0005]    Although there are some similarities in the physical layer interfaces (PHY) of WLAN and WiMAX systems, the medium access control (MAC) layers specified by the respective standards differ significantly. In an 802.11 WLAN, the MAC layer typically uses contention, as in Ethernet networks: Mobile stations compete for the resources of access points on a random basis. By contrast, the 802.16 MAC typically uses scheduling, in which the mobile station is allocated a time slot by the base station. The time slot can enlarge and constrict, but it remains assigned to the subscriber station, meaning that other subscribers are not supposed to use it and must take their turn. 
         [0006]    Other broadband wireless standards are also in development. Examples include the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE), 3GPP2 Evolution-Data Optimized (EVDO) Rev C and the IEEE 802.20 High Speed Mobile Broadband Wireless Access (MBWA) specifications. 
       SUMMARY OF THE INVENTION 
       [0007]    WLAN and WIMAX are complementary technologies: While WIMAX provides broad, long-range coverage with moderate bandwidth over wide areas, WLAN provides local coverage at low cost and very high bandwidth. Embodiments of the present invention that are described hereinbelow provide wireless terminals that can communicate with both WLAN access points and WiMAX base stations. Although these embodiments relate specifically to certain features of the 802.11 and 802.16 families of standards, the principles of the present invention may be extended, mutatis mutandis, to integration of short- and long-range wireless data networks of other types, such as short-range Bluetooth networks and long-range cellular data networks, such as 3GPP LTE. 
         [0008]    There is therefore provided, in accordance with an embodiment of the present invention, apparatus for communication, including: 
         [0009]    at least one RF receiver circuit, which is coupled to receive first and second radio frequency (RF) signals that are transmitted respectively over different, first and second wireless networks and carry first and second data modulated in accordance with different, first and second network protocols, and which is arranged to downconvert the first and second RF signals so as to output first and second downconverted signals; 
         [0010]    a baseband processing circuit, including processing components that are coupled to receive and process the first and second downconverted signals so as to extract the first and second data from the signals, the processing components having a first configuration for demodulating the first downconverted signals in accordance with the first network protocol and a second configuration for demodulating the second downconverted signals in accordance with the second network protocol. 
         [0011]    There is additionally provided, in accordance with an embodiment of the present invention, a method for communication, including: 
         [0012]    receiving first and second radio frequency (RF) signals that are transmitted respectively over different, first and second wireless networks and carry first and second data modulated in accordance with different, first and second network protocols; 
         [0013]    downconverting the first and second RF signals so as to generate first and second downconverted signals; 
         [0014]    processing the first and second downconverted signals so as to extract the first and second data from the signals using a baseband processing circuit, which includes processing components having a first configuration for demodulating the first downconverted signals in accordance with the first network protocol and a second configuration for demodulating the second downconverted signals in accordance with the second network protocol. 
         [0015]    In a disclosed embodiment, the baseband processing circuit includes a memory for holding both the first and the second data simultaneously. Typically, the first and second data respectively include first and second data frames, and processing the first and second downconverted signals includes tagging the first and second data frames in the memory with different, respective first and second identifying tags. 
         [0016]    Additionally or alternatively, the processing components include at least one programmable component, and the baseband processing circuit includes a memory for holding program code to drive the at least one programmable component, the program code including first code for driving the at least one programmable component in the first configuration and second code for driving the at least one programmable component in the second configuration. Typically, the first and second network protocols include first and second medium access control (MAC) protocols, respectively, and the at least one programmable component includes a MAC controller, which is driven by the program code to operate in accordance with either of the first and second MAC protocols. 
         [0017]    In a disclosed embodiment, processing the first and second downconverted signals includes driving the MAC controller using the first code in the first configuration to scan for a second connection to the second wireless network while receiving the first data over a first connection to the first wireless network, and after finding the second connection, to receive the second data over the second connection in the second configuration while maintaining the first connection at a reduced level of functionality relative to the first configuration. Driving the MAC controller may include unloading the first code and loading the second code into the memory upon finding the second connection, and unloading the second code and loading the first code into the memory when the second connection is lost and the baseband processing circuit resumes the first configuration. 
         [0018]    In some embodiments, processing the first and second downconverted signals includes scanning for a second connection to the second wireless network while receiving the first data over a first connection to the first wireless network in the first configuration, and upon finding the second connection, receiving the second data over the second connection in the second configuration. Processing the first and second downconverted signals may include maintaining the first connection at a reduced level of functionality relative to the first configuration while receiving the second data over the second connection in the second configuration, and returning to the first configuration when the second connection is lost. 
         [0019]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a schematic, pictorial illustration showing a multi-network system for wireless data communications, in accordance with an embodiment of the present invention; 
           [0021]      FIG. 2  is a diagram that schematically illustrates movement of a mobile terminal through coverage areas of WiMAX and WLAN networks, in accordance with an embodiment of the present invention; 
           [0022]      FIG. 3  is a block diagram that schematically shows elements of a dual-function mobile terminal, in accordance with an embodiment of the present invention; 
           [0023]      FIG. 4  is a block diagram that schematically shows details of baseband processing components in a dual-function mobile terminal, in accordance with an embodiment of the present invention; 
           [0024]      FIG. 5  is a state diagram that schematically illustrates a method for dual-function operation of a mobile terminal, in accordance with an embodiment of the present invention; 
           [0025]      FIG. 6  is a signal timing diagram that schematically illustrates methods for controlling timing of WiMAX and WLAN transmissions, in accordance with an embodiment of the present invention; 
           [0026]      FIG. 7  is a flow chart that schematically illustrates a method for controlling scanning for WLAN availability by a dual-function mobile terminal, in accordance with an embodiment of the present invention; 
           [0027]      FIG. 8A  is a signal timing diagram that schematically illustrates a method for detecting WLAN availability, in accordance with an embodiment of the present invention; and 
           [0028]      FIG. 8B  is a flow chart that schematically illustrates a method for detecting and connecting to a WLAN, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
       [0029]      FIG. 1  is a schematic, pictorial illustration of a multi-network wireless communication system  20 , in accordance with an embodiment of the present invention. In this system, a wireless terminal  22  communicates with both WLAN access points  24  and WiMAX base stations  26 . Depending on the location and operating parameters of terminal  22 , the terminal may access the Internet and various network services over either a WLAN or a WiMAX link, and may in some cases be handed over from WLAN to WIMAX, and vice versa. Although  FIG. 1  shows a certain type of wireless terminal by way of illustration, the embodiments described hereinbelow are applicable to substantially any sort of mobile computing and communication device that has the appropriate multi-network communication capabilities. The term “wireless terminal” as used in the present patent application and in the claims should therefore be understood broadly to refer to any and all suitable sorts of consumer electronics, computing and communication devices in which the principles of the present invention may be implemented. 
         [0030]    In some embodiments of the present invention, terminal  22  communicates over both WLAN and WiMAX networks using the same antenna, radio frequency (RF) transceiver, and baseband processing circuits. Integrating WLAN and WIMAX functionality in this manner is possible and desirable because both networks use OFDM technology over similar radio frequencies and bandwidths. Sharing the circuit components in this manner can reduce the cost and size of the terminal. 
         [0031]    Sharing resources in a single terminal between WLAN and WiMAX functionalities can lead to resource conflicts, however, particularly in view of the differences between the MAC protocols mandated by the WLAN and WiMAX networks. Each protocol was designed with the assumption of full availability of the antenna, RF transceiver and baseband functionality. Therefore, some embodiments of the present invention provide methods for controlling the timing of transmission and reception by terminal  22  so as to avoid conflict between WLAN and WiMAX communications. Such methods are particularly useful in managing the use of shared circuit components, but they can be advantageous, as well, even in dual-function terminals having separate WLAN and WIMAX circuits that may be active simultaneously. 
         [0032]    In some embodiments of the present invention, terminal  22  is capable of roaming between WLAN and WiMAX networks without interrupting application-level functions. For example, the terminal may be handed over from a WLAN access point to a WiMAX base station, or vice versa, in the middle of a communication session (such as a VoIP telephone call). To facilitate this sort of handover, the terminal scans for one network while it is connected to and communicating over the other. The scanning is coordinated in order to avoid comprising quality of service (QoS) requirements of real-time applications (such as VoIP) and to minimize power consumption. 
         [0033]    Although the embodiments described hereinbelow relate specifically to coexistence between WiMAX and WLAN functionalities within the same mobile terminal, the principles of the present invention may similarly be applied in multi-function terminals that support other broadband wireless technologies, such as Bluetooth and technologies mandated by the IEEE 802.20, 3GPP LTE or 3GPP2 EVDO Rev C specification. These other technologies may be supported in addition to or instead of the IEEE 802.11 and IEEE 802.16 support functions that are described hereinbelow. A mobile terminal supporting Bluetooth functions, in addition to IEEE 802.11 and IEEE 802.16 functions, is described, for example, in U.S. Provisional Patent Application 60/803,192, filed May 25, 2006, which is assigned to the assignee of the present patent application, and whose disclosure is incorporated herein by reference. 
         [0034]      FIG. 2  is a diagram that schematically illustrates movement of terminal  22  through coverage areas  30  and  32  of WiMAX and WLAN networks, respectively, in accordance with an embodiment of the present invention. The WiMAX network typically covers a large area  30 , in which WLAN coverage areas  32 , commonly referred to as “hotspots,” are distributed. Within WLAN coverage areas  32 , it is generally desirable that terminal  22  communicate with a WLAN access point, for reasons of enhanced bandwidth and reduced cost and power consumption. 
         [0035]    WLAN coverage areas  32  are typically surrounded by a transition region  34  that is characterized by lower-quality WLAN coverage. As terminal  22  moves along a “roam-in” path  36  from WiMAX coverage area  30  into one of WLAN coverage areas  32 , terminal  22  automatically senses that it has entered the WLAN coverage area and connects to the WLAN. The timing of the handover is usually not critical, since WiMAX coverage is generally maintained within the WLAN coverage areas. (Exceptions may occur inside certain buildings.) 
         [0036]    On the other hand, as terminal  22  moves along a roam-out path  38 , exiting from WLAN coverage area  32 , it is desirable that terminal  22  sense immediately that it has passed into transition region  34  and switch over to WiMAX communication before it has lost its connection with the WLAN access point. For this purpose, terminal  22  may re-establish its connection with the WiMAX base station as soon as it enters region  34 , or it may even maintain a connection with the WIMAX base station while it is in region  32 . The WiMAX communication under these circumstances is coordinated so as to minimize the additional power consumption and avoid interference with WLAN data communication. Methods and device architectures that can be used to facilitate these sorts of interleaved communication models and inter-network handovers are described further hereinbelow. 
       Hardware Architecture 
       [0037]      FIG. 3  is a block diagram that schematically shows elements of terminal  22 , in accordance with an embodiment of the present invention. The terminal comprises a dual-function wireless modem  40 , which serves a host processor  42 . The modem comprises dual antennas  44  and  46 , which are connected to dual RF receivers  48  and  50  in a multiple-input multiple-output (MIMO) configuration, as is mandated by the WiMAX Forum. Each RF receiver downconverts the received signals using a frequency input provided by a frequency synthesis circuit  51 . This circuit may comprise a single frequency synthesizer or, optionally, dual synthesizers  52  and  54 , for reasons explained hereinbelow. The outputs of RF receivers  48  and  50  (which may be either at intermediate frequency (IF) or I/Q baseband signals) are processed by a baseband receiver circuit  56 , which outputs a decoded stream of digital data via a host interface  58  to host  42 . Host data, as well as program code, are stored in a host memory  66 . 
         [0038]    For uplink transmission, host  42  passes data and instructions via host interface  58  to a baseband transmitter circuit  60 . This circuit outputs IF or I/Q baseband signals to a RF transmitter  62 , which is coupled via a switch  64  to at least one of antennas  46 . Frequency synthesis circuit  51  provides a frequency input for use by transmitter  62  in upconversion. 
         [0039]    Since both IEEE 802.11 and IEEE 802.16e use OFDM schemes in the same frequency range, RF receivers  48 ,  50  and transmitter  62 , as well as baseband circuits  56  and  60 , may be used for processing both WLAN and WiMAX transmissions. In one embodiment, a mode controller  68  switches the RF and baseband circuits between WLAN and WiMAX operation on a schedule determined by a timer  69 . (Although the mode controller and timer are separated from baseband circuits  56  and  60  in  FIG. 3  for the sake of clarity, these elements are actually a part of the baseband circuitry of terminal  22  and may be implemented, for example, as part of the MAC control circuits described below. Details of the baseband circuitry are shown in  FIG. 4 .) In other words, mode controller  68  switches the RF and baseband circuits back and forth between WiMAX and WLAN operation for short periods so as to establish and maintain contact with both networks, as described further hereinbelow. A single frequency synthesizer  52  is sufficient for this embodiment. 
         [0040]    In an alternative embodiment, dual synthesizers  52  and  54  may be used to enable two basic modes of operation:
       For normal WiMAX operation in MIMO configuration, the RF receivers are both used to receive and process WiMAX signals at the same frequency.   On the other hand, when terminal  22  is receiving and processing WLAN signals, one of the RF receivers may be used as a WLAN receiver, while the other RF receiver is used to receive WiMAX signals. In this mode, synthesizers  52  and  54  provide the respective RF receivers with different frequency inputs, one tuned to the WiMAX base station frequency and the other to the WLAN access point frequency.
 
This latter, “hybrid” mode of operation enables terminal  22  to remain in contact with the WiMAX base station during WLAN operation in areas  32  ( FIG. 2 ), so as to ensure smooth handover to WiMAX operation in transition region  34 . This mode may also be used intermittently during WiMAX operation in order to detect WLAN transmissions. Baseband receiver circuit  56  is likewise flexibly configurable to support either MIMO WiMAX operation or hybrid WLAN/WiMAX operation.
       
 
         [0043]      FIG. 4  is a block diagram that schematically shows details of baseband circuits  56  and  60 , in accordance with an embodiment of the present invention. This diagram illustrates how other elements (in addition to the RF receivers) may be shared by IEEE 802.11 and IEEE 802.16 functions of terminal  22 . Specifically, the functional blocks shown in the figure meet the requirements of both IEEE 802.11 OFDM and IEEE 802.16 OFDMA operation, as defined in the applicable standards. Although these blocks are separated in the figure for conceptual clarity, in practice they may typically be integrated together on a single integrated circuit chip or chip set. Alternatively or additionally, certain of these functions may be performed in software on a suitable programmable processor. 
         [0044]    The description of  FIG. 4  that follows will focus on how elements of circuits  56  and  60  may be configured to process both IEEE 802.11 and IEEE 802.16 signals. Implementation of the other elements and features of these circuits will be apparent to those skilled in the WLAN and WiMAX art and is beyond the scope of the present invention. For compatibility with legacy single-carrier WLAN access points, which may operate in IEEE 802.11b or mixed-mode IEEE 802.11g networks, terminal  22  may comprise an additional 802.11b-compatible module (not shown). Alternatively or additionally, some of the elements of the circuits shown in  FIG. 4  may also be adapted for single-carrier operation. 
         [0045]    Incoming signals from RF receivers  48  and  50  (IF or baseband I/Q) are digitized by dual-channel analog/digital (A/D) converters  70  (one channel per antenna), which typically operate at 10-bit resolution, each processing 40 MSPS. A dual-channel downlink preprocessor  72 , typically processing 10 MHz signal bandwidth per channel (or 20 MHz for single-channel), down-converts IF samples to baseband, if necessary, and performs preliminary filtering and resampling functions. A dual-channel, 1024-bin Fast Fourier Transform (FFT) processor  74  (2048 bins single-channel) transforms the time-domain samples in each channel to the frequency domain. The frequency-domain samples are stored in a tri-port random access memory (RAM)  76  for further processing. 
         [0046]    A programmable channel estimation (CE) processor  78  reads and processes the time-domain samples from preprocessor  72 , as well as the frequency-domain samples in RAM  76 , in order to determine channel coefficients for equalization and MIMO processing. The CE processor is provided with two sets of firmware—one for WIMAX signals and the other for WLAN—which are loaded into the program memory of the processor according to the type of signals to be processed. For WLAN signals, the CE processor typically uses preamble-based acquisition and pilot-based signal tracking. 
         [0047]    A frequency interpolator  80  processes the channel coefficients that are output by CE processor  78 , and inputs the coefficients to a frequency-domain equalization (FDE) and MIMO processor  82 . This processor, like the CE processor, is programmable, with different firmware for WiMAX and WLAN processing. For WIMAX, MIMO processing may be used for enhanced interference cancellation. For WLAN operation, MIMO or SIMO (single-input multiple-output) processing may be used for 802.11n reception and for improving 802.11g performance, respectively. In either case, for each received symbol, processor  82  outputs an array of digital values, corresponding to the bits encoded on each sub-carrier. 
         [0048]    For WiMAX OFDMA signals, a slot assembler  84  extracts the digital values belonging to the time/frequency slot that is assigned to terminal  22 . A decoder  86  performs convolutional code (CC) or convolutional turbo code (CTC) decoding of the data values, as mandated by the IEEE 802.16e standard. Typically, decoder  86  applies hybrid automatic repeat request (HARQ) error control with incremental redundancy (IR) and Chase combining, as are known in the art. The functions of the slot assembler and HARQ combiner are not required for WLAN operation, and these blocks are therefore inactive in processing of WLAN data. 
         [0049]    A programmable downlink (DL) controller  88  performs MAC functions according to the applicable IEEE 802.11 or IEEE 802.16 protocol, as appropriate. Controller  88  then passes the data payloads of the frames that is receives to host  42  via host interface  58 . Similarly, a programmable uplink (UL) controller  90  performs MAC functions on uplink data that are generated for transmission by host  42 . Although controllers  88  and  90  are shown, for the sake of clarity, as separate unit, in practice a single MAC control circuit may be used for both DL and UL functions. Like processors  78  and  82 , controllers  88  and  90  are driven by firmware, which is chosen and loaded according to the type of signals that terminal  22  is receiving. Depending on the mode of operation (WLAN or WiMAX) downlink controller  88  passes appropriate synchronization and control signals to uplink controller  90 , such as uplink map (UL-MAP) signals identifying slot assignments in WiMAX and channel feedback instructions. Controllers  88  and  90  typically use at least one on-chip memory  92  for storing data (frame buffer) and program code. Techniques that enable efficient sharing of this memory between WLAN and WiMAX functions are described hereinbelow. The downlink and uplink controllers may also share a hardware accelerator (HWA)  94  for encryption and decryption according to the applicable Data Encryption Standard (DES) or Advanced Encryption Standard (AES). 
         [0050]    Uplink WiMAX data frames generated by UL controller  90  are encoded by a CC/CTC encoder  96 , and are then input to a MIMO modulator  98 . A modulation controller  100  determines the modulation scheme and bit allocation that are to be used. Modulator  98  is programmable in firmware for either WiMAX or WLAN transmission. In the latter case, modulator  98  allocates the data bits to sub-carriers, using forward error correction (FEC), as mandated by the IEEE 802.11g standard. An inverse FFT (IFFT) processor  102  converts the multi-bin frequency-domain samples that are output by modulator  98  to the time domain. An uplink post-processor  104  performs digital filtering and, if necessary, up-converts the samples to IF, following which digital/analog (D/A) converters generate analog IF or baseband I/Q signals for output to RF transmitter  62 . 
       Sharing MAC Memories 
       [0051]    As noted above, uplink and downlink MAC controllers  88  and  90  use memory  92  to hold their operating programs and data. For rapid memory access, it is advantageous that the memory be located on the same chip as the MAC controllers. To minimize chip size, the memory footprint should be as small as possible. To achieve this objective, it is desirable that the same memory be used for both WLAN and WiMAX functions, without unduly increasing memory size over what would be required for either WLAN or WiMAX operation alone. 
         [0052]    A major part of the data memory used by controllers  88  and  90  is the frame buffer. To permit this memory to be shared, each frame in the buffer may be marked with a tag (one bit) that indicates whether the frame belongs to the WLAN or WiMAX frame sequence. When a frame is written to the buffer, a buffer manager (which may be functionally integrated into one or more of the components accessing memory  92 ) tags the frame appropriately. Based on this tag, the buffer manager is able to output the appropriate frames to host interface  58  or uplink controller  90  depending on whether WLAN or WiMAX processing is called for. 
         [0053]      FIG. 5  is a state diagram that schematically illustrates a method for sharing program memory between WLAN and WiMAX operating modes, in accordance with an embodiment of the present invention. This method is built on the ability of terminal  22  to manage its WLAN and WiMAX functionalities at different levels. Typically, the levels may depend on whether the terminal is being served by WLAN access point  24  or WiMAX base station  26 , or is in transition from one type of service to the other, as explained hereinabove with reference to  FIG. 2 . For example, the functionalities may be partitioned as follows: 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 FUNCTIONALITY LEVELS 
               
             
          
           
               
                 Level 
                 Function 
                 WLAN 
                 WiMAX 
               
               
                   
               
               
                 1 
                 Scanning* 
                 Scan frequencies, 
                 Functionality to 
               
               
                   
                   
                 handle probe 
                 keep the link alive 
               
               
                   
                   
                 request-response 
                 (periodic ranging, 
               
               
                   
                   
                 and parse access 
                 sleep control, 
               
               
                   
                   
                 point capabilities 
                 etc.) 
               
               
                 2 
                 Connection 
                 Establish 
                 Reduced WIMAX 
               
               
                   
                   
                 connection with 
                 functionality 
               
               
                   
                   
                 access point 
               
               
                 3 
                 Normal 
                 Full functionality 
                 Full functionality 
               
               
                   
                 operation 
               
               
                   
               
               
                 (*Some of these scanning functions are described in greater detail hereinbelow.) 
               
             
          
         
       
     
         [0054]    At full functionality (Level 3) processors  88  and  90  require that the complete package of MAC software code be loaded into memory  92 . As the functionality level decreases, however, parts of the software may be removed from memory  92  and held off-chip, in host memory  66 , for example. Since processors  88  and  90  may operate at Level 3 for either WLAN or WiMAX communication, but not necessarily both simultaneously, the amount of on-chip program memory that is required to support dual-function WLAN/WiMAX operation can be substantially less than twice the amount of program memory that would be required in a single-mode terminal. 
         [0055]    Referring now to the details of  FIG. 5 , in a normal WiMAX operation state  110 , terminal carries out normal data communications with base station  26 , while scanning for possible connections to WLAN access points. Typically, while in state  110 , terminal  22  scans for access points using the probe request/probe response protocol provided by the IEEE 802.11 standard. In this state, terminal  22  operates at WiMAX Level 3 and WLAN Level 1. Therefore, only a part of the complete WLAN software is loaded into memory  92 . 
         [0056]    Upon receiving a WLAN probe response, terminal  22  checks the access point capabilities in order to determine its suitability for a connection. Once the terminal determines that the WLAN access point is qualified, it moves to a WLAN connection establishment state  112 . For the purposes of establishing the connection, the terminal loads additional WLAN software into memory  92 , while unloading some of the WiMAX software to make room. As a result, both WiMAX and WLAN functionalities now operate at Level 2. State  112  is short-lived, in order to avoid interrupting application-level communications by the terminal. 
         [0057]    Once the connection with the WLAN access point is established, terminal  22  shifts to a normal WLAN operation state  114 , in which the full complement of WLAN software is loaded into memory  92 , and the terminal operates at WLAN Level 3. WIMAX operation (and memory consumption) is reduced to Level 1. At this level, memory  92  contains the minimal amount of WiMAX code that is needed to keep a connection alive to base station  26 . This keep-alive function is useful, as explained above, in order to facilitate a smooth handover to WiMAX operation when terminal  22  moves out of WLAN service area  32  into transition region  34  ( FIG. 2 ). 
         [0058]    When terminal  22  roams out of WLAN coverage area  32 , the terminal enters a WiMAX resumption state  116 . In this state, the terminal communicates with base station  26  in order to resume normal WiMAX operation. The terminal loads the Level 3 WiMAX code back into memory  92  while unloading the WLAN code back down to Level 1. Normal WiMAX operation and WLAN scanning then continue in state  110 , as described above. 
       Coordinated Timing of WLAN and WIMAX Communication 
       [0059]      FIG. 6  is a signal timing diagram that schematically illustrates signals used in controlling timing of WiMAX and WLAN transmissions, in accordance with an embodiment of the present invention. The methods of timing control that are described hereinbelow are useful particularly in wireless terminals that use the same radio and baseband processing resources to transmit and receive both WiMAX and WLAN signals. Thus, these methods will be described, by way of illustration, with reference to terminal  22 . Alternatively or additionally, these methods may be applied in dual-function terminals with separate (but coordinated) WiMAX and WLAN radio and baseband circuits, as well as dual-function terminals with different resource-sharing schemes from those described above. 
         [0060]    As shown in  FIG. 6 , as long as terminal  22  is within range of a WiMAX base station, it receives timing signals that dictate the synchronization of WiMAX frames  120 . The default frame period is 5 ms. On the other hand, as explained above, WLAN access points generally operate asynchronously, and permit stations in the WLAN to send uplink signals at will, subject to signaling and backoff time constraints. Terminal  22  therefore controls the timing of its uplink transmissions in synchronization with the WIMAX frame clock, in such a manner that the terminal may interleave WiMAX and WLAN transmission and reception while minimizing interference and potential data loss. This interleaving permits the terminal to scan for and connect to WLAN access points while in the midst of data communications with a WiMAX base station, as well to keep its WiMAX connection alive during WLAN data communication in order to facilitate smooth handover to WiMAX when the terminal roams out of the WLAN service area. 
         [0061]    To reserve certain frames  120  for WLAN communications and prevent WiMAX base station  26  from transmitting downlink signals to terminal  22  during these frames, terminal  22  sends a reduced availability message to the base station. In the embodiment describes hereinbelow, the terminal uses the sleep mechanism defined by the IEEE 802.16e standard (particularly section 6.3.21 in IEEE 802.16-2005). Alternatively, terminal  22  may use other mechanisms to signal reduced availability, such as a scanning mechanism (in which the terminal requests certain scanning intervals), or other messages that may be defined for this purpose in future standards. 
         [0062]    The sleep mechanism is defined by the WiMAX standard as a power-saving technique, to reduce the duty cycle during which the terminal (referred to as a mobile station, or MS, in WiMAX standards) must listen for downlink signals. To invoke the mechanism, the terminal transmits a sleep request (SLP-REQ) signal to the base station, identifying the frames during which the terminal will be sleeping and will therefore not receive downlink signals. In the example shown in  FIG. 6 , SLP class 1 is used to define sleep windows of four successive sleep frames  122 , while SLP class 2 is used to define periodic windows of two sleep frames  122 . The sleep windows may then be used for WLAN transmission and reception. The periodic, shorter sleep windows may be preferable for real-time traffic, such as VoIP, whereas the longer SLP class 1 windows may be advantageous for Internet data communications. In accordance with WiMAX standards, terminal  22  is able to control the length of the sleep intervals with a granularity of one WiMAX frame  120 . 
         [0063]    WiMAX class 1 sleep may be interleaved conveniently with the WLAN power save polling mode (PS-Poll) as defined by the IEEE 802.11 standard, section 11.2. In this mode, terminal  22  makes use of beacons  124 , accompanied by a delivery traffic indication message (DTIM), that are periodically broadcast by access point  24 . The terminal senses these beacons, and sets the timing of WiMAX sleep frames  122  so that the sleep interval begins shortly before the next beacon  124  is expected from the WLAN access point, as shown in  FIG. 6 . According to the IEEE 802.11 standard, the beacon interval is 102.4 ms, so that the timing of beacons  124  will drift relative to WiMAX frames  120 . To compensate for the drift, terminal  22  may send a new SLP-REQ message to base station  26  from time to time (typically once every few seconds), requesting a new sleep start frame number. Alternatively, the SLP-REQ message mandated by the 802.16 standard may be modified (together with suitable modification of the base station) to support non-integer sleep periods. 
         [0064]    Upon receiving beacon  124  during one of sleep frames  122 , terminal  22  responds by transmitting a PS-Poll uplink signal  134  to the access point. In response, the WLAN access point transmits a downlink data signal  136  to the terminal. Long WiMAX sleep intervals are desirable in this operational mode, in order to leave sufficient time for the delay of WLAN responses that is mandated by the distributed coordination function (DCF) of the IEEE 802.11 standard. Depending on the length of the sleep interval, there may be time to exchange several uplink and/or downlink packets over the WLAN between the terminal and the access point before the terminal resumes WiMAX operation. 
         [0065]    To reinitiate WiMAX data transmission following the class 1 sleep interval, terminal  22  transmits an uplink bandwidth request (BW-REQ) signal  130 . Base station  26  responds by transmitting a downlink signal  132  to the terminal, following which a data exchange may take place. Terminal  22  transmits the bandwidth request with sufficient time before the next sleep interval to ensure that the data exchange with the base station will be completed before the sleep interval begins. 
         [0066]    For VoIP communication (and other real-time applications), terminal  22  may use a reservation message to reserve periodic bandwidth resources for WIMAX real-time communications, coupled with sleep class 2 to reserve respective frames  120  for WLAN communications. Bandwidth reservation messages that may be used for this purposes are provided, for example, by the WiMAX unsolicited grant service (UGS), real time polling service (rtPS) or extended real time polling service (ertPS), as defined in 802.16-2005, section 6.3.5.2 In the example shown in  FIG. 6 , the terminal reserves two frames out of every four-frame period for WiMAX, and the other two frames (during WiMAX class 2 sleep) for WLAN. In this mode of WiMAX operation, base station  26  transmits a downlink signal  126  to terminal  22  immediately following the end of the final sleep frame  122  in each sleep interval. The terminal responds to the base station with an uplink signal  128  in the next frame  120 , before going back to sleep. 
         [0067]    For WLAN real-time transmission during the WiMAX class 2 sleep intervals, terminal  22  may use a terminal-initiated transmission mechanism, such as the unscheduled automatic power save delivery (U-APSD) mechanism of the WLAN multimedia (WMM) power save mode, defined by IEEE standard 802.11e, section 11.2.1. Using this mechanism, the terminal does not wait for access point beacons, but rather transmits an uplink signal  138 , containing a trigger frame and data, shortly after the beginning of the WiMAX sleep interval. Access point  24  then responds with a downlink signal  140 . Alternatively, during the WiMAX sleep interval (class 1 or class 2), terminal  22  may transmit a WLAN probe request signal, to which access point  24  responds with a probe response, followed by a data exchange. 
         [0068]    Thus, using the mechanism shown in  FIG. 6  with class 2 sleep and U-APSD, terminal  22  is able to transmit at least one uplink data packet and receive at least one downlink data packet every 20 ms via each of WLAN access point  24  and WiMAX base station  26 . As a result, the terminal is able to keep a VoIP call alive during handover between the two networks and may, if necessary, carry on two simultaneous calls, one on each network. (Under these circumstances, however, it may be necessary to keep both the WiMAX and WLAN processing chains fully active in terminal  22 , so that the memory swapping technique described above with reference to  FIG. 5  may not be practical.) Alternatively, terminal  22  may use the above schemes to scan for connections or to maintain a connection to one of the networks while carrying on a data communication session (real-time or Internet data) over the other network. 
         [0069]    Additionally or alternatively, terminal  22  may use “microsleep” intervals within a given WiMAX frame  120  (i.e., WiMAX time slots in which there are no sub-carriers mapped to the terminal) for ad-hoc WLAN communications. Thus, in the example shown in  FIG. 6 , the terminal uses the U-APSD mechanism described above to transmit additional WLAN uplink data  142  and receive additional WLAN downlink data  144  during the interval between WiMAX downlink signals  126  and uplink signals  128 . 
         [0070]    Further alternatively or additionally, other patterns of wake and sleep intervals, of greater or lesser durations, may be used. For example, terminal  22  may allocate certain WIMAX sleep frames  122  to search for other WLAN access points while the terminal maintains connections both to the WiMAX base station and to the WLAN access point that is currently serving the terminal. Other standard mechanisms, such as the IEEE 802.11e hybrid coordination function (HCF), may also be used controlling the WLAN operation of the terminal. Likewise, other IEEE 802.16 standard mechanisms, such as using scan intervals, may be applied in addition or alternatively to using sleep intervals. 
       Power Saving Techniques 
       [0071]    In the embodiment described above, standard power-saving modes are used in order to facilitate interleaved dual-network communication by terminal  22 . Incidentally, the use of these methods can also help to reduce power consumption and extend battery life of the terminal. Terminal  22  may switch off RF receivers  48 ,  50  and transmitter  62  during sleep and other periods of inactivity, and may thus save substantial amounts of power. The need to maintain connections to both WLAN and WiMAX simultaneously, however, may often cause terminal  22  to consume more power than comparable single-network terminals. A number of methods for further reducing power consumption by terminal  22  are described hereinbelow. 
         [0072]      FIG. 7  is a flow chart that schematically illustrates a method for controlling scanning by terminal  22  for available WLAN access points, in accordance with an embodiment of the present invention. This method may be used particularly when terminal  22  is operating outside service areas  32  of WLAN access points ( FIG. 2 ), in order to locate access points to which the terminal may connect. In accordance with this method, terminal periodically scans for available access points, either by active scanning (probe request/probe response) or polling, as described above. Once in every scan period, the terminal scans for an access point on one of the predefined WLAN frequency channels. If no access point is found in a given scan, the terminal moves on to scan the next frequency channel in the next scan period. Increasing the scanning rate (i.e., shortening the period between scans) increases the probability of finding an access point but also increases battery consumption. 
         [0073]    Using the method of  FIG. 7 , the scanning rate is adjusted according to the expected usefulness of the scan, so as to arrive at an optimal trade-off between performance and battery life of terminal  22 . For this purpose, the terminal detects one or more indications of its own mobility, at a mobility detection step  150 . The simplest and most straightforward method for measuring mobility is to determine directly the velocity of movement of the terminal by taking the differences between successive measurements of a positioning device, such as a GPS receiver (not shown), that is built into the terminal. Alternatively, the terminal may detect changes in WiMAX channel characteristic that are indicative of movement, such as changes in channel estimation by CE processor  78  ( FIG. 4 ); gain variance; Doppler effect on radio frequency; or substantially any other PHY-related parameter that is influenced by movement of the terminal. MAC-level indications of cell handovers may also be used as a mobility-related trigger. Other possible mobility indicators may be generated when terminal  22  senses that it is in a desktop charging cradle (low mobility) or a motor vehicle cradle (high mobility). 
         [0074]    Terminal  22  assesses the mobility indicator(s) to determine its current mobility level, at a mobility assessment step  152 . If mobility is in a middle range, indicative of pedestrian movement (for example, in the range of 1-6 km/h), terminal  22  typically maintains a regular rate of scanning for WLAN access points, at a regular scan step  154 . For example, scanning for a new channel at about one scan per second will permit the terminal to connect with an access point within 5-10 sec of entering its service area. On the other hand, if mobility is below some minimum threshold, indicating that terminal  22  is stationary (or nearly so), the chances of finding a new access point in a given scan are low. Therefore, in this situation, the terminal reduces its rate of scanning for access points, at a rate reduction step  156 . Power consumption due to scanning is thus reduced. 
         [0075]    During rapid motion (in a vehicle, for example, at 10 km/h or more), it is unlikely that terminal  22  will remain in the service area of an access point long enough to establish communications. Therefore, there is little to be gained by scanning for WLAN access points, and the terminal may stop the scanning function when the mobility is above a maximum threshold, at a shut-off step  158 . Thus, no energy at all is expended on unnecessary scanning. 
         [0076]    Although the method of  FIG. 7  is described specifically with reference to dual function WiMAX/WLAN operation, the principles of this method may also be applied to other types of multi-function mobile terminals, such as cellular telephones with a WLAN interface for use in a converged cellular/VoIP network. 
         [0077]      FIGS. 8A and 8B  schematically illustrate a method for detecting and connecting to a WLAN access point, in accordance with an embodiment of the present invention.  FIG. 8A  is a signal timing diagram illustrating the principles of the method, while  FIG. 8B  is a flow chart showing the steps in the method. This method is particularly useful in a dual-function terminal, such as terminal  22 , in which rapidity of connection to the WLAN is not crucial, while reducing power consumption is highly desirable. Alternatively, the method may be used for reducing power consumption in scanning for an access point using mobile stations of other types. 
         [0078]    As shown in  FIG. 8A , in order to scan for an access point, the terminal transmits a probe request  160 . If an access point receives the probe request, it may transmit a probe response  164  at any point during a listen time  162 . The listen time depends on the access point but may be as long as 2-3 ms. During this period, the power consumption of the terminal is increased relative to its idle level, since the terminal must supply power to the circuits of its RF receivers while listening. These circuits tend to consume much more power than the digital processing components of the terminal. 
         [0079]    In order to reduce this power consumption, after transmitting the probe request at a probe transmission step  170 , terminal  22  samples the WLAN channel for the probe response, at a sampling step  172 , as shown in  FIG. 8B . A sequence of sampling gates  166  is shown, for example, in  FIG. 8A . The terminal turns on the RF receiver circuits intermittently for short periods, such as a period of 10 μs in every 100 μs interval during the listen time, rather than listening continuously. The power consumed by the RF circuits during the listen time is thus reduced, relative to continuous listening, by a factor that scales roughly with the reduction in duty cycle. A duty cycle of 10% or less is particularly effective for power saving, but any duty cycle less than about 50% can be useful in this regard. 
         [0080]    The digital receiver circuits of terminal  22  process the samples produced by the RF receiver in order to determine whether they contain an access point probe response, at a sample assessment step  174 . The sampled signal itself may not provide sufficient data to permit the terminal to decode the probe response and thus determine conclusively that it has found an access point. Instead, the terminal may compute a metric based on the samples indicating the likelihood that a probe response was received. The metric may be based on a number of factors, such as signal energy, repetition (indicative of a frame preamble), or modulation characteristics (such as a Barker code or characteristics of complementary code keying (CCK) or OFDM in the signal). If the metric is low, terminal concludes that it has not found an access point and goes on to transmit a new probe request at step  170 . 
         [0081]    If the metric is above some detection threshold, however, terminal  22  next attempts to detect the modulation of the probe response, at a modulation detection step  176 . At this step, the terminal may turn up the RF receiver to full-power, continuous operation in order to lock onto and decode the probe response from the access point. Step  176  may take place immediately after step  174 , so as to operate on the same access point signal that was sampled at step  172 . To facilitate detection of this sort and save power, terminal  22  may transmit the probe request at step  170  at 11 Mbps using CCK modulation (shortest permissible packet), and instruct the access point to answer with a direct-sequence spread spectrum (DSSS) response at 1 Mbps (longest possible packet). Alternatively, after detecting a probable probe response at step  174 , the terminal may transmit another probe request and then perform step  176  on the next probe response issued by the access point. 
         [0082]    The above steps may take place while terminal  22  is in data communication with WiMAX base station  26 , during the WiMAX sleep intervals ( FIG. 6 ). After the terminal successfully detects and demodulates the probe response from WLAN access point  24  at step  176 , it begins WLAN data communication with the access point, at a WLAN data transmission step  178 . Steps  176  and  178  may be accompanied by loading WLAN firmware into baseband circuits  56  and  60 , while unloading the WiMAX firmware, as described above with reference to  FIG. 5 . Alternatively, terminal  22  may carry on communications with WLAN access point  24  while continuing to communicate with the WiMAX base station. 
         [0083]    Although terminal  22  and the methods of operation of the terminal described above are directed specifically at dual-function WiMAX/WLAN (IEEE 802.16/IEEE 802.11) operation, the principles of the present invention may also be applied, mutatis mutandis, to other types of multi-function mobile terminals. For example, the device designs and methods described above may be adapted for use with long- and short-range wireless networks based on other standards, as well as for use in devices that interoperate with three or more different types of networks, such as WiMAX, WLAN and Bluetooth. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.