- Table 4.1: The US cellular bands.. - now examine the physical layer of the radio interface. - The forward link consists of the base station (BS) transmitter, the radio channel and the MS receiver. - The pilot carrier of the serving cells is also used by the MS as a coherent reference in the demodulation process and in the reverse link power control algorithm.. - The full block diagram of the cdmaOne BS transmitter is shown in Figure 4.2. - Figure 4.1: The Walsh Hadamard transform (WHT) matrix of order 64.. - requirements of the channel. - The character- istics of the low pass filters are shown in Figure 4.3 in the form of a response mask taken from the specifications [1, 2]. - In the diagram, S ( f ) is the frequency response of the filter.. - Figure 4.4: The phase constellation at the BS transmitter.. - We have described the construction of the pilot channel as it has been shown in the spec- ifications. - Having described the construction of the pilot channel we now examine its functions.. - One of the main functions of the pilot channel is to allow the MS to detect and identify the BSs. - It also carries the pilot PN offset of the cell (PILOT PN), the contents of the long code generator (LC STATE, this will be described later) and the system time (SYS TIME).. - the structure of the sync channel message is shown in Figure 4.6. - Figure 4.6: The sync channel structure.. - Figure 4.8: The content of the sync channel message.. - In the case of the paging channel, the construction of the long code mask is shown in Figure 4.10. - Figure 4.9: The long code generator.. - The start of the Walsh code, i.e. - marks of system time, regardless of the pilot PN offset. - The MS transmits its preferred slot cycle period to the network in the form of the three bit SLOT CYCLE INDEX. - The construction of the 42-bit public long code mask is shown in Figure 4.13, where ESN signifies the electronic serial number of the MS. - The actual position is defined by the last four bits of the scrambling sequence that was used in the previous power control group. - In the example in Figure 4.14, the last four bits of the scrambling sequence are 1101. - We also note that the MS receiver will always know the position of the power control bits. - (1011 =11) two power control bits starting position of the are used to define the. - Figure 4.14: The position of the power control bits. - always transmitted at full power regardless of the channel bit rate.. - Having described the construction of the forward link IS-95 traffic channel on the radio path, we now examine its frame structure. - In the case of the 9.6 kb/s frame, the CRC is 12 bits in length and is defined by the following generator polynomial:. - The block diagram of the CDMA-PCS Rate Set 2 traffic channel is shown in Figure 4.17.. - In frames where the MM bit is set to a ‘1’, the next four bits form the frame mode (FM) field and they are used to indicate the division between the different data types. - where τ p j represents the propagation delay from the jth BS to the MS, α j is the attenuation experienced by the signal from the jth BS and B is the total number of BSs in the vicinity of the MS.. - When this signal is applied to the input of the searcher, the values of ˆ r Ii ( t. - The magnitude of the noise contaminating ˆ r Ii ( t. - (This has been likened to the garden rake, hence the name.) The different signal paths are combined using a maximal ratio combining scheme whereby the signal on each path is weighted according to its received power prior to combining.. - c ( t ) and a logical ‘0’ is represented by the inverse of the code, c 2 ( t. - the length of the code) and W is the bandwidth of the transmitted signal (i.e. - This shows that the received signal in the kth path has been weighted by the amplitude of the channel tap, α k , and the phase change, θ k , has been removed.. - If we ignore the use of the channel taps, a k ( t. - The result of the correlation (i.e. - The integration (correlation) occurs over the entire length of the respective delayed replica of c 1 ( t ) and the results of the correlation become available at different times. - The reverse link radio path consists of the mobile station (MS) transmitter, the radio channel and the base station (BS) receiver. - We will now describe each of the reverse link channels in detail.. - A block diagram of the cdmaOne transmitter supporting the access channel is shown in Figure 4.24. - The index of the Walsh code, W x , is chosen according to the formula. - Figure 4.24: A block diagram of the cdmaOne access channel.. - The format of the long code generator has already been described in the previous section which dealt with the forward link channel. - The contents of the long code generator are determined from the information carried by the sync channel, and the same generator is used for the reverse and forward links. - The MS randomly selects one of the access channels associated with its paging channel by setting the ACN field in the long code mask according to the output of a pseudo-random number generator. - The paging channel number (PCN) in the long code mask gives the identity of the forward link paging channels used by the MS. - Together, the PCN and the ACN uniquely identify one of the possible 7 32 reverse link access channels. - We note that the PNI and PNQ codes used by the MS on the reverse link always have a zero offset, regardless of the PN offset used by the BS on the forward link.. - The specifications of the baseband filters are identical to those used on the forward link. - This simplifies the design of the MS transmitter output stages.. - The maximum length of the access channel message is 880 bits (i.e. - The maximum size of the access channel message is limited by the parameter MAX CAP SZ which is transmitted to the MS on the paging channel as part of the access parameters message. - The NUM STEP parameter is transmitted as part of the access parameters message on the paging channel. - This parameter must be adjusted according to the transmitted power of the BS and the required power at the BS receiver. - The transmitted power of the nth access probe in the access probe sequence, MS T Xn , is giv en by. - In the event that the test fails, the MS delays the transmission of the access probe sequence and proceeds to test the next slot. - registration, call origination) and the overload class of the MS. - where ACC TMO is a system parameter transmitted to the MS as part of the access pa- rameters message. - Figure 4.31: Block diagram of the reverse link traffic channel used in IS-95.. - four copies of the same data) to produce a data rate of 28.8 ksymbols/s. - The index of the modulation symbol is selected according to Equation (4.57), in the same manner as that used on the access channel. - In other words, the duty cycle of the MS transmissions is adjusted according to the input data rate as shown in Table 4.15.. - The bursts that are erased are chosen based on the state of the long code generator in the previous frame. - EXored) by the output of the long code generator. - We note that the MS will always use the zero offset versions of the PNI and PNQ codes.. - Having discussed the construction of the IS-95 reverse link traffic channel we now con- sider its frame structure. - The formation of the traffic channel frames used on the IS-95 reverse link are identical to those used on the forward link. - Our discussion of the reverse link traffic channel has been limited to the IS-95 system, but it also applies equally to the CDMA-PCS system. - From this point, the remainder of the Rate Set 2 traffic channel (i.e. - We note that, since the reverse link traffic channels are defined by the 2 42 1 bit long code mask, the number of channels is not limited to the number of Walsh codes as in the case of the forward link. - The BS receiver will use a RAKE receiver to demodulate the reverse link signal from a particular MS and the basic discussion of the RAKE receiver contained in the previous. - Figure 4.34 gives a possible block diagram of the BS receiver for a single reverse link channel. - Figure 4.34: Block diagram of the BS receiver.. - and one of the 64 possible Walsh codes, w x ( t. - (4.72) where w x ( t ) is any one of the 64 possible Walsh codes.. - are then correlated with each of the 64 possible c ix ( t ) and c qx ( t ) sequences. - Therefore the search receiver is able to ‘find’ the MS by sweeping τ r over a range of possible values and noting the values of τ r that cause the output of one of its bank of 64 correlators, i.e. - We note that this analysis is a gross over-simplification of the situation in the full cdmaOne system. - however, it has been used to demonstrate the basic principles of the BS search receiver. - When this signal is applied to the input of the searcher, the values of ˆ r Iix ( t. - 4.3 Control of the Radio Resources. - We will examine the cell selection procedure performed by the MS at switch-on, and the behaviour of the MS whilst in idle mode. - The operation of the network and the MS during an access attempt will be described. - In the idle mode, the MS must monitor the paging channel of the current BS to detect incoming calls and it must also monitor the pilot channels of surrounding cells to ensure that the MS is always ‘camped-on’ the most appropriate BS. - the Pilot PN offset) of the neighbouring BSs. - Whilst in the idle mode, the MS continually monitors the received signal strength of the pilot channel in neighbouring cells. - The message contains the identity of the MS and a field to indicate the reason for the message (e.g. - Details of window size are transmitted as part of the system parameters message on the forward link paging channel. - multipath components 20 chips either side of the earliest usable multipath for each pilot in the set. - The MS is allocated a forward link channel at each of the BSs, and the information trans- mitted on each channel is the same (except for the power control bits). - RAKE finger) will be able to ‘see’ the signals arriving at all the antennae in each of the sectors and the receiver will always select the four best paths, regardless of the antenna on which they arrive. - The new BS searches for, and acquires, the signal from the MS, and then it commences transmission of the appropriate forward link traffic to the MS. - Figure 4.39 gives an example of the message exchanges between the BS and MS during a typical soft handover. - The MS then disables its transmitter and attempts acquisition of the newly assigned channel
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