移动无线通信第11章

1 Chapter 10 10.1 A chip rate of 3.84Mcps corresponds to a chip interval of (1/3.84) μsec = 0.26 μsec. As noted at the end of chapter 2, in introducing the RAKE receiver concept, delay spreads greater than the chip interval allow individual multipath rays to be resolved. A delay spread of 0.26 μsec corresponds to a path difference of 0.26μsec x 3x108m/sec = 78

m. At a chip rate of 1.2288 Mcps, the chip interval is (1/1.2288) μsec = 0.81μ sec, corresponding to a path difference of 0.81x300 = 240 m.

10.2 (a) Say a bit rate of R bps, with a bit width of 1/R is initially used. Now double the bit rate to 2R but collect successive pairs of bits and transmit them in parallel, with a width of 1/R each. Each of the two bit steams transmitted at R bps is now equivalent to the one initial stream at R bps. Chip-encoding each of these two bit streams at the same chip rate as the initial R bps bit stream results in the same spreading gain being made available. The transmission bandwidth is the same in the two cases as well. But it is clear that the two bit streams must be separately distinguishable. This is done by choosing orthogonal codes, similar to the orthogonal codes discussed at the beginning of section 6.2 in chapter 6. (Calling two separate code sequences ci and cj, orthogonality means ci • cj = 0, i ≠j .) But, as noted in (6.2), pseudorandom codes used in practice are not completely orthogonal, and, just as in the case of different users using pseudorandom orthogonal codes discussed in previous descriptions of CDMA systems, interference will result. Since the interference due to incomplete orthogonality is, in this case, due to interference between codes of the same user, this is called self-interference. Although the example used here is one of doubling the bit rate, it is clear that the same analysis applies to more than two parallel bit streams, or to multicode transmission in general. (See (b) following.) (b) Serial-to-parallel conversion: collect three successive bits, transmitted at a rate of 3R and transmit them in parallel, widening each bit to 1/R, hence at a rate of R bps. (c) Each bit stream must be separately decoded, hence a RAKE receiver is required for each. Summing multiple quasi-uncorrelated bit streams and using the sum to drive the modulator must result in a wider envelope variation at the output.

10.3 The object with a RAKE receiver is to receive separate, distinguishable replicas of a signal. This means, as discussed in chapter 2, using a digital signal much narrower than the delay spread. This is accomplished in CDMA systems by the chip-encoding process. Referring back to (2.48) and (2-49) in chapter 2, this results in a signal whose transmission bandwidth is greater than the coherence bandwidth, or, equivalently, a condition under which frequency-selective fading results.

10.4 (a) The use of rate-1/2 convolutional encoding, with tail bits required, plus any additional error correction required, results in a transmission rate more than double the actual data rate. A local-area environment, with much shorter propagation distances, provides higher received-power, hence higher received-energy conditions, than does a wider-area environment. Hence higher bit rates are possible. (See Sec. 6.4 of chapter 6.) (b) Multicode transmission using six orthogonal codes after serial-to-parallel conversion allows six times the bit rate of 960 kbps to be used. (See problem 10.2 (a) 2

above.) But note that the chip rate is 3.84 Mcps. The spreading gain for a channel transmitting at 960 kbps is then 3840/960 = 4. ( c) As noted in chapters 5 and 6, PSK provides better bit-error performance in interference and noise than does QPSK. The base station, transmitting in the downlink direction, can use higher-power transmission than a mobile in the uplink direction. It can therefore use QPSK transmission, allowing higher bit rates to be used in the downlink direction.

10.5 People engaged in real-time interactive conversation cannot tolerate delays longer than 50-100 msec in transmission of the information packets sent between users. (This was noted many years ago with satellite transmission of voice.) A similar problem arises when successive packets encounter variable delays. Received packets must be aligned to have the same spacing as when transmitted. This means delaying the first packet enough to align all successive packets following. If the realignment delay is too high, this again creates an intolerable problem. In the streaming class case, without two-side interaction taking place (examples- streaming music or video), delay in transmission generally poses no problem. But reconstructing the original bit stream from successive packets transmitted does pose a problem if successive packets are delayed differentially. E-mail is generally considered non-real-time, however: Users generally accept a few seconds of delay in asking for replies to messages transmitted.