11.3.4 Reverse link of IS-95

rows are read according to a pattern providing adjacency of PCG repeating each other (due to encoded symbol repetition) whenever the raw datastream rates are smaller than 9.6 kbps (i.e. 4.8, 2.4 and 1.2 kbps). For example, when the raw rate is 4.8 kbps every even PCG contains repetition of the same interleaved code symbols as the previous; for the datastream rate 2.4 kbps the groups with numbers 4i þ 2, 4i þ 3, 4i þ 4 are replicas of the group number 4i þ 1, etc. Such an order is convenient for further lowering of the average transmitted power proportionally to the rate reduction, which is realized in the reverse link slightly differently as compared to the forward link (see below).

The codestream from the interleaver is fed to the 64-ary orthogonal modulator, meaning that every 6-symbol block treated as a binary 6-digit number selects one of 64 orthogonal signals (a Walsh function of this number). This gives an extra coding gain (up to three times asymptotically; see Section 2.6) above that of convolutional coding. Since every 6 input binary symbols are now replaced by 64 binary symbols, the stream of chips at the orthogonal modulator output becomes 64/6 ¼ 32/3 times faster (307.2 kcps). Let us stress that Walsh functions in the reverse link bear no channelization functions and only implement spread spectrum orthogonal coding for data transmission, as discussed in Section 2.7.3.

The next step in forming a traffic channel is spreading, i.e. modulo 2 summation of the binary symbol stream after the orthogonal modulator with the offset long-code sequence. Since the long code is a chip stream with rate 1.2288 Mcps, there are 4 longcode chips per Walsh symbol or 256 long-code chips per Walsh signal at the orthogonal modulator output. The BS receiver after despreading uses a 64-channel correlator bank, each channel being tuned to one of the Walsh signals, and decides in favour of the Walsh signal referencing the correlator with the strongest response. As such, a processing interval covers the whole Walsh function duration, i.e. 256 long-code chips, and the spreading factor of the reverse link appears to be 256. A long code is unique, strictly specified by the standard, and it is user-specific masks (time-offsets of the code) that secure CDMA separation of different users. Thus, we encounter here the strategy of asynchronous CDMA discussed in Section 7.4. Of course, these offsets should be properly assigned to have no risk of synchronous arrival at the BS of signals from two MSs migrating freely over the whole coverage zone. The value of offset is a current MS identity rendered to it by the network similarly to the channel carrier in FDMA. In parallel with the CDMA channelization, the user-specific mask provides the encryption (scrambling) of the stream after orthogonal modulator. Due to the enormous length of the long code, it is not an easy task for an unauthorized interceptor, who does not know the user’s mask, to synchronize a local long-code generator to the intercepted signal and despread (thereby descramble and decrypt) the data.

The operation of mapping logical f0, 1g symbols onto the real modulation alphabet f 1g is the same as before and does not require any special comment.

One of the primary requirements for MS handset is a long enough battery lifetime. From this angle a linear power amplifier consuming higher average power is less attractive than a nonlinear (operating in a keying mode) one. That is the reason why in the MS transmitter average power reduction is achieved not by lowering an instant power, but alternatively by transmitting only one PCG of all those that replicate each other. For example, with a raw rate 2.4 kbps there are quadruplets of identical PCGs and only one of them is transmitted, while the transmitter is cut off during the three

20 ms frame = 16 PCG

PCG 1.25 ms

Figure 11.6 Example MS transmission pattern for the raw rate 4.8 kbps

others. Certainly, this makes MS emission discontinuous. Randomization of the positions of discarded PCGs (data burst randomization) enables better averaging of discontinuous MAI at the BS. The pseudorandom pattern of gating off the PCG inside any frame is determined by the last 14 chips of a user’s mask, i.e. offset long-code replica, at the end of the previous frame. The rule defined in the specification for every particular raw rate recalculates values of these chips (as binary digits) into the positions of erased PCGs. Figure 11.6, where identical numbers mark replicated PCGs, and shaded and dashed rectangles correspond to transmitted and erased PCGs, shows an example transmission pattern for raw rate 4.8 kbps. We may treat the sequence after the data burst randomizer as ternary with symbols f1g and zero corresponding to active and pausing transmitter, respectively.

11.3.4.2 Access channel

MS uses an access channel when responding to a notification about an incoming call in the idle state and when it either needs to register on the network or initiate a call. The procedures of framing, convolutional encoding, orthogonal modulation, interleaving and long-code spreading in the access channel are basically similar to those of the reverse traffic channel. Of course, no voice data is transmitted through this channel, so no rate/power control according to voice activity is performed. One of the specific features of the access channel relates to initiating access by MS. Not knowing precisely the propagation conditions in the reverse link, the MS starts by sending probe signals of low strength, gradually increasing the signal level with every next attempt until it obtains the BS confirmation that the connection is established. Probe signals are sent in a burst mode with randomized intervals to reduce the probability of colliding requests from several users, because it is not impossible that at the access stage different MSs have the same long code masks.

11.3.4.3 Reverse link modulation

MS can never use both traffic and access channels simultaneously, therefore there is no need for channel signal summation at the modulator input as there was in the forward channel. Traffic or access channel output is immediately fed in parallel into in-phase/ quadrature branches, differing from those of Figure 11.4 in the following details. First, offsetting short codes PN-1 and PN-2 to identify BS is now needless (every mobile has its own unique identity—long code mask—all over the coverage zone), and even inconvenient